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A literature compilation of 1136 low‐Ca pyroxene compositions from chondrules from 12 primitive type 2–3 carbonaceous, ordinary and enstatite chondrite groups define unique regions on an Al2O3 and Cr2O3 diagram when compared to low‐Ca pyroxenes from equilibrated type 4‐6 chondrites. Measured compositions of 100 low‐Ca pyroxenes from comet Wild 2 and a giant cluster IDP of probable cometary origin are similar to each other and fall in the type 2–3 chondrite chondrule region suggesting that most of the pyroxenes likely formed in the solar nebula like conventional chondrules. The data imply that most low Ca‐pyroxenes from comet Wild 2 and the giant cluster IDP formed from igneous crystallization processes and did not experience significant thermal metamorphism, indicating that the low‐Ca pyroxenes were unlikely incorporated into large parent bodies prior to accretion in their respective comet bodies. An intriguing group of nine low‐Ca pyroxenes from comet Wild 2 with low Cr and Al that fall where type 4–6 chondrites are located are interpreted as products of condensation. The compositional data combined with previously measured oxygen isotopes on 17 low‐Ca pyroxenes support earlier conclusions that comet samples have links with carbonaceous, ordinary, and possibly enstatite chondrite groups. Our results provide additional evidence that comets accreted materials from multiple chondrule reservoirs throughout the solar nebula. [ABSTRACT FROM AUTHOR]

High-precision secondary ion mass spectrometry ( SIMS) was employed to investigate oxygen three isotopes of phenocrysts in 35 chondrules from the Yamato (Y) 82094 ungrouped 3.2 carbonaceous chondrite. Twenty-one of 21 chondrules have multiple homogeneous pyroxene data (∆17O 3 SD analytical uncertainty: 0.7‰); 17 of 17 chondrules have multiple homogeneous pyroxene and plagioclase data. Twenty-one of 25 chondrules have one or more olivine data matching coexisting pyroxene data. Such homogeneous phenocrysts (1) are interpreted to have crystallized from the final chondrule melt, defining host O-isotope ratios; and (2) suggest efficient O-isotope exchange between ambient gas and chondrule melt during formation. Host values plot within 0.7‰ of the primitive chondrule mineral ( PCM) line. Seventeen chondrules have relict olivine and/or spinel, with some δ17O and δ18O values approaching −40‰, similar to CAI or AOA-like precursors. Regarding host chondrule data, 22 of 34 have Mg#s of 98.8-99.5 and ∆17O of −3.9‰ to −6.1‰, consistent with most Acfer 094, CO, CR, and CV chondrite chondrules, and suggesting a common reduced O-isotope reservoir devoid of 16O-poor H2O. Six Y-82094 chondrules have ∆17O near −2.5‰, with Mg#s of 64-97, consistent with lower Mg# chondrules from Acfer 094, CO, CR, and CV chondrites; their signatures suggest precursors consisting of those forming Mg# ~99, ∆17O: −5‰ ± 1‰ chondrules plus 16O-poor H2O, at high dust enrichments. Three type II chondrules plot slightly above the PCM line, near the terrestrial fractionation line (∆17O: ~+0.1‰). Their O-isotopes and olivine chemistry are like LL3 type II chondrules, suggesting they sampled ordinary chondrite-like chondrule precursors. Finally, three Mg# >99 chondrules have ∆17O of −6.7‰ to −8.1‰, potentially due to 16O-rich refractory precursor components. The predominance of Mg# ~99, ∆17O: −5‰ ± 1‰ chondrules and a high chondrule-to-matrix ratio suggests bulk Y-82094 characteristics are closely related to anhydrous dust sampled by most carbonaceous chondrite chondrules. [ABSTRACT FROM AUTHOR]

