Garnet-field Melting and Late-stage Refertilization in 'Residual' Abyssal Peridotites from the Central Indian Ridge

The role of residual garnet during melting beneath mid-oceanridges has been the subject of many recent investigations. Toaddress this issue from the perspective of melting residues,we obtained major and trace element mineral chemistry of residualabyssal peridotites from the Central Indian Ridge. Many clinopyroxeneshave ratios of middle to heavy rare earth elements (MREE/HREE)that are too low to be explained by melting in the stabilityfield of spinel peridotite alone. Several percent of meltingmust have occurred at higher pressures in the garnet peridotitestability field. Application of new trace element partitioningmodels, which predict that HREE are compatible in high-pressureclinopyroxene, cannot fully explain the fractionation of theMREE from the HREE. Further, many samples show textural andchemical evidence for refertilization, such as relative enrichmentsof highly incompatible trace elements with respect to moderatelyincompatible trace elements. Therefore, highly incompatibleelements, which are decoupled from major and moderately incompatibletrace elements, are useful to assess late-stage processes, suchas melt entrapment, melt–rock reaction and veining. Moderatelyincompatible trace elements are less affected by such late-stageprocesses and thus useful to infer the melting history of abyssalperidotites. KEY WORDS: abyssal peridotites; mantle melting; garnet     

Heterogeneous distribution of 26Al at the birth of the solar system: Evidence from refractory grains and inclusions

We review recent results on O‐ and Mg‐isotope compositions of refractory grains (corundum, hibonite) and calcium, aluminum‐rich inclusions (CAIs) from unequilibrated ordinary and carbonaceous chondrites. We show that these refractory objects originated in the presence of nebular gas enriched in ¹⁶O to varying degrees relative to the standard mean ocean water value: the Δ¹⁷O_SMOW value ranges from approximately ?16‰ to ?35‰, and recorded heterogeneous distribution of ²⁶Al in their formation region: the inferred (²⁶Al/²⁷Al)₀ ranges from approximately 6.5 × 10^?5 to <2 × 10^?6. There is no correlation between O‐ and Mg‐isotope compositions of the refractory objects: ²⁶Al‐rich and ²⁶Al‐poor refractory objects have similar O‐isotope compositions. We suggest that ²⁶Al was injected into the ²⁶Al‐poor collapsing protosolar molecular cloud core, possibly by a wind from a neighboring massive star, and was later homogenized in the protoplanetary disk by radial mixing, possibly at the canonical value of ²⁶Al/²⁷Al ratio (approximately 5 × 10^?5). The ²⁶Al‐rich and ²⁶Al‐poor refractory grains and inclusions represent different generations of refractory objects, which formed prior to and during the injection and homogenization of ²⁶Al. Thus, the duration of formation of refractory grains and CAIs cannot be inferred from their ²⁶Al‐²⁶Mg systematics, and the canonical (²⁶Al/²⁷Al)₀ does not represent the initial abundance of ²⁶Al in the solar system; instead, it may or may not represent the average abundance of ²⁶Al in the fully formed disk. The latter depends on the formation time of CAIs with the canonical ²⁶Al/²⁷Al ratio relative to the timing of complete delivery of stellar ²⁶Al to the solar system, and the degree of its subsequent homogenization in the disk. The injection of material containing ²⁶Al resulted in no observable changes in O‐isotope composition of the solar system. Instead, the variations in O‐isotope compositions between individual CAIs indicate that O‐isotope composition of the CAI‐forming region varied, because of coexisting of ¹⁶O‐rich and ¹⁶O‐poor nebular reservoirs (gaseous and/or solid) at the birth of the solar system, or because of rapid changes in the O‐isotope compositions of these reservoirs with time, e.g., due to CO self‐shielding in the disk.