A partial melting control on the Zn isotope composition of basalts
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Keywords: zinc isotopes, fractionation, partial melting, boninites, MORB, basalt
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Abstract
Figures and Tables
Figure 1 Bulk rock (a) Zn abundance and (b) isotopic composition as a function of MgO content for basaltic rocks. Data sources are given in Figure S-1, with Baffin Island bulk rock picrite data and the magmatic olivine compositions (assuming a possible range of olivine MgO contents) from McCoy-West et al. (2018). Martian shergottite and nakhlite data (shown as points and as a field) are from Paniello et al. (2012). New MORB data presented here (solid circles) are distinguished from published data (solid stars). |
Figure 2 (a) Zinc isotopic composition and (b) Zn abundance versus estimated degree of partial melting for basaltic rocks (Table 2) versus models for partial melting of harzburgite, lherzolite and metasomatised peridotite, with dots conforming to 1 % increments of melting. Shown in upper panel are the estimated bulk silicate Earth (BSE) average (solid line) and standard deviation (shaded regions) values from Chen et al. (2013) and Sossi et al. (2018). |
Table 1 Zinc isotope and abundance data for boninites and mid-ocean ridge basalts. Major element data and ages for boninites are from Ishizuka et al. (2011, 2014) and for MORB are from Le Roux (2000) and Deng et al. (2018). *Replicate analyses are reported for these samples. |
Table 2 Zinc isotope, abundance data, melting type and extent for terrestrial basalts. |
| Figure 1 | Figure 2 | Table 1 | Table 2 |
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Introduction
Zinc stable isotopes have utility in the study of basaltic rocks, from the examination of contributions of potential mantle and crustal reservoirs with distinct 66Zn/64Zn ratios (e.g., Beunon et al., 2020
Beunon, H., Mattielli, N., Doucet, L.S., Moine, B., Debret, B. (2020) Mantle heterogeneity through Zn systematics in oceanic basalts: Evidence for a deep carbon cycling. Earth-Science Reviews 205, 103174. https://doi.org/10.1016/j.earscirev.2020.103174
), to identification of volatile element loss in planets (e.g., Paniello et al., 2012Paniello, R.C., Day, J.M.D., Moynier, F. (2012) Zinc isotopic evidence for the origin of the Moon. Nature 490, 376–379. https://doi.org/10.1038/nature11507
). Zinc consists of five stable isotopes (64Zn [natural abundance 48.6], 66Zn [27.9], 67Zn [4.1], 68Zn [18.1], and 70Zn [0.6]) which are typically reported in per mil variations relative to a standard (δxZn = [((xZn/64Zn)sample/(xZn/64Zn)JMC-Lyon standard − 1) × 1000], where x typically refers to mass 66 or 68). Studies of basaltic magmatic differentiation series have demonstrated that Zn isotopes are not substantially modified (<0.1 ‰) during fractional crystallisation processes (Chen et al., 2013Chen, H., Savage, P.S., Teng, F.-Z., Helz, R.T., Moynier, F. (2013) Zinc isotope fractionation during magmatic differentiation and the isotopic composition of the bulk Earth. Earth and Planetary Science Letters 369–370, 32–42. https://doi.org/10.1016/j.epsl.2013.02.037
), but that significant fractionation (≥0.1 ‰) in δ66Zn is likely to occur during mantle partial melting (Wang et al., 2017Wang, Z.-Z., Liu, S.-A., Liu, J., Huang, J., Xiao, Y., Chu, Z.-Y., Zhao, X.-M., Tang, L. (2017) Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth’s upper mantle. Geochimica et Cosmochimica Acta 198, 151–167. https://doi.org/10.1016/j.gca.2016.11.014
). Partial melting processes can explain δ66Zn variations in arc lavas (Huang et al., 2018Huang, J., Zhang, X.-C., Chen, S., Tang, L., Wörner, G., Yu, H., Huang, F. (2018) Zinc isotopic systematics of Kamchatka-Aleutian arc magmas controlled by mantle melting. Geochimica et Cosmochimica Acta 238, 85–101. https://doi.org/10.1016/j.gca.2018.07.012
), whereas it has been argued that δ66Zn variations in some mantle-derived basaltic rocks reflect contributions from distinct sources, including recycled carbonate (Beunon et al., 2020Beunon, H., Mattielli, N., Doucet, L.S., Moine, B., Debret, B. (2020) Mantle heterogeneity through Zn systematics in oceanic basalts: Evidence for a deep carbon cycling. Earth-Science Reviews 205, 103174. https://doi.org/10.1016/j.earscirev.2020.103174
; Liu et al., 2022Liu, S.A., Qu, Y.R., Wang, Z.Z., Li, M.L., Yang, C., Li, S.G. (2022) The fate of subducting carbon tracked by Mg and Zn isotopes: A review and new perspectives. Earth-Science Reviews 228, 104010. https://doi.org/10.1016/j.earscirev.2022.104010
).Intraplate volcanic rocks, including ocean island basalts (OIB), were first used to estimate a bulk silicate Earth (BSE) Zn isotopic composition (δ66ZnBSE = +0.28 ± 0.05 ‰; all uncertainties 2 s.d.; Chen et al., 2013
Chen, H., Savage, P.S., Teng, F.-Z., Helz, R.T., Moynier, F. (2013) Zinc isotope fractionation during magmatic differentiation and the isotopic composition of the bulk Earth. Earth and Planetary Science Letters 369–370, 32–42. https://doi.org/10.1016/j.epsl.2013.02.037
), followed by direct analysis of fertile peridotites (+0.30 ± 0.03 ‰; Doucet et al., 2016Doucet, L.S., Mattielli, N., Ionov, D.A., Debouge, W., Golovin, A.V. (2016) Zn isotopic heterogeneity in the mantle: A melting control? Earth and Planetary Science Letters 451, 232–240. https://doi.org/10.1016/j.epsl.2016.06.040
). Subsequently, peridotites and komatiites have been shown to have indistinguishable Zn isotopic compositions (+0.16 ± 0.06 ‰) and so were interpreted to represent a consistent δ66ZnBSE (Sossi et al., 2018Sossi, P.A., Nebel, O., O’Neill, H.S.C., Moynier, F. (2018) Zinc isotope composition of the Earth and its behaviour during planetary accretion. Chemical Geology 477, 73–84. https://doi.org/10.1016/j.chemgeo.2017.12.006
), with slightly more elevated estimates also reported from modern and ancient mantle melts (+0.20 ±0.03 ‰; McCoy-West et al., 2018McCoy-West, A.J., Fitton, J.G., Pons, M.L., Inglis, E.C., Williams, H.M. (2018) The Fe and Zn isotope composition of deep mantle source regions: Insights from Baffin Island picrites. Geochimica et Cosmochimica Acta 238, 542–562. https://doi.org/10.1016/j.gca.2018.07.021
; Doucet et al., 2020Doucet, L.S., Laurent, O., Ionov, D.A., Mattielli, N., Debaille, V., Debouge, W. (2020) Archean lithospheric differentiation: Insights from Fe and Zn isotopes. Geology 48, 1028–1032. https://doi.org/10.1130/G47647.1
), with a similar value established for depleted mid-ocean ridge basalt (MORB) mantle (δ66ZnDMM = +0.20 ± 0.05 ‰; Wang et al., 2017Wang, Z.-Z., Liu, S.-A., Liu, J., Huang, J., Xiao, Y., Chu, Z.-Y., Zhao, X.-M., Tang, L. (2017) Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth’s upper mantle. Geochimica et Cosmochimica Acta 198, 151–167. https://doi.org/10.1016/j.gca.2016.11.014
). Observations of Zn isotope fractionation during partial melting (Wang et al., 2017Wang, Z.-Z., Liu, S.-A., Liu, J., Huang, J., Xiao, Y., Chu, Z.-Y., Zhao, X.-M., Tang, L. (2017) Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth’s upper mantle. Geochimica et Cosmochimica Acta 198, 151–167. https://doi.org/10.1016/j.gca.2016.11.014
; Huang et al., 2018Huang, J., Zhang, X.-C., Chen, S., Tang, L., Wörner, G., Yu, H., Huang, F. (2018) Zinc isotopic systematics of Kamchatka-Aleutian arc magmas controlled by mantle melting. Geochimica et Cosmochimica Acta 238, 85–101. https://doi.org/10.1016/j.gca.2018.07.012
) leave several outstanding questions prior to utilisation of Zn isotopes as tracers of distinct mantle and crustal reservoir contributions, and for estimating the BSE composition, particularly how Zn isotope variations differ in different melting regimes, such as during adiabatic decompression and during water-assisted mantle partial melting.Limited data are available for Zn isotopes in MORB samples and data have yet to be reported for boninites. Boninites are potentially useful samples for understanding the behaviour of Zn during partial melting within the mantle. Unlike MORB, which are formed during adiabatic decompression melting, and hotspot volcanic rocks (OIB, komatiites) that are likely produced through thermal anomalies in the mantle, boninites are widely accepted to form from low pressure melting of previously depleted mantle sources by flux melting of water (e.g., Cameron et al., 1979
Cameron, W.E., Nisbet, E.G., Dietrich, V.J. (1979) Boninites, komatiites and ophiolitic basalts. Nature 280, 550–553. https://doi.org/10.1038/280550a0
; Crawford et al., 1989Crawford, A.J., Falloon, T.J., Green, D.H. (1989) Classification, petrogenesis and tectonic setting of boninites. In: Crawford, A.J. (Ed.) Boninites and Related Rocks. Unwin Hyman, London, 1–49.
