Archean age and radiogenic source for the world’s oldest emeralds
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Abstract
Figures and Tables
Figure 1 (a) Map of major dated emerald deposits, with the studied deposits highlighted in red (Muzo) and green (Gravelotte). Listed ages come from 87Rb-87Sr whole rock dating (Australia), U-Pb apatite dating (Austria), Ar-Ar mica dating (Afghanistan, Brazil, Canada, Colombia, Egypt, Madagascar, Pakistan and Zambia) or inferred from regional terrane ages (USA). Deposit ages are taken from Groat et al. (2002) and the compilations of Giuliani et al. (2019) and Alonso-Perez and Day (2021). (b) Muzo sample 97472-1, dime for scale. (c) Gravelotte sample GE-3, dime for scale. | Figure 2 Sample REE abundance patterns normalised to continental crust (CC) (Rudnick and Gao, 2014). | Figure 3 (a) Sample 1/Sr (in μg/g) plotted against 87Sr/86Sri ratios. Error bars shown for 87Sr/86Sr are 2 s.e. The three most radiogenic Gravelotte emeralds were likely reset and were excluded from the isochron (“Gravelotte excluded”; see main text for details). (b) 87Rb/86Sr plotted against 87Sr/86Sr for Gravelotte Emeralds. Error bars are 2 s.e. for the 87Sr/86Sr ratios and 7.07 % for 87Rb/86Sr ratios. The straight line is an errorchron (MSWD = 11) for the Gravelotte emeralds. (c) Rb-Sr data for Muzo (this study) and Peñas Blancas emeralds (Vidal et al., 1992). The straight line is an isochron (MSWD = 1.8) for both datasets. All isochron calculations were performed using Isoplot 3.0 (Ludwig, 2003). | Table 1 Rubidium-strontium isotope systematics of emeralds and replicate digestions of USGS standard reference material BHVO-2, with initial 87Sr/86Sr ratios for the Gravelotte samples at 700 Ma (Pan-African) and 2960 Ma (Deposit Age). |
Figure 1 | Figure 2 | Figure 3 | Table 1 |
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Introduction
Emeralds are a variety of beryl (Be3Al2Si6O18) that are valued for their vibrant green colour, derived from the chromophore elements V, Cr and Fe. Emeralds are thought to form by the interaction of Be-enriched magmatic or aqueous fluids or hydrous evolved melts with V- and Cr-rich ultramafic or sedimentary country rocks (e.g., Giuliani et al., 2019
Giuliani, G., Groat, L.A., Marshall, D., Fallick, A.E., Branquet, Y. (2019) Emerald Deposits: A Review and Enhanced Classification. Minerals, 105, 1–63. https://doi.org/10.3390/min9020105
). Emeralds are important gemstones for examining crustal processes because, compared to other gems, they contain high abundances of trace elements that can reveal their provenance (e.g., Aurisicchio et al., 2018Aurisicchio, C., Conte, A.M., Medeghini, L., Ottolini, L., De Vito, C. (2018) Major and trace element geochemistry of emerald from several deposits: Implications for genetic models and classification schemes. Ore Geology Reviews 94, 351–366. https://doi.org/10.1016/j.oregeorev.2018.02.001
; Giuliani et al., 2019Giuliani, G., Groat, L.A., Marshall, D., Fallick, A.E., Branquet, Y. (2019) Emerald Deposits: A Review and Enhanced Classification. Minerals, 105, 1–63. https://doi.org/10.3390/min9020105
) and petrogenesis (Alonso-Perez and Day, 2021Alonso-Perez, R., Day, J.M.D. (2021) Rare earth element and incompatible trace element abundances in emeralds reveal their formation environments. Minerals 11, 513, 1–21. https://doi.org/10.3390/min11050513
). Among trace elements, Rb and Sr can occur at concentrations >10 μg/g in bulk emeralds. Despite high concentrations of Rb and Sr, only a single study has utilised the commonly employed 87Rb-87Sr (t1/2 = 4.72 × 1010 yr−1) isotope system to study emeralds (Vidal et al., 1992Vidal, Ph., Lasnier, B., Poirot, J.-P. (1992) Determination of the age and origin of emeralds using rubidium-strontium analysis. Journal of Gemmology 23, 198–200. https://doi.org/10.15506/JoG.1992.23.4.198
), showing that old emeralds (∼600 Ma) can host extremely radiogenic 87Sr/86Sr ratios (>65) due to the high 87Rb/86Sr ratios in some samples (up to ∼11,000).Reported ages for emerald deposits have dominantly been obtained from the 40K-40Ar or 40Ar-39Ar ages of coexisting micas (Cheilletz et al., 1993
Cheilletz, A., Feraud, G., Giuliani, G., Ruffet, G. (1993) Emerald dating through 40Ar/39Ar step-heating and laser spot analysis of syngenetic phlogopite. Earth and Planetary Science Letters 120, 473–485. https://doi.org/10.1016/0012-821X(93)90258-B
; Svadlenak, 2015Svadlenak, E. (2015) 40Ar/39Ar Ages and Trace Element Variations in Colombian Emeralds. [Honors Baccalaureate Thesis], Corvallis, Oregon State University.
