CO2 transfer between the upper mantle and the atmosphere: temporary storage in the lower continental crust

The CO₂ atmospheric content has shown large variations over geological times. High contents (up to one order of magnitude more than present-day values) ultimately correspond to discrete episodes of mantle degassing, either juvenile, or subduction-related (carbon recycling). A number of arguments (e.g. the continuous volume increase of carbonate-bearing sediments with time) suggest that, throughout the Earth's history, juvenile CO₂ has formed a major contribution to the global carbon budget of the Earth.
The absence of a direct relationship between major volcanic episodes and the average CO₂ atmospheric content suggests that volcanoes might not be the only way by which mantle CO₂ is transported to the surface. It is proposed that large quantities of juvenile CO₂ could temporarily be stored in the lower continental crust during major episodes of granulite formation. These are primarily caused by magmatic underplating and they result in a vertical accretion of the crust by accumulation of CO₂-bearing, mantle-derived magmas. Most of the CO₂ migrates through the crust during post-metamorphic evolution and isostatic restoration of the normal continental thickness. However, large quantities of CO₂ can still be present in some areas, notably as high-density fluids enclosed in minerals.     

Characterization of insoluble organic matter in primitive meteorites by microRaman spectroscopy

Abstract— We have analyzed the chemically and isotopically well‐characterized insoluble organic matter (IOM) extracted from 51 unequilibrated chondrites (8 CR, 9 CM, 1 CI, 3 ungrouped C, 9 CO, 9 CV, 10 ordinary, 1 CB and 1 E chondrites) using confocal imaging Raman spectroscopy. The average Raman properties of the IOM, as parameterized by the peak characteristics of the so‐called D and G bands, which originate from aromatic C rings, show systematic trends that are correlated with meteorite (sub‐) classification and IOM chemical compositions. Processes that affect the Raman and chemical properties of the IOM, such as thermal metamorphism experienced on the parent bodies, terrestrial weathering and amorphization due to irradiation in space, have been identified. We established separate sequences of metamorphism for ordinary, CO, oxidized, and reduced CV chondrites. Several spectra from the most primitive chondrites reveal the presence of organic matter that has been amorphized. This amorphization, usually the result of sputtering processes or UV or particle irradiation, could have occurred during the formation of the organic material in interstellar or protoplanetary ices or, less likely, on the surface of the parent bodies or during the transport of the meteorites to Earth. D band widths and peak metamorphic temperatures are strongly correlated, allowing for a straightforward estimation of these temperatures.     

Mineral associations and character of isotopically anomalous organic material in the Tagish Lake carbonaceous chondrite

We report a coordinated analytical study of matrix material in the Tagish Lake carbonaceous chondrite in which the same small (?20 μm) fragments were measured by secondary ion mass spectrometry (SIMS), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), electron energy-loss spectroscopy (EELS), and X-ray absorption near-edge spectroscopy (XANES). SIMS analysis reveals H and N isotopic anomalies (hotspots), ranging from hundreds to thousands of nanometers in size, which are present throughout the fragments. Although the differences in spatial resolution of the SIMS techniques we have used introduce some uncertainty into the exact location of the hotspots, in general, the H and N isotopic anomalies are spatially correlated with C enrichments, suggesting an organic carrier. TEM analysis, enabled by site-specific extraction using a focused-ion-beam scanning-electron microscope, shows that the hotspots contain an amorphous component, Fe-Ni sulfides, serpentine, and mixed-cation carbonates. TEM imaging reveals that the amorphous component occurs in solid and porous forms, EDS indicates that it contains abundant C, and EELS and XANES at the C K edge reveal that it is largely aromatic. This amorphous component is probably macromolecular C, likely the carrier of the isotopic anomalies, and similar to the material extracted from bulk samples as insoluble organic matter. However, given the large sizes of some of the hotspots, the disparity in spatial resolution among the various techniques employed in our study, and the phases with which they are associated, we cannot entirely rule out that some of the isotopic anomalies are carried by inorganic material, e.g., sheet silicates. The isotopic composition of the organic matter points to an initially primitive origin, quite possibly within cold interstellar clouds or the outer reaches of the solar protoplanetary disk. The association of organic material with secondary phases, e.g., serpentine and carbonates, suggests that the organic matter was susceptible to parent-body processing, and thus, isotopic dilution.     