). These rocks should therefore have low Zn and relatively isotopically light δ66Zn due to their origin from refractory mantle similar to harzburgite (∼+0.16 ‰; Doucet et al., 2016Doucet, L.S., Mattielli, N., Ionov, D.A., Debouge, W., Golovin, A.V. (2016) Zn isotopic heterogeneity in the mantle: A melting control? Earth and Planetary Science Letters 451, 232–240. https://doi.org/10.1016/j.epsl.2016.06.040
; Wang et al., 2017Wang, Z.-Z., Liu, S.-A., Liu, J., Huang, J., Xiao, Y., Chu, Z.-Y., Zhao, X.-M., Tang, L. (2017) Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth’s upper mantle. Geochimica et Cosmochimica Acta 198, 151–167. https://doi.org/10.1016/j.gca.2016.11.014
). An alternative possibility exists that boninites might show Zn isotope variations due to variable additions of pelagic sedimentary components or altered igneous rocks that are known to have affected them from Sr-Nd-Hf-Pb isotope studies (e.g., Ishizuka et al., 2020Ishizuka, O., Taylor, R.N., Umino, S., Kanayama, K. (2020) Geochemical Evolution of Arc and Slab Following Subduction Initiation: a Record from the Bonin Islands, Japan. Journal of Petrology 61, egaa050. https://doi.org/10.1093/petrology/egaa050
). The first boninite data are presented from Nakoudojima Island and the Bonin Ridge (Ishizuka et al., 2011Ishizuka, O., Tani, K., Reagan, M.K., Kanayama, K., Umino, S., Harigane, Y., Sakamoto, I., Miyajima, Y., Yuasa, M., Dunkley, D.J. (2011) The timescales of subduction initiation and subsequent evolution of an oceanic island arc. Earth and Planetary Science Letters 306, 229–240. https://doi.org/10.1016/j.epsl.2011.04.006
, 2014Ishizuka, O., Umino, S., Taylor, R.N., Kanayama, K. (2014) Evidence for Hydrothermal Activity in the Earliest Stages of Intraoceanic Arc Formation: Implications for Ophiolite-Hosted Hydrothermal Activity. Economic Geology 109, 2159–2178. https://doi.org/10.2113/econgeo.109.8.2159
), Japan, along with a new dataset for MORB from three ocean basins (Atlantic, Indian, Pacific) that greatly expands existing MORB Zn isotope data, to further examine Zn behaviour during partial melting processes.top
Methods and Results
Methods for the separation and analysis of Zn abundances and isotopic composition are described in detail in the Supplementary Information. Five boninite samples from the Izu-Bonin Mariana Arc, Japan, were analysed for Zn isotope and elemental abundances (Table 1) for which bulk rock major- and trace-element data has been published previously (Ishizuka et al., 2011
Ishizuka, O., Tani, K., Reagan, M.K., Kanayama, K., Umino, S., Harigane, Y., Sakamoto, I., Miyajima, Y., Yuasa, M., Dunkley, D.J. (2011) The timescales of subduction initiation and subsequent evolution of an oceanic island arc. Earth and Planetary Science Letters 306, 229–240. https://doi.org/10.1016/j.epsl.2011.04.006
, 2014Ishizuka, O., Umino, S., Taylor, R.N., Kanayama, K. (2014) Evidence for Hydrothermal Activity in the Earliest Stages of Intraoceanic Arc Formation: Implications for Ophiolite-Hosted Hydrothermal Activity. Economic Geology 109, 2159–2178. https://doi.org/10.2113/econgeo.109.8.2159
). These samples were erupted during the Eocene (44–48 Ma), span a range of MgO (4.4 to 15.2 wt. %) and SiO2 contents (55.2 to 62.4 wt. %), and have trace element and isotopic compositions consistent with partial melts originating from depleted mantle sources. Boninites have between 20 and 52 ppm Zn and δ66Zn from +0.17 ± 0.06 to +0.25 ± 0.06 ‰. Zinc isotope compositions are mass dependent and replicate analyses of KH07-2 D43 (Bonin Ridge) and 10100205 (Nakoudojima Island) provided identical results within uncertainties. Boninites have lower Zn abundances than MORB, OIB and most komatiites, and their average δ66Zn (+0.21 ± 0.06 ‰, 2 s.d.) is relatively homogeneous, similar to those measured for komatiites (Fig. S-1). For a given MgO content, boninites have the lowest Zn abundances and lie at the lowest values for δ66Zn of all basaltic rocks (Fig. 1).Table 1 Zinc isotope and abundance data for boninites and mid-ocean ridge basalts. Major element data and ages for boninites are from Ishizuka et al. (2011
Ishizuka, O., Tani, K., Reagan, M.K., Kanayama, K., Umino, S., Harigane, Y., Sakamoto, I., Miyajima, Y., Yuasa, M., Dunkley, D.J. (2011) The timescales of subduction initiation and subsequent evolution of an oceanic island arc. Earth and Planetary Science Letters 306, 229–240. https://doi.org/10.1016/j.epsl.2011.04.006
, 2014)Ishizuka, O., Umino, S., Taylor, R.N., Kanayama, K. (2014) Evidence for Hydrothermal Activity in the Earliest Stages of Intraoceanic Arc Formation: Implications for Ophiolite-Hosted Hydrothermal Activity. Economic Geology 109, 2159–2178. https://doi.org/10.2113/econgeo.109.8.2159
and for MORB are from Le Roux (2000)Le Roux, P.J. (2000) The geochemistry of selected mid-ocean ridge basalts from the Southern mid-Atlantic ridge (40°-55°S). PhD Thesis, University of Cape Town. https://open.uct.ac.za/handle/11427/4207
and Deng et al. (2018)Deng, Z., Moynier, F., Sossi, P.A., Chaussidon, M. (2018) Bridging the depleted MORB mantle and the continental crust using titanium isotopes. Geochemical Perspectives Letters 9, 11–15. https://doi.org/10.7185/geochemlet.1831
.| Sample | Location | Rock Type | Age (Ma) |
Uncer. | SiO2 (wt. %) |
MgO (wt. %) |
Zn (ppm) |
δ66Zn (‰) |
±2σ | δ68Zn (‰) |
±2σ | n |
| 10100202 | Nakoudojima Island | Boninite | 47.81 | 0.17 | 55.6 | 13.4 | 45.3 | 0.22 | 0.02 | 0.40 | 0.03 | 3 |
| 10100215 | Nakoudojima Island | Boninite | 55.2 | 15.2 | 50.7 | 0.17 | 0.06 | 0.34 | 0.09 | 4 | ||
| 10100213 | Nakoudojima Island | Bronzite Andesite | 57.1 | 7.1 | 19.9 | 0.25 | 0.06 | 0.45 | 0.10 | 4 | ||
| 10100205* | Nakoudojima Island | Basaltic Dike | 58.2 | 6.7 | 49.8 | 0.20 | 0.07 | 0.40 | 0.05 | 4 | ||
| 49.3 | 0.18 | 0.04 | 0.37 | 0.04 | 4 | |||||||
| KH07-2 D43* | Bonin Ridge | Submarine boninite | 44.78 | 0.16 | 62.4 | 4.4 | 51.8 | 0.21 | 0.05 | 0.43 | 0.10 | 4 |
| 46.0 | 0.24 | 0.05 | 0.46 | 0.07 | 4 | |||||||