) (Fig. 1a), not of the emeralds themselves. Given the often polymetamorphic nature of emerald deposits, such ages may only represent the minimum age for emerald mineralisation. For the presumed oldest emerald deposit in the world, Gravelotte-Leydsdorp (henceforth Gravelotte), South Africa, the assumed age of ∼2.97 Ga comes from indirect U-Pb zircon dating of associated rocks (Poujol, 2001Poujol, M. (2001) U-Pb isotopic evidence for episodic granitoid emplacement in the Murchison greenstone belt, South Africa. Journal of African Earth Sciences 33, 155–163. https://doi.org/10.1016/S0899-5362(01)90096-X
; Lum et al., 2016Lum, J.E., Viljoen, K.S., Cairncross, B. (2016) Mineralogical and geochemical characteristics of emeralds from the Leydsdorp area, South Africa. South African Journal of Geology 119, 2, 359–378. https://doi.org/10.2113/gssajg.119.2.359
). Gravelotte emeralds have been shown to have geochemical signatures consistent with an S-type granite protolith (Alonso-Perez and Day, 2021Alonso-Perez, R., Day, J.M.D. (2021) Rare earth element and incompatible trace element abundances in emeralds reveal their formation environments. Minerals 11, 513, 1–21. https://doi.org/10.3390/min11050513
). Their association with the Pan-African metamorphic belt and other emeralds with similar compositions in Zambia and Madagascar may indicate that they date to only ∼900 to 600 Ma (Rino et al., 2008Rino, S., Kon, Y., Sato, W., Maruyama, S., Santosh, M., Zhao, D. (2008) The Grenvillian and Pan-African orogens: World’s largest orogenies through geologic time, and their implications on the origin of superplume. Gondwana Research 14, 51–72. https://doi.org/10.1016/j.gr.2008.01.001
). To determine if Gravelotte emeralds are indeed Archean in age, we report Rb-Sr isotope systematics and trace element abundances for them, as well as for the younger Muzo deposit in Colombia. An Archean age for Gravelotte emeralds would indicate that collisional tectonics and hydrothermal alteration of the continental crust was taking place at ∼3 Ga.top
Samples and Methods
Eight cut Gravelotte emeralds (GE-1 to GE-8) were analysed and all showed dark inclusions (Fig. 1c, Supplementary Information), likely biotite that is abundant in Gravelotte emeralds (Lum et al., 2016
Lum, J.E., Viljoen, K.S., Cairncross, B. (2016) Mineralogical and geochemical characteristics of emeralds from the Leydsdorp area, South Africa. South African Journal of Geology 119, 2, 359–378. https://doi.org/10.2113/gssajg.119.2.359
). In addition, two samples from the same lot of uncut emeralds from the Muzo deposit, Colombia, were analysed (MGMH#97472-1 and MGMH#97472-2; Fig. 1b). Samples were digested using protocols outlined previously in Alonso Perez and Day (2021)Alonso-Perez, R., Day, J.M.D. (2021) Rare earth element and incompatible trace element abundances in emeralds reveal their formation environments. Minerals 11, 513, 1–21. https://doi.org/10.3390/min11050513
at the Scripps Isotope Geochemistry Laboratory (SIGL, Scripps Institution of Oceanography). Digestions necessarily integrated the trace element and Rb-Sr isotopic signatures of the inclusions and their emerald lattices. A sample solution aliquot was analysed for major and trace element concentrations. Strontium separation chemistry was then performed using a two-step column chromatography procedure, after which purified Sr cuts were run on a ThermoScientific Triton thermal ionisation mass spectrometer at the SIGL. Details of the analytical methods and samples are provided in the Supplementary Information.top
Results
Major and trace element abundances and 87Rb-87Sr data are reported in Tables 1 and S-1 and the abundance data are consistent with those previously reported for their respective deposits (Alonso-Perez and Day, 2021
Alonso-Perez, R., Day, J.M.D. (2021) Rare earth element and incompatible trace element abundances in emeralds reveal their formation environments. Minerals 11, 513, 1–21. https://doi.org/10.3390/min11050513
) (Fig. 2). Gravelotte samples GE-1 through GE-5 have broadly similar REE patterns (normalised to continental crust, CC) with variable negative Ce anomalies, high La/YbCC (1.9 ± 1.1) and low Ho/YbCC (0.57 ± 0.19), consistent with their classification as Type IA emeralds (Alonso-Perez and Day, 2021Alonso-Perez, R., Day, J.M.D. (2021) Rare earth element and incompatible trace element abundances in emeralds reveal their formation environments. Minerals 11, 513, 1–21. https://doi.org/10.3390/min11050513
). Emeralds GE-6 and GE-8 have flatter REE patterns with similar Ho/YbCC (0.46 ± 0.14) but lower La/YbCC (0.56 ± 0.12), and GE-7 is distinct in having elevated LREE/HREE (La/YbCC = 22; Ho/YbCC = 1). Samples GE-7 and GE-8 are heavily included, with dark fracture-filling material (Supplementary Information).Table 1 Rubidium-strontium isotope systematics of emeralds and replicate digestions of USGS standard reference material BHVO-2, with initial 87Sr/86Sr ratios for the Gravelotte samples at 700 Ma (Pan-African) and 2960 Ma (Deposit Age).