Extreme Cr-rich nano-oxides in the CI chondrite Orgueil - Implication for a late supernova injection into the solar system

Systematic variations in ⁵⁴Cr/⁵²Cr ratios between meteorite classes ( [Trinquier et al., 2007] and [Qin et al., 2010a]) point to large scale spatial and/or temporal isotopic heterogeneity in the solar protoplanetary disk. Two explanations for these variations have been proposed, with important implications for the formation of the Solar System: heterogeneous seeding of the disk with dust from a supernova, or energetic-particle irradiation of dust in the disk. The key to differentiating between them is identification of the carrier(s) of the ⁵⁴Cr anomalies. Here we report the results of our recent NanoSIMS imaging search for the ⁵⁴Cr-rich carrier in the acid-resistant residue of the CI chondrite Orgueil. A total of 10 regions with extreme ⁵⁴Cr-excesses (δ⁵⁴Cr values up to 1500‰) were found. Comparison between SEM, Auger and NanoSIMS analyses showed that these ⁵⁴Cr-rich regions are associated with one or more sub-micron (typically less than 200 nm) Cr oxide grains, most likely spinels. Because the size of the NanoSIMS primary O⁻ ion beam is larger than the typical grain size on the sample mount, the measured anomalies are lower limits, and we estimate that the actual ⁵⁴Cr enrichments in three grains are at least 11 times Solar and in one of these may be as high as 50 times Solar. Such compositions strongly favor a Type II supernova origin. The variability in bulk ⁵⁴Cr/⁵²Cr between meteorite classes argues for a heterogeneous distribution of the ⁵⁴Cr carrier in the solar protoplanetary disk following a late supernova injection event. Such a scenario is also supported by the O-isotopic distribution and variable abundances in different planetary materials of other presolar oxide and silicate grains from supernovae.     

Hydrogen and major element concentrations on 433 Eros: Evidence for an L‐ or LL‐chondrite‐like surface composition

A reanalysis of NEAR X‐ray/gamma‐ray spectrometer (XGRS) data provides robust evidence that the elemental composition of the near‐Earth asteroid 433 Eros is consistent with the L and LL ordinary chondrites. These results facilitated the use of the gamma‐ray measurements to produce the first in situ measurement of hydrogen concentrations on an asteroid. The measured value,  ppm, is consistent with hydrogen concentrations measured in L and LL chondrite meteorite falls. Gamma‐ray derived abundances of hydrogen and potassium show no evidence for depletion of volatiles relative to ordinary chondrites, suggesting that the sulfur depletion observed in X‐ray data is a surficial effect, consistent with a space‐weathering origin. The newfound agreement between the X‐ray, gamma‐ray, and spectral data suggests that the NEAR landing site, a ponded regolith deposit, has an elemental composition that is indistinguishable from the mean surface. This observation argues against a pond formation process that segregates metals from silicates, and instead suggests that the differences observed in reflectance spectra between the ponds and bulk Eros are due to grain size differences resulting from granular sorting of ponded material.     

Deuterium enrichments in chondritic macromolecular material—Implications for the origin and evolution of organics, water and asteroids

Here we report the elemental and isotopic compositions of the insoluble organic material (IOM) isolated from several previously unanalyzed meteorites, as well as the reanalyses of H isotopic compositions of some previously measured samples (Alexander et al., 2007). The IOM in ordinary chondrites (OCs) has very large D enrichments that increase with increasing metamorphism and decreasing H/C, the most extreme δD value measured being almost 12,000‰. We propose that such large isotopic fractionations could be produced in the OC parent bodies through the loss of isotopically very light H₂ generated when Fe was oxidized by water at low temperatures (<200 °C). We suggest that similar isotopic fractionations were not generated in the IOM of CV and CO chondrites with similar metamorphic grades and IOM H/C ratios because proportionately less water was consumed during metamorphism, and the remaining water buffered the H isotopic composition of the IOM even a H was being lost from it.Hydrogen would also have been generated during the alteration of CI, CM and CR carbonaceous chondrites. The IOM in these meteorites exhibit a considerable range in isotopic compositions, but all are enriched in D, as well as ¹⁵N, relative to terrestrial values. We explore whether these enrichments could also have been produced by the loss of H₂, but conclude that the most isotopically anomalous IOM compositions in meteorites from these groups are probably closest to their primordial values. The less isotopically anomalous IOM has probably been modified by parent body processes. The response of IOM to these processes was complex and varied, presumably reflecting differences in conditions within and between parent bodies.The D enrichments associated with H₂ generation, along with exchange between D-rich IOM and water in the parent bodies, means that it is unlikely that any chondrites retain the primordial H isotopic composition of the water ice that they accreted. The H isotopic compositions of the most water-rich chondrites, the CMs and CIs, are probably the least modified and their compositions (δD ? −25‰) suggest that their water did not form at large radial distances from the Sun where ice is predicted to be very D-rich. Yet models to explain the O isotopic composition of inner Solar System bodies require that large amounts of ice were transported from the outer to the inner Solar System.     