| SWIFT DR32-1-3g | South West Indian Ridge | MORB | 66.2 | 0.29 | 0.03 | 0.61 | 0.06 | 6 | ||||
| SWIFT DR04-2-3g | South West Indian Ridge | MORB | 6.2 | 0.26 | 0.06 | 0.51 | 0.13 | 4 | ||||
| SWIFT DR06-3-6g | South West Indian Ridge | MORB | 6.6 | 78.6 | 0.30 | 0.06 | 0.61 | 0.08 | 5 | |||
| MD57 D2-8 | Central Indian Ridge | MORB | 5.8 | 60.4 | 0.31 | 0.03 | 0.62 | 0.09 | 6 | |||
| MD57 D7-5 | Central Indian Ridge | MORB | 8.1 | 50.2 | 0.29 | 0.04 | 0.56 | 0.07 | 6 | |||
| MD57 D9-4 | Central Indian Ridge | MORB | 59.8 | 0.26 | 0.05 | 0.52 | 0.11 | 5 | ||||
| EW9309 10D-3g | Mid Atlantic Ridge, 40–50°S | MORB | 50.7 | 7.9 | 56.5 | 0.31 | 0.03 | 0.58 | 0.04 | 6 | ||
| EW9309 27D-1g | Mid Atlantic Ridge, 40–50°S | MORB | 49.0 | 8.3 | 40.3 | 0.26 | 0.06 | 0.56 | 0.11 | 5 | ||
| EW9309 3D | S Atlantic (Discovery) | MORB | 50.6 | 7.0 | 0.31 | 0.02 | 0.61 | 0.07 | 3 | |||
| EW9309 20D* | S Atlantic (Discovery) | MORB | 50.8 | 6.9 | 0.29 | 0.09 | 0.59 | 0.23 | 3 | |||
| 0.24 | 0.03 | 0.47 | 0.03 | 2 | ||||||||
| RD87 DR18-102 | North Atlantic (Dosso) | MORB | 7.4 | 54.7 | 0.26 | 0.05 | 0.49 | 0.10 | 6 | |||
| RD87 DR24 | North Atlantic (Dosso) | MORB | 56.4 | 0.27 | 0.06 | 0.50 | 0.11 | 6 | ||||
| RD87 DR29-107 | North Atlantic (Dosso) | MORB | 44.8 | 0.27 | 0.01 | 0.54 | 0.04 | 4 | ||||
| DIVA1 12-2 | Mid Atlantic Ridge, 37–38°N | MORB | 49.6 | 0.35 | 0.03 | 0.72 | 0.04 | 6 | ||||
| DIVA1 13-3 | Mid Atlantic Ridge, 37–38°N | MORB | 7.6 | 60.2 | 0.33 | 0.04 | 0.62 | 0.07 | 6 | |||
| DIVA1 15-5 | Mid Atlantic Ridge, 37–38°N | MORB | 5.9 | 75.9 | 0.27 | 0.06 | 0.52 | 0.11 | 5 | |||
| PAC2 DR32-1g | Pacific-Antarctic Ridge | MORB | 6.2 | 90.6 | 0.24 | 0.01 | 0.49 | 0.03 | 2 | |||
| PAC2 DR37-3g | Pacific-Antarctic Ridge | MORB | 75.3 | 0.24 | 0.06 | 0.48 | 0.09 | 4 | ||||
| PAC2 DR38-1g | Pacific-Antarctic Ridge | MORB | 7.6 | 70.1 | 0.27 | 0.02 | 0.55 | 0.06 | 4 | |||
| SEARISE1 DR04 | East Pacific Rise | MORB | 50.0 | 6.8 | 92.8 | 0.25 | 0.03 | 0.48 | 0.04 | 5 | ||
| SEARISE2 DR03 | East Pacific Rise | MORB | 51.2 | 7.3 | 62.0 | 0.28 | 0.06 | 0.54 | 0.09 | 6 | ||
| CYP 12-34 | East Pacific Rise | MORB | 50.4 | 8.2 | 65.8 | 0.27 | 0.05 | 0.51 | 0.08 | 5 | ||
*Replicate analyses are reported for these samples.
Figure 1 Bulk rock (a) Zn abundance and (b) isotopic composition as a function of MgO content for basaltic rocks. Data sources are given in Figure S-1, with Baffin Island bulk rock picrite data and the magmatic olivine compositions (assuming a possible range of olivine MgO contents) from McCoy-West et al. (2018)
McCoy-West, A.J., Fitton, J.G., Pons, M.L., Inglis, E.C., Williams, H.M. (2018) The Fe and Zn isotope composition of deep mantle source regions: Insights from Baffin Island picrites. Geochimica et Cosmochimica Acta 238, 542–562. https://doi.org/10.1016/j.gca.2018.07.021
. Martian shergottite and nakhlite data (shown as points and as a field) are from Paniello et al. (2012)Paniello, R.C., Day, J.M.D., Moynier, F. (2012) Zinc isotopic evidence for the origin of the Moon. Nature 490, 376–379. https://doi.org/10.1038/nature11507
. New MORB data presented here (solid circles) are distinguished from published data (solid stars).Zinc isotope data are reported for 22 mid-ocean ridge basalts spanning the Atlantic, Indian and Pacific Ocean basins in Table 1. Zinc isotope compositions fall along a mass-dependent slope and span a range in Zn abundances (40 to 93 ppm) yet a restricted range of δ66Zn (+0.28 ± 0.06 ‰; 2 s.d.), with no evidence for systematic variation between ocean basins. These results agree well with the limited range in δ66Zn reported for MORB from the Atlantic and Indian oceans (Fig. S-1; Wang et al., 2017
Wang, Z.-Z., Liu, S.-A., Liu, J., Huang, J., Xiao, Y., Chu, Z.-Y., Zhao, X.-M., Tang, L. (2017) Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth’s upper mantle. Geochimica et Cosmochimica Acta 198, 151–167. https://doi.org/10.1016/j.gca.2016.11.014
), although the Zn abundances span a wider range.top
Discussion
Boninite magmatism and the cause of low Zn and δ66Zn. Partial melting occurs within shallow forearc regions during release of volatiles from the subducting slab into the mantle wedge and drives boninite magmatism (e.g., Ishizuka et al., 2011
Ishizuka, O., Tani, K., Reagan, M.K., Kanayama, K., Umino, S., Harigane, Y., Sakamoto, I., Miyajima, Y., Yuasa, M., Dunkley, D.J. (2011) The timescales of subduction initiation and subsequent evolution of an oceanic island arc. Earth and Planetary Science Letters 306, 229–240. https://doi.org/10.1016/j.epsl.2011.04.006
, 2014Ishizuka, O., Umino, S., Taylor, R.N., Kanayama, K. (2014) Evidence for Hydrothermal Activity in the Earliest Stages of Intraoceanic Arc Formation: Implications for Ophiolite-Hosted Hydrothermal Activity. Economic Geology 109, 2159–2178. https://doi.org/10.2113/econgeo.109.8.2159
; Shervais et al., 2021Shervais, J.W., Reagan, M.K., Godard, M., Prytulak, J., Ryan, J.G., et al. (2021) Magmatic Response to Subduction Initiation, Part II: Boninites and Related Rocks of the Izu-Bonin Arc From IODP Expedition 352. Geochemistry, Geophysics, Geosystems 22, e2020GC009093. https://doi.org/10.1029/2020GC009093
). Such melting produces high MgO (>8 wt. %), SiO2 (52–63 wt. %), and low TiO2 (<0.5 wt. %) melts distinct from komatiites, MORB, OIB or any other type of basaltic rock (e.g., Crawford et al., 1989Crawford, A.J., Falloon, T.J., Green, D.H. (1989) Classification, petrogenesis and tectonic setting of boninites. In: Crawford, A.J. (Ed.) Boninites and Related Rocks. Unwin Hyman, London, 1–49.
), and which can be reproduced in melting experiments mimicking hydrous melting conditions of refractory harzburgite-like lithologies (Van der Laan et al., 1989Van der Laan, S.R., Flower, M.J.F., Koster van Groos, A.F. (1989) Experimental evidence for the origin of boninites: near-liquidus phase relations to 7.5 kbar. In: Crawford, A.J. (Ed.) Boninites and Related Rocks. Unwin Hyman, London, 112–147.