Sample | Rb (μg/g) | Sr (μg/g) | 87Rb/86Sr | 2 s.d. | 87Sr/86Sr | 2 s.e. | (87Sr/86Sr)700 Ma | (87Sr/86Sr)2960 Ma |
GE-1 | 8.62 | 1.10 | 24.5 | 1.7 | 1.867963 | 0.000029 | 1.63 | 0.85 |
GE-2 | 1.99 | 0.26 | 25.0 | 1.8 | 2.272476 | 0.000066 | 2.03 | 1.23 |
GE-3a | 8.19 | 0.87 | 29.0 | 2.0 | 1.685235 | 0.000016 | 1.41 | 0.48 |
GE-3b | 11.8 | 3.87 | 9.07 | 0.6 | 1.249522 | 0.000020 | 1.16 | 0.87 |
GE-3c | 14.1 | 2.73 | 15.6 | 1.1 | 1.473472 | 0.000048 | 1.32 | 0.82 |
GE-3d | 12.0 | 2.32 | 15.6 | 1.1 | 1.373425 | 0.000014 | 1.22 | 0.73 |
GE-3e | 12.8 | 3.56 | 10.7 | 0.8 | 1.281804 | 0.000010 | 1.18 | 0.84 |
GE-4 | 10.4 | 0.37 | 102 | 7.2 | 3.68916 | 0.00057 | 2.71 | −0.54 |
GE-5 | 10.7 | 0.73 | 49.5 | 3.5 | 2.67065 | 0.00012 | 2.19 | 0.61 |
GE-6 | 0.52 | 0.08 | 30.1 | 2.1 | 8.1644 | 0.0071 | 7.87 | 6.91 |
GE-7 | 0.83 | 0.09 | 38.7 | 2.7 | 6.0647 | 0.0024 | 5.69 | 4.46 |
GE-8 | 4.69 | 0.39 | 49.7 | 3.5 | 5.27496 | 0.00056 | 4.80 | 3.21 |
97472-1 | 0.28 | 0.14 | 5.85 | 0.4 | 0.72131 | 0.00032 | ||
97472-2 | 1.98 | 0.16 | 34.4 | 2.4 | 0.737875 | 0.000010 | ||
BHVO-2 | 9.11 | 396 | 0.065 | 0.005 | 0.703456 | 0.000005 | ||
BHVO-2 | 9.11 | 396 | 0.065 | 0.005 | 0.703453 | 0.000004 |
Gravelotte emeralds have 87Rb/86Sr and measured 87Sr/86Sr from 9.1 to 102 and from 1.25 to 8.16, respectively, more extreme than those for Muzo (87Rb/86Sr = 5.9–34.4, 87Sr/86Sr = 0.7213–0.7379). Notably, the Sr concentrations of the samples (0.08–3.87 μg/g) range to higher values than in situ data for global emeralds (0.02–0.11 μg/g; Aurisicchio et al., 2018
Aurisicchio, C., Conte, A.M., Medeghini, L., Ottolini, L., De Vito, C. (2018) Major and trace element geochemistry of emerald from several deposits: Implications for genetic models and classification schemes. Ore Geology Reviews 94, 351–366. https://doi.org/10.1016/j.oregeorev.2018.02.001
), indicating that a significant portion of the measured Sr was likely hosted in inclusions. Published in situ data for Colombian emeralds specifically (0.04–0.10 μg/g Sr; Aurisicchio et al., 2018Aurisicchio, C., Conte, A.M., Medeghini, L., Ottolini, L., De Vito, C. (2018) Major and trace element geochemistry of emerald from several deposits: Implications for genetic models and classification schemes. Ore Geology Reviews 94, 351–366. https://doi.org/10.1016/j.oregeorev.2018.02.001
) are similarly lower than the new bulk Muzo data (0.14–0.16 μg/g Sr). Inclusion digestion likely also led to the lower Rb/Sr of the studied samples (2–28) compared to in situ Rb/Sr (13–6020; Aurisicchio et al., 2018Aurisicchio, C., Conte, A.M., Medeghini, L., Ottolini, L., De Vito, C. (2018) Major and trace element geochemistry of emerald from several deposits: Implications for genetic models and classification schemes. Ore Geology Reviews 94, 351–366. https://doi.org/10.1016/j.oregeorev.2018.02.001
). The method employed here integrates the inclusions and emerald lattices and assumes that any inclusions are cogenetic with the emeralds.Assuming similar origins for the Muzo and proximal Peñas Blancas deposits (Vidal et al., 1992
Vidal, Ph., Lasnier, B., Poirot, J.-P. (1992) Determination of the age and origin of emeralds using rubidium-strontium analysis. Journal of Gemmology 23, 198–200. https://doi.org/10.15506/JoG.1992.23.4.198
), these samples plot on an isochron (MSWD = 1.6) with an age of 48.7 ± 3.2 Ma and 87Sr/86Sri = 0.71724 ± 0.00057 (Fig. 3c). The Gravelotte data do not plot on a single isochron. Samples GE-1, GE-2, GE-3a–e, GE-4 and GE-5 show a positive correlation between 87Rb/86Sr and 87Sr/86Sr, whereas GE-6, GE-7 and GE-8 show extremely radiogenic 87Sr/86Sr ratios (5.27–8.16) and only moderate 87Rb/86Sr (30–50), plotting away from the other samples. The spatial relationships between these three emeralds and the other samples within the deposit are unknown. Excluding the anomalous emeralds (GE-6–8) on the basis of their distinct low-Al contents and REE patterns yields an errochron (MSWD = 11) with an age of 2883 ± 131 Ma and (87Sr/86Sr)i = 0.841 ± 0.031 (Fig. 3b).top
Discussion
Dating emeralds using Rb-Sr isotope systematics. Emeralds extend to high 87Sr/86Sr (>65; Vidal et al., 1992
Vidal, Ph., Lasnier, B., Poirot, J.-P. (1992) Determination of the age and origin of emeralds using rubidium-strontium analysis. Journal of Gemmology 23, 198–200. https://doi.org/10.15506/JoG.1992.23.4.198
