¹Robbin Visser,¹Timm John,²Martin J.Whitehouse,³Markus Patzek,³Addi Bischoff
Earth and Planetary Science Letters 547, 116440 Link to Article [https://doi.org/10.1016/j.epsl.2020.116440]
¹Freie Universität Berlin, Institut für Geologische Wissenschaften, Berlin, Germany
²Swedish Museum of Natural History, Stockholm, Sweden
³Institut für Planetologie, University of Münster, Münster, Germany
Copyright Elsevier

A key process in the early solar system that significantly affects the further evolution and transport of highly volatile elements throughout the solar system hydrothermal parent body alteration. To determine whether hydrothermal alteration in outer solar system parent bodies occurred more or less simultaneously or due to a sequence of multiple different events, we investigated low-temperature hydrothermally altered CM and CI chondrites along with volatile-rich CM-like clasts and C1 clasts with abundant mineral phases that contain volatiles. In this respect, C1 clasts are particularly important as they closely resemble the CI chondrites but originate from isotopically different parent bodies. Specifically, we applied the SIMS-based Mn/Cr in situ dating technique to carbonates, a common hydrothermally formed phase in low-temperature hydrothermally altered meteorites. The Mn/Cr ages of dolomites in CI chondrites and C1 clasts as well as calcites in CM chondrites and CM-like clasts reveal that nearly all carbonates in low-temperature hydrothermally altered clasts and chondrites were formed within a brief period between 2-6 Ma after CAI formation. Given this sharp separation, and that hardly any material contains carbonates formed later than ∼6 Ma after CAI formation, hydrothermal alteration likely occurred near-contemporaneously among different parent bodies in the outer solar system. Further, the timing of hydrothermal alteration matches peak heating of ²⁶Al decay that ceased at ∼5 Ma after CAI formation. Hereby, these results are consistent with a model in which the carbonates in low-temperature hydrothermally altered parent bodies precipitated from the fluid produced by melting ice. The results also show that other potential heating events (e.g., impacts) only negligibly contributed to creating environments where fluid-mediated dissolution and precipitation of carbonates was possible. Additionally, the isotopic (H, O, Cr, and S) differences between C1 clasts and CI chondrites are most likely not caused by differences in timing of hydrothermal aqueous alteration and, thus, are best explained by spatially different isotopic reservoirs.

^1,2M.D.Suttle,^3,4Z.Dionnet,⁵I.Franchi,^1,6L.Folco,⁵J.Gibson,⁵R.C.Greenwood,³A.Rotundi,⁷A.King,²S.S.Russell
Earth and Planetary Science Letters 546, 116444 Link to Article [https://doi.org/10.1016/j.epsl.2020.116444]
¹Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy
²Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, UK
³DIST-Università di Napoli “Parthenope”, Centro Direzionale Isola C4, 80143 Naples, Italy
⁴INAF-IAPS, via Fosso del Cavaliere 100, 00133 Rome, Italy
⁵School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
⁶CISUP, Centro per l’Integrazione della Strumentazione dell’Università di Pisa, Lungarno Pacinotti 43, 56126 Pisa, Italy
⁷Psiche beamline, Synchrotron SOLEIL, Orne des Meurisiers, France
Copyright Elsevier

Bulk oxygen isotope data has the potential to match extraterrestrial samples to parent body sources based on distinctive
O and
O ratios. We analysed 10 giant (>500 μm) micrometeorites using combined micro-Computer Tomography (μCT) and O-isotope analysis to pair internal textures to inferred parent body groups. We identify three ordinary chondrite particles (L and LL groups), four from CR chondrites and the first micrometeorite from the enstatite chondrite (EH4) group. In addition, two micrometeorites are from hydrated carbonaceous chondrite parent bodies with 16O-poor isotopic compositions and plot above the terrestrial fractionation line. They experienced intense aqueous alteration, contain pseudomorphic chondrules and are petrographically similar to the CM1/CR1 chondrites. These micrometeorites may be members of the newly established CY chondrites and/or derived from the enigmatic “Group 4” micrometeorite population, previously identified by Yada et al., 2005 [GCA, 69:5789-5804], Suavet et al., 2010 [EPSL, 293:313-320] (and others). One of our 16O-poor micrometeorite plots on the same isotopic trendline as the CO, CM and CY chondrites – “the CM mixing line” (with a slope of ∼0.7 and a
O intercept of -4.23‰), this implies a close relationship and potentially a genetic link to these hydrated chondrites. If position along the CM mixing line reflects the amount of 16O-poor (heavy) water-ice accreted onto the parent body at formation, then the CY chondrites and these 16O-poor micrometeorites must have accreted at least as much water-ice as CM chondrites but potentially more. In addition, thermal metamorphism could have played a role in further raising the bulk O-isotope compositions through the preferential loss of isotopically light water during phyllosilicate dehydration. The study of micrometeorites provides insights into asteroid belt diversity through the discovery of material not currently sampled by larger meteorites, perhaps as a result of atmospheric entry biases preventing the survival of large blocks of friable hydrated material.

