¹Chi Ma, ²Alexander N. Krot, ²Kazuhide Nagashima
American Mineralogist 102, 1556-1560 Link to Article [https://doi.org/10.2138/am-2017-6032]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96822, U.S.A.
Copyright: The Mineralogical Society of America

Addibischoffite (IMA 2015-006), Ca2Al6Al6O20, is a new calcium aluminate mineral that occurs with hibonite, perovskite, kushiroite, Ti-kushiroite, spinel, melilite, anorthite, and FeNi-metal in the core of a Ca-Al-rich inclusion (CAI) in the Acfer 214 CH3 carbonaceous chondrite. The mean chemical composition of type addibischoffite measured by electron probe microanalysis is (wt%) Al2O3 44.63, CaO 15.36, SiO2 14.62, V2O3 10.64, MgO 9.13, Ti2O3 4.70, FeO 0.46, total 99.55, giving rise to an empirical formula of (Ca2.00)(Al2.55Mg1.73V1.3+08Ti3+0.50Ca0.09Fe2+0.05)∑6.01(Al4.14Si1.86)O20. The general formula is Ca2(Al,Mg,V,Ti)6(Al,Si)6O20. The end-member formula is Ca2Al6Al6O20. Addibischoffite has the P1̄ aenigmatite structure with a = 10.367 Å, b = 10.756 Å, c = 8.895 Å, α = 106.0°, β = 96.0°, γ = 124.7°, V = 739.7 Å3, and Z = 2, as revealed by electron backscatter diffraction. The calculated density using the measured composition is 3.41 g/cm3. Addibischoffite is a new member of the warkite (Ca2Sc6Al6O20) group and a new refractory phase formed in the solar nebula, most likely as a result of crystallization from an 16O-rich Ca, Al-rich melt under high-temperature (~1575 °C) and low-pressure (~10−4 to 10−5 bar) conditions in the CAI-forming region near the protosun, providing a new puzzle piece toward understanding the details of nebular processes. The name is in honor of Addi Bischoff, cosmochemist at University of Münster, Germany, for his many contributions to research on mineralogy of carbonaceous chondrites, including CAIs in CH chondrites.

¹Kent C. Condie, ²Charles K. Shearer
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.034]
¹Department of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801, USA
²Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, USA
Copyright Elsevier

The distribution of high field strength incompatible element ratios Zr/Nb, Nb/Th, Th/Yb and Nb/Yb in terrestrial oceanic basalts prior to 2.7 Ga suggests the absence or near-absence of an enriched mantle reservoir. Instead, most oceanic basalts reflect a variably depleted mantle source similar in composition to primitive mantle. In contrast, basalts from hydrated mantle sources (like those associated with subduction) exist from 4 Ga onwards. The gradual appearance of enriched mantle between 2 and 3 Ga may reflect the onset and propagation of plate tectonics around the globe. Prior to 3 Ga, Earth may have been in a stagnant-lid regime with most basaltic magmas coming from a rather uniform, variably depleted mantle source or from a non-subduction hydrated mantle source. It was not until the extraction of continental crust and accompanying propagation of plate tectonics that “modern type” enriched and depleted mantle reservoirs developed. Consistent with the absence of plate tectonics on the Moon is the near absence of basalts derived from depleted (DM) and enriched (EM) mantle reservoirs as defined by the four incompatible element ratios of this study. An exception are Apollo 17 basalts, which may come from a mixed source with a composition similar to primitive mantle as one end member and a high-Nb component as the other end member. With exception of Th, which requires selective enrichment in at least parts of the martian mantle, most martian meteorites can be derived from sources similar to terrestrial primitive mantle or by mixing of enriched and depleted mantle end members produced during magma ocean crystallization. Earth, Mars and the Moon exhibit three very different planetary evolution paths. The mantle source regions for Mars and the Moon are ancient and have HFS element signatures of magma ocean crystallization well-preserved, and differences in these signatures reflect magma ocean crystallization under two distinct pressure regimes. In contrast, plate tectonics on Earth has destroyed most or all of the magma ocean crystallization geochemical record, or less likely, the terrestrial magma ocean may not have been strongly fractionated during crystallization. The rather uniform incompatible element ratio record in pre-2 Ga oceanic terrestrial basalts requires vigorous mixing of most of the mantle between magma ocean crystallization and about 4 Ga, the onset of the preserved greenstone record.