) due to the greater affinity of Rb over Sr for beryl and the Precambrian ages of some samples, making the 87Rb-87Sr system useful for dating their mineralisation. The newly reported data facilitate the first direct dating of the Gravelotte emerald deposit and offers age constraints on the economically important Colombian Western Emerald Zone.Gravelotte samples GE-6, GE-7 and GE-8 have radiogenic Sr contents that are unsupported by their 87Rb/86Sr values. On the 1/Sr versus 87Sr/86Sr plot (Fig. 3a), these samples have the lowest Sr contents and highest Sr isotopic compositions of the data set. These three samples also have low, non-stochiometric Al2O3 contents (4.7–7.4 wt. %), indicating that they likely contain significant fractions of low-Al fluid and mineral inclusions, and so may have been susceptible to resetting. For the three samples to have been displaced from the isochron, they must have lost 83 % (GE-6), 70 % (GE-7), and 54 % (GE-8) of their Rb or gained radiogenic Sr. Rubidium is likely hosted either in saline fluid inclusions or within the channels formed by the beryl structure (Groat et al., 2008
Groat, L.A., Giuliani, G., Marshall, D.D., Turner, D. (2008) Emerald deposits and occurrences: A review. Ore Geology Reviews 34, 87–112. https://doi.org/10.1016/j.oregeorev.2007.09.003
). Consequently, emerald Sr isotopic compositions may be susceptible to metamorphic disturbance. The non-coherence of GE-6, GE-7 and GE-8 indicates that analyses of closely associated crystals within a deposit will be beneficial in future studies. Such Rb-Sr isotopic disturbances may also be useful for examining metamorphic heating events affecting emeralds. In the case of Gravelotte, GE-6, GE-7 and GE-8 do not yield meaningful ages, but, assuming Rb loss rather than 87Sr gain, their high measured 87Sr/86Sr values might suggest relatively recent, rather than ancient, disturbances. These samples are excluded from further discussion of ages.Archean Gravelotte emeralds. The remaining undisturbed Gravelotte samples yield a nine-point errorchron (MSWD = 11) with an age of 2883 ± 131 Ma and a radiogenic initial 87Sr/86Sr of 0.841 ± 0.031 (Fig. 3b). Although the high MSWD indicates additional uncertainty on the age, the measured initial 87Sr/86Sr of the samples lies within the permissible range of Archean continental crust (Table 1). The scatter of the data is likely due to 87Sr/86Sr heterogeneity at the time of deposit formation and possible minor isotopic disequilibrium between Sr-rich inclusions and the emerald lattice during mineralisation. The errorchron age is within the uncertainties of the 2.969 Ga age of coexisting pegmatites (Poujol, 2001
Poujol, M. (2001) U-Pb isotopic evidence for episodic granitoid emplacement in the Murchison greenstone belt, South Africa. Journal of African Earth Sciences 33, 155–163. https://doi.org/10.1016/S0899-5362(01)90096-X
). This Archean 87Rb-87Sr age confirms that the Gravelotte deposit was formed in the Archean, and not by regional metamorphism during the Pan-African Orogeny ∼900–600 Ma because the initial 87Sr/86Sr ratios are too radiogenic at 700 Ma (1.16–2.71) to be sourced from reasonable crustal protoliths. This age confirms that emerald formation, and corresponding extensive fractionation and alteration of the CC, was occurring in the Archean.Rubidium-strontium isotopes in Colombian emeralds. Only two Muzo samples were measured, but Vidal et al. (1992)
Vidal, Ph., Lasnier, B., Poirot, J.-P. (1992) Determination of the age and origin of emeralds using rubidium-strontium analysis. Journal of Gemmology 23, 198–200. https://doi.org/10.15506/JoG.1992.23.4.198
measured Rb-Sr isotopes in samples from the Peñas Blancas deposit, also in the Colombian Western Emerald Zone. The combined isochron (MSWD = 1.6) for both deposits yields an age of 48.7 ± 3.2 Ma with 87Sr/86Sri = 0.71724 ± 0.00057 (Fig. 3c). This age is intermediate between the two groups of Muzo muscovite Ar-Ar ages at ∼62 Ma and ∼30 Ma (Svadlenak, 2015Svadlenak, E. (2015) 40Ar/39Ar Ages and Trace Element Variations in Colombian Emeralds. [Honors Baccalaureate Thesis], Corvallis, Oregon State University.
