Satoko Ishimaru

Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid‐Atlantic Ridge 30°N

Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.

Calcic amphiboles in peridotite xenoliths from Avacha volcano, Kamchatka, and their implications for metasomatic conditions in the mantle wedge

Abstract Highly metasomatized parts of peridotite xenoliths from Avacha volcano, Kamchatka, Russia, characteristically contain calcic amphiboles, especially tremolites. They are rich in metasomatic pyroxenes with high Mg-number (= Mg/(Mg+Fe) atomic ratio), up to 0.94–0.98, and contain Cr-poor aluminous spinels. They have the spinel lherzolite mineral assemblage and equilibrium temperatures of 900–1000 °C or higher, beyond the stability field of tremolite. The tremolite was therefore retrogressively formed after the peak of high-temperature metasomatism. The high-Mg-number, low-alkali environment facilitates the formation of tremolite instead of Al-rich calcic amphiboles. A sulphur-bearing silicic melt derived from a slab is a probable agent involved in the metasomatism. High fO2 recorded in the highly metasomatized peridotites is consistent with this process. This type of metasomatism can produce high-Mg-number peridotites and pyroxenites with low-Cr-number spinel within the mantle wedge where the Mg-number of silicates is positively correlated with the Cr-number of spinel in ambient peridotites.

Arsenide in a metasomatized peridotite xenolith as a constraint on arsenic behavior in the mantle wedge

Abstract An arsenide (low-Ni, high-Co löllingite) was found in a peridotite xenolith, which is strongly metasomatized by slab-derived melt or fluid from Avacha volcano, located in the Kamchatka arc. This is the first finding of a mantle arsenide within a fresh peridotite xenolith that may have been precipitated from metasomatic fluid/melt ultimately derived from the subducting slab. The löllingite is very low in Ni, suggesting low Ni-Fe partitioning between arsenide and olivine at mantle conditions. This is in contrast to sulfide, which favors Ni over Fe. An As-bearing fluid/melt thus plays some role in the metasomatic Fe enrichment in the mantle wedge. Supply of As is one of the characteristics of the upper mantle beneath the volcanic front.