Incorporation mechanism of Fe and Al into bridgmanite in a subducting mid-ocean ridge basalt and its crystal chemistry

The compositional difference between subducting slabs and their surrounding lower-mantle can yield the difference in incorporation mechanism of Fe and Al into bridgmanite between both regions, which should cause heterogeneity in physical properties and rheology of the lower mantle. However, the precise cation-distribution has not been examined in bridgmanites with Fe- and Al-contents expected in a mid-ocean ridge basalt component of subducting slabs. Here we report on Mg0.662Fe0.338Si0.662Al0.338O3 bridgmanite single-crystal characterized by a combination of single-crystal X-ray diffraction, synchrotron 57Fe-Mössbauer spectroscopy and electron probe microanalysis. We find that the charge-coupled substitution AMg2+ + BSi4+ ↔ AFe3+(high-spin) + BAl3+ is predominant in the incorporation of Fe and Al into the practically eightfold-coordinated A-site and the sixfold-coordinated B-site in bridgmanite structure. The incorporation of both cations via this substitution enhances the structural distortion due to the tilting of BO6 octahedra, yielding the unusual expansion of mean <A–O> bond-length due to flexibility of A–O bonds for the structural distortion, in contrast to mean <B–O> bond-length depending reasonably on the ionic radius effect. Moreover, we imply the phase-transition behavior and the elasticity of bridgmanite in slabs subducting into deeper parts of the lower mantle, in terms of the relative compressibility of AO12 (practically AO8) and BO6 polyhedra.

Petrological Implications of Seafloor Hydrothermal Alteration of Subducted Mid-Ocean Ridge Basalt

Determining the mineralogical changes occurring in subducted oceanic crust is key to understanding short- and long-term geochemical cycles. Although numerous studies have explored the mineral assemblages that form in mid-ocean ridge basalt (MORB) at different depths below the Earth’s surface, it is widely recognized that seafloor hydrothermal alteration of the uppermost portion of the oceanic crust can change its composition between a ridge and a trench prior to subduction. In this study, we use petrological modelling to explore the effects of different types of pre-subduction hydrothermal alteration on the phase changes that occur during seafloor alteration of MORB-like compositions during subduction along an average Phanerozoic geotherm. We consider a representative composition of altered oceanic crust, as well as extreme end-member scenarios (pervasive spilitization, chloritization, and epidotization). Our models show that epidotization and chloritization of MORB strongly affects phase equilibria at different depths, whereas spilitization and an average style of alteration produce relatively fewer changes on the mineral assemblage to those expected in a pristine MORB. Devolatilization of MORB during subduction occurs mostly in the forearc region, although the type and extent of alteration strongly control the depth and magnitude of fluid released. Altered compositions carry significantly more H2O to sub- and postarc depths than unaltered compositions; the H2O carrying capacity of unaltered and altered compositions is further enhanced during subduction along colder geotherms. Extremely localized areas affected by epidotization can transport up to 22 times more H2O than unaltered MORB and up to two times more than average altered oceanic crust compositions to depths beyond the arc. Regardless of the extent and style of alteration, the stability of hydrous phases, such as epidote and phengite (important trace element carriers), is expanded to greater pressure and temperature conditions. Thus, hydrothermal alteration of the subducted oceanic slab-top represents a viable, and probably common, mechanism that enhances geochemical recycling between the Earth’s hydrosphere and shallow interior.

Geoquímica dos diques máficos de Brumado, porção sudeste do Bloco Gavião, Bahia, Brasil

No sudoeste do estado da Bahia, entre as cidades de Brumado e Caetité e mais a norte próximo às cidades de Tanque Novo, Paramirim, Botuporã, Macaúbas, Novo Horizonte e Ibitiara, ocorrem diques máficos formados em ambiente intraplaca. São toleítos continentais apresentando valores de índice de diferenciação em magnésio (mg#) entre 0,25 e 0,45, indicativos de líquidos magmáticos evoluídos. Plagioclásio e augita predominaram no processo magmático evolutivo, cujos diagramas não mostram alterações significativas por processos secundários. Os diques máficos originaram-se de fonte pouco heterogênea tipo Enriched Mid-Ocean Ridge Basalt (E-MORB), provavelmente com leve contribuição de fluidos tipo Ocean Island Basalt (OIB). Os dados geoquímicos sugerem influência significativa da crosta continental inferior na origem da fonte mantélica, não havendo sinais de importante contribuição sedimentar.