), and younger than the 87Rb-87Sr age of the Peñas Blancas deposit alone (∼61 ± 5 Ma; Vidal et al., 1992Vidal, Ph., Lasnier, B., Poirot, J.-P. (1992) Determination of the age and origin of emeralds using rubidium-strontium analysis. Journal of Gemmology 23, 198–200. https://doi.org/10.15506/JoG.1992.23.4.198
), suggesting the recent disturbance of the Muzo deposit but not the Peñas Blancas deposit. This is consistent with Muzo sample 97472-2 plotting to the right of the isochron (within the error envelope), indicating the possible late gain of Rb at ∼30 Ma (Fig. 3c). There is a need for further Rb-Sr isotope dating of individual Colombian crystals and associated minerals, especially as it is currently unknown if Peñas Blancas and Muzo emeralds formed contemporaneously from isotopically similar fluids. Furthermore, the Eastern Emerald Zone deposits of Colombia have more coherent muscovite Ar-Ar ages (65–60 Ma; Svadlenak, 2015Svadlenak, E. (2015) 40Ar/39Ar Ages and Trace Element Variations in Colombian Emeralds. [Honors Baccalaureate Thesis], Corvallis, Oregon State University.
) and are therefore important targets for 87Rb-87Sr dating.Using 87Sr/86Sr in emeralds to understand geological processes. Initial Sr isotopic compositions can reveal information on the sources of studied materials. In the case of the Gravelotte and Colombian Western Emerald Zone deposits, the age differences are matched by differences in 87Sr/86Sri. For the Colombian emeralds, the initial 87Sr/86Sr of 0.71724 is more radiogenic than typical Andean Northern Volcanic Zone lavas (∼0.7045; e.g., Errazuriz-Henao et al., 2019
Errazuriz-Henao, C., Gomez-Tuena, A., Duque-Trujillo, J., Weber, M. (2019) The role of subducted sediments in the formation of intermediate mantle-derived magmas from the Northern Colombian Andes. Lithos, 336–337, 151–168. https://doi.org/10.1016/j.lithos.2019.04.007
), but falls within the range of Colombian plutonic rocks (0.7165–0.778; McCourt et al., 1984McCourt, W.J., Aspden, J.A., Brook, M. (1984) New geological and geochronological data from the Colombian Andes: continental growth by multiple accretion. Journal of the Geological Society 141, 831–845. https://doi.org/10.1144/gsjgs.141.5.0831
). The fluids that formed the Western Emerald Zone deposit were therefore likely sourced from the local CC, consistent with previous conclusions (Ottaway et al., 1994Ottaway, T.L., Wicks, F.J., Bryndzia, L.T., Kyser, T.K., Spooner E.T.C. (1994) Formation of the Muzo hydrothermal emerald deposit in Colombia. Nature 369, 552–554. https://doi.org/10.1038/369552a0
). The Muzo CC-like REE patterns and initial Sr isotope compositions therefore support the ultimate derivation of those sediments from uplifted and eroded basement rocks.In contrast with Colombian emeralds, the South African Gravelotte emeralds are characterised by high 87Sr/86Sri of ∼0.841 ± 0.031, more radiogenic than the average values of 3 Ga CC, which only range up to ∼0.73 (Goldstein, 1988
Goldstein, S.L. (1988) Decoupled evolution of Nd and Sr isotopes in the continental crust and the mantle. Nature 336, 733–738. https://doi.org/10.1038/336733a0
). The 87Sr/86Sri values of gneisses and plutons from the Barberton Greenstone belt, also in the Kaapvaal Craton but unrelated to Gravelotte, range from 0.7011 (3.35 Ga Barberton granodiorites) to 0.88 in younger (2.24 ± 0.36 Ga) Kaapvaal Craton Nhlangano gneisses (Barton et al., 1983Barton Jr., J.M., Hunter, D.R., Jackson, M.P.A., Wilson, A.C. (1983) Geochronologic and Sr-isotopic studies of certain units in the Barberton granite-greenstone terrane, Swaziland. South African Journal of Geology 86, 71–80.
). Critically, some of the older plutons have high Rb/Sr (>10). These comparisons indicate that Gravelotte 87Sr/86Sri values are not unreasonable. Such conclusions await data on cogenetic rocks from Gravelotte. Both the likely presence of highly differentiated lithologies at Gravelotte at 3 Ga and the inferred presence of pre-existing ancient high-Rb/Sr material supports the formation of mature CC-like rocks by the Mesoarchean at the latest.It has previously been shown that Gravelotte emeralds show an upturned HREE pattern diagnostic of emeralds from Type IA deposits, e.g., those associated with sediment melting during continent-continent collision (Alonso-Perez and Day, 2021
Alonso-Perez, R., Day, J.M.D. (2021) Rare earth element and incompatible trace element abundances in emeralds reveal their formation environments. Minerals 11, 513, 1–21. https://doi.org/10.3390/min11050513
). Our results for the Gravelotte emeralds confirm an Archean heritage, supporting the melting of S-type protoliths in the Mesoarchean and the concept that emerald geochemistry is useful for understanding collisional tectonics and plate tectonic processes (Alonso-Perez and Day, 2021Alonso-Perez, R., Day, J.M.D. (2021) Rare earth element and incompatible trace element abundances in emeralds reveal their formation environments. Minerals 11, 513, 1–21. https://doi.org/10.3390/min11050513
).In conclusion, new Rb-Sr isotope data show that Gravelotte is the oldest known emerald deposit and indicates that pegmatite formation was occurring at ∼3 Ga. Fluid inclusion-rich, high-Rb/Sr emeralds lost significant Rb in a later thermal metamorphic event after the in-growth of significant quantities of 87Sr. This conclusion awaits further work on the diffusion of Sr and Rb in the beryl structure. Future Sr isotope studies, either in situ or of bulk emeralds isolated from their inclusions, are a necessary step in proliferating the use of the Rb-Sr isotope system in understanding beryl genesis. Given the ability of the Rb-Sr isotope system to directly date emerald mineralisation, more global deposits should be targeted. The Archean age and radiogenic 87Sr/86Sri value of the Gravelotte deposit suggests that emerald deposits in Precambrian terranes may be underexplored.