; Falloon and Danyushevsky, 2000Falloon, T.J., Danyushevsky, L.V. (2000) Melting of Refractory Mantle at 1.5, 2 and 2.5 GPa under Anhydrous and H2O-undersaturated Conditions: Implications for the Petrogenesis of High-Ca Boninites and the Influence of Subduction Components on Mantle Melting. Journal of Petrology 41, 257–283. https://doi.org/10.1093/petrology/41.2.257
). Forearc mantle is dominated by highly melt-depleted (residues after >20 % partial melting) harzburgite mantle formed by both modern and ancient melt events and is characterised by low Zn contents (Day and Brown, 2021Day, J.M.D., Brown, D.B. (2021) Ancient Melt-Depletion in Fresh to Strongly Serpentinized Tonga Trench Peridotites. Journal of Petrology 62, egab088. https://doi.org/10.1093/petrology/egab088
), which can only be further melted by additions of fluids or excess temperatures. The fluid-assisted melting of such shallow depleted residues within the melting region of the nascent Izu-Bonin-Mariana subduction system in the Eocene is widely accepted as the cause of boninite magmatism at that time (Ishizuka et al., 2011Ishizuka, O., Tani, K., Reagan, M.K., Kanayama, K., Umino, S., Harigane, Y., Sakamoto, I., Miyajima, Y., Yuasa, M., Dunkley, D.J. (2011) The timescales of subduction initiation and subsequent evolution of an oceanic island arc. Earth and Planetary Science Letters 306, 229–240. https://doi.org/10.1016/j.epsl.2011.04.006
; Shervais et al., 2021Shervais, J.W., Reagan, M.K., Godard, M., Prytulak, J., Ryan, J.G., et al. (2021) Magmatic Response to Subduction Initiation, Part II: Boninites and Related Rocks of the Izu-Bonin Arc From IODP Expedition 352. Geochemistry, Geophysics, Geosystems 22, e2020GC009093. https://doi.org/10.1029/2020GC009093
).Doucet et al. (2016)
Doucet, L.S., Mattielli, N., Ionov, D.A., Debouge, W., Golovin, A.V. (2016) Zn isotopic heterogeneity in the mantle: A melting control? Earth and Planetary Science Letters 451, 232–240. https://doi.org/10.1016/j.epsl.2016.06.040
and Wang et al. (2017)Wang, Z.-Z., Liu, S.-A., Liu, J., Huang, J., Xiao, Y., Chu, Z.-Y., Zhao, X.-M., Tang, L. (2017) Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth’s upper mantle. Geochimica et Cosmochimica Acta 198, 151–167. https://doi.org/10.1016/j.gca.2016.11.014
presented data for refractory harzburgites from a variety of continental lithospheric mantle localities for δ66Zn, showing that they have generally similar Zn abundances to forearc harzburgites (∼40 ppm) and lower δ66Zn (∼0.16 ‰) than lherzolites (∼0.28 ‰; Fig. 1). Furthermore, analyses of minerals from spinel lherzolites indicate that olivine and orthopyroxene have lower δ66Zn than clinopyroxene, or the main carrier of Zn, spinel, which is characterised by heavy δ66Zn (Wang et al., 2017Wang, Z.-Z., Liu, S.-A., Liu, J., Huang, J., Xiao, Y., Chu, Z.-Y., Zhao, X.-M., Tang, L. (2017) Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth’s upper mantle. Geochimica et Cosmochimica Acta 198, 151–167. https://doi.org/10.1016/j.gca.2016.11.014
). From these constraints, it is likely that boninite Zn systematics are a direct consequence of flux melting of low-Zn depleted harzburgites at low pressure.Two potential issues are whether Zn can be affected by subducted pelagic sedimentary components or altered igneous rocks, and the possibility of mineral accumulation processes modifying Zn isotope systematics of boninites. Nakoudojima boninites have 87Sr/86Sri (0.7041–0.7049), 143Nd/144Ndi (0.5127–0.5129) and Pb isotope systematics consistent with variable contamination from pelagic sediment (Ishizuka et al., 2020
Ishizuka, O., Taylor, R.N., Umino, S., Kanayama, K. (2020) Geochemical Evolution of Arc and Slab Following Subduction Initiation: a Record from the Bonin Islands, Japan. Journal of Petrology 61, egaa050. https://doi.org/10.1093/petrology/egaa050
). Drill core samples from the Bonin Ridge (e.g., KH07-2 D43) have more MORB-like 87Sr/86Sri of 0.7031–0.7037 and 143Nd/144Ndi of 0.5131–0.5132 (Ishizuka et al., 2011Ishizuka, O., Tani, K., Reagan, M.K., Kanayama, K., Umino, S., Harigane, Y., Sakamoto, I., Miyajima, Y., Yuasa, M., Dunkley, D.J. (2011) The timescales of subduction initiation and subsequent evolution of an oceanic island arc. Earth and Planetary Science Letters 306, 229–240. https://doi.org/10.1016/j.epsl.2011.04.006
). Despite the limited number of boninite samples studied, the large isotopic variations in Sr-Nd-Pb, yet limited range in Zn isotope ratios, suggest Zn in boninites is likely to be relatively unaffected by subducting sediment or altered igneous rock components. An alternative source for low δ66Zn is accumulation of olivine with isotopically light Zn (McCoy-West et al., 2018McCoy-West, A.J., Fitton, J.G., Pons, M.L., Inglis, E.C., Williams, H.M. (2018) The Fe and Zn isotope composition of deep mantle source regions: Insights from Baffin Island picrites. Geochimica et Cosmochimica Acta 238, 542–562. https://doi.org/10.1016/j.gca.2018.07.021
; Sossi et al., 2018Sossi, P.A., Nebel, O., O’Neill, H.S.C., Moynier, F. (2018) Zinc isotope composition of the Earth and its behaviour during planetary accretion. Chemical Geology 477, 73–84. https://doi.org/10.1016/j.chemgeo.2017.12.006
). This is not the case for boninites examined here, which do not contain significant accumulative olivine. Compared with komatiites, OIB, alkali basalts, basanites, and nephelinites, boninites plot at the lowest Zn and δ66Zn for a given MgO of any terrestrial basaltic partial melt (Fig. 1). New data for MORB extend the range of Zn to abundances that can be as low as in boninites. This may indicate the presence of harzburgite source components in some MORB.Mantle partial melting controls on Zn isotope fractionation.Due to their genesis in an unusual tectonic setting, boninites represent some of the largest-degree partial melts of all modern mantle-derived melts (Supplementary Information, Table 2). There are relationships between δ66Zn and Zn and the extent of partial melting in basaltic rocks (Fig. 2). With decreasing extents of partial melting, both δ66Zn and Zn concentrations tend to increase. An increase in incompatible (D < 1) Zn is expected at lower extents of partial melting, but the cause(s) of δ66Zn increases are less clear. For example, this relationship could indicate obfuscation of components enriched in Zn with high δ66Zn, such as recycled carbonates, at higher degrees of partial melting (Beunon et al., 2020
Beunon, H., Mattielli, N., Doucet, L.S., Moine, B., Debret, B. (2020) Mantle heterogeneity through Zn systematics in oceanic basalts: Evidence for a deep carbon cycling. Earth-Science Reviews 205, 103174. https://doi.org/10.1016/j.earscirev.2020.103174
). Alternatively, such variations may result from melting variably depleted or fertile mantle peridotite without the obvious presence of recycled components, noting the difficulty in melting refractory mantle sources at low degrees of partial melting. To examine these possibilities, a family of non-modal partial melting models were constructed for Zn using previous modelling compilations (see Table S-1). The first assumes a fertile mantle garnet lherzolite composition (green lines) similar to that considered previously (e.g., Sossi et al., 2018Sossi, P.A., Nebel, O., O’Neill, H.S.C., Moynier, F. (2018) Zinc isotope composition of the Earth and its behaviour during planetary accretion. Chemical Geology 477, 73–84. https://doi.org/10.1016/j.chemgeo.2017.12.006
); the second is a refractory mantle spinel harzburgite (red lines), and the third is a metasomatised mantle composition (blue lines) (Fig. 2).Table 2 Zinc isotope, abundance data, melting type and extent for terrestrial basalts.