Introduction: Unraveling the origins of particles in comet Wild 2 is the primary goal of the NASA Stardust (SD) mission that returned samples of a comet believed to have formed in the Kuiper Belt (KB). Some relatively wellpreserved particles have chondrule-like textures and chondrule-like oxygen isotope compositions supporting a high temperature inner Solar System (SS) origin [1-3]. However, ~1/2 of all Wild 2 particles are single monomineralic olivine or pyroxene or simple mixtures of the two [4], thus many SD grains lack petrographic and petrologic context information which complicates efforts to understand their origins. Here we use the minor elements Al, Cr and Mn in SD low-Ca pyroxenes to provide insights into their source regions. To facilitate this study we compiled nearly 600 Lpx chemical analyses from the literature from chondrites of petrographic types 3-6 from 8 groups including CM, CO, CV, CR, H, L, LL and Acfer 094 [5-14]. To our knowledge this represents the largest compilation of Lpx compositions from such a diverse group of chondrites to date. Our study provides evidence that most Lpx grains in the Wild 2 samples formed by igneous processes similar to chondrules in primitive chondrites (type 3) and some likely formed by gas-solid condensation in the nebula. The data imply that the Lpx comet grains did not experience significant thermal metamorphism and thus were not incorporated into parent bodies prior to comet Wild 2 and were likely transported directly to the KB after formation in the nebula. Techniques/Methods: Sixty five Lpx grains from 16 SD tracks, ~1/2 of which were isolated monomineralic fragments, were analyzed for major and minor elements by standard TEM/EDX techniques using a Tecnai TF20 TEM/STEM located at the University of Washington. To prepare the grains for TEM analyses, the samples were flattened in aerogel keystones or bulb wall fragments and embedded in acrylic or epoxy resin and microtomed to <70nm thickness and mounted on Cu or Au TEM grids supporting 10nm C films. Results: A literature compilation of 499 analyses of Lpx minerals from chondrules in type 3 chondrites (8 groups) shows a distinct separation from Lpx minerals in types 4-6 chondrites (82 analyses) [5-14] on a Al2O3 vs Cr2O3 plot. In chondrules from type 3 chondrites, Lpx falls above 0.4 wt% Cr2O3 and has variable Al2O3 while Lpx in types 4-6 chondrites is metamorphically depleted in both Al and Cr. Within the type 3 groups, Lpx in ordinary chondrites (OC) can be distinguished from carbonaceous chondrites (CC) because of generally lower Al2O3 concentrations. Al and Cr systematics provide evidence for the origin of many Lpx grains in Wild 2 samples similar to chondrules. Fifty five Wild 2 Lpx minerals (Mg# 77-99, MnOave = 0.8 wt%) fall in the type 3 region while 10 Lpx (9 of 10 have Mg# > 99 and MnO < 0.25 wt%) plot in the depleted Cr and Al types 4-6 region, but the latter is not a result of metamorphism. Discussion: Elevated Cr and Al in 55 Lpx grains (15 tracks) from Wild 2 demonstrates that these pyroxenes are similar to Lpx in chondrules from type 3 OC and CC groups. Oxygen isotopes previously measured on 8 of these Wild 2 Lpx grains (Al2O3 > 0.9 wt%) have negative Δ17O values (ave= -2.3 ‰) showing CC affinities and may be aligned with CR chondrites [15-17]. Because of their compositional purity (low Fe abundances and low Al, Cr and Mn) the 9 of the 10 remaining Wild 2 Lpx grains are not likely associated with equilibrated chondrites (types 4 -6) which have high Fe and Mn abundances, but are more consistent with a gas-to-solid condensate origin. Unlike Lpx in equilibrated OCs the low Cr and Al in these Wild 2 Lpx grains is not due to diffusion or feldspar growth but likely was inherited during condensation. Oxygen isotope analyses previously obtained on 3 of the Wild 2 Lpx grains show that two have large negative Δ17O values (Δ17O = -21.3 to -22.3 ‰) [15], a result that is also consistent with a condensation origin. Conclusions: Minor element abundances in isolated and other Lpx grains from Wild 2 indicate: 1) most Lpx from Wild 2 likely originated in CC chondrule-forming regions with a smaller proportion from OC chondrule regions, 2) Wild 2 Lpx did not experience significant thermal metamorphism suggesting the grains did not reside in parent bodies prior to transport to the outer SS and 3) ~15% of Wild 2 Lpx plausibly have condensate origins. Oxygen isotopes previously obtained on some of the Wild 2 grains in this study support these conclusions [15-17]. [ABSTRACT FROM AUTHOR]

