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Volume 29

About the cover:  False colour, electron backscatter diffraction, band contrast image of ≥3.99 billion year old zircon crystal from the Acasta Gneiss, Canada, revealing internal microstructure. Colours reflect different degrees of crystallinity (as represented by the diffraction pattern quality), with red highlighting a metamict structure, yellow and green different generations of more crystalline overgrowth. In Letter 2332, Kirkland et al. use ion imaging to demonstrate that isolated pockets within the ancient metamict zircon core still preserve primary isotope systematics. Recognition of primary isotope signatures in ancient zircon may help to better understand early crust production processes on Earth.
Image credit: Jack Gillespie and the Microscopy and Microanalysis Facility, John De Laeter Centre, Curtin University. Download high-resolution cover.

Reliability of Raman analyses of CO2-rich fluid inclusions as a geobarometer at Kīlauea
Abstract:
Interpreting signals of volcanic unrest requires knowledge of the architecture of the magmatic system, particularly the depths at which magmas are stored. Such information can be vital to help predict changes in eruptive style and vigour. However, popular petrological tools to assess magma storage depths (e.g., melt inclusions) are costly, present large uncertainties, and are too slow for real time monitoring. Here, we evaluate the reliability of Raman Spectroscopy measurements of CO2-dominated fluid inclusions as a geobarometer relative to microthermometry and melt inclusion barometry. We calculate storage pressures for 102 olivine-hosted fluid inclusions from the 2018 Lower East Rift Zone eruption of Kīlauea, which are statistically indistinguishable to those determined from melt inclusions. We show that calibrated Raman spectroscopy yields densities within 5–10 % of microthermometry for CO2-dominated fluid inclusions (<10 mol % H2O) but is a far more suitable method for systems like Kīlauea dominated by shallow magma storage. Overall, pressures determined from fluid inclusions by Raman spectroscopy are robust and require only a fraction of the time and resources of melt inclusion studies.

C.L. DeVitre, P.E. Wieser

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Geochem. Persp. Let. (2024) 29, 1–8 | https://doi.org/10.7185/geochemlet.2404 | Published 31 January 2024

Mantle depletion recorded by olivine and plagioclase megacrysts in oceanic basalts
Abstract:
Earth’s oceanic crust is largely formed by melting of the upper mantle where it upwells beneath mid-ocean ridges, providing a geographically widespread elemental and isotopic ‘sample’ of Earth’s interior. Radiogenic isotope variations in oceanic basalts are commonly attributed to compositional heterogeneity in Earth’s upper mantle, albeit reduced by mixing and reaction during melt ascent. Nevertheless, many mid-ocean ridge basalts are biased towards incompatible element enriched radiogenic isotope compositions, and questions remain as to whether their chemistry is indeed representative of the underlying mantle. Here we present Pb isotope data for plagioclase megacrysts (and olivine-megacryst hosted inclusions) from oceanic basalts that crystallised in the lower oceanic crust or mantle, from incompatible element depleted melts. Our data show that the plagioclase megacrysts and olivine grew from melts with substantially less radiogenic Pb compositions than their host lava. High Ca plagioclase megacrysts are common in oceanic basalts, suggesting that depleted melts in the lower crust or mantle may also be widespread, acting to balance the enriched isotope compositions seen in many erupted basalts.

K.W. Burton, I.J. Parkinson, D.A. Neave

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Geochem. Persp. Let. (2024) 29, 9–13 | https://doi.org/10.7185/geochemlet.2405 | Published 15 February 2024

Chondritic chlorine isotope composition of acapulcoites and lodranites
Abstract:
Bulk rock chondrites and Earth’s reservoirs share a common chlorine isotopic value, while more differentiated bodies such as the Moon or Vesta record significant chlorine isotopic fractionation in their Ca phosphates. As such, an important but scarcely studied parameter is the effect of melting and differentiation processes on chlorine concentration and isotopic composition of a planetesimal. Here we report chlorine abundances and isotopic compositions for apatite in a range of primitive achondrites, acapulcoites and lodranites. These meteorites originated from a parent body that experienced some partial melting, allowing an assessment of chlorine behaviour during the early stages of planetary evolution in the inner Solar System. Overall, while bulk rock estimates of F and Cl abundances are indicative of degassing during the early stages of partial melting, no chlorine isotopic fractionation is recorded in apatite. Consequently, acapulcoites and lodranites retain their chondritic precursor isotopic signature for chlorine.

A. Stephant, M. Anand, C. Carli, X. Zhao, J. Davidson, T. Cuppone, G. Pratesi, I.A. Franchi

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Geochem. Persp. Let. (2024) 29, 14–19 | https://doi.org/10.7185/geochemlet.2406 | Published 16 February 2024

Corrigendum to “Titanium isotope constraints on the mafic sources and geodynamic origins of Archean crust” by Hoare et al., 2023
Abstract:

L. Hoare, L.J.A. Rzehak, S. Kommescher, M. Jansen, M.T. Rosing, T. Nagel, M.-A. Millet, J.E. Hoffmann, R.O.C. Fonseca

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Geochem. Persp. Let. (2024) 29, 26–26 | https://doi.org/10.7185/geochemlet.2342 | Published 23 February 2024

Imaging of boron in altered mantle rocks illuminates progressive serpentinisation episodes
Abstract:
Serpentinised mantle rocks reflect the cumulative sum of multiple alteration events, but to date, identifying distinct serpentinisation episodes has remained challenging due to limited knowledge of the spatial distribution of tracers of fluid-rock exchange. Here we present novel high spatial resolution (∼10 μm) boron, nickel, calcium, and lithium concentration maps combined with in situ boron isotope analyses of strongly serpentinised mantle peridotites from the Troodos ophiolite, Cyprus. Our maps indicate strongly heterogenous boron concentrations with high boron concentrations in early formed serpentine replacing olivine but much lower boron contents in mesh-textured serpentine and bastitic pyroxene. Late stage crosscutting serpentine veins have very low boron concentrations. In contrast, boron isotope measurements, made at coarser scales, are remarkably uniform (mean value +11.9 ± 3.2 ‰, 1σ, n = 49). We interpret the high boron serpentine as reflecting the partial preservation of an early pervasive serpentinisation episode by fluids with high boron concentrations sourced from the dehydration of the subducting Cyprus slab. Subsequent serpentine phases with moderate to low boron reflect progressive recrystallisation and leaching by low boron concentration meteoric waters.

A.D. Evans, C.D. Standish, J.A. Milton, A.G. Robbins, D. Craw, G.L. Foster, D.A.H. Teagle

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Geochem. Persp. Let. (2024) 29, 20–25 | https://doi.org/10.7185/geochemlet.2407 | Published 23 February 2024