Yumiko Harigane

Primitive layered gabbros from fast-spreading lower oceanic crust

Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks—in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas—provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.

Diversity of melt conduits in the Izu-Bonin-Mariana forearc mantle: Implications for the earliest stage of arc magmatism

ABSTRACT Magmatic processes during the earliest stage of subduction initiation are still not well understood. We examined peridotites recovered from an exhumed crust-mantle section exposed along the landward slopes of the northern Izu-Bonin Trench using the Japan Agency for Marine-Earth Science and Technology9s remotely operated vehicle KAIKO7000II . Based on the Cr# [Cr/(Cr + Al) atomic ratio] of spinel, two distinctive groups, (1) high-Cr# (>0.8) dunite and (2) medium-Cr# (0.4–0.6) dunite, occur close to each other and are associated with refractory harzburgite. Two distinctive melts were in equilibrium with these dunites: a boninitic melt for the high-Cr# dunite and a mid-oceanic ridge basalt (MORB)–like melt for the medium-Cr# dunite. The TiO 2 content of the latter melt is lower than typical MORB compositions. We suggest that the medium-Cr# dunite was a melt conduit for a basalt recently reported from the Mariana forearc that was erupted at the inception of subduction. The wide range of variation in the Cr#s of spinels in dunites from the Izu-Bonin-Mariana forearc probably reflects changing melt compositions from MORB-like melts to boninitic melts in the forearc setting due to an increase of slab-derived hydrous fluids and/or melts during subduction initiation.

Peridotites from a ductile shear zone within back‐arc lithospheric mantle, southern Mariana Trench: Results of a Shinkai 6500 dive

[1]xa0Two N–S fault zones in the southern Mariana fore arc record at least 20 km of left-lateral displacement. We examined the eastward facing slope of one of the fault zones (the West Santa Rosa Bank fault) from 6469 to 5957 m water depth using the submersible Shinkai 6500 (YK06-12 Dive 973) as part of a cruise by the R/V Yokosuka in 2006. The dive recovered residual but still partly fertile lherzolite, residual lherzolite, and dunite; the samples show mylonitic, porphyroclastic, and coarse, moderately deformed secondary textures. Crystal-preferred orientations of olivine within the peridotites show a typical [100](010) pattern, with the fabric intensity decreasing from rocks with coarse secondary texture to mylonites. The sampled peridotites therefore represent a ductile shear zone within the lithospheric mantle of the overriding plate. Peridotites were probably exposed in association with a tear in the subducting slab, previously inferred from bathymetry and seismicity. Furthermore, although the dive site is located in the fore arc close to the Mariana Trench, spinel compositions within the sampled peridotites are comparable to those from the Mariana Trough back arc, suggesting that back-arc basin mantle is exposed along the West Santa Rosa Bank fault.

Multi-scale magnetic mapping of serpentinite carbonation

Peridotite carbonation represents a critical step within the long-term carbon cycle by sequestering volatile CO2 in solid carbonate. This has been proposed as one potential pathway to mitigate the effects of greenhouse gas release. Most of our current understanding of reaction mechanisms is based on hand specimen and laboratory-scale analyses. Linking laboratory-scale observations to field scale processes remains challenging. Here we present the first geophysical characterization of serpentinite carbonation across scales ranging from km to sub-mm by combining aeromagnetic observations, outcrop- and thin section-scale magnetic mapping. At all scales, magnetic anomalies coherently change across reaction fronts separating assemblages indicative of incipient, intermittent, and final reaction progress. The abundance of magnetic minerals correlates with reaction progress, causing amplitude and wavelength variations in associated magnetic anomalies. This correlation represents a foundation for characterizing the extent and degree of in situ ultramafic rock carbonation in space and time.Peridotite carbonation plays an important role in the carbon cycle. Here, the authors present a geophysical characterization of serpentinite carbonation from km to mm scale and confirm that the abundance of magnetic minerals provides a strong correlation with the overall carbonation reaction process.

Al-Zoning of Serpentine Aggregates in Mesh Texture Induced by Metasomatic Replacement Reactions

Serpentinization of oceanic lithosphere commonly proceeds with the development of mesh texture. Examination of a serpentinized harzburgite and a plagioclase-bearing wehrlite revealed conspicuous zoning of Al in a serpentine mesh texture, with Al-rich cores and Al-poor rims, as well as Al-rich veins, indicating local transport of Al from plagioclase and pyroxene during serpentinization. To reveal the influences of Al on the reaction mechanisms and textural development during serpentinization, we conducted hydrothermal experiments in the olivine (Ol)–plagioclase (Pl)–H2O system and analyzed the variations in mineralogy and microtexture of olivine replacements as a function of distance from the Ol–Pl boundary. The Al–Si metasomatic zone, where Al-serpentine and a minor amount of Ca-saponite were formed, was developed in the Ol-hosted region close to the Ol–Pl boundary. Far from the Ol–Pl boundary, Al-free serpentine, brucite, and magnetite were formed, indicating the progress of an ‘isochemical’ reaction (separate from water). The aggregates of Al-serpentine after olivine in the metasomatic zone showed a characteristic zoning of Al. Microtextural evidence indicates that the zoning was produced in response to the migration of an Al metasomatic front that involved an early-stage of serpentinization with an Al-free solution, and the subsequent pseudomorphic replacement of olivine and simultaneous development of overgrowths as the amount of Al increased in the solution. The Al-bearing aqueous solution caused the formation of olivine pseudomorphs and this contrasts with the lack of preservation of original olivine outlines in the isochemical zones. Comparisons of zoning in natural and experimentally produced mesh textures suggest that Al-poor rims in the mesh texture form at the start of the serpentinization process, followed by the coupled formation of Al-rich mesh cores and Al-rich veins. Our experimental results indicate that Al-zoning in the mesh texture represent the transition from a closed to an open system during serpentinization.