Oxygen isotope ratios and rare earth element (REE) concentrations provide independent tests of competing models of injection v. anatexis for the origin of migmatites from amphibolite and granulite facies metasedimentary rocks of the Adirondack Mountains, New York. Values of δ18O and REE profiles were measured by ion microprobe in garnet–zircon pairs from 10 sample localities. Prior U–Pb SIMS dating of zircon grains indicates that inherited cores (1.7–1.2 Ga) are surrounded by overgrowths crystallized during the Grenville orogenic cycle (∼1.2–1.0 Ga). Cathodoluminescence imaging records three populations of zircon: (i) featureless rounded ‘whole grains’ (interpreted as metamorphic or anatectic), and rhythmically zoned (igneous) cores truncated by rims that are either (ii) discordant rhythmically zoned (igneous) or (iii) unzoned (metamorphic or anatectic). These textural interpretations are supported by geochronology and oxygen isotope analysis. In both the amphibolite facies NW Adirondacks and the granulite facies SE Adirondacks, δ18O(Zrc) values in overgrowths and whole zircon are highly variable for metamorphic zircon (6.1–13.4‰; n = 95, 10 μm spot). In contrast, garnet is typically unzoned and δ18O(Grt) values are constant at each locality, differing only between leucosomes and corresponding melanosomes. None of the analysed metamorphic zircon–garnet pairs attained oxygen isotope equilibrium, indicating that zircon rims and garnet are not coeval. Furthermore, REE profiles from zircon rims indicate zircon growth in all regions was prior to significant garnet growth. Thus, petrological estimates from garnet equilibria (e.g. P–T) cannot be associated uncritically with ages determined from zircon. The unusually high δ18O values (>10‰) in zircon overgrowths from leucocratic layers are distinctly different from associated metaigneous rocks (δ18O(Zrc) < 10‰) indicating that these leucosomes are not injected magmas derived from known igneous rocks. Surrounding melanosomes have similarly high δ18O(Zrc) values, suggesting that leucosomes are related to surrounding melanosomes, and that these migmatites formed by anatexis of high δ18O metasedimentary rocks. [ABSTRACT FROM AUTHOR]

Ion microprobe analysis with a sub-micrometer diameter spot reveals a sharp, 2 µm gradient in oxygen isotope ratio proving that oxygen diffusion in zircon is slow even under prolonged high-grade metamorphism. The data are consistent with an oxygen diffusion coefficient of 10-23.5±1 cm²/s. Furthermore, this gradient is found in a zircon that contains clear textural evidence of recrystallization in nearby regions. This finding shows that through careful textural and chemical analysis, primary information can be extracted from a zircon that has also undergone partial recrystallization. The oxygen isotope ratios found in zircon have been used to infer magmatic and pre-magmatic histories, including the presence of liquid water on the surface of earliest Earth. Recently, these interpretations have been questioned with the assertion that zircon may not retain its primary oxygen isotope signature through metamorphism. The slow diffusion confirmed by these results supports interpretations that assume preservation of magmatic compositions. [ABSTRACT FROM AUTHOR]

Detailed electron- and ion-microprobe analysis of diopsides extracted from a block of marble collected in the Mt. Marcy anorthosite massif (Adirondack Highlands, New York) shows that single crystals have preserved Al-Si zoning features from early contact metamorphism later modified by regional metamorphism. Diopsides show Al and Si complementary zoning from core to rim; average Al concentration in rims is constant (0.11 pfu), but values in the cores vary with crystals from 0.06 to 0.14 pfu. A δ18O profile measured with the ion microprobe shows no zoning. Modeling of the Al-diffusion profiles affected by regional metamorphism of the Grenville orogeny shows that original Al zoning was sharp and corresponds to a rapid change of crystal-growth conditions during early contact metamorphism: (1) diopside cores crystallized with variable Al contents due to restricted fluid circulation or differing sedimentary composition, (2) then pervasive fluid infiltration crystallized the homogeneous rim of diopside, and (3) core to rim Al zoning was later smoothed by granulite-facies metamorphism. Homogeneity of δ18O in core and mantle (near rim) of diopsides suggests that the crystals were isotopically homogeneous prior to and during regional metamorphism. Values of Al-Si diffusion coefficient deduced from modeling are in agreement with low-temperature extrapolation of experimental data. [ABSTRACT FROM AUTHOR]