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New hydrothermal experiments in rapid-quench pressure vessels have been performed to investigate the effect of redox state on the serpentinization reaction. The experimental hydrogen fugacity (fH2) was controlled by monitoring the mobility of H2 in the reacting system (internal vs. external fH2 control). This was achieved by using either Au (H2 impermeable) or AgPd (H2 permeable) capsules and Ar pressurizing gas to control fH2. The experiments were performed with either San Carlos olivine powders or Åheim dunite chips. Water / rock mass ratios of 1–2, a total pressure of 50 MPa , and temperatures of 300 and 350 °C were investigated. Experimental durations of 30, 45, or ≈80 d were imposed. Serpentine production is observed in almost all experiments but is favored at 300 °C under external fH2 control. The serpentine–magnetite assemblage is observed in Au capsules (high fH2) at 300 °C, while the serpentine–hematite(–magnetite) is observed in AgPd capsules (low fH2). At 350 °C, less H2 is produced than at 300 °C and the serpentine–hematite(–magnetite) assemblage is present in both Au and AgPd capsules. Brucite is absent and this is interpreted to reflect both the initially oxidizing conditions and relatively low serpentine production in our experiments. Differences in product phase assemblages found in this study imply that natural serpentinization reaction mechanisms vary with redox conditions, and consequences for H2 production fluxes and rates can be expected. The high- fH2 (reduced) internally controlled experiments simulate low-permeability "closed" oceanic hydrothermal systems. The low- fH2 (oxidized) externally controlled experiments are analogous to "open" oceanic hydrothermal systems where serpentinization is driven by tectonically aided infiltration of an external fluid. [ABSTRACT FROM AUTHOR]

The ungrouped carbonaceous chondrite (CC) Bells has long been considered petrographically similar to CM chondrites based on its matrix abundance and degree of aqueous alteration, but also shows significant isotopic affinities to CR chondrites. Its taxonomic status is thus important for clarifying the relationship of the CRHB (formerly "CR") clan with other CCs. In this study, we measured the oxygen isotopic compositions of olivines in type I chondrules and isolated olivine grains in Bells. Bells olivines mostly have ∆17O > −4‰, similar to CR chondrites but unlike other CCs that are rich in refractory inclusions, in which chondrules are generally richer in 16O. Therefore, Bells is a CR chondrite (albeit an anomalous one), most similar to the rare, matrix‐rich CRs like Al Rais. These chondrites (i) may not necessarily derive from the same primary parent body as mainstream CRs, (ii) bear witness to significant variations of the matrix/chondrule ratio within the CRHB clan, and (iii) may be a good analog for samples retrieved by the space mission OSIRIS‐REx. [ABSTRACT FROM AUTHOR]

The timing of the formation of the first solids in the solar system remains poorly constrained. Micrometer-scale, high-precision magnesium (Mg) isotopic analyses demonstrate that Earth, refractory inclusions, and chondrules from primitive meteorites formed from a reservoir in which short-lived aluminum-26 ([sup.26]Al and Mg isotopes were homogeneously distributed at [+ or -] 10%. This level of homogeneity validates the use of [sup.26]Al as a precise chronometer for early solar system events. High-precision chondrule [sup.26]Al isochrons show that several distinct chondrule melting events took place from ~1.2 million years (My) to ~4 My after the first solids condensed from the solar nebula, with peaks between ~1.5 and ~3 My, and that chondrule precursors formed as early as [0.87.sup.+0.19.sub.-0.16] My after.