^1,2Richard A. F. Grieve,^1,2Gordon R. Osinski
Meteorits & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13542]
¹Department of Earth Sciences, University of Western Ontario, London, Ontario, N6A 5B7 Canada
²Institute for Earth and Space Exploration, University of Western Ontario, London, Ontario, N6A 5B7 Canada
Published by arrangement with John Wiley & Sons

Observational and logical arguments are presented for the lithology formerly named the Garson Member of the Onaping Formation being the clast‐bearing, fine‐grained, chilled Upper Contact Unit (UCU) of the Sudbury Igneous Complex (SIC) in the Garson region of the Sudbury impact structure. It differs considerably, however, from the UCU in the North Range of the SIC with respect to the character of its clasts. Namely, the clasts are essentially monomict (quartzites), much larger (up to 100 m across), and much more abundant (up to 80% in places). These differences indicate a different source than “fallback” material for the clasts in the UCU in the Garson region. Their character requires a “coherent,” singular source that was topographically above the SIC melt pool. Such a source would correspond to that of an emergent peak ring of fractured target rocks. The clasts are identified as Huronian Mississagi quartzite, which is estimated to have been at a nominal depth of 7.5 ± 2.5 km at the time of impact. This provides a constraint on the depth of origin of the peak ring. This depth estimate is closest to the lower depth estimate from current numerical models of Sudbury and the similar‐sized Chicxulub impact structures.

¹M.Matthes,²J.A.van Orman,¹T.Kleine
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.07.009]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
²Department of Earth, Environmental and Planetary Sciences, Case Western Reserve University, Cleveland, OH USA
Copyright Elsevier

To better constrain the crystallization and cooling history of the IIIAB iron meteorite parent body, we report new ¹⁰⁷Pd-¹⁰⁷Ag data for metal and troilite samples from the IIIAB iron Cape York, and combine these data with a numerical model for the diffusive exchange of ¹⁰⁷Ag between metal and troilite. We find that the Pd-Ag closure temperature for iron meteorites varies between 500 and 700 °C, and for most irons typically is between 550 and 650 °C. The closure temperature not only depends on cooling rate, grain size, and bulk Ni content, but also on the abundance and distribution of troilite nodules. Specifically, metal in direct contact to troilite has a lower closure temperature than more distant metal. Consistent with this, our new Pd-Ag data show that metals adjacent to troilites have lower Ag contents and plot on shallower Pd-Ag isochrons than more distant metals. These disparate Pd-Ag systematics in metal as a function of distance to troilite provide a new means to determine cooling rates for iron meteorites. Using this approach, we obtained a cooling rate of 67–202 °C/Ma for Cape York, which is in good agreement with metallographic cooling rates for IIIAB irons. This cooling rate combined with the precise Pd-Ag age of Cape York of 5.0±0.4 Ma after solar system formation reveals that the IIIAB core completely solidified at 2.6±1.3 Ma after solar system formation. This rapid crystallization was most likely facilitated by collisional disruption of the IIIAB parent body, which removed most of the insulating mantle and exposed its core.

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.

¹Joseph Razzell Hollis,^1,2Schelin Ireland,¹William Abbey,³Rohit Bhartia,¹Luther W.Beegle
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113969]
¹NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States
²University of Hawaii at Mānoa, Mānoa, HI, United States
³Photon Systems Inc., Covina, CA, United States
Copyright Elsevier

We have measured the deep-ultraviolet (DUV) Raman spectra of a number of evaporite minerals that are relevant to lacustrine and fluvial environments found on Earth and Mars, and show that DUV Raman can provide detailed information on elemental composition and crystal structure. The minerals included three borates, eight carbonates, and seven sulfates, with each class of mineral exhibited very distinct spectra under 248.6 nm excitation, dominated by the various internal vibrations of the borate, carbonate or sulfate oxyanion. Peak positions were shifted by markedly lower wavenumbers vs positions reported in the literature for longer wavelengths, a phenomenon we ascribe to the effect of pre-resonance with the oxyanion. Within each class, minerals consisting different metallic cations could be distinguished by the position of the dominant vibrational mode and the relative intensities of the minor modes, ascribed to the electrostatic impact of the cation on the vibrational behavior of the oxyanion. There was also evidence that DUV Raman can reveal minor metallic components even if they are not apparent in XRD, as two of three calcite (CaCO₃) samples exhibited a shoulder on the dominant peak consistent with perturbation by Mg. UV absorption by Fe^2+/3+ was a major factor in determining measurable signal, with Fe-rich minerals exhibiting weak/undetectable spectra. Understanding the spectra of these evaporite minerals will be essential to interpreting and identifying complex mineral samples using this technique, and represents an important addition to the spectral standards library of DUV Raman spectroscopy.