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Acknowledgements
Financial support for this work came from the Scripps Institution of Oceanography and the Mineralogical and Geological Museum, Harvard University (MGMH). We are grateful to S. and A. Pouroulis for their generous provision of samples and excellent knowledge of the Gravelotte deposit. This work was improved by comments from reviewers Elis Hoffmann and Rainer Thomas, editor Horst Marschall and the earlier comments of David Chew.
Editor: Horst R. Marschall
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References
Alonso-Perez, R., Day, J.M.D. (2021) Rare earth element and incompatible trace element abundances in emeralds reveal their formation environments. Minerals 11, 513, 1–21. https://doi.org/10.3390/min11050513
Show in context
Emeralds are important gemstones for examining crustal processes because, compared to other gems, they contain high abundances of trace elements that can reveal their provenance (e.g., Aurisicchio et al., 2018; Giuliani et al., 2019) and petrogenesis (Alonso-Perez and Day, 2021).
View in article
Gravelotte emeralds have been shown to have geochemical signatures consistent with an S-type granite protolith (Alonso-Perez and Day, 2021).
View in article
Deposit ages are taken from Groat et al. (2002) and the compilations of Giuliani et al. (2019) and Alonso-Perez and Day (2021).
View in article
Samples were digested using protocols outlined previously in Alonso Perez and Day (2021) at the Scripps Isotope Geochemistry Laboratory (SIGL, Scripps Institution of Oceanography).
View in article
Major and trace element abundances and 87Rb-87Sr data are reported in Tables 1 and S-1 and the abundance data are consistent with those previously reported for their respective deposits (Alonso-Perez and Day, 2021) (Fig. 2).
View in article
Gravelotte samples GE-1 through GE-5 have broadly similar REE patterns (normalised to continental crust, CC) with variable negative Ce anomalies, high La/YbCC (1.9 ± 1.1) and low Ho/YbCC (0.57 ± 0.19), consistent with their classification as Type IA emeralds (Alonso-Perez and Day, 2021).
View in article
It has previously been shown that Gravelotte emeralds show an upturned HREE pattern diagnostic of emeralds from Type IA deposits, e.g., those associated with sediment melting during continent-continent collision (Alonso-Perez and Day, 2021).
View in article
Our results for the Gravelotte emeralds confirm an Archean heritage, supporting the melting of S-type protoliths in the Mesoarchean and the concept that emerald geochemistry is useful for understanding collisional tectonics and plate tectonic processes (Alonso-Perez and Day, 2021).
View in article
Aurisicchio, C., Conte, A.M., Medeghini, L., Ottolini, L., De Vito, C. (2018) Major and trace element geochemistry of emerald from several deposits: Implications for genetic models and classification schemes. Ore Geology Reviews 94, 351–366. https://doi.org/10.1016/j.oregeorev.2018.02.001
Show in context
Emeralds are important gemstones for examining crustal processes because, compared to other gems, they contain high abundances of trace elements that can reveal their provenance (e.g., Aurisicchio et al., 2018; Giuliani et al., 2019) and petrogenesis (Alonso-Perez and Day, 2021).
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Notably, the Sr concentrations of the samples (0.08–3.87 μg/g) range to higher values than in situ data for global emeralds (0.02–0.11 μg/g; Aurisicchio et al., 2018), indicating that a significant portion of the measured Sr was likely hosted in inclusions.
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Published in situ data for Colombian emeralds specifically (0.04–0.10 μg/g Sr; Aurisicchio et al., 2018) are similarly lower than the new bulk Muzo data (0.14–0.16 μg/g Sr).
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Inclusion digestion likely also led to the lower Rb/Sr of the studied samples (2–28) compared to in situ Rb/Sr (13–6020; Aurisicchio et al., 2018).
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Barton Jr., J.M., Hunter, D.R., Jackson, M.P.A., Wilson, A.C. (1983) Geochronologic and Sr-isotopic studies of certain units in the Barberton granite-greenstone terrane, Swaziland. South African Journal of Geology 86, 71–80.
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The 87Sr/86Sri values of gneisses and plutons from the Barberton Greenstone belt, also in the Kaapvaal Craton but unrelated to Gravelotte, range from 0.7011 (3.35 Ga Barberton granodiorites) to 0.88 in younger (2.24 ± 0.36 Ga) Kaapvaal Craton Nhlangano gneisses (Barton et al., 1983).
