¹M.D.Suttle,²A.Greshake,^1,3A.J.King,¹P.F.Schofield,⁴A.Tomkins,¹S.S.Russell
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.11.008]
¹Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, UK
²Museum für Naturkunde, Leibniz-Institut für Evolutions und Biodiversitätsforschung, Invalidenstraße 43, 10115 Berlin, Deutschland
³Planetary and Space Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK
⁴School of Earth, Atmosphere and Environment, Melbourne, Victoria, Australia
Copyright Elsevier

We provide the first detailed analysis of the carbonaceous chondrite Dhofar (Dho) 1988. This meteorite find was recovered in 2011 from the Zufar desert region of Oman and initially classified as a C2 ungrouped chondrite. Dho 1988 is a monomict breccia composed of millimetre-sized clasts, between which large (∼50-250µm) intermixed sulphide-Ca-carbonate veins formed. It has high sulphide abundances (∼14 vol%), medium-sized chondrules (avg. 530µm, N=33), relatively low chondrule/CAI abundances (<20 area%), a heavy bulk O-isotope composition (δ¹⁷O=9.12‰, δ¹⁸O=19.46‰) and an aqueously altered and then dehydrated alteration history. These characteristics are consistent with the newly defined Yamato-type (CY) carbonaceous chondrite group, suggesting this meteorite should be reclassified as a CY chondrite.

Dho 1988 experienced advanced aqueous alteration (petrologic subtype 1.3 in the scheme of Howard et al., [2015]). Alteration style and extent are similar to the CM chondrite group, with the matrix having been replaced by tochilinite-cronstedtite intergrowths and chondrules progressively pseudomorphed by phyllosilicates, sulphides and in one instance Ca-carbonates. However, departures from CM-like alteration include the replacement of chondrule cores with Al-rich, Na-saponite and upon which Cr-spinel and Mg-ilmenite grains precipitated. These late-stage aqueous alteration features are common among the CY chondrites. Fractures in Dho 1988 that are infilled by phyllosilicates, sulphides and carbonates attest to post-brecciation aqueous alteration. However, whether aqueous alteration was also active prior to brecciation remains unclear. Veins are polymineralic with a layered structure, allowing their relative chronology to be reconstructed: intermixed phyllosilicate-sulphide growth transitioned to sulphide-carbonate deposition. We estimate temperatures during aqueous alteration to have been between 110°C<T<160°C, based on the co-formation of Na-saponite and tochilinite.

Dho 1988 was later overprinted by thermal metamorphism. Peak temperatures are estimated between 700°C and 770°C, based on the thermal decomposition of phyllosilicates (both serpentine and saponite) combined with the survival of calcite. As temperatures rose during metamorphism the thermal decomposition of pyrrhotite produced troilite. Sulphur gas was liberated in this reaction and flowed through the chondrite reacting with magnetite (previously formed during aqueous alteration) to form a second generation of troilite grains. The presence of both troilite and Ni-rich metal in Dho 1988 (and other CY chondrites) demonstrate that conditions were constrained at the iron-troilite buffer.

¹J.R.Darling et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.11.007]
¹School of the Environment, Geography and Geosciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth, PO1 3QL, United Kingdom
Copyright Elsevier