¹Nicolas D. Greber, ¹Nicolas Dauphas, ²Igor S. Puchtel, ³Beda A. Hofmann, ⁴Nicholas T. Arndt
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.033]
¹Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60615, USA
²Department of Geology, University of Maryland, College Park, MD 20742, USA
³Naturhistorisches Museum der Burgergemeinde Bern, Bernastrasse 15, Bern, Switzerland
⁴Université Grenoble Alpes, Institute Science de la Terre (ISTerre), CNRS, F-38041 Grenoble, France
Copyright Elsevier

Titanium isotopes are potential tracers of processes of evaporation/condensation in the solar nebula and magmatic differentiation in planetary bodies. To gain new insights into the processes that control Ti isotopic variations in planetary materials, 25 komatiites, 15 chondrites, 11 HED-clan meteorites, 5 angrites, 6 aubrites, a martian shergottite, and a KREEP-rich impact melt breccia have been analyzed for their mass-dependent Ti isotopic compositions, presented using the δ49Ti notation (deviation in permil of the 49Ti/47Ti ratio relative to the OL-Ti standard). No significant variation in δ 49Ti is found among ordinary, enstatite, and carbonaceous chondrites, and the average chondritic δ49Ti value of +0.004 ± 0.010 ‰ is in excellent agreement with the published estimate for the bulk silicate Earth, the Moon, Mars, and the HED and angrite parent-bodies. The average δ49Ti value of komatiites of -0.001 ± 0.019 ‰ is also identical to that of the bulk silicate Earth and chondrites. OL-Ti has a Ti isotopic composition that is indistinguishable from chondrites and is therefore a suitable material for reporting δ49Ti values. Previously published isotope data on another highly refractory element, Ca, show measurable variations among chondrites. The decoupling between Ca and Ti isotope systematics most likely occurred during condensation in the solar nebula.

Aubrites exhibit significant variations in δ49Ti, from -0.07 to +0.24 ‰. This is likely due to the uniquely reducing conditions under which the aubrite parent-body differentiated, allowing chalcophile Ti3+ and lithophile Ti4+ to co-exist. Consequently, the observed negative correlation between δ49Ti values and MgO concentrations among aubrites is interpreted to be the result of isotope fractionation driven by the different oxidation states of Ti in this environment, such that isotopically heavy Ti4+ was concentrated in the residual liquid during magmatic differentiation.

Finally, KREEPy impact melt breccia Sau 169 exhibits a heavy δ49Ti of +0.330 ± 0.034 ‰ which is interpreted to result from Ti isotopic fractionation during ilmenite precipitation in the late stages of lunar magma ocean crystallization. A Rayleigh distillation calculation assuming a crystallization temperature of 1175°C predicts that a δ49Ti value of +0.330 ‰ is achieved after removal of 94% of the Ti in ilmenite with an ilmenite-melt Ti isotopic fractionation of –0.12‰.

