Every large eruption of nonbasaltic magma taps a magma reservoir that is thermally and compositionally zoned. Most small eruptions also tap parts of heterogeneous and evolving magmatic systems. Several kinds of compositionally zoned ash flow tuffs provide examples of preemptive gradients in T and ƒO2, in chemical and isotopic composition, and in the variety, abundance, and composition of phenocrysts. Such gradients help to constrain the mechanisms of magmatic differentiation operating in each system. Roofward decreases in both T and phenocryst content suggest water concentration gradients in magma chambers. Wide compositional gaps are common features of large eruptions, proving the existence of such gaps in a variety of magmatic systems. Nearly all magmatic systems are ‘fundamentally basaltic’ in the sense that mantle-derived magmas supply heat and mass to crustal systems that evolve a variety of compositional ranges. Feedback between crustal melting and interception of basaltic intrusions focuses and amplifies magmatic anomalies, suppresses basaltic volcanism, produces and sustains crustal magma chambers, and sometimes culminates in large-scale diapirism. Degassing of basalt crystallizing in the roots of these systems provides a flux of He, CO2, S, halogens, and other components, some of which may influence chemical transport in the overlying, more silicic zones. Basaltic magmas become andesitic by concurrent fractionation and assimilation of partial melts over a large depth range during protracted upward percolation in a plexus of crustal conduits. Zonation in the andesitic-dacitic compositional range develops subsequently within magma chambers, primarily by crystal fractionation. Some dacitic and rhyolitic liquids may separate from less-silicic parents by means of ascending boundary layers along the walls of convecting magma chambers. Many rhyolites, however, are direct partial melts of crustal rocks, and still others fractionate from crystal-rich intermediate parents. The zoning of rhyolitic magma is accomplished predominantly by liquid state thermodiffusion and volatile complexing; liquid structural gradients may be important, and thermal gradients across magma chamber boundary layers are critical. Intracontinental silicic batholiths form where extensional tectonism favors coalescence of crustal partial melts instead of hybridization with the intrusive basaltic magma. Cordilleran batholiths, however, result from prolonged diffuse injection of the crust by basalt that hybridizes, fractionates, and preheats the crust with pervasive mafic to intermediate forerunners, culminating in large-scale diapiric mobilization of partially molten zones from which granodioritic magmas separate. Much of the variability among magmatic systems probably reflects the depth variation of relative rates of transport of magma, heat, and volatile components, as controlled in turn by the orientation and relative magnitudes of principal stresses in the lithosphere, the thickness and composition of the affected crust, and variations in the rate and longevity of basaltic magma supply. Extension of the lithosphere may reduce the susceptibility of basaltic magmas to hybridization in the crust, but it can also enhance the role of mantle-derived volatiles in chemical transport.

Gradients in silicic magma chambers: Implications for lithospheric magmatism

Environmental isotopes in lake studies

Liquid-vapor fractionation of oxygen and hydrogen isotopes of water from the freezing to the critical temperature

Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin

Volatiles are lost from the Earth’s mantle to the atmosphere, hydrosphere and crust through a combination of subaerial and submarine volcanic and magmatic activity. These volatiles can be primordial in origin, trapped in the mantle since planetary accretion, produced in situ, or they may be recycled—re-injected into the mantle via material originally at the surface through the subduction process. Quantifying the absolute and relative contributions of these various volatile sources bears fundamental information on a number of issues in the Earth Sciences ranging from the evolution of the atmosphere and hydrosphere to the nature and scale of chemical heterogeneity in the Earth’s mantle.

Noble gases have a pivotal role to play in addressing the volatile mass balance between the Earth’s interior and exterior reservoirs. The primordial isotope 3He provides an unambiguous measure of the juvenile volatile flux from the mantle (Craig et al. 1975). As such, it provides a means to calibrate other volatiles of geological and geochemical interest. A prime example is the CO2 flux at mid-ocean ridges (MOR): by combining estimates of the 3He flux at MOR with measurements of the CO2/3He ratio in oceanic basalts, Marty and Jambon (1987) derived an estimate of the CO2 flux from the (upper) mantle.