The elemental and chlorine isotope compositions of calcium-phosphate minerals are key recorders of the volatile inventory of Mars, as well as the planet’s endogenous magmatic and hydrothermal history. Most martian meteorites have clear evidence for exogenous impact-generated deformation and metamorphism, yet the effects of these shock metamorphic processes on chlorine isotopic records contained within calcium phosphates have not been evaluated. Here we test the effects of a single shock metamorphic cycle on chlorine isotope systematics in apatite from the highly shocked, enriched shergottite Northwest Africa (NWA) 5298. Detailed nanostructural (EBSD, Raman and TEM) data reveals a wide range of distributed shock features. These are principally the result of intensive plastic deformation, recrystallization and/or impact melting. These shock features are directly linked with chemical heterogeneities, including crosscutting microscale chlorine-enriched features that are associated with shock melt and iron-rich veins. NanoSIMS chlorine isotope measurements of NWA 5298 apatite reveal a range of δ37Cl values (-3 to 1 ‰; 2σ uncertainties <0.9 ‰) that is almost as large as all previous measurements of basaltic shergottites, and the measured δ37Cl values can be readily linked with different nanostructural states of targeted apatite. High spatial resolution atom probe tomography (APT) data reveal that chlorine-enriched and defect-rich nanoscale boundaries have highly negative δ37Cl values (mean of -15 ± 8 ‰). Our results show that shock metamorphism can have significant effects on chemical and chlorine isotopic records in calcium phosphates, principally as a result of chlorine mobilization during shock melting and recrystallization. Despite this, low-strain apatite domains have been identified by EBSD, and yield a mean δ37Cl value of -0.3 ± 0.6 ‰ that is taken as the best estimate of the primary chlorine isotopic composition of NWA 5298. The combined nanostructural, microscale-chemical and nanoscale APT isotopic approach gives the ability to better isolate and identify endogenous volatile-element records of magmatic and near-surface processes as well as exogenous, shock-related effects.

^1,2,3M.D.Suttle,^2,4L.Folco,^1,3M.J.Genge,⁵I.A.Franchi,^2,6F.Campanale,⁶E.Mugnaioli,⁶X.Zhao
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.11.006]
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
³Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, South Kensington, London, SW7 2AZ, UK
⁴CISUP, Centro per l’Integrazione della Strumentazione dell’Università di Pisa, Lungarno Pacinotti 43, 56126 Pisa, Italy
⁵School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
⁶Center for Nanotechnology Innovation @NEST, Istituto Italiano di Tecnologia (IIT), Piazza San Silvestro 12, 56127 Pisa, Italy
Copyright Elsevier

We analysed the heterogenous fine-grained (sub-μm) matrix of a small (58×93μm), unmelted and minimally heated (<350°C) micrometeorite (CP94-050-052) recovered from Antarctic blue ice. This particle contains some unaltered highly primitive phases, including refractory anhydrous high-Mg silicates and submicron crystalline needle-shaped acicular grains interpreted as enstatite whiskers. The particle also contains an abundance of micron-sized Fe-rich grains, which span a compositional and textural continuum between amorphous oxygen-rich silicate and poorly crystalline Fe-rich phyllosilicate (cronstedtite). These Fe-rich grains are here interpreted as secondary phases formed by aqueous alteration. Their inferred anhydrous precursors were likely primitive “GEMS-like” amorphous Fe-Mg-silicates. This micrometeorite’s bulk chemical composition and mineralogy suggest either a carbonaceous chondrite or cometary origin. However, the particle’s average O-isotope composition (δ17O: -12.4‰ [±5.0‰], δ18O: -24.0‰ [±2.3‰] and Δ17O at +0.1‰ [±4.8‰] is distinct from all previously measured chondritic materials. Instead this value is intermediate between primitive chondritic materials and the composition of Antarctic water – strongly implying that the particle was heavily affected by Antarctic alteration. Analysis of the micrometeorite’s H-isotopes reveals low deuterium abundances (δD: -217‰ to -173‰ [±43-47‰]) paired with high H abundances (and thus high water contents [<25wt.%]). Although both water contents and H-isotope compositions overlap with those reported in CM chondrites, the datapoints measured from CP94-050-052 extend to more extreme values. Further supporting the idea that the aqueous alteration that affected this micrometeorite operated under different environmental conditions to asteroidal settings. These data collectively demonstrate partial isotopic exchange with light (δ18O-poor, δD-poor) terrestrial fluids whilst the micrometeorite resided in Antarctica. Although this micrometeorite may have been aqueously altered whilst on its parent body this cannot be conclusively demonstrated due to the extent of the weathering overprint. Antarctic alteration operated at significantly higher water-to-rock ratios than chondritic settings. Despite these differences the extent of secondary replacement and the duration of alteration were limited with mafic silicates remaining unaffected. The combined alteration conditions for this particle likely operated over short timescales (<24hrs), under mildly alkaline conditions (∼pH8) and at low temperatures (<50°C), this could have occurred during the micrometeorite’s extraction from blue ice.