¹Jean-David Bodénan,²Natalie A.Starkey,³Sara S.Russell,²Ian P.Wright,²Ian A.Franchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.06.034]
¹ETH Zürich, Institute für Geochemie und Petrologie, Clausiusstrasse 25, 8092, Zürich, Switzerland
²Planetary and Space Sciences, School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom
³Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom
Copyright Elsevier

A ∼175 µm refractory inclusion, A-COR-01 from one of the least altered carbonaceous chondrites, ALHA 77307 (CO3.0), has been found to bear unique characteristics that indicate that it is one of the first solids to have formed at the very birth of the solar system while isotopic reservoirs were still evolving rapidly. Its core is composed mainly of hibonite and corundum, the two phases predicted to condense first from a gas of solar composition, and like many common types of Calcium-, Aluminium-rich Inclusions (CAIs) is surrounded by a rim of diopside.

Core minerals in A-COR-01 are very ¹⁶O-rich (Δ¹⁷O_Core = -32.5 ± 3.3 (2SD) ‰) while those in the rim display an O isotopic composition (Δ¹⁷O_Rim = -24.8 ± 0.5 (2SD) ‰) indistinguishable from that found in the vast majority of the least altered CAIs. These observations indicate that this CAI formed in a very ¹⁶O-rich reservoir and either recorded the subsequent evolution of this reservoir or the transit to another reservoir. The origin of A-COR-01in a primitive reservoir is consistent with the very low content of excess of radiogenic ²⁶Mg in its core minerals corresponding to the inferred initial ²⁶Al/²⁷Al ratio ((²⁶Al/²⁷Al)₀ = (1.67 ± 0.31) × 10^-7), supporting a very early formation before injection and/or homogenisation of ²⁶Al in the protoplanetary disk. Possible reservoir evolution and short-lived radionuclide (SLRs) injection scenarios are discussed and it is suggested that the observed isotope composition resulted from mixing of a previously un-observed early reservoir with the rest of the disk.

¹Josh Wimpenny,¹Naomi Marks,¹ Kim Knight,¹Lars Borg,²James Badro,¹Rick Ryerson
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.07.006]
¹Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
²Université de Paris, Institut de physique du globe de Paris, CNRS, 75005 Paris, France
Copyright Elsevier

Renewed interest in gallium isotope systematics has stemmed from the fact that Ga is moderately volatile and is hypothesized to undergo kinetic fractionation during evaporation. Here, we present the first Ga isotope data from terrestrial volatile depleted samples including a suite of experimentally heated rhyolitic soils, fallout melt glass, and splash-form tektites from the Australasian strewn field (hereafter termed australite tektites). The Ga in these samples is isotopically heavy compared to Ga in terrestrial basalts and estimates for the composition of the bulk silicate Earth (BSE). For each sample suite the isotopic fractionation of Ga scales with the degree of Ga depletion, consistent with isotopic fractionation caused by evaporation.

The rapid experimental heating of rhyolitic soil to temperatures ranging between 1600-2200 ^oC resulted in volatile loss from the starting soil. Based on the fraction of Ga that was evaporated and the degree of Ga isotopic fractionation between starting soil and experimental samples, we calculate a fractionation factor (α) of 0.99891 ± 0.00024. This is within uncertainty of the fractionation factor we previously calculated for Zn isotopes in the same sample suite (0.99879 ± 0.00013). Although Ga isotopic data from nuclear fallout melt glass is less coherent, the Ga isotope systematics are generally consistent with a suppressed fractionation factor of approximately 0.9995-0.9998 during evaporation, which is also similar to the behavior of Zn systematics. Thus, although the fractionation factors obtained from the laser heating experiments and fallout melt glass are different, in both cases Ga and Zn behave similarly, as evidenced by the covariation of δ⁷¹Ga and δ⁶⁶Zn in these samples.

The behavior of Ga isotopes in australite tektites is more difficult to constrain because we do not know the location of the impact site and hence the chemical composition of the target rocks. Nevertheless, based on the composition of more volatile rich Muong-Nong type tektites, we estimate that evaporative fractionation of Ga occurs with an α between 0.9998 and 0.9987; broadly consistent with data from the laser heating experiments and nuclear fallout glass. There is no correlation between δ⁷¹Ga and δ⁶⁶Zn values in australite tektites which is likely to reflect inherited isotopic heterogeneity from weathered precursor material in combination with varying extents of evaporative loss during tektite formation.

Gallium isotope ratios in mare basalts are generally isotopically heavy compared to basalts from Earth. Individual mare basalts have δ⁷¹Ga and δ⁶⁶Zn values that do not correlate, contrary to data from the laser levitation experiments and nuclear fallout glass. This suggests that δ⁷¹Ga and/or δ⁶⁶Zn values were fractionated by geologic processes after the Moon had accreted.

^1,2,3E.S.Steenstra,²E.Kelderman,³ J.Berndt,³S.Klemme,¹E.S.Bullock,²W.van Westrenen
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.07.002]
¹The Earth and Planets Laboratory, Carnegie Institution of Science, Washington D.C., USA
²Faculty of Science, Vrije Universiteit Amsterdam, the Netherlands
³Institute of Mineralogy, University of Münster, Germany
Copyright Elsevier

The Earth may have formed at very reducing conditions through the accretion of (a) large reduced and differentiated impactor(s). Segregation of Fe-S liquids within these bodies would have left a geochemical mark on the mantles of reduced impactors and on the proto-Earth’s mantle. Here, we study the geochemical consequences of highly reduced accretion of the Earth by large impactors. New insights into the partitioning of trace elements between Fe-S liquid and silicate melt at (highly) reduced conditions (ΔIW = –5 to +1) were obtained by performing 21 high pressure experiments at 1 GPa and 1683–2283 K. The observed Fe-S liquid-silicate melt partitioning behavior is in agreement with thermodynamic models that predict a significant role for O in Fe-S liquid and S in the silicate melt.

The experimental results were combined with literature data to obtain new and/or revised thermodynamic parameterizations that quantify the effects of composition and redox state on the elemental distribution between Fe-S liquids and highly reduced silicate melts. The results were used to assess which elements would most likely retain the geochemical signature of accretion of reduced impactors. Under the assumption of instantaneous core merging, impact delivery to the proto-Earth’s mantle was found to be significant (>10% of present-day BSE concentrations) only for S, Zn, Se, Te and Tl, whereas the abundances of the other elements remain largely unaffected.

The results also show that present-day BSE S/Se, Se/Te, Tl/S and potentially In/Zn as well as their absolute abundances are inconsistent with their delivery by (a) large, highly reduced chondritic differentiated impactor(s) during terrestrial accretion. Continued core-mantle equilibration in the proto-Earth, volatility-related loss and/or post-accretion sulfide liquid segregation in the terrestrial magma ocean would further increase or not affect these discrepancies. We conclude that a significant contribution of (a) large (>10% of Earth’s mass) reduced and differentiated chondritic impactor(s) during accretion of the Earth is not reflected in the present-day S, Zn, Se, Te and Tl systematics of the terrestrial mantle. This suggests that significant overprinting of the primordial BSE S/Se, Se/Te and S/Tl signature could have occurred and/or (2) that the S/Se and Se/Te ratios were set by accretion of more oxidised CI-like materials.