Richard

Carlson

The surface chemistry of pyrrhotites from intact particles directly collected from asteroid (162173) Ryugu was investigated by micro-Raman spectroscopy. The Raman peak characteristic to pyrrhotite was observed at around 115 cm-1 in Ryugu pyrrhotites, similar to freshly cleaved surfaces of terrestrial pyrrhotites. Additional Raman bands centered at around 220, 275, and 313 cm-1 with broadened features were also detected from the Ryugu pyrrhotites. The set of Raman bands at 220 and 275 cm-1 was assigned to typical Fe-S stretching vibrations of nu 2 (225 cm-1 ) and nu 1 (275 cm-1 ). These bands are not clearly observed in bulk crystals of pyrrhotite but appear in its nanoparticulate phase. These bands are ordinarily seen in amorphous monosulfides that formed under low oxygen fugacity ( f O 2 ) conditions in nature, indicating that the structural alteration of pyrrhotite surfaces occurred heterogeneously on the nanoscale under low f O 2 conditions. Further, the Raman band at 313 cm-1 was attributed to a characteristic tetrahedral bonding of Fe(III) in the lattice of Fe II 1-3x Fe III 1-2x S, followed by the local breakdown of the crystal lattice structures from planar bonding with Fe(II). In addition, some areas of the Ryugu pyrrhotite grains showed corroded structures with iridescence. Furthermore, assemblages of magnetite particles were also preferentially observed on small areas of the likely-dissolved pyrrhotite crystals in phyllosilicate matrices. These characteristic features in the Raman spectra and in corroded structures of Ryugu pyrrhotites record changes in the local environmental conditions via aqueous alteration. The corrosion of pyrrhotite crystals followed by the preferential formation of magnetite particles by asteroidal water is the likely product of dissolution of Fe(II) from the pyrrhotite surface and its oxidative precipitation in microchemical environments on the Ryugu parent body.

The scarcity of rocks preserved from the first billion years (Gy) of Earth's history hinders our ability to study the nature of the earliest crust. Rare >4.0-Gy-old zircons confirm that felsic crust was present within 500 million years of Earth's formation. Given that most of that ancient crust has been destroyed, geochemical and isotopic tracers applied to rocks from the oldest sections of continents can be used to provide insights into the nature of the predecessor crust. Evidence from Earth's oldest rocks and minerals suggests multiple early mantle depletion episodes, possibly linked to the formation of an initial, dominantly mafic, crust. This early crust was the precursor to evolved rocks that now constitute considerable portions of Earth's oldest surviving crust.

The Columbia River Basalt Group (CRBG) is the youngest continental flood basalt province, proposed to be sourced from the deep-seated plume that currently resides underneath Yellowstone National Park. If so, the earliest erupted basalts from this province, such as those in the Picture Gorge Basalt (PGB), aid in understanding and modeling plume impingement and the subsequent evolution of basaltic volcanism. Using geochemical and isotopic data, this study explores potential mantle sources and magma evolution of the PGB. Long known geochemical signatures of the PGB include overall large ion lithophile element (LILE) enrichment and relative depletion of high field strength elements (HFSE) typical of other CRBG main-phase units. Basaltic samples of the PGB have Sr-87/Sr-86 ratios on the low end of the range displayed by other CRBG lavas and mantle-like delta O-18 values. The relatively strong enrichment of LILE and depletion of HFSE coupled with depleted isotopic signatures suggest a metasomatized upper mantle as the most likely magmatic source for the PGB. Previous geochemical modeling of the PGB utilized the composition of two high-MgO primitive dikes exposed in the northern portion of the Monument Dike swarm as parental melt. However, fractionation of these dike compositions cannot generate the compositional variability illustrated by basaltic lavas and dikes of the PGB. This study identifies a second potential parental PGB composition best represented by basaltic flows in the extended spatial distribution of the PGB. This composition also better reflects the lowest stratigraphic flows identified in the previously mapped extent of the PGB. Age data reveal that PGB lavas erupted first and throughout eruptions of main-phase CRBG units (Steens, Imnaha, Grande Ronde Basalt). Combining geochemical signals with these age data indicates cyclical patterns in the amounts of contributing mantle components. Eruption of PGB material occurred in two pulses, demonstrated by a similar to 0.4 Ma temporal gap in reported ages, 16.62 to 16.23 Ma. Coupling ages with observed geochemical signals, including relative elemental abundances of LILE, indicates increased influence of a more primitive, potentially plume-like source with time.

