Accretion and Preservation of D-rich Organic Particles in Carbonaceous Chondrites: Evidence for Important Transport in the Early Solar System Nebula

We have acquired NanoSIMS images of the matrices of CI, CM, and CR carbonaceous chondrites to study, in situ, the organic matter trapped during the formation of their parent bodies. D/H ratio images reveal the occurrence of D-rich hot spots, constituting isolated organic particles. Not all the organic particles are D-rich hot spots, indicating that at least two kinds of organic particles have been accreted in the parent bodies. Ratio profiles through D-rich hot spots indicate that no significant self-diffusion of deuterium occurs between the D-rich organic matter and the depleted hydrous minerals that are surrounding them. This is not the result of a physical shielding by any constituent of the chondrites. Ab initio calculations indicate that it cannot be explained by isotopic equilibrium. Then it appears that the organic matter that is extremely enriched in D does not exchange with the hydrous minerals, or this exchange is so slow that it is not significant over the 4.5 billion year history on the parent body. If we consider that the D-rich hot spots are the result of an exposure to intense irradiation, then it appears that carbonaceous chondrites accreted organic particles that have been brought to different regions of the solar nebula. This is likely the result of important radial and vertical transport in the early solar system

Oxygen-isotope and trace element constraints on the origins of silica-rich melts in the subarc mantle

Peridotitic xenoliths in basaltic andesites from Batan island in the Luzon arc contain silica-rich (broadly dacitic) hydrous melt inclusions that were likely trapped when these rocks were within the upper mantle wedge underlying the arc. These melt inclusions have been previously interpreted to be slab-derived melts. We tested this hypothesis by analyzing the oxygen isotope compositions of these inclusions with an ion microprobe. The melt inclusions from Batan xenoliths have δ 18OVSMOW values of 6.45 ± 0.51‰. These values are consistent with the melts having been in oxygen isotope exchange equilibrium with average mantle peridotite at temperatures of ≥875°C. We suggest the δ 18O values of Batan inclusions, as well as their major and trace element compositions, can be explained if they are low-degree melts (or differentiation products of such melts) of peridotites in the mantle wedge that had previously undergone extensive melt extraction followed by metasomatism by small amounts (several percent or less) of slab-derived components. A model based on the trace element contents of Batan inclusions suggests that this metasomatic agent was an aqueous fluid extracted from subducted basalts and had many characteristics similar to slab-derived components of the sources of arc-related basalts at Batan and elsewhere. Batan inclusions bear similarities to “adakites,” a class of arc-related lava widely considered to be slab-derived melts. Our results suggest the alternative interpretation that at least some adakite-like liquids might be generated from low-degree melting of metasomatized peridotites

Stable isotopic characteristics and magmatic history of meta-igneous rocks, Adirondacks, NY

The Adirondack Highlands contain dominantly meta-igneous rocks including the anorthosite-granite association and a wide variety of unrelated orthogneisses from gabbroic to granitic composition. The internal variation of oxygen isotopes within Adirondack meta-igneous suites related to the Marcy anorthosite shows that elevated δ^(18)O is a compositionally dependent characteristic of the protolith. Analysis of δ^(18)O in previously unsampled compositions in this suite has led to a stable isotopic, petrographic and major element description of a continuous range of rock types from two-pyroxene, oxide rich monzonitic gneiss to mesoperthite, quartz rich granite gneiss within the Tupper and Saranac sheets on the NW margin of the Marcy anorthosite. This expanded data set has allowed the formulation of a semi-quantitative model, based both in thermodynamics and mass balance, for the AFC magmatic evolution of this suite. A sampling traverse of the Snowy Mountain dome anorthosite and related halo of granitic gneiss in the southern Adirondack Highlands shows the concentric spatial distribution of δ^(18)O in granites around anorthosite, constrains the extent of interaction between anorthosite and granite, and provides an additional example of crustal interaction in the magmatic evolution of this suite. This evidence, combined with previous work (Morrison and Valley, 1988), documents extensive crustal interaction in region-wide anorthosite-granite plutonism prior to the peak of Grenville metamorphism

Micro-Analysis of D/H Ratios in Mantle Minerals by Carrier-Gas Mass Spectrometry

Isotopic analysis of water released by vacuum heating of hydrous minerals and sub-marine basaltic glasses indicates that they vary in δD_(SMOW) between ca. +10 and -115 per mil (e.g., [1]). However, with few exceptions variations within that range are not well correlated with other geochemical and geologic properties and it is debated whether they reflect isotopic heterogeneity in the mantle, fractionations produced during ascent to the surface, or sub-solidus alteration. This ambiguity is at least partly due to the large sample size and slow rate of conventional analyses, which precludes linking isotopic ranges to grain-scale petrographic variables or constructing large data bases (many 10’s of samples) in a reasonable period of time. Ion microprobe methods provide one solution to these problems, although they suffer from analytical uncertainties nominally 5 to 10 times worse than conventional measurements and large fractionations that can be a source of systematic error

Mantle heterogeneity during the formation of the North Atlantic Igneous Province: Constraints from trace element and Sr-Nd-Os-O isotope systematics of Baffin Island picrites

