Melting Relations of Brown-Hornblende Mylonite from St. Paul's Rocks under Water-Saturated and Water-Undersaturated Conditions to 30 Kilobars

Crystalline brown-hornblende mylonite, with a composition similar to some basanites and nephelin-ites, was crushed and reacted with excess water and with no water added (about 2% water present) in sealed platinum capsules in a piston-cylinder apparatus from 10-30 kb. The solidus with excess water, and liquidus curves for excess water and for 2% water, are presented, together with stability limits (within 50°C brackets) in the melting intervals for amphibole, plagioclase, clinopyroxene, garnet, olivine, rutile, nepheline, and zoisite. With increasing water content and consequent decreasing liquidus temperature on the water-undersaturated liquidus surface, the number of liquidus and near-liquidus minerals increases, the field of olivine extends to higher pressures, and garnet becomes stabilized at pressures above 20 kb. Orthopyroxene was not found. Amphibole is stable at the liquidus between 10 and 21 kb with excess water at 1,075°C, and between 10 and 16 kb with 2% water at 1,175°C. These results combined with previous interpretations are consistent with the following tentative petrogenetic history for St. Paul's Rocks. A water-undersaturated olivine-basanite magma rose from a depth of about 100 km at a temperature of 1,200-1,300°C, and differentiated until it reached a depth somewhere between 45-70 km, where the magma had reached the composition of brown-hornblende mylonite. At a temperature between 1,050°C and 1,000°C amphibole was the dominant mineral being precipitated. Exsolution of water from the magma became fixed in the mantle peridotite as amphibole, and locally may have caused incipient melting of the peridotite. This process may have initiated upward movement of the heterogeneous assemblage of peridotite and largely crystallized basanite magma, which rose as a near-solid diapiric intrusion with temperature possibly remaining as high as 1,000°C up to 7 km deep, where intense mylonitization began during the final ascent and shallow emplacement

Melting Interval of Peridotite with 5.7 per Cent Water to 30 Kilobars

Crystalline pargasite-rich spinel peridotite mylonite from St. Paul's Rocks containing 5.7% water bound in hydrous minerals was reacted in sealed platinum capsules in a piston-cylinder apparatus from 10 - 30 kb. At 10 kb the subsolidus assemblage is amphibole, olivine, orthopyroxene, clinopyroxene and spinel, an amphibole lherzolite; with increasing pressure garnet appears at 18 kb, spinel and amphibole disappear at about 25 kb; the resulting high pressure assemblage is that of a garnet lherzolite. The solidus was located in the presence of water-rich vapor, but vapor dissolves completely in the liquid at higher temperatures, and the liquid becomes water-undersaturated. Stability limits in the melting interval were determined for amphibole, garnet, spinel, clinopyroxene, orthopyroxene, and olivine (the liquidus mineral). The results are consistent with a published conclusion that St. Paul's Rocks is a diapiric, solid-state mantle intrusion initially mobilized at a depth between 45 km and 70 km near 1,000 - 1,050°C. An estimate of the solidus of peridotite with 0.2% water is presented and compared with other studies. At intermediate pressures this solidus is determined by the breakdown of amphibole. Discrepancies among results of the various studies probably arise at least in part from experimental problems involved in the complex systems

Melting relations of NaAlSi_3O_8 to 30 Kb in the presence of H_2O:CO_2 = 50:50 vapor

Synthetic albite was reacted with oxalic acid dihydrate under oxidizing conditions in an internally-heated pressure vessel from 1 to 4 kb and in a piston-cylinder apparatus from 10 to 30 kb. Breakdown of the oxalic acid in subsolidus runs produced a vapor with H_2O:CO_2 = 50:50 in moles (X^v_(H_2O) = 0.5, neglecting solubility of solids in the vapor). The curve determined for the beginning of melting under these conditions defines a contour on the divariant solidus surface for the system NaAlSi_3O_8-H_2O-CO_2 for X^v_(H_2O) = 0.5, between the published limits for X^v_(H_2O) = 1.0 and X^v_(H_2O) = O. Addition of 50 mole percent of relatively insoluble CO_2 to NaAlSi_3O_8-H_2O at 15 kb total pressure raises the solidus temperature from 660° to 830°C. The results give no indication of increased solubility of CO_2 in NaAlSi_3O_8 liquids at pressures above 15 kb, in contrast with published results for a basalt liquid