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Cheilletz, A., Feraud, G., Giuliani, G., Ruffet, G. (1993) Emerald dating through 40Ar/39Ar step-heating and laser spot analysis of syngenetic phlogopite. Earth and Planetary Science Letters 120, 473–485. https://doi.org/10.1016/0012-821X(93)90258-B
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Reported ages for emerald deposits have dominantly been obtained from the 40K-40Ar or 40Ar-39Ar ages of coexisting micas (Cheilletz et al., 1993; Svadlenak, 2015) (Fig. 1a), not of the emeralds themselves.
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Errazuriz-Henao, C., Gomez-Tuena, A., Duque-Trujillo, J., Weber, M. (2019) The role of subducted sediments in the formation of intermediate mantle-derived magmas from the Northern Colombian Andes. Lithos, 336–337, 151–168. https://doi.org/10.1016/j.lithos.2019.04.007
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For the Colombian emeralds, the initial 87Sr/86Sr of 0.71724 is more radiogenic than typical Andean Northern Volcanic Zone lavas (∼0.7045; e.g., Errazuriz-Henao et al., 2019), but falls within the range of Colombian plutonic rocks (0.7165–0.778; McCourt et al., 1984).
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Goldstein, S.L. (1988) Decoupled evolution of Nd and Sr isotopes in the continental crust and the mantle. Nature 336, 733–738. https://doi.org/10.1038/336733a0
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In contrast with Colombian emeralds, the South African Gravelotte emeralds are characterised by high 87Sr/86Sri of ∼0.841 ± 0.031, more radiogenic than the average values of 3 Ga CC, which only range up to ∼0.73 (Goldstein, 1988).
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Groat, L.A., Marshall, D.D., Giuliani, G., Murphy, D.C., Piercey, S.J., Jambor, J.L., Mortensen, J.K., Ercit, T.S., Gault, R.A., Mattey, D.P., Schwarz, D., Maluski, H., Wise, M.A., Wengzynowski, W., Eaton, D.W. (2002) Mineralogical and geochemical study of the regal ridge emerald showing, southeastern Yukon. Canadian Mineralogist 40, 1313–1338. https://doi.org/10.2113/gscanmin.40.5.1313
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Deposit ages are taken from Groat et al. (2002) and the compilations of Giuliani et al. (2019) and Alonso-Perez and Day (2021).
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Groat, L.A., Giuliani, G., Marshall, D.D., Turner, D. (2008) Emerald deposits and occurrences: A review. Ore Geology Reviews 34, 87–112. https://doi.org/10.1016/j.oregeorev.2007.09.003
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For the three samples to have been displaced from the isochron, they must have lost 83 % (GE-6), 70 % (GE-7), and 54 % (GE-8) of their Rb or gained radiogenic Sr. Rubidium is likely hosted either in saline fluid inclusions or within the channels formed by the beryl structure (Groat et al., 2008).
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Giuliani, G., France-Lanord, C., Cheilletz, A., Coget, P., Branquet, Y., Laumomnier, B. (2000) Sulfate Reduction by Organic Matter in Colombian Emerald Deposits: Chemical and Stable Isotope (C, O, H) Evidence. Economic Geology 95, 1129–1153. https://doi.org/10.2113/gsecongeo.95.5.1129
Giuliani, G., Groat, L.A., Marshall, D., Fallick, A.E., Branquet, Y. (2019) Emerald Deposits: A Review and Enhanced Classification. Minerals, 105, 1–63. https://doi.org/10.3390/min9020105
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Emeralds are thought to form by the interaction of Be-enriched magmatic or aqueous fluids or hydrous evolved melts with V- and Cr-rich ultramafic or sedimentary country rocks (e.g., Giuliani et al., 2019).
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Emeralds are important gemstones for examining crustal processes because, compared to other gems, they contain high abundances of trace elements that can reveal their provenance (e.g., Aurisicchio et al., 2018; Giuliani et al., 2019) and petrogenesis (Alonso-Perez and Day, 2021).
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Deposit ages are taken from Groat et al. (2002) and the compilations of Giuliani et al. (2019) and Alonso-Perez and Day (2021).
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Ludwig, K.R. (2003) ISOPLOT 3.00: A geochronological toolkit for Microsoft Excel. Berkeley Geochronology Center Special Publication 4, 1–70.
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The straight line is an isochron (MSWD = 1.8) for both datasets. All isochron calculations were performed using Isoplot 3.0 (Ludwig, 2003).
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Lum, J.E., Viljoen, K.S., Cairncross, B. (2016) Mineralogical and geochemical characteristics of emeralds from the Leydsdorp area, South Africa. South African Journal of Geology 119, 2, 359–378. https://doi.org/10.2113/gssajg.119.2.359
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For the presumed oldest emerald deposit in the world, Gravelotte-Leydsdorp (henceforth Gravelotte), South Africa, the assumed age of ∼2.97 Ga comes from indirect U-Pb zircon dating of associated rocks (Poujol, 2001; Lum et al., 2016).
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Eight cut Gravelotte emeralds (GE-1 to GE-8) were analysed and all showed dark inclusions (Fig. 1c, Supplementary Information), likely biotite that is abundant in Gravelotte emeralds (Lum et al., 2016).
