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Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC(3)). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70–80% are ureilites (achondrites) and 20–30% are various types of chondrites. Asteroid 2008 TC(3) was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC(3). AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (

We report petrologic studies and oxygen isotope analyses of normal and anomalous eucrites, termed eucrite-type achondrites. Petrologically anomalous eucrite-type achondrites can have normal oxygen isotope compositions, and vice versa. Two basaltic eucrites with normal oxygen isotope compositions contain pyroxenes with anomalous Fe/Mn engendered by parent body processes acting on normal eucrites: solid-state reduction by S gas in EET 87542, and reduction during crystallization by magmatic S in QUE 94484. Cataclastic basaltic breccias PCA 82502 and PCA 91007 are paired (petrology, anomalous oxygen). Although isotopically like Pasamonte, they are petrologically distinct. We confirm the petrological and isotopic anomalies of cumulate gabbro EET 92023; likely formed by impact melting of mixed cumulate and basaltic materials. Many main group eucrites include plagioclases that retain near-liquidus compositions despite metamorphic overprinting. Stannern group eucrites contain more sodic plagioclase, which is consistent with the melt hybridization hypothesis for Stannern group magma formation. The lack of more calcic plagioclase suggests reactive exchange of the anorthite component of the primary melt with the albitic component of the crust. Asteroids that are modestly different in composition can produce virtually indistinguishable basalts, providing a ready explanation for the eucrite-type achondrite suite. Small stochastic variations in petrologic evolution can cause substantial differences in rocks produced on an asteroid.

Northwest Africa (NWA) 1500 is an ultramafic meteorite dominated by coarse (∼100- 500 μm) olivine (95-96%), augite (2-3%), and chromite (0.6-1.6%) in an equilibrated texture. Plagioclase (0.7-1.8%) occurs as poikilitic grains (up to ∼3 mm) in vein-like areas that have concentrations of augite and minor orthopyroxene. Other phases are Cl-apatite, metal, sulfide, and graphite. Olivine ranges from Fo 65-73, with a strong peak at Fo 68-69. Most grains are reverse- zoned, and also have ∼10-30 μm reduction rims. In terms of its dominant mineralogy and texture, NWA 1500 resembles the majority of monomict ureilites. However, it is more ferroan than known ureilites (Fo ≥75) and other mineral compositional parameters are out of the ureilite range as well. Furthermore, neither apatite nor plagioclase have ever been observed, and chromite is rare in monomict ureilites. Nevertheless, this meteorite may be petrologically related to the rare augite-bearing ureilites and represent a previously unsampled part of the ureilite parent body (UPB). The Mn/Mg ratio of its olivine and textural features of its pyroxenes are consistent with this interpretation. However, its petrogenesis differs from that of known augite- bearing ureilites in that: 1) it formed under more oxidized conditions; 2) plagioclase appeared before orthopyroxene in its crystallization sequence; and 3) it equilibrated to significantly lower temperatures (800-1000 °C, from two-pyroxene and olivine-chromite thermometry). Formation under more oxidized conditions and the appearance of plagioclase before orthopyroxene could be explained if it formed at a greater depth on the UPB than previously sampled. However, its significantly different thermal history (compared to ureilites) may more plausibly be explained if it formed on a different parent body. This conclusion is consistent with its oxygen isotopic composition, which suggests that it is an ungrouped achondrite. Nevertheless, the parent body of NWA 1500 may have been compositionally and petrologically similar to the UPB, and may have had a similar differentiation history.

Basaltic glasses from the Mid-Cayman Rise (MCR) spreading center have a significant range of compositions, from 7.7% to 5.4% MgO. Well-defined trends in the major element and trace element contents of these glasses are consistent with their development by up to 50% crystallization of parental magmas with only a small range of compositional variability. Calculated liquid lines of descent and 1 atm phase equilibria studies of primitive MCR glasses, as well as petrographic relations, indicate that olivine and plagioclase (±spinel) crystallize from MCR liquids at low pressures until the residual liquid has

Five differences are noted between the compositions of minerals in oceanic cumulates and minerals that crystallize from mid-ocean ridge basalts (MORBs) at 1 atm. These differences are (1) Mg numbers of clinopyroxene (CPX) in oceanic cumulates are often higher (>88) than found in experimental studies of MORBs at 1 atm; (2) Mg numbers of orthopyroxene (OPX) in oceanic cumulates are often higher (>81) than found in experimental studies of MORBs at 1 atm; (3) the low-Ca pyroxene in oceanic cumulates (with some notable exceptions) is OPX rather than (inverted) pigeonite; (4) Na2O contents of CPX in oceanic cumulates are often higher (0.3 to 0.8% Na2O) than in 1-atm experiments or phenocrysts in MORBs (typically 0.15 to 0.35% Na2O); and (5) TiO2 contents of CPX in oceanic cumulates are often higher than in 1-atm experiments. These characteristics are consistent with crystallization of many oceanic gabbros at moderate pressures (3–10 kbar). The Na2O and TiO2 contents of liquids calculated to be in equilibrium with CPXs from oceanic cumulates generally are comparable to the abundances of these oxides in MORBs. The notable exception is the suite of cumulates from Deep Sea Drilling Project site 334, which appears to have crystallized at low pressures from strongly depleted Na2O-poor, TiO2-poor, SiO2-rich basalts. We suggest that such strongly depleted basaltic liquids are a widespread, but minor, component in mid-ocean ridge petrogenesis even though such liquids have never been directly sampled.

Oxygen isotopic variations across the outer margins and Wark–Lovering (WL) rims of a diverse suite of six coarse-grained Types A and B refractory inclusions from both oxidized and reduced CV3 chondrites suggest that CAIs originated from a 16 O-rich protosolar gas reservoir and were later exposed to both relatively 17,18 O-rich and 16 O-rich reservoirs. The O-isotope profiles of CAIs can be explained by changes in the composition of gas near the protoSun or the migration of CAIs through a heterogeneous nebula. Variability within the inclusion interiors appears to have been set prior to WL rim growth. Modeling the isotopic zoning profiles as diffusion gradients between inclusion interiors and edges establishes a range of permissible time–temperature combinations for their exposure in the nebula. At mean temperatures of 1400 K, models that match the isotope gradients in the inclusions yield timescales ranging from 5 × 10 3 to 3 × 10 5 years. Assuming CAIs originated with a relatively 16 O-rich (protosolar) isotopic composition, differences among the melilite interiors and the isotopic gradients in their margins imply the existence of a number of isotopically distinct reservoirs. Evidence at the edges of some CAIs for subsequent isotopic exchange may relate to the beginning of rim formation. In the WL rim layers surrounding the interiors, spinel is relatively 16 O-rich but subtly distinct among different CAIs. Melilite is often relatively 16 O-poor, but rare relatively 16 O-rich grains also exist. Pyroxene generally exhibits intermediate O-isotope compositions and isotopic zoning. Olivine in both WL and accretionary rims, when present, is isotopically heterogeneous. The extreme isotopic heterogeneity among and within individual WL rim layers and in particular, the observed trends of outward 16 O-enrichments, suggest that rims surrounding CAIs contained in CV3 chondrites, like the inclusions themselves, formed from a number of isotopically distinct gas reservoirs. Collectively, these results support numerical protoplanetary disk models in which CAIs were transported between several distinct nebular reservoirs multiple times prior to accretion onto a parent body.