Sr-Nd-Os-O isotope and major and trace element data from ~62 Ma picrites from Baffin Island constrain the composition of mantle sources sampled at the inception of North Atlantic Igneous Province (NAIP) magmatism. We recognize two compositional types. Depleted (N-type) lavas have low 87Sr/86Sri (0.702990–0.703060) and 187Os/188Osi (0.1220–0.1247) and high 143Nd/144Ndi (0.512989–0.512999) and are depleted in incompatible elements relative to primitive mantle. Enriched (E-type) lavas have higher 87Sr/86Sri (0.703306–0.703851) and 187Os/188Osi (0.1261–0.1303), lower 143Nd/144Ndi (0.512825–0.512906), and incompatible element concentrations similar to, or more enriched than, primitive mantle. There is also a subtle difference in oxygen isotope composition; E-type lavas are marginally lower in δ18Oolivine value (5.16–4.84‰) than N-type lavas (5.15–5.22‰). Chemical and isotopic variations between E- and N-type lavas are inconsistent with assimilation of crust and/or subcontinental lithospheric mantle and appear to instead reflect mixing between melts derived from two distinct mantle sources. Strontium-Nd-O isotope compositions and incompatible trace element abundances of N-type lavas suggest these are largely derived from the depleted upper mantle. The 187Os/188Osi ratios of N-type lavas can also be explained by such a model but require that the depleted upper mantle had γOs of approximately −5 to −7 at 62 Ma. This range overlaps the lowest γOs values measured in abyssal peridotites. Baffin Island lava compositions are also permissive of a model involving recharging of depleted upper mantle with 3He-rich material from the lower mantle (Stuart et al., Nature, 424, 57–59, 2003), with the proviso that recharge had no recognizable effect on the lithophile trace element and Sr-Nd-Os-O isotope composition. The origin of the enriched mantle component sampled by Baffin Island lavas is less clear but may be metasomatized and high-temperature-altered recycled oceanic lithosphere transported within the proto Iceland plume. Differences between Baffin Island lavas and modern Icelandic basalts suggest that a range of enriched and depleted mantle sources have been tapped since the inception of magmatism in the province. Similarities between Baffin Island lavas erupted and those of similar age from East and West Greenland also suggest that the enriched component in Baffin Island lavas may have been sampled by lavas erupted over a wide geographic range

High-temperature environments of human evolution in East Africa based on bond ordering in paleosol carbonates

Many important hominid-bearing fossil localities in East Africa are in regions that are extremely hot and dry. Although humans are well adapted to such conditions, it has been inferred that East African environments were cooler or more wooded during the Pliocene and Pleistocene when this region was a central stage of human evolution. Here we show that the Turkana Basin, Kenya—today one of the hottest places on Earth—has been continually hot during the past 4 million years. The distribution of ^(13)C-^(18)O bonds in paleosol carbonates indicates that soil temperatures during periods of carbonate formation were typically above 30 °C and often in excess of 35 °C. Similar soil temperatures are observed today in the Turkana Basin and reflect high air temperatures combined with solar heating of the soil surface. These results are specific to periods of soil carbonate formation, and we suggest that such periods composed a large fraction of integrated time in the Turkana Basin. If correct, this interpretation has implications for human thermophysiology and implies a long-standing human association with marginal environments

Neutron-rich Chromium Isotope Anomalies in Supernova Nanoparticles

Neutron-rich isotopes with masses near that of iron are produced in Type Ia and II supernovae (SNeIa and SNeII). Traces of such nucleosynthesis are found in primitive meteorites in the form of variations in the isotopic abundance of ^(54)Cr, the most neutron-rich stable isotope of chromium. The hosts of these isotopic anomalies must be presolar grains that condensed in the outflows of SNe, offering the opportunity to study the nucleosynthesis of iron-peak nuclei in ways that complement spectroscopic observations and can inform models of stellar evolution. However, despite almost two decades of extensive search, the carrier of ^(54)Cr anomalies is still unknown, presumably because it is fine grained and is chemically labile. Here, we identify in the primitive meteorite Orgueil the carrier of ^(54)Cr anomalies as nanoparticles (3.6 × solar). Such large enrichments in ^(54)Cr can only be produced in SNe. The mineralogy of the grains supports condensation in the O/Ne-O/C zones of an SNII, although a Type Ia origin cannot be excluded. We suggest that planetary materials incorporated different amounts of these nanoparticles, possibly due to late injection by a nearby SN that also delivered ^(26)Al and ^(60)Fe to the solar system. This idea explains why the relative abundance of ^(54)Cr and other neutron-rich isotopes vary between planets and meteorites. We anticipate that future isotopic studies of the grains identified here will shed new light on the birth of the solar system and the conditions in SNe

Oxygen-isotopic composition and high-resolution secondary ion mass spectrometry imaging of Martian carbonate in Lafayette meteorite

Carbonate from SNC meteorites gives insight into a variety of processes on and/or beneath the surface of Mars. In Lafayette, carbonate occurs in unusually intimate association with hydrous phases when compared with other carbonate-bearing SNCs [1]. We have measured the ^(18)O/^(16)O ratio of carbonate in the alteration veins of Lafayette using the magnetic sector ion microprobe. In addition, we obtained isotope images of major- and minor-element cations in veinlets with the focused ion beam (FIB) quadrupole SIMS. The spot size of the FIB probe used in this study (~0.03 µm) provides an opportunity for high spatial resolution chemical imaging, which is ideal for examining features in the fine-grained alteration assemblage