Computer Science
Scientific paper
May 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50..711s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 50, Issue 5, pp.711-724
Computer Science
41
Scientific paper
Solubilities of oxides in H 2 O and H 2 O-CO 2 fluids in equilibrium with peridotites and with several single minerals have been determined at 15-20 kbar pressure and 600-1100°C. Solutes from H 2 O fluids are rich in normative quartz and feldspars, rather than in alkalies alone, and are not enriched in Mg, Fe, or Ti. Fluids in equilibrium with phlogopite peridotite contain much more solute than fluids in equilibrium with amphibole peridotite. Typical reactions for solution of phlogopite peridotite and amphibole peridotite in H 2 O fluid are, respectively, in weight proportions: 1.05 phlogopite + 1.33 orthopyroxene + 8.62 H 2 O = 9.62 fluid + 1.38 olivine0.59 amphibole + 1.64 orthopyroxene + 0.05 spinel + 32.33 H 2 O = 33.33 fluid + 1.85 olivine + 0.43 diopside Experiments on amphibole peridotite and on Jadeite peridotite indicate that solutes in H 2 O-CO 2 fluids have molar Na / Al > 1, unlike H 2 O solutes. By analogy, H 2 O-CO 2 solute in equilibrium with phlogopite peridotite is less peraluminous than H 2 O solutes. These relations suggest alkali-CO 3 complexing in fluids. Solute contents of H 2 O-CO 2 fluids are far less than of H 2 O fluids, however, even for low CO 2 / H 2 O ratios. Fluids that equilibrate with peridotitic wallrocks in subsolidus continental lithosphere, at depths in excess of about 70 km, are constrained by phase equilibria to be H 2 O-rich and to coexist with phlogopite and carbonate. Upward-moving solute-undersaturated hydrous fluids may leach that portion of the lithosphere; the greatest relative depletion, among major elements, will be in K. Fluids must remain saturated during precipitation reaction with wallrocks, however, and hence large fluid/rock ratios are required for mantle metasomatism at depths below 70 km. Melts are more efficient metasomatic agents, in the sense that only small ratios of crystallizing melt/rock are required. Hydrous fluids that flow upward through the phlogopite-carbonate region of the lithosphere will precipitate, at depths shallower than about 70 km, by amphibolitization-carbonatization of peridotite wallrocks. That process will add small amounts of major elements, notably K, relative to amphibole produced (on the order of 1 K 2 O to 3700 amphibole). Nevertheless, integrated over time, the process may produce a metasomatized zone that represents a possible source for alkaline magmatism.
Eggler David H.
Schneider Mark E.
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