Oxygen 3-isotope ratios of magnetite and carbonates in aqueously altered carbonaceous chondrites provide important clues to understanding the evolution of the fluid in the asteroidal parent bodies. We conducted oxygen 3-isotope analyses of magnetite, dolomite, and breunnerite in two sections of asteroid Ryugu returned samples, A0058 and C0002, using a secondary ion mass spectrometer (SIMS). Magnetite was analyzed by using a lower primary ion energy that reduced instrumental biases due to the crystal orientation effect. We found two groups of magnetite data identified from the SIMS pit morphologies: (1) higher delta O-18 (from 3 parts per thousand to 7 parts per thousand) and Delta O-17 (similar to 2 parts per thousand) with porous SIMS pits mostly from spherulitic magnetite, and (2) lower delta O-18 (similar to -3 parts per thousand) and variable Delta O-17 (0 parts per thousand-2 parts per thousand) mostly from euhedral magnetite. Dolomite and breunnerite analyses were conducted using multi-collection Faraday cup detectors with precisions <= 0.3 parts per thousand. The instrumental bias correction was applied based on carbonate compositions in two ways, using Fe and (Fe + Mn) contents, respectively, because Ryugu dolomite contains higher amounts of Mn than the terrestrial standard. Results of dolomite and breunnerite analyses show a narrow range of Delta O-17; 0.0 parts per thousand-0.3 parts per thousand for dolomite in A0058 and 0.2 parts per thousand-0.8 parts per thousand for dolomite and breunnerite in C0002. The majority of breunnerite, including large >= 100 mu m grains, show systematically lower delta O-18 (similar to 21 parts per thousand) than dolomite (25 parts per thousand-30 parts per thousand and 23 parts per thousand-27 parts per thousand depending on the instrumental bias corrections). The equilibrium temperatures between magnetite and dolomite from the coarse-grained lithology in A0058 are calculated to be 51 +/- 11 degrees C and 78 +/- 14 degrees C, depending on the instrumental bias correction scheme for dolomite; a reliable temperature estimate would require a Mn-bearing dolomite standard to evaluate the instrumental bias corrections, which is not currently available. These results indicate that the oxygen isotope ratios of aqueous fluids in the Ryugu parent asteroid were isotopically heterogeneous, either spatially, or temporary. Initial water ice accreted to the Ryugu parent body might have Delta O-17 > 2 parts per thousand that was melted and interacted with anhydrous solids with the initial Delta O-17 < 0 parts per thousand. In the early stage of aqueous alteration, spherulitic magnetite and calcite formed from aqueous fluid with Delta O-17 similar to 2 parts per thousand that was produced by isotope exchange between water (Delta O-17 > 2 parts per thousand) and anhydrous solids (Delta O-17 < 0 parts per thousand). Dolomite and breunnerite, along with some magnetite, formed at the later stage of aqueous alteration under higher water-to-rock ratios where the oxygen isotope ratios were nearly at equilibrium between fluid and solid phases. Including literature data, delta O-18 of carbonates decreased in the order calcite, dolomite, and breunnerite, suggesting that the temperature of alteration might have increased with the degree of aqueous alteration.

In order to gain insights on the conditions of aqueous alteration on asteroid Ryugu and the origin of water in the outer solar system, we developed the measurement of water content in magnetite at the micrometer scale by secondary ion mass spectrometry (NanoSIMS) and determined the H and Si content of coarse-grained euhedral magnetite grains (polyhedral magnetite) and coarse-grained fibrous (spherulitic) magnetite from the Ryugu polished section A0058-C1001. The hydrogen content in magnetite ranges between similar to 900 and similar to 3300 wt ppm equivalent water and is correlated with the Si content. Polyhedral magnetite has low and homogenous silicon and water content, whereas fibrous magnetite shows correlated Si and water excesses. These excesses can be explained by the presence of hydrous Si-rich amorphous nanoinclusions trapped during the precipitation of fibrous magnetite away from equilibrium and testify that fibrous magnetite formed from a hydrous gel with possibly more than 20 wt% water. An attempt to determine the water content in sub-mu m framboids indicates that additional calibration and contamination issues must be addressed before a safe conclusion can be drawn, but hints at elevated water content as well. The high water content in fibrous magnetite, expected to be among the first minerals to crystallize at low water-rock ratio, points to the control of water content by local conditions of magnetite precipitation rather than large-scale alteration conditions. Systematic lithological variations associated with water-rich and water-poor magnetite suggest that the global context of alteration may be better understood if local water concentrations are compared with millimeter-scale distribution of the various morphologies of magnetite. Finally, the high water content in the magnetite precursor gel indicates that the initial O isotopic composition in alteration water must not have been very different from that of the earliest magnetite crystals.