¹A.Galiano et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113959]
¹INAF-IAPS, Rome, Italy
Copyright Elsevier

The Near-Earth Asteroid 162,173 Ryugu (1999 JU3) was investigated by the JAXA Hayabusa2 mission from June 2018 to November 2019. The data acquired by NIRS3 spectrometer revealed a dark surface with a positive near-infrared spectral slope. In this work we investigated the spectral slope variations across the Ryugu surface, providing information about physical/chemical properties of the surface.

We analysed the calibrated, thermally and photometrically corrected NIRS3 data, and we evaluated the spectral slope between 1.9 μm and 2.5 μm, whose values extend from 0.11 to 0.28 and the mean value corresponds to 0.163±0.022. Starting from the mean value of slope and moving in step of 1 standard deviation (0.022), we defined 9 “slope families”, the Low-Red-Slope families (LR1, LR2 and LR3) and the High-Red-Sloped families (HR1, HR2, HR3, HR4, HR5, HR6). The mean values of some spectral parameters were estimated for each family, such as the reflectance factor at 1.9 μm, the spectral slope, the depth of bands at 2.7 μm and at 2.8 μm. A progressive spectral reddening, darkening and weakening/narrowing of OH bands is observed moving from the LR families to the HR families.

We concluded that the spectral variability observed among families is the result of the thermal metamorphism experienced by Ryugu after the catastrophic disruption of its parent body and space weathering processes that occurred on airless bodies as Ryugu, such as impact cratering and solar wind irradiation. As a consequence, the HR1, LR1, LR2 and LR3 families, corresponding to equatorial ridge and crater rims, are the less altered regions on Ryugu surface, which experienced the minor alteration and OH devolatilization; the HR2, HR3, HR4, HR5 families, coincident with floors and walls of impact craters, are the most altered areas, result of the three processes occurring on Ryugu. The strong reddening of the HR6 family (coincident with Ejima Saxum) is likely due to the fine-sized material covering the large boulder.

^1,2M.D.Suttle,^2,3L.Folco,^2,4M.J.Genge,¹S.S.Russell
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113956]
¹Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK
²Dipartimento di Scienze della Terra, Università di Pisa, 56126 Pisa, Italy
³CISUP, Centro per l’Integrazione della Strumentazione dell’Università di Pisa, Lungarno Pacinotti 43, 56126 Pisa, Italy
⁴Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, South Kensington, London SW7 2AZ, UK
Copyright Elsevier

Comets are typically considered to be pristine remnants of the early solar system. However, by definition they evolve significantly over their lifetimes through evaporation, sublimation, degassing and dust release. This occurs once they enter the inner solar system and are heated by the Sun. Some comets (e.g. 1P/Halley, 9P/Tempel and Hale-Bopp) as well as chondritic porous cosmic dust – released from comets – show evidence of minor aqueous alteration resulting in the formation of phyllosilicates, carbonates or other secondary phases (e.g. Cu-sulphides, amphibole and magnetite). These observations suggest that (at least some) comets experienced limited interaction with liquid water under conditions distinct from the alteration histories of hydrated chondritic asteroids (e.g. the CM and CR chondrites).

This synthesis paper explores the viability of perihelion-induced heating as a mechanism for the generation of highly localised subsurface liquid water and thus mild aqueous alteration in periodic comets. We draw constraints from experimental laboratory studies, numerical modelling, spacecraft observations and microanalysis studies of cometary micrometeorites. Both temperature and pressure conditions necessary for the generation and short-term (hour-long) survival of liquid water are plausible within the immediate subsurface (<0.5 m depth) of periodic comets with small perihelia (<1.5 A.U.), low surface permeabilities and favourable rotational states (e.g. high obliquities and/or slow rotational periods). We estimate that solar radiant heating may generate liquid water and perform aqueous alteration reactions in 3–9% of periodic comets. An example of an ideal candidate is 2P/Encke which has a small perihelion (0.33 A.U.), a high obliquity and a short orbital period. This comet should therefore be considered a high priority candidate in future spectroscopic studies of comet surfaces. Small quantities of phyllosilicate generated by aqueous alteration may be important in cementing together grains in the subsurface of older dormant comets, thereby explaining observations of unexpectedly high tensile strength in some bodies.

Most periodic comets which currently pass close to the Sun are dormant, having experienced surface heating, significant cometary activity and dust release in the past. These bodies may be responsible for the partially hydrated cometary micrometeorites we find at the Earth’s surface and their aqueous alteration histories may have been produced by perihelion-induced subsurface heating. This is in contrast to radiogenic and impact heating that operated during the early solar system on asteroids. This study has implications for the alteration history of the active asteroid Phaethon, the target of JAXA’s DESTINY+ mission.