Figure 2 (a) Zinc isotopic composition and (b) Zn abundance versus estimated degree of partial melting for basaltic rocks (Table 2) versus models for partial melting of harzburgite, lherzolite and metasomatised peridotite, with dots conforming to 1 % increments of melting. Shown in upper panel are the estimated bulk silicate Earth (BSE) average (solid line) and standard deviation (shaded regions) values from Chen et al. (2013)
Chen, H., Savage, P.S., Teng, F.-Z., Helz, R.T., Moynier, F. (2013) Zinc isotope fractionation during magmatic differentiation and the isotopic composition of the bulk Earth. Earth and Planetary Science Letters 369–370, 32–42. https://doi.org/10.1016/j.epsl.2013.02.037
and Sossi et al. (2018)Sossi, P.A., Nebel, O., O’Neill, H.S.C., Moynier, F. (2018) Zinc isotope composition of the Earth and its behaviour during planetary accretion. Chemical Geology 477, 73–84. https://doi.org/10.1016/j.chemgeo.2017.12.006
.Of interest for boninite petrogenesis is the refractory mantle harzburgite model. Assuming a starting composition similar to harzburgites presented by Doucet et al. (2016)
Doucet, L.S., Mattielli, N., Ionov, D.A., Debouge, W., Golovin, A.V. (2016) Zn isotopic heterogeneity in the mantle: A melting control? Earth and Planetary Science Letters 451, 232–240. https://doi.org/10.1016/j.epsl.2016.06.040
this model yields Zn and δ66Zn values within uncertainty of measured boninite compositions and expected degrees of partial melting (Fig. 2). The model supports the low Zn and δ66Zn of boninites reflecting partial melting of refractory forearc mantle peridotites. MORB lie slightly above the partial melting estimates for spinel harzburgite, yet are also consistent with generation from a relatively refractory mantle source, as would be expected for melts of DMM. Conversely, the fertile lherzolite mantle composition of Doucet et al. (2016)Doucet, L.S., Mattielli, N., Ionov, D.A., Debouge, W., Golovin, A.V. (2016) Zn isotopic heterogeneity in the mantle: A melting control? Earth and Planetary Science Letters 451, 232–240. https://doi.org/10.1016/j.epsl.2016.06.040
can reproduce the Zn abundances observed in OIB and komatiites, but not the δ66Zn of komatiites, where a low δ66Zn source, more akin to refractory harzburgite, better matches komatiite data. OIB span a range of compositions that could be explained by mixtures of both refractory and fertile peridotite sources. Conversely, low-degree partial melts like alkali basalts, basanites and nephelinites from eastern China (Wang et al., 2018Wang, Z.-Z., Liu, S.-A., Chen, L.-H., Li, S.-G., Zeng, G. (2018) Compositional transition in natural alkaline lavas through silica-undersaturated melt–lithosphere interaction. Geology 46, 771–774. https://doi.org/10.1130/G45145.1
) require fertile melt compositions, but their high Zn abundances also permit contributions from hybridised metasomatised peridotite mantle sources.Mantle-derived basaltic rocks can span ∼0.4 ‰ variation in δ66Zn. In the absence of other evidence, such as O-Ca-Sr-Nd-Os-Pb isotopic variability reflecting distinct mantle or crustal components, much of the variation in Zn isotopes can be attributed to different extents of partial melting of variably fertile and refractory peridotite mantle sources. Studies using Zn isotopes to examine potential carbonate or enriched recycled mantle sources in OIB and related rocks should consider the potential for partial melting control on Zn isotope fractionation. Relatively low degree partial melts are likely to sample more extreme end member compositions for Zn. The highest degree partial melts examined are MORB, boninites and komatiites, and these necessarily sample the largest regions of mantle and so are most likely to express mantle composition. Based on data trends and models, these magmatic rocks appear to sample mantle sources akin to harzburgite mantle for δ66Zn. In this sense, high degree partial melts from ancient (komatiites) and modern (boninites, MORB) magmatism conform to the value determined by Sossi et al. (2018)
Sossi, P.A., Nebel, O., O’Neill, H.S.C., Moynier, F. (2018) Zinc isotope composition of the Earth and its behaviour during planetary accretion. Chemical Geology 477, 73–84. https://doi.org/10.1016/j.chemgeo.2017.12.006
of +0.16 ± 0.06 ‰, consistent with BSE being perhaps up to +0.2 ‰ (e.g., McCoy-West et al., 2018McCoy-West, A.J., Fitton, J.G., Pons, M.L., Inglis, E.C., Williams, H.M. (2018) The Fe and Zn isotope composition of deep mantle source regions: Insights from Baffin Island picrites. Geochimica et Cosmochimica Acta 238, 542–562. https://doi.org/10.1016/j.gca.2018.07.021
; Doucet et al., 2020Doucet, L.S., Laurent, O., Ionov, D.A., Mattielli, N., Debaille, V., Debouge, W. (2020) Archean lithospheric differentiation: Insights from Fe and Zn isotopes. Geology 48, 1028–1032. https://doi.org/10.1130/G47647.1
).It has been shown that basaltic rocks from the Moon have δ66Zn values ∼1 ‰ heavier and Zn abundances >20 times less than terrestrial basalts, interpreted to reflect volatile loss during lunar formation (Paniello et al., 2012
Paniello, R.C., Day, J.M.D., Moynier, F. (2012) Zinc isotopic evidence for the origin of the Moon. Nature 490, 376–379. https://doi.org/10.1038/nature11507
). Such large differences between basalts from the Earth and Moon cannot be reconciled by differences in extents of partial melting. Paniello et al. (2012)Paniello, R.C., Day, J.M.D., Moynier, F. (2012) Zinc isotopic evidence for the origin of the Moon. Nature 490, 376–379. https://doi.org/10.1038/nature11507
also presented Zn isotope data for martian meteorites (Fig. 1). Shergottites are relatively high-degree basaltic partial melts of depleted and enriched mantle sources in Mars, while nakhlites have been considered akin to rejuvenated lava, which are formed by lower degrees of partial melting, meaning the closest analogues to these samples are plume-derived melts on Earth (Day et al., 2018Day, J.M.D., Tait, K.T., Udry, A., Moynier, F., Liu, Y., Neal, C.R. (2018) Martian magmatism from plume metasomatized mantle. Nature Communications 9, 4799. https://doi.org/10.1038/s41467-018-07191-0
). Martian meteorites have low Zn contents and δ66Zn in the upper range of terrestrial plume-derived lavas (OIB, komatiites) for a given MgO content. Such results are consistent with lower Zn contents in bulk silicate Mars and may suggest a slightly heavier bulk silicate Mars value for δ66Zn compared to Earth. This conclusion needs to be confirmed by further analyses of martian meteorites.top
Acknowledgements
Reviews by L. Doucet and an anonymous referee are appreciated. An Institut de Physique du Globe de Paris Visiting Professor position (JMDD), ERC grant agreement No. 101001282 (METAL), the UnivEarthS Labex program (numbers: ANR-10-LABX-0023 and ANR-11-IDEX-0005-02), the IPGP multidisciplinary program PARI, and the Region île-de-France DIM ACAV + and SESAME Grants no. 12015908 supported this work.
Editor: Anat Shahar
top
References
Beunon, H., Mattielli, N., Doucet, L.S., Moine, B., Debret, B. (2020) Mantle heterogeneity through Zn systematics in oceanic basalts: Evidence for a deep carbon cycling. Earth-Science Reviews 205, 103174. https://doi.org/10.1016/j.earscirev.2020.103174
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Zinc stable isotopes have utility in the study of basaltic rocks, from the examination of contributions of potential mantle and crustal reservoirs with distinct 66Zn/64Zn ratios (e.g., Beunon et al., 2020), to identification of volatile element loss in planets (e.g., Paniello et al., 2012).
View in article
Partial melting processes can explain δ66Zn variations in arc lavas (Huang et al., 2018), whereas it has been argued that δ66Zn variations in some mantle-derived basaltic rocks reflect contributions from distinct sources, including recycled carbonate (Beunon et al., 2020; Liu et al., 2022).
View in article
For example, this relationship could indicate obfuscation of components enriched in Zn with high δ66Zn, such as recycled carbonates, at higher degrees of partial melting (Beunon et al., 2020).