Measuring the level of interstellar inheritance in the solar protoplanetary disk

The timing and extent to which the initial interstellar material was thermally processed provide fundamental constraints for models of the formation and early evolution of the solar protoplanetary disk. We argue that the nonsolar (solar Δ¹⁷O ≈ ?29‰) and near‐terrestrial (Δ¹⁷O ≈ 0‰) O‐isotopic compositions of the Earth and most extraterrestrial materials (Moon, Mars, asteroids, and comet dust) were established very early by heating of regions of the disk that were modestly enriched (dust/gas ≥ 5–10 times solar) in primordial silicates (Δ¹⁷O ≈ ?29‰) and water‐dominated ice (Δ¹⁷O ≈ 24‰) relative to the gas. Such modest enrichments could be achieved by grain growth and settling of dust to the midplane in regions where the levels of turbulence were modest. The episodic heating of the disk associated with FU Orionis outbursts were the likely causes of this early thermal processing of dust. We also estimate that at the time of accretion the CI chondrite and interplanetary dust particle parent bodies were composed of ~5–10% of pristine interstellar material. The matrices of all chondrites included roughly similar interstellar fractions. Whether this interstellar material avoided the thermal processing experienced by most dust during FU Orionis outbursts or was accreted by the disk after the outbursts ceased to be important remains to be established.     

X‐ray fluorescence measurements of the surface elemental composition of asteroid 433 Eros

Abstract— We report major element ratios determined for the S‐class asteroid 433 Eros using remote‐sensing x‐ray fluorescence spectroscopy with the near‐Earth asteroid rendezvous Shoemaker x‐ray spectrometer (XRS). Data analysis techniques and systematic errors are described in detail. Data acquired during five solar flares and during two extended “quiet Sun” periods are presented; these results sample a representative portion of the asteroid's surface. Although systematic uncertainties are potentially large, the most internally consistent and plausible interpretation of the data is that Eros has primitive Mg/Si, Al/Si, Ca/Si and Fe/Si ratios, closely similar to H or R chondrites. Global differentiation of the asteroid is ruled out. The S/Si ratio is much lower than that of chondrites, probably reflecting impact‐induced volatilization and/or photo‐ or ion‐induced sputtering of sulfur at the surface of the asteroid. An alternative explanation for the low S/Si ratio is that it reflects a limited degree of melting with loss of an FeS‐rich partial melt. Size‐sorting processes could lead to segregation of Fe‐Ni metal from silicates within the regolith of Eros; this could indicate that the Fe/Si ratios determined by the x‐ray spectrometer are not representative of the bulk Eros composition.     

The NEAR‐Shoemaker x‐ray/gamma‐ray spectrometer experiment: Overview and lessons learned

Abstract— The near‐Earth asteroid rendezvous (NEAR)‐Shoemaker remote‐sensing x‐ray/gamma‐ray spectrometer (XGRS) completed more than a year of operation in orbit and on the surface of 433 Eros. Elemental compositions for a number of regions on the surface of Eros have been derived from analyses of the characteristic x‐ray and gamma‐ray emission spectra. The NEAR XGRS detection system was included as part of the interplanetary network (IPN) for the detection and localization of gamma‐ray bursts (GRBs). Preliminary results for both the elemental composition of the surface of Eros and the detection of GRBs have been obtained. In addition to the science results, the design and operation of the NEAR XGRS is considered. Significant information important for the design of future remote sensing XGRS systems has been obtained and evaluated. We focus on four factors that became particularly critical during NEAR: (1) overall spacecraft design, (2) selection of materials, (3) increase of the signal‐to‐noise ratio and (4) knowledge of the incident solar x‐ray spectrum.