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McCourt, W.J., Aspden, J.A., Brook, M. (1984) New geological and geochronological data from the Colombian Andes: continental growth by multiple accretion. Journal of the Geological Society 141, 831–845. https://doi.org/10.1144/gsjgs.141.5.0831
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For the Colombian emeralds, the initial 87Sr/86Sr of 0.71724 is more radiogenic than typical Andean Northern Volcanic Zone lavas (∼0.7045; e.g., Errazuriz-Henao et al., 2019), but falls within the range of Colombian plutonic rocks (0.7165–0.778; McCourt et al., 1984).
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Ottaway, T.L., Wicks, F.J., Bryndzia, L.T., Kyser, T.K., Spooner E.T.C. (1994) Formation of the Muzo hydrothermal emerald deposit in Colombia. Nature 369, 552–554. https://doi.org/10.1038/369552a0
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The fluids that formed the Western Emerald Zone deposit were therefore likely sourced from the local CC, consistent with previous conclusions (Ottaway et al., 1994).
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Poujol, M. (2001) U-Pb isotopic evidence for episodic granitoid emplacement in the Murchison greenstone belt, South Africa. Journal of African Earth Sciences 33, 155–163. https://doi.org/10.1016/S0899-5362(01)90096-X
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For the presumed oldest emerald deposit in the world, Gravelotte-Leydsdorp (henceforth Gravelotte), South Africa, the assumed age of ∼2.97 Ga comes from indirect U-Pb zircon dating of associated rocks (Poujol, 2001; Lum et al., 2016).
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The errorchron age is within the uncertainties of the 2.969 Ga age of coexisting pegmatites (Poujol, 2001).
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Rino, S., Kon, Y., Sato, W., Maruyama, S., Santosh, M., Zhao, D. (2008) The Grenvillian and Pan-African orogens: World’s largest orogenies through geologic time, and their implications on the origin of superplume. Gondwana Research 14, 51–72. https://doi.org/10.1016/j.gr.2008.01.001
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Their association with the Pan-African metamorphic belt and other emeralds with similar compositions in Zambia and Madagascar may indicate that they date to only ∼900 to 600 Ma (Rino et al., 2008).
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Rudnick, R.L., Gao, S. (2014) 4.1 - Composition of the Continental Crust. In: Holland, H.D., Turkenian, K.K. (Eds.) Treatise on Geochemistry. 2nd edition, Elsevier, Amsterdam, 1–51. https://doi.org/10.1016/B978-0-08-095975-7.00301-6
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Sample REE abundance patterns normalised to continental crust (CC) (Rudnick and Gao, 2014).
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Svadlenak, E. (2015) 40Ar/39Ar Ages and Trace Element Variations in Colombian Emeralds. [Honors Baccalaureate Thesis], Corvallis, Oregon State University.
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Reported ages for emerald deposits have dominantly been obtained from the 40K-40Ar or 40Ar-39Ar ages of coexisting micas (Cheilletz et al., 1993; Svadlenak, 2015) (Fig. 1a), not of the emeralds themselves.
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This age is intermediate between the two groups of Muzo muscovite Ar-Ar ages at ∼62 Ma and ∼30 Ma (Svadlenak, 2015), and younger than the 87Rb-87Sr age of the Peñas Blancas deposit alone (∼61 ± 5 Ma; Vidal et al., 1992), suggesting the recent disturbance of the Muzo deposit but not the Peñas Blancas deposit.
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Furthermore, the Eastern Emerald Zone deposits of Colombia have more coherent muscovite Ar-Ar ages (65–60 Ma; Svadlenak, 2015) and are therefore important targets for 87Rb-87Sr dating.
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Vidal, Ph., Lasnier, B., Poirot, J.-P. (1992) Determination of the age and origin of emeralds using rubidium-strontium analysis. Journal of Gemmology 23, 198–200. https://doi.org/10.15506/JoG.1992.23.4.198
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Despite high concentrations of Rb and Sr, only a single study has utilised the commonly employed 87Rb-87Sr (t1/2 = 4.72 × 1010 yr−1) isotope system to study emeralds (Vidal et al., 1992), showing that old emeralds (∼600 Ma) can host extremely radiogenic 87Sr/86Sr ratios (>65) due to the high 87Rb/86Sr ratios in some samples (up to ∼11,000).
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Assuming similar origins for the Muzo and proximal Peñas Blancas deposits (Vidal et al., 1992), these samples plot on an isochron (MSWD = 1.6) with an age of 48.7 ± 3.2 Ma and 87Sr/86Sri = 0.71724 ± 0.00057 (Fig. 3c).
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(c) Rb-Sr data for Muzo (this study) and Peñas Blancas emeralds (Vidal et al., 1992).
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Emeralds extend to high 87Sr/86Sr (>65; Vidal et al., 1992) due to the greater affinity of Rb over Sr for beryl and the Precambrian ages of some samples, making the 87Rb-87Sr system useful for dating their mineralisation.
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Only two Muzo samples were measured, but Vidal et al. (1992) measured Rb-Sr isotopes in samples from the Peñas Blancas deposit, also in the Colombian Western Emerald Zone.
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This age is intermediate between the two groups of Muzo muscovite Ar-Ar ages at ∼62 Ma and ∼30 Ma (Svadlenak, 2015), and younger than the 87Rb-87Sr age of the Peñas Blancas deposit alone (∼61 ± 5 Ma; Vidal et al., 1992), suggesting the recent disturbance of the Muzo deposit but not the Peñas Blancas deposit.
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Supplementary Information
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