View in article
Cameron, W.E., Nisbet, E.G., Dietrich, V.J. (1979) Boninites, komatiites and ophiolitic basalts. Nature 280, 550–553. https://doi.org/10.1038/280550a0
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Unlike MORB, which are formed during adiabatic decompression melting, and hotspot volcanic rocks (OIB, komatiites) that are likely produced through thermal anomalies in the mantle, boninites are widely accepted to form from low pressure melting of previously depleted mantle sources by flux melting of water (e.g., Cameron et al., 1979; Crawford et al., 1989).
View in article
Chen, H., Savage, P.S., Teng, F.-Z., Helz, R.T., Moynier, F. (2013) Zinc isotope fractionation during magmatic differentiation and the isotopic composition of the bulk Earth. Earth and Planetary Science Letters 369–370, 32–42. https://doi.org/10.1016/j.epsl.2013.02.037
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Studies of basaltic magmatic differentiation series have demonstrated that Zn isotopes are not substantially modified (<0.1 ‰) during fractional crystallisation processes (Chen et al., 2013), but that significant fractionation (≥0.1 ‰) in δ66Zn is likely to occur during mantle partial melting (Wang et al., 2017).
View in article
Intraplate volcanic rocks, including ocean island basalts (OIB), were first used to estimate a bulk silicate Earth (BSE) Zn isotopic composition (δ66ZnBSE = +0.28 ± 0.05 ‰; all uncertainties 2 s.d.; Chen et al., 2013), followed by direct analysis of fertile peridotites (+0.30 ± 0.03 ‰; Doucet et al., 2016).
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Chen et al. (2013); Wang et al. (2017).
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Shown in upper panel are the estimated bulk silicate Earth (BSE) average (solid line) and standard deviation (shaded regions) values from Chen et al. (2013) and Sossi et al. (2018).
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Crawford, A.J., Falloon, T.J., Green, D.H. (1989) Classification, petrogenesis and tectonic setting of boninites. In: Crawford, A.J. (Ed.) Boninites and Related Rocks. Unwin Hyman, London, 1–49.
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Unlike MORB, which are formed during adiabatic decompression melting, and hotspot volcanic rocks (OIB, komatiites) that are likely produced through thermal anomalies in the mantle, boninites are widely accepted to form from low pressure melting of previously depleted mantle sources by flux melting of water (e.g., Cameron et al., 1979; Crawford et al., 1989).
View in article
Such melting produces high MgO (>8 wt. %), SiO2 (52–63 wt. %), and low TiO2 (<0.5 wt. %) melts distinct from komatiites, MORB, OIB or any other type of basaltic rock (e.g., Crawford et al., 1989), and which can be reproduced in melting experiments mimicking hydrous melting conditions of refractory harzburgite-like lithologies (Van der Laan et al., 1989; Falloon and Danyushevsky, 2000).
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Day, J.M.D., Brown, D.B. (2021) Ancient Melt-Depletion in Fresh to Strongly Serpentinized Tonga Trench Peridotites. Journal of Petrology 62, egab088. https://doi.org/10.1093/petrology/egab088
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Forearc mantle is dominated by highly melt-depleted (residues after >20 % partial melting) harzburgite mantle formed by both modern and ancient melt events and is characterised by low Zn contents (Day and Brown, 2021), which can only be further melted by additions of fluids or excess temperatures.
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Day, J.M.D., Tait, K.T., Udry, A., Moynier, F., Liu, Y., Neal, C.R. (2018) Martian magmatism from plume metasomatized mantle. Nature Communications 9, 4799. https://doi.org/10.1038/s41467-018-07191-0
Show in context
Shergottites are relatively high-degree basaltic partial melts of depleted and enriched mantle sources in Mars, while nakhlites have been considered akin to rejuvenated lava, which are formed by lower degrees of partial melting, meaning the closest analogues to these samples are plume-derived melts on Earth (Day et al., 2018).
View in article
Deng, Z., Moynier, F., Sossi, P.A., Chaussidon, M. (2018) Bridging the depleted MORB mantle and the continental crust using titanium isotopes. Geochemical Perspectives Letters 9, 11–15. https://doi.org/10.7185/geochemlet.1831
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Major element data and ages for boninites are from Ishizuka et al. (2011, 2014) and for MORB are from Le Roux (2000) and Deng et al. (2018).
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Doucet, L.S., Mattielli, N., Ionov, D.A., Debouge, W., Golovin, A.V. (2016) Zn isotopic heterogeneity in the mantle: A melting control? Earth and Planetary Science Letters 451, 232–240. https://doi.org/10.1016/j.epsl.2016.06.040
Show in context
Intraplate volcanic rocks, including ocean island basalts (OIB), were first used to estimate a bulk silicate Earth (BSE) Zn isotopic composition (δ66ZnBSE = +0.28 ± 0.05 ‰; all uncertainties 2 s.d.; Chen et al., 2013), followed by direct analysis of fertile peridotites (+0.30 ± 0.03 ‰; Doucet et al., 2016).
View in article
These rocks should therefore have low Zn and relatively isotopically light δ66Zn due to their origin from refractory mantle similar to harzburgite (∼+0.16 ‰; Doucet et al., 2016; Wang et al., 2017).
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Doucet et al. (2016) and Wang et al. (2017) presented data for refractory harzburgites from a variety of continental lithospheric mantle localities for δ66Zn, showing that they have generally similar Zn abundances to forearc harzburgites (∼40 ppm) and lower δ66Zn (∼0.16 ‰) than lherzolites (∼0.28 ‰; Fig. 1).
View in article
Of interest for boninite petrogenesis is the refractory mantle harzburgite model. Assuming a starting composition similar to harzburgites presented by Doucet et al. (2016) this model yields Zn and δ66Zn values within uncertainty of measured boninite compositions and expected degrees of partial melting (Fig. 2).
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Conversely, the fertile lherzolite mantle composition of Doucet et al. (2016) can reproduce the Zn abundances observed in OIB and komatiites, but not the δ66Zn of komatiites, where a low δ66Zn source, more akin to refractory harzburgite, better matches komatiite data.
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Doucet, L.S., Laurent, O., Ionov, D.A., Mattielli, N., Debaille, V., Debouge, W. (2020) Archean lithospheric differentiation: Insights from Fe and Zn isotopes. Geology 48, 1028–1032. https://doi.org/10.1130/G47647.1
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Subsequently, peridotites and komatiites have been shown to have indistinguishable Zn isotopic compositions (+0.16 ± 0.06 ‰) and so were interpreted to represent a consistent δ66ZnBSE (Sossi et al., 2018), with slightly more elevated estimates also reported from modern and ancient mantle melts (+0.20 ±0.03 ‰; McCoy-West et al., 2018; Doucet et al., 2020), with a similar value established for depleted mid-ocean ridge basalt (MORB) mantle (δ66ZnDMM = +0.20 ± 0.05 ‰; Wang et al., 2017).
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In this sense, high degree partial melts from ancient (komatiites) and modern (boninites, MORB) magmatism conform to the value determined by Sossi et al. (2018) of +0.16 ± 0.06 ‰, consistent with BSE being perhaps up to +0.2 ‰ (e.g., McCoy-West et al., 2018; Doucet et al., 2020).
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Falloon, T.J., Danyushevsky, L.V. (2000) Melting of Refractory Mantle at 1.5, 2 and 2.5 GPa under Anhydrous and H2O-undersaturated Conditions: Implications for the Petrogenesis of High-Ca Boninites and the Influence of Subduction Components on Mantle Melting. Journal of Petrology 41, 257–283. https://doi.org/10.1093/petrology/41.2.257
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Such melting produces high MgO (>8 wt. %), SiO2 (52–63 wt. %), and low TiO2 (<0.5 wt. %) melts distinct from komatiites, MORB, OIB or any other type of basaltic rock (e.g., Crawford et al., 1989), and which can be reproduced in melting experiments mimicking hydrous melting conditions of refractory harzburgite-like lithologies (Van der Laan et al., 1989; Falloon and Danyushevsky, 2000).
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Herzberg, C. (1992) Depth and degree of melting of komatiites. Journal of Geophysical Research: Solid Earth 97, 4521–4540. https://doi.org/10.1029/91JB03066
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Sossi et al. (2018); Herzberg (1992).