The approach of using ratios (involving noble gas isotopes) has also been extended to island arcs. Marty et al. (1989) found significantly higher CO2/3He ratios in arc-related geothermal fluids than observed at mid-ocean ridges, consistent with addition of slab-derived CO2 to the mantle wedge. Sano and Williams (1996) scaled the CO2 flux to 3He, showing that the output of CO2 at subduction zones was comparable in magnitude to that at spreading ridges. Therefore, for CO2 at least, subduction zones also …

Noble gases and volatile recycling at subduction zones

Granite and quartz were crushed under moist conditions in the temperature range of 25–270°C, in order to obtain the information on the mechanism operative for H2 degassing along active faults. It was found that the amounts of H2 gas released by crushing granite and quartz increased with temperature up to around 200°C and then decreased suddenly. The results indicate that H2 gas is generated by a chemical reaction between water and such radicals as Si · and Si—O · formed on the fresh surfaces when the Si—O—Si bonds are destroyed by crushing and that the existence of a maximum for amount of H2 gas is due to the decrease of the lifetime of Si—O · radical at the temperature higher than about 200°C. The results lend credence to the possibility that H2 gas is generated in the temperature range of 25–270°C along the active fault through the reaction between groundwater and the fresh surfaces of underground rocks formed by the fault movements.

H2 generation by reaction between H2O and crushed rock: An experimental study on H2 degassing from the active fault zone

Direct measurements of D/H and 18O/16O fractionation factors between vapor and liquid water in the temperature range from 10 to 40.DEG.C.

The deuterium content (δDSMOW) of the water extracted from coexisting biotite and hornblende in granitic rocks was measured. The relationship between δD of biotite and hornblende suggested an isotopic equilibrium in the rocks of four granitic bodies in the Kitakami mountains (Northeast Japan), and the maximum temperature of isotopic equilibration of biotite and hornblende was estimated to be 700-1, 000°C, provided that the minimum temperature was 450-550°C. The δD value obtained for the water with which these minerals equilibrated was -29_??_-37‰ and is definitely enriched in deuterium relative to the present local surface water. On the other hand, the relationship between δD of biotite and hornblende in the Ryoke metamorphic belt contiguous to the Median Dislocation Line (Southwest Japan) indicated an isotopic disequilibrium. In this case a complicated history of formation of these granitic rocks in terms of the interaction of water and hydrous silicates was inferred.

D/h fractionation of coexisting biotite and hornblende in some granitic rock masses

The hydrogen isotopic fractionation factor between brucite and water has been determined in the temperature range of 100°–510° C. Brucite is always depleted in deuterium relative to the coexisting water, and the degree of depletion becomes larger with decreasing temperature. The fractionation factor changes smoothly in the temperature range of 144°–510° C and its temperature dependence was obtained by the method of least square fit in the following form: 103Inα=8.72×106
T
−2−3.86×104
T
−1+14.5 However, a marked decrease of about 5‰ was observed at 100°–144° C. The D/H fractionation factor for the brucite-water system is not similar to that for serpentine-water system presented by Sakai and Tsutsumi (1978), though all the hydroxyl ions coordinate to magnesium ion in both minerals. This discrepancy cannot be attributed to hydrogen bonding but to distortion of Mg-octahedron of serpentine, in which the Mg-OH bonding length is shorter than the sum of ionic radius of Mg2+ and O2− and there is no distortion in brucite. It is indicated that aside from hydrogen bonding, the structure effect also controls the D/H fractionation between hydrous mineral and water.

Hydrogen isotopic fractionation factor between brucite and water in the temperature range from 100° to 510° C

D/H ratios of phlogopites and amphiboles from rocks of possible mantle origin and also those of water from (glass?) inclusions in olivines of the olivine nodule and peridotites have been determined. The mantle water seems to have aδD value of —85±10‰ on the basis of results of inclusions in the nodule-olivine.

Sadao Matsuo

Papers

H2 generation by reaction between H2O and crushed rock: An experimental study on H2 degassing from the active fault zone

Direct measurements of D/H and 18O/16O fractionation factors between vapor and liquid water in the temperature range from 10 to 40.DEG.C.

D/h fractionation of coexisting biotite and hornblende in some granitic rock masses

Hydrogen isotopic fractionation factor between brucite and water in the temperature range from 100° to 510° C

Hydrogen isotope composition of deep-seated water