The key to the origin of carbonatite and kimberlite lies in the system CaOMg0-Si0 2 -C0 2 . Increase in pressure causes a carbonation reaction in the peridotite assemblage as follows: forsterite + clinopyroxene + C 0 2 ^ orthopyroxene + carbonate (Ca:Mg::70:30). This reaction passes through 15 kb-960°C with slope 45 b/°C and terminates at an invariant point near 25 kb-1200°C, where melting begi...

Tectonically exposed mantle peridotite in the Oman Ophiolite is variably serpentinized and carbonated. Networks of young carbonate veins are prevalent in highly serpentinized peridotite, particularly near low-temperature alkaline springs emanating from the peridotite. An unusual feature in some samples is the coexistence of serpentine and quartz, which is not commonly observed in serpentinites....

Strontium, carbon, and oxygen isotope data and radiocarbon ages document at least 30,000 years of hydrothermal activity driven by serpentinization reactions at Lost City. Serpentinization beneath this off-axis field is estimated to occur at a minimum rate of 1.2 x 10(-4) cubic kilometers per year. The access of seawater to relatively cool, fresh peridotite, coupled with faulting, volumetric exp...

Lower mantle seismic scatterers are often interpreted in terms of subducted basaltic crust, which should contain free-silica phases that are seismically fast. However, free-silica phases are unstable in a peridotite lower mantle, as they react with ferropericlase (magnesiowustite) to form silicate perovskite. Nonetheless, conservative estimates and recent measurements of diffusion coefficients ...

Near-surface reaction of CO2-bearing fluids with silicate minerals in peridotite and basalt forms solid carbonate minerals. Such processes form abundant veins and travertinedeposits, particularly in associationwith tectonically exposed mantle peridotite. This is important in the global carbon cycle, in weathering, and in understanding physical-chemical interaction during retrograde metamorphism...

Komatiites from Alexo and Pyke Hill in the Archean Abitibi greenstone belt provide petrological evidence for an early Earth carbon and water cycle, ingassing in the cool Hadean and outgassing in the hot Archean. The komatiites have SiO2 contents that are lower than those expected of advanced volatile-free melting of mantle peridotite. The SiO2 misfit cannot be plausibly accounted for by variati...

Two dives of the DSV Shinkai 6500 in the Mariana Trough back-arc basin in the western Pacific sampled back-arc basin mantle exposures. Reports of peridotite exposures in back-arc basin setting are very limited and the lack of samples has hindered our understanding of this important aspect of lithospheric evolution. The Mariana Trough is a slow-spreading ridge, and ultramafic exposures with asso...

The seawater Mg/Ca ratio increased significantly from ~ 80 Ma to present, as suggested by studies of carbonate veins in oceanic basalts and of fluid inclusions in halite. We show here that reactions of mantle-derived peridotites with seawater along slow spreading mid-ocean ridges contributed to the post-Cretaceous Mg/Ca increase. These reactions can release to modern seawater up to 20% of the y...

This paper provides a summary of postcruise scientific results from Ocean Drilling Program (ODP) Leg 209 available to date, building upon shipboard observations and syntheses summarized in the Leg 209 Initial Results volume. During Leg 209, 19 holes were drilled at 8 sites along the Mid-Atlantic Ridge from 14°43′ to 15°44′N, mainly in residual mantle peridotite intruded by gabbroic rocks, in or...

This paper presents results of modelling reaction between peridotite and fractionating tholeiitic basalt in simple and complex silicate systems. Reactions are outlined in appropriate binary and ternary silicate systems. In these simple systems, the result of reactions between 'basalt' and 'peridotite' may be treated as a combination of Fe-Mg exchange and mass transfer reactions at constant Fe/M...