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Huang, J., Zhang, X.-C., Chen, S., Tang, L., Wörner, G., Yu, H., Huang, F. (2018) Zinc isotopic systematics of Kamchatka-Aleutian arc magmas controlled by mantle melting. Geochimica et Cosmochimica Acta 238, 85–101. https://doi.org/10.1016/j.gca.2018.07.012
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Partial melting processes can explain δ66Zn variations in arc lavas (Huang et al., 2018), whereas it has been argued that δ66Zn variations in some mantle-derived basaltic rocks reflect contributions from distinct sources, including recycled carbonate (Beunon et al., 2020; Liu et al., 2022).
View in article
Observations of Zn isotope fractionation during partial melting (Wang et al., 2017; Huang et al., 2018) leave several outstanding questions prior to utilisation of Zn isotopes as tracers of distinct mantle and crustal reservoir contributions, and for estimating the BSE composition, particularly how Zn isotope variations differ in different melting regimes, such as during adiabatic decompression and during water-assisted mantle partial melting.
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Ishizuka, O., Tani, K., Reagan, M.K., Kanayama, K., Umino, S., Harigane, Y., Sakamoto, I., Miyajima, Y., Yuasa, M., Dunkley, D.J. (2011) The timescales of subduction initiation and subsequent evolution of an oceanic island arc. Earth and Planetary Science Letters 306, 229–240. https://doi.org/10.1016/j.epsl.2011.04.006
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The first boninite data are presented from Nakoudojima Island and the Bonin Ridge (Ishizuka et al., 2011, 2014), Japan, along with a new dataset for MORB from three ocean basins (Atlantic, Indian, Pacific) that greatly expands existing MORB Zn isotope data, to further examine Zn behaviour during partial melting processes.
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Five boninite samples from the Izu-Bonin Mariana Arc, Japan, were analysed for Zn isotope and elemental abundances (Table 1) for which bulk rock major- and trace-element data has been published previously (Ishizuka et al., 2011, 2014).
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Major element data and ages for boninites are from Ishizuka et al. (2011, 2014) and for MORB are from Le Roux (2000) and Deng et al. (2018).
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Partial melting occurs within shallow forearc regions during release of volatiles from the subducting slab into the mantle wedge and drives boninite magmatism (e.g., Ishizuka et al., 2011, 2014; Shervais et al., 2021).
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The fluid-assisted melting of such shallow depleted residues within the melting region of the nascent Izu-Bonin-Mariana subduction system in the Eocene is widely accepted as the cause of boninite magmatism at that time (Ishizuka et al., 2011; Shervais et al., 2021).
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Drill core samples from the Bonin Ridge (e.g., KH07-2 D43) have more MORB-like 87Sr/86Sri of 0.7031–0.7037 and 143Nd/144Ndi of 0.5131–0.5132 (Ishizuka et al., 2011).
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Ishizuka, O., Umino, S., Taylor, R.N., Kanayama, K. (2014) Evidence for Hydrothermal Activity in the Earliest Stages of Intraoceanic Arc Formation: Implications for Ophiolite-Hosted Hydrothermal Activity. Economic Geology 109, 2159–2178. https://doi.org/10.2113/econgeo.109.8.2159
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The first boninite data are presented from Nakoudojima Island and the Bonin Ridge (Ishizuka et al., 2011, 2014), Japan, along with a new dataset for MORB from three ocean basins (Atlantic, Indian, Pacific) that greatly expands existing MORB Zn isotope data, to further examine Zn behaviour during partial melting processes.
View in article
Five boninite samples from the Izu-Bonin Mariana Arc, Japan, were analysed for Zn isotope and elemental abundances (Table 1) for which bulk rock major- and trace-element data has been published previously (Ishizuka et al., 2011, 2014).
View in article
Major element data and ages for boninites are from Ishizuka et al. (2011, 2014) and for MORB are from Le Roux (2000) and Deng et al. (2018).
View in article
Partial melting occurs within shallow forearc regions during release of volatiles from the subducting slab into the mantle wedge and drives boninite magmatism (e.g., Ishizuka et al., 2011, 2014; Shervais et al., 2021).
View in article
Ishizuka, O., Taylor, R.N., Umino, S., Kanayama, K. (2020) Geochemical Evolution of Arc and Slab Following Subduction Initiation: a Record from the Bonin Islands, Japan. Journal of Petrology 61, egaa050. https://doi.org/10.1093/petrology/egaa050
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An alternative possibility exists that boninites might show Zn isotope variations due to variable additions of pelagic sedimentary components or altered igneous rocks that are known to have affected them from Sr-Nd-Hf-Pb isotope studies (e.g., Ishizuka et al., 2020).
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Nakoudojima boninites have 87Sr/86Sri (0.7041–0.7049), 143Nd/144Ndi (0.5127–0.5129) and Pb isotope systematics consistent with variable contamination from pelagic sediment (Ishizuka et al., 2020).
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Le Roux, P.J. (2000) The geochemistry of selected mid-ocean ridge basalts from the Southern mid-Atlantic ridge (40°-55°S). PhD Thesis, University of Cape Town. https://open.uct.ac.za/handle/11427/4207
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Major element data and ages for boninites are from Ishizuka et al. (2011, 2014) and for MORB are from Le Roux (2000) and Deng et al. (2018).
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Liu, S.A., Qu, Y.R., Wang, Z.Z., Li, M.L., Yang, C., Li, S.G. (2022) The fate of subducting carbon tracked by Mg and Zn isotopes: A review and new perspectives. Earth-Science Reviews 228, 104010. https://doi.org/10.1016/j.earscirev.2022.104010
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Partial melting processes can explain δ66Zn variations in arc lavas (Huang et al., 2018), whereas it has been argued that δ66Zn variations in some mantle-derived basaltic rocks reflect contributions from distinct sources, including recycled carbonate (Beunon et al., 2020; Liu et al., 2022).
View in article
McCoy-West, A.J., Fitton, J.G., Pons, M.L., Inglis, E.C., Williams, H.M. (2018) The Fe and Zn isotope composition of deep mantle source regions: Insights from Baffin Island picrites. Geochimica et Cosmochimica Acta 238, 542–562. https://doi.org/10.1016/j.gca.2018.07.021
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Subsequently, peridotites and komatiites have been shown to have indistinguishable Zn isotopic compositions (+0.16 ± 0.06 ‰) and so were interpreted to represent a consistent δ66ZnBSE (Sossi et al., 2018), with slightly more elevated estimates also reported from modern and ancient mantle melts (+0.20 ±0.03 ‰; McCoy-West et al., 2018; Doucet et al., 2020), with a similar value established for depleted mid-ocean ridge basalt (MORB) mantle (δ66ZnDMM = +0.20 ± 0.05 ‰; Wang et al., 2017).
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Data sources are given in Figure S-1, with Baffin Island bulk rock picrite data and the magmatic olivine compositions (assuming a possible range of olivine MgO contents) from McCoy-West et al. (2018).
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An alternative source for low δ66Zn is accumulation of olivine with isotopically light Zn (McCoy-West et al., 2018; Sossi et al., 2018).
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In this sense, high degree partial melts from ancient (komatiites) and modern (boninites, MORB) magmatism conform to the value determined by Sossi et al. (2018) of +0.16 ± 0.06 ‰, consistent with BSE being perhaps up to +0.2 ‰ (e.g., McCoy-West et al., 2018; Doucet et al., 2020).
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Paniello, R.C., Day, J.M.D., Moynier, F. (2012) Zinc isotopic evidence for the origin of the Moon. Nature 490, 376–379. https://doi.org/10.1038/nature11507
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Zinc stable isotopes have utility in the study of basaltic rocks, from the examination of contributions of potential mantle and crustal reservoirs with distinct 66Zn/64Zn ratios (e.g., Beunon et al., 2020), to identification of volatile element loss in planets (e.g., Paniello et al., 2012).
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Martian shergottite and nakhlite data (shown as points and as a field) are from Paniello et al. (2012).
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It has been shown that basaltic rocks from the Moon have δ66Zn values ∼1 ‰ heavier and Zn abundances >20 times less than terrestrial basalts, interpreted to reflect volatile loss during lunar formation (Paniello et al., 2012).
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Paniello et al. (2012) also presented Zn isotope data for martian meteorites (Fig. 1).
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Shervais, J.W., Reagan, M.K., Godard, M., Prytulak, J., Ryan, J.G., et al. (2021) Magmatic Response to Subduction Initiation, Part II: Boninites and Related Rocks of the Izu-Bonin Arc From IODP Expedition 352. Geochemistry, Geophysics, Geosystems 22, e2020GC009093. https://doi.org/10.1029/2020GC009093
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Partial melting occurs within shallow forearc regions during release of volatiles from the subducting slab into the mantle wedge and drives boninite magmatism (e.g., Ishizuka et al., 2011, 2014; Shervais et al., 2021).
