The deep carbon cycle and melting in Earth's interior

http://www2.ess.ucla.edu/~manning/pdfs/m04.pdf

The chemistry of subduction-zone fluids

The polarized optical spectra of the ions Ti3+, V3+, Cr3+, Mn3+, Co3+, and Ni3+ in corundum single crystals have been studied at temperatures from 4.2° to 1200°K. A theory of the band strength based on the point‐charge model and p‐d mixing has been developed and applied to the data with results in fair agreement with experiment. The effects of temperature show that the vibrational‐electronic contribution to band strength is quite small at low temperature but may be appreciable at high temperatures. The crystal‐field parameters have been calculated as convergent lattice sums. The observed trigonal‐field parameter has the opposite sign from that calculated by the point‐charge model if the impurity ion is assumed to occupy an Al3+ ion position in the lattice, but has the same sign as calculated for an ion 0.1 A displaced along the c3 axis toward the empty octahedral site. Details of the spectra have been interpreted as showing that the surroundings of an ion are distorted in some electronic states.

Optical spectra of transition-metal ions in corundum

Hydrous melting driven by changes in H2O storage capacity may occur in a variety of settings in the mantle, including in oceanic basalt sources and in deeper regions above and below the transition zone. The 50–200 ppm H2O in the upper mantle likely derives from a blend of sources that may include residues of hydrous partial melting, either in the deep mantle and/or beneath arcs or oceanic islands. Relative to the large storage capacity in the transition zone, low storage capacities above and below may lead to hydrous melting for material upwelling through 410 km or downwelling through 670 km. The apparently very low storage capacity of the lower mantle (<20 ppm H2O) may force melting even if downwelling rocks have normal upper mantle H2O (50–200 ppm) concentrations. Very low storage capacity in the lower mantle, if verified experimentally, presents a challenge to the view that the H2O-rich sources of oceanic island basalts reside in the lower mantle.

Water, Melting, and the Deep Earth H 2 O Cycle

Inferring upper-mantle temperatures from seismic velocities

Water transport into the deep mantle and formation of a hydrous transition zone

Phase relations in hydrous MORB at 18-28 GPa: implications for heterogeneity of the lower mantle

Minor amounts of alkalies (Na and K) can reduce drastically the solidus temperatures of carbonated silicate mantle, by as much as 400–500 °C Low-degree melting of carbonated peridotite and eclogite at pressures of 3–10 GPa produces Na- and K-bearing carbonatite melt Mass-balance calculations of samples obtained below apparent solidi show clear deficits of alkalies, suggesting the presence of minor alkali-rich liquid or solid carbonate phases Here we determine the true solidi in Na- and K-bearing carbonate systems and report the stability of alkaline carbonate phases Melting of subducting alkaline carbonates would likely occur at transition zone depths to produce mobile carbonatite melt diapirs that migrate upward, modifying and oxidizing the upper mantle and initiating volcanism at the surface

Solidus of alkaline carbonatite in the deep mantle

Phase relations and melt compositions in CMAS-pyrolite-H2O system up to 25 GPa

We have experimentally investigated a K-bearing altered mid-ocean ridge basalt (MORB) composition to which 10% CaCO3 was added (GA1+10%cc), at temperatures of 1050-1400oC and pressures of 3•5-5•5 GPa. Experiments were conducted in piston-cylinder apparatus in Pt-Gr (Pt with inner graphite) and Au-Pd capsules. Sub-solidus assemblages for both sets of experiments contain clinopyroxene, garnet, carbonate, rutile, coesite and K-feldspar. Apatite was observed only in the Pt-Gr experiments. Melting behaviour in experiments using different capsule materials contrasted markedly. Experiments in Pt-Gr capsules showed the silicate solidus to be at temperatures less than 1100oC at 3•5GPa and less than 1050oC at 4•5-5•0 GPa.These are similar (3•5GPa) or lower (4•5-5•0 GPa) temperatures compared with the carbonate solidus (1075-1125oC at 3•5-5•0 GPa). Melts in the Pt-Gr runs evolve with increasing degree of melting from K-rich silicate melts at the lowest degree of melting to carbonate-silicate immiscible liquids and silicate-carbonate melts at intermediate degrees of melting, and finally to silicate melts at the highest degrees of melting. Experiments in Au-Pd capsules were performed only at 5•0GPa. The carbonate solidus is between 1200 and 1225oC (at least 100oC higher than in the experiments in Pt-Gr capsules at the same pressure-temperature conditions).The first melts to be produced are carbonatitic and exhibit increasing SiO2 content with increasing temperature.This contrast in melting behaviour is explained by the relatively rapid diffusion of H through the Pt-Gr capsules, resulting in formation of H2O, and thus dramatically depressing both the silicate and the carbonate solidi in the Pt-Gr experiments compared with those in the Au-Pd experiments. This presumably reflects the lower permeability of Au-Pd to H, resulting in a much lower H2O/CO2 ratio in the Au-Pd encapsulated experiments. The presence of water in the melt was demonstrated by Fourier transform infrared (FTIR) spectroscopic analysis of one Pt-Gr experiment, indicating ~0•5wt % H2O in the bulk composition. Further confirmation that H2O plays such a role in the Pt-Gr experiments was provided by an additional experiment performed in a Au-Pd capsule with ~10 wt % H2Ospecifically added. In this experiment immiscible carbonate and silicate melts were observed. Carbonate- silicate liquid immiscibility is considered to occur as a result of the H2O present in the system. These results can be applied to natural systems in several ways. First, the presence of a small amount of either silicate melt or H2O-fluid in the system will act as a ‘flux’, depressing the carbonate solidus to much lower temperatures than inanhydrous systems. Second, the full trend in melt evolution from silicate-rich to carbonate-rich melts, which is also observed in inclusions in diamonds, can be explained by melting of K- and CO2-bearing, water-undersaturated MORB compositions. In cratonic environments low-degree silicate and immiscible silicate and carbonate melts will metasomatize the overlying mantle in different ways, producing, in the first instance, Si enrichment and crystallization of additional orthopyroxene, phlogopite, pyrope-rich garnet and consuming olivine, and, in the second case, carbonate metasomatism, with additional magnesite-dolomite, clinopyroxene and apatite. Both metasomatic styles have been described in natural peridotite xenoliths from the cratonic lithosphere.

/pdf/an-experimental-study-of-carbonated-eclogite-at-3-5-5-5-gpa-3ck06nyu5g.pdf

Konstantin D. Litasov

Papers

Water transport into the deep mantle and formation of a hydrous transition zone

Phase relations in hydrous MORB at 18-28 GPa: implications for heterogeneity of the lower mantle

Solidus of alkaline carbonatite in the deep mantle

Phase relations and melt compositions in CMAS-pyrolite-H2O system up to 25 GPa

An Experimental Study of Carbonated Eclogite at 3·5–5·5 GPa—Implications for Silicate and Carbonate Metasomatism in the Cratonic Mantle