Water-undersaturated Conditions Research Articles

Thermal aspects of the origin of Hebridean Tertiary acid magmas. I. An experimental study of partial fusion of Lewisian gneisses and Torridonian sediments

AbstractThermal aspects of the conflicting hypotheses that the acid magmas of the British Tertiary Igneous Province were generated by either fractional crystallization of basic magmas or partial fusion of country rocks around basic intrusions are investigated by combining petrological and experimental data. Sparse large xenoliths (up to 12 m2 outcrops) of Lewisian gneiss occur in the ferrodiorite member of the Marscoite Suite in the Western Redhills Eocene intrusive complex of Skye. Most of the blocks are of granodioritic-tonalitic gneiss which is very similar to the grey gneisses of the northern and southern regions of the mainland Lewisian. One is a two-pyroxene basic granulite, closely resembling in petrography and composition the granulite-facies basic gneisses of the central region of the Scottish mainland Lewisian. During Tertiary thermal metamorphism the xenoliths recrystallized patchily to pyroxene hornfelses and up to 50% of melting occurred in any parts of blocks that were rich in alkali feldspar and quartz. Microprobe analyses of intergrown Ca-rich and Ca-poor pyroxenes in a hornfelsed ultramafic mica pyroxenite lens within a leucogneiss block give a calculated temperature of 950 °C for the Tertiary metamorphism. Melting experiments at 1 kb PH2O were carried out on leucocratic and mesocratic Lewisian gneisses (from the xenoliths), Torridonian arkose and shale. Extensive melting of the leucogneiss and arkose, to peraluminous liquids, occurs within a few degrees above their solidus (715 °C). The silicate liquidus of the leucogneiss is at 930 °C. Microprobe analyses of the glass products of melting experiments (mostly of leucogneiss) show that liquid composition trends during progressive melting are consistent with theoretical equilibria in the synthetic system Ab-Or-An-Qz-H2O. Partial fusion in the Skye Lewisian gneiss xenoliths took place at a water vapour pressure of a few hundred bars, probably under water-undersaturated conditions.

The peculiarities of magma rise in presence of water

The viscosity of basalts (quartz and olivine tholeiite) was studied under pressure in dry conditions and in the presence of water. In dry conditions at 1400°C when pressure increases to 20 kbars the viscosity reduces by a factor of 2. In conditions of water saturation of basalt melts at 5 kbar the viscosity is smaller by a factor of ∼ 50 than that in dry conditions. In the water undersaturated conditions when water content is fixed (3.3% H2O) in melt the viscosity considerably decreases with pressure and takes intermediate value between those in dry and water saturated conditions. Experimental data recently obtained permit us to consider the peculiarities of physical properties of magma in the presence of water on a new base. Ascending magma can reach critical velocities of transition to the turbulent regime under negligible pressure drop, as a result of low viscosity. It is known at present that water influences on the viscosity of acidic melt under pressure of 1–8 kbars and at temperatures between 800–1200°C. Various authors gave physico-chemical evaluation of the dynamics of granite melts on the basis of these data. The viscosity of basalt melts and their dynamics under normal pressure is also well-known. The known new experimental data of basaltic melt viscosity under pressure in dry conditions (Kushiro et al., 1976;Khitarov et al., 1978) and in the presence of water (Khitarov et al., 1976) embrace broader intervals of physico-chemical conditions as on the pressure (up to 20–30 kbar) as well on the content of water (from 3% up to 12 %). These data permitted to evaluate on a new base the dynamics of magmatic melts under pressure.

Water content of a granite magma deduced from the sequence of crystallization determined experimentally with water-undersaturated conditions

The sequence of crystallization in a biotite-granite from the Bohus batholith of Norway and Sweden, deduced from its texture, was magnetite, plagioclase, microcline, quartz, and finally biotite. Several sequences of crystallization were determined experimentally at 2 kb in the presence of varying only for H_2O contents below 1.2% by weight. The rock was fused to a homogeneous glass, and each experiment included samples of finely crushed rock and glass. The samples were reacted in Ag-Pd capsules with measured H_2O content in coldseal pressure vessels with NNO buffer. With excess H_2O (more than 6.5%) the crystallization interval extends from 865° C to 705° C. In the H_2O-deficient region, the solidus temperature remains unchanged as long as a trace of vapor is present, but the liquidus temperature increases as H_2O content decreases; with 0.8 % H_2O the liquidus temperature is 1125° C, the crystallization interval is 420° C, and a separate aqueous vapor phase is evolved only a few degrees above the solidus at 705° C. The biotite phase boundary increases slightly from 845° C with excess H_2O to 875° C with 1% H_2O, and it intersects the steep phase boundaries for quartz and feldspars; the sequence of crystallization changes at each intersection point. Similar diagrams at various pressures for related rock compositions involving muscovite, biotite and amphibole will provide grids useful in defining limits for the water content of granitic and dioritic magmas. Applications are considered for the Bohus batholith, other granitic rocks, and rhyolites. The Bohus magma could have been formed by crustal anatexis as a mobile assemblage of H_2O-undersaturated liquid and residual crystals with initial total H_2O content less than 1.2%, or it could have been derived by fractionation of a more basic parent with low H_2O content from mantle or subduction zone, but it could not have been derived from a primary andesite generated from mantle peridotite. We consider it unlikely that the H_2O content of large granitic magma bodies exceeds about 1.5% H_2O; these magmas are H_2O-undersaturated through most of their histories. Uprise and progressive crystallization of magma bodies produces H_2O-saturation around margins and in the upper regions of magma chambers. H_2O-saturated rhyolitic and dacitic magmas with phenocrysts can be tapped from the upper parts of the magma chambers.

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.

Experimental melting studies on a model upper mantle composition at high pressure under water-saturated and water-undersaturated conditions

The solidus of a model pyrolite composition is sensitively dependent on water content and has been determined experimentally up to 40 kb, for water-saturated (6% H 2O) and water-undersaturated (0.2% H 2O) conditions. Pargasitic hornblende is a major subsolidus phase to 29 kb and its breakdown at higher pressure has the effect of sharply depressing the solidus for (pyrolite+0.2% H 2O) from ∼1150°C to∼1020°C between 25 and 29 kb. Experiments have been carried out above the solidus to determine the nature of the partial melting process, particularly the nature and composition of the residual phases at a specific pressure, temperature and water content. The presence of siliceous (>58% SiO 2), low-magnesium glasses, broadly of andesitic or dacitic character, in experiments quenched at both 10 kb and 20 kb, is shown to be due to growth during quenching of olivine, clinopyroxene, amphibole and mica. However, it is possible in some experiments to use the compositions of the starting mix and analyzed residual phases to calculate the composition of the equilibrium liquid and degree of melting at the particular condition. High degrees of melting under water-saturated conditions at 10 kb yield magnesian, quartz-normative basaltic andesites ∼10%Qz, (1100°C, 28% melting) to quartz tholeiite magmas 5–7% Qz, (1200°C, 32.5% melting), and at 20 kb, yield olivine tholeiite magmas (1100°C, 27–30% melting). Andesitic or dacitic magmas are not products of equilibrium partial melting of pyrolite at P > 10kb under water-saturated conditions but may be derived from parental olivine-poor tholeiites, quartz tholeiites or basaltic andesites ( P < 10kb) by crystal fractionation. Parental magmas of the island arc tholeiitic magma series may originate by partial melting of upper mantle peridotite (pyrolite or residual peridotite of the lithosphere) under water-saturated conditions at ∼5–20kb.