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The fluid-assisted melting of such shallow depleted residues within the melting region of the nascent Izu-Bonin-Mariana subduction system in the Eocene is widely accepted as the cause of boninite magmatism at that time (Ishizuka et al., 2011; Shervais et al., 2021).
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Shervais et al. (2021).
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Sossi, P.A., Nebel, O., O’Neill, H.S.C., Moynier, F. (2018) Zinc isotope composition of the Earth and its behaviour during planetary accretion. Chemical Geology 477, 73–84. https://doi.org/10.1016/j.chemgeo.2017.12.006
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Subsequently, peridotites and komatiites have been shown to have indistinguishable Zn isotopic compositions (+0.16 ± 0.06 ‰) and so were interpreted to represent a consistent δ66ZnBSE (Sossi et al., 2018), with slightly more elevated estimates also reported from modern and ancient mantle melts (+0.20 ±0.03 ‰; McCoy-West et al., 2018; Doucet et al., 2020), with a similar value established for depleted mid-ocean ridge basalt (MORB) mantle (δ66ZnDMM = +0.20 ± 0.05 ‰; Wang et al., 2017).
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An alternative source for low δ66Zn is accumulation of olivine with isotopically light Zn (McCoy-West et al., 2018; Sossi et al., 2018).
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The first assumes a fertile mantle garnet lherzolite composition (green lines) similar to that considered previously (e.g., Sossi et al., 2018); the second is a refractory mantle spinel harzburgite (red lines), and the third is a metasomatised mantle composition (blue lines) (Fig. 2).
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Sossi et al. (2018); Herzberg (1992).
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Shown in upper panel are the estimated bulk silicate Earth (BSE) average (solid line) and standard deviation (shaded regions) values from Chen et al. (2013) and Sossi et al. (2018).
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In this sense, high degree partial melts from ancient (komatiites) and modern (boninites, MORB) magmatism conform to the value determined by Sossi et al. (2018) of +0.16 ± 0.06 ‰, consistent with BSE being perhaps up to +0.2 ‰ (e.g., McCoy-West et al., 2018; Doucet et al., 2020).
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Van der Laan, S.R., Flower, M.J.F., Koster van Groos, A.F. (1989) Experimental evidence for the origin of boninites: near-liquidus phase relations to 7.5 kbar. In: Crawford, A.J. (Ed.) Boninites and Related Rocks. Unwin Hyman, London, 112–147.
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Such melting produces high MgO (>8 wt. %), SiO2 (52–63 wt. %), and low TiO2 (<0.5 wt. %) melts distinct from komatiites, MORB, OIB or any other type of basaltic rock (e.g., Crawford et al., 1989), and which can be reproduced in melting experiments mimicking hydrous melting conditions of refractory harzburgite-like lithologies (Van der Laan et al., 1989; Falloon and Danyushevsky, 2000).
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Wang, Z.-Z., Liu, S.-A., Liu, J., Huang, J., Xiao, Y., Chu, Z.-Y., Zhao, X.-M., Tang, L. (2017) Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth’s upper mantle. Geochimica et Cosmochimica Acta 198, 151–167. https://doi.org/10.1016/j.gca.2016.11.014
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Studies of basaltic magmatic differentiation series have demonstrated that Zn isotopes are not substantially modified (<0.1 ‰) during fractional crystallisation processes (Chen et al., 2013), but that significant fractionation (≥0.1 ‰) in δ66Zn is likely to occur during mantle partial melting (Wang et al., 2017).
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Subsequently, peridotites and komatiites have been shown to have indistinguishable Zn isotopic compositions (+0.16 ± 0.06 ‰) and so were interpreted to represent a consistent δ66ZnBSE (Sossi et al., 2018), with slightly more elevated estimates also reported from modern and ancient mantle melts (+0.20 ±0.03 ‰; McCoy-West et al., 2018; Doucet et al., 2020), with a similar value established for depleted mid-ocean ridge basalt (MORB) mantle (δ66ZnDMM = +0.20 ± 0.05 ‰; Wang et al., 2017).
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Observations of Zn isotope fractionation during partial melting (Wang et al., 2017; Huang et al., 2018) leave several outstanding questions prior to utilisation of Zn isotopes as tracers of distinct mantle and crustal reservoir contributions, and for estimating the BSE composition, particularly how Zn isotope variations differ in different melting regimes, such as during adiabatic decompression and during water-assisted mantle partial melting.
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These rocks should therefore have low Zn and relatively isotopically light δ66Zn due to their origin from refractory mantle similar to harzburgite (∼+0.16 ‰; Doucet et al., 2016; Wang et al., 2017).
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These results agree well with the limited range in δ66Zn reported for MORB from the Atlantic and Indian oceans (Fig. S-1; Wang et al., 2017), although the Zn abundances span a wider range.
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Doucet et al. (2016) and Wang et al. (2017) presented data for refractory harzburgites from a variety of continental lithospheric mantle localities for δ66Zn, showing that they have generally similar Zn abundances to forearc harzburgites (∼40 ppm) and lower δ66Zn (∼0.16 ‰) than lherzolites (∼0.28 ‰; Fig. 1).
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Furthermore, analyses of minerals from spinel lherzolites indicate that olivine and orthopyroxene have lower δ66Zn than clinopyroxene, or the main carrier of Zn, spinel, which is characterised by heavy δ66Zn (Wang et al., 2017).
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This study; Wang et al. (2017).
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Chen et al. (2013); Wang et al. (2017).
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Wang, Z.-Z., Liu, S.-A., Chen, L.-H., Li, S.-G., Zeng, G. (2018) Compositional transition in natural alkaline lavas through silica-undersaturated melt–lithosphere interaction. Geology 46, 771–774. https://doi.org/10.1130/G45145.1
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Conversely, low-degree partial melts like alkali basalts, basanites and nephelinites from eastern China (Wang et al., 2018) require fertile melt compositions, but their high Zn abundances also permit contributions from hybridised metasomatised peridotite mantle sources.
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Wang et al. (2018).
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Wang et al. (2018).
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Wang et al. (2018).
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Supplementary Information
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Figures
Figure 1 Bulk rock (a) Zn abundance and (b) isotopic composition as a function of MgO content for basaltic rocks. Data sources are given in Figure S-1, with Baffin Island bulk rock picrite data and the magmatic olivine compositions (assuming a possible range of olivine MgO contents) from McCoy-West et al. (2018)
McCoy-West, A.J., Fitton, J.G., Pons, M.L., Inglis, E.C., Williams, H.M. (2018) The Fe and Zn isotope composition of deep mantle source regions: Insights from Baffin Island picrites. Geochimica et Cosmochimica Acta 238, 542–562. https://doi.org/10.1016/j.gca.2018.07.021
. Martian shergottite and nakhlite data (shown as points and as a field) are from Paniello et al. (2012)Paniello, R.C., Day, J.M.D., Moynier, F. (2012) Zinc isotopic evidence for the origin of the Moon. Nature 490, 376–379. https://doi.org/10.1038/nature11507
. New MORB data presented here (solid circles) are distinguished from published data (solid stars).
Figure 2 (a) Zinc isotopic composition and (b) Zn abundance versus estimated degree of partial melting for basaltic rocks (Table 2) versus models for partial melting of harzburgite, lherzolite and metasomatised peridotite, with dots conforming to 1 % increments of melting. Shown in upper panel are the estimated bulk silicate Earth (BSE) average (solid line) and standard deviation (shaded regions) values from Chen et al. (2013)
Chen, H., Savage, P.S., Teng, F.-Z., Helz, R.T., Moynier, F. (2013) Zinc isotope fractionation during magmatic differentiation and the isotopic composition of the bulk Earth. Earth and Planetary Science Letters 369–370, 32–42. https://doi.org/10.1016/j.epsl.2013.02.037
and Sossi et al. (2018)Sossi, P.A., Nebel, O., O’Neill, H.S.C., Moynier, F. (2018) Zinc isotope composition of the Earth and its behaviour during planetary accretion. Chemical Geology 477, 73–84. https://doi.org/10.1016/j.chemgeo.2017.12.006
.