¹Jemma Davidson,¹Conel M.O’D.Alexander,²Rhonda M.Stroud,³Henner Busemann,¹Larry R.Nittler
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.08.032]
¹Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington DC, 20015-1305, USA
²Naval Research Laboratory Code 6366, 4555 Overlook Ave. SW, Washington, DC 20375, USA
³Institute of Petrology and Geochemistry, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
Copyright Elsevier

Here we report the relative degrees of thermal metamorphism for five Antarctic Ornans-like carbonaceous (CO) chondrites, including Dominion Range (DOM) 08006, as determined from the Cr-content of their FeO-rich (ferroan) olivine. These five CO3 chondrites complete the previously poorly-defined CO3.00 to 3.2 chondrite metamorphic trend. DOM 08006 appears to be a highly primitive CO chondrite of petrologic type 3.00. We report the detailed mineralogy and petrography of DOM 08006 using a coordinated, multi-technique approach.

The interchondrule matrix in DOM 08006 consists of unequilibrated mixtures of silicate, metal, and sulfide minerals and lacks Fe-rich rims on silicates indicating that DOM 08006 has only experienced minimal, if any, thermal metamorphism. This is also reflected by the Co/Ni ratios of Ni-rich and Ni-poor metal, a sensitive indicator of thermal metamorphism, and the presence of euhedral chrome-spinel grains, which typically become subhedral to anhedral during progressive metamorphism. DOM 08006 matrix shows minor evidence for aqueous alteration and while the presence of magnetite surrounding metal in chondrules indicates that there has been some interaction with fluid, much metal remains and none of the sulfides analyzed show evidence of being formed by aqueous alteration. Furthermore, the plagioclase of ∼50% of chondrules analyzed show resolvable excess silica indicating that these chondrules have experienced minimal, if any, reprocessing in the CO parent body.

Noble gas data for DOM 08006 show that it contains the highest concentrations of trapped ³⁶Ar and ¹³²Xe of all CO chondrites analyzed to date, further indicating that DOM 08006 is the most primitive CO chondrite known. The cosmic ray exposure age of DOM 08006 is estimated to be ∼19 Ma.

The minimally altered nature of DOM 08006 demonstrates that it is an extremely important sample for providing valuable insight into early Solar System conditions. At a total weight of 667 g, a significant amount of material is available for a wide array of future studies.

^1,2Adam R.Sarafian,^1,3Sune G.Nielsen,^3,4Horst R.Marschall,³Glenn A.Gaetani,⁵Kevin Righter,⁶Eve L.Berger
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.08.023]
¹NIRVANA Laboratories, Woods Hole Oceanographic Institution, Woods Hole, MA 02540, USA
²Corning Research and Development Corporation, Corning, NY 14873
³Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02540, USA
⁴Institut für Geowissenschaften, Goethe Universtät Frankfurt, 60438 Frankfurt am Main, Germany
⁵NASA-JSC, Mailcode XI2, 2101 NASA Pkwy, Houston, TX 77058, USA
⁶GeoControl Systems Inc. – Jacobs JETS Contract –NASA JSC, USA
Copyright Elsevier

The processes that controlled accretion of water and volatiles to the inner solar system remain enigmatic, because it is difficult to determine the absolute concentrations of volatile elements in planetary bodies. In this contribution we study rare unequilibrated eucrites derived from the asteroid 4 Vesta, to determine the water and fluorine content of this asteroid by measuring the volatile content of pyroxene. Common thermal metamorphism in most equilibrated eucrites would have diffusively reset magmatic volatile contents. The unequilibrated eucrites are, therefore, the most suitable samples to determine primary magmatic volatile contents of 4 Vesta. We find H₂O and F contents in pyroxenes of 4–11 µg/g and 0.12–0.23 µg/g. We also determine a H₂O partition coefficient for clinopyroxene and melt equilibrated at 0.1 MPa of D_H2O = 0.1, which is higher than values previously reported for higher pressures. The higher compatibility of H₂O in this experiment could partially be due to high OH/H₂O ratio at the low total water contents in this experimental charge, but only further more detailed experiments will fully delineate the reasons for the more compatible behavior for water at lower pressures. However, given the lack of H₂O partitioning data at low pressures we conclude that our 0.1 MPa experiment is the most appropriate to calculate magmatic water contents for melt in equilibrium with eucrite pyroxene. After using appropriate partition coefficients we calculate melt concentrations of 50–70 µg/g H₂O and 1.5–2.4 µg/g F. In turn, these are converted into bulk 4 Vesta water and F contents of 10–70 µg/g H₂O and 0.3–2 µg/g F by assuming eucrite formation via either mantle partial melting or extraction from a magma ocean. We also measure the D/H of the clinopyroxenes and show that these are identical to the results of previous studies that reported D/H in eucrite apatite. These values match those found in carbonaceous chondrites suggesting that water in 4 Vesta accreted from carbonaceous chondrites and not from cometary material.