^1,2My E.I. Riebe, ¹Liliane Huber, ³Knut Metzler, ¹Henner Busemann, ¹Stefanie M. Luginbuehl, ¹Matthias M.M. Meier, ¹Colin Maden, ¹Rainer Wieler
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.035]
¹Institute of Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, CH-8092 Zürich, Switzerland
²Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington D.C. 2015-1305, USA
³Institut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Strasse 10, D-48149 Münster, Germany
Copyright Elsevier

Whether or not some meteorites retain a record of irradiation by a large flux of energetic particles from the early sun in the form of excesses of cosmic-ray produced noble gases in individual crystals or single chondrules is a topic of ongoing debate. Here, we present He and Ne isotopic data for individual chondrules in Murchison, a chondritic regolith breccia of the CM group. We separated 27 chondrules from a clastic matrix portion and 26 chondrules from an adjacent single so-called “primary accretionary rock” (Metzler et al., 1992). All chondrules from the primary rock fragment are expected to share a common regolith history, whereas chondrules from the clastic matrix were stirred in the regolith independently of each other. All “primary rock chondrules” and 23 of the “matrix chondrules” have very similar concentrations of cosmogenic 3He and 21Ne, corresponding to a cosmic-ray exposure age to galactic cosmic rays (GCR) of ∼1.3 – 1.9 Ma, in the range of Murchison’s meteoroid exposure age determined with cosmogenic radionuclides. Four clastic matrix chondrules contain excesses of cosmogenic 3He and 21Ne, corresponding to nominal 4π exposure ages of ∼4 to ∼29 Ma, with a Ne isotopic composition as expected for production by GCR. If the fraction of excess cosmogenic gas bearing chondrules in the primary rock and clastic matrix were the same, we would expect this result with a statistical probability of only 0.5 – 2.7%. Therefore, the exposure age distributions for Murchison chondrules in primary rock and clastic matrix are very likely different. Such a difference is expected if the excess cosmogenic gas was acquired by some of the matrix chondrules in the regolith, but not if chondrules were irradiated in the solar nebula by the early sun before they accreted on the Murchison parent body. Therefore, Murchison does not provide evidence for irradiation by a high fluence of energetic particles from the early sun. By inference, this statement likely holds for the other regolithic meteorites for which large occasional excesses of cosmogenic noble gases have been reported. Considering pre-irradiation in a regolith (2π exposure), the pre-exposure times for these four chondrules are at least between some 4 and 40 Ma near the very surface of the parent body, and even longer if they were buried deeper in the regolith.

¹Josiah B. Lewis, ¹Christine Floss, ¹Frank Gyngard
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.008]
¹Laboratory for Space Sciences, Physics Department, Washington University, St. Louis, MO, USA
Copyright Elsevier

Meteoritic nanodiamonds carry noble gases with anomalies in their stable isotopes that have drawn attention to their potentially presolar origin. Measurements of 12C/13C isotope ratios of presolar nanodiamonds are essential to understanding their origins, but bulk studies do not show notable deviations from the solar system 12C/13C ratio.

We implemented a technique using secondary ion mass spectrometry with maximized spatial resolution to measure carbon isotopes in the smallest clusters of nanodiamonds possible. We measured C and Si from clusters containing as few as 1000 nanodiamonds, the smallest clusters of nanodiamonds measured to date by traditional mass spectrometry. This allowed us to investigate many possible complex compositions of the nanodiamonds, both through direct methods and statistical analysis of the distributions of observed isotopic ratios.

Analysis of the breadth of distributions of carbon isotopic ratios for a number of ∼1000-nanodiamond aggregates indicates that the 12C/13C ratio may be drawn from multiple Gaussian distributions about different isotopic ratios, which implies the presence of presolar material. The mean isotopic ratio is consistent with the solar system value, so presolar components are required to be either low in concentration, or to have a mean ratio close to that of the solar system. Supernovae are likely candidates for the source of such a presolar component, although asymptotic giant branch stars are not excluded.

A few aggregates show deviations from the mean 12C/13C ratio large enough to be borderline detections of enrichments in 13C. These could be caused by the presence of a small population of nanodiamonds formed from sources that produce extremely 13C-rich material, such as J-stars, novae, born-again asymptotic giant branch stars, or supernovae. Of these possible sources, only supernovae would account for the anomalous noble gases carried in the nanodiamonds.