Search

Available constraints on metamorphic reaction rates derived from the study of natural systems are similar to, or slightly lower than, the bulk strain rates measured in the same rocks. Here, we explore whether this apparent relationship is merely coincidence or due to a more fundamental mechanistic link between reaction and strain. Grain boundary migration accommodated dislocation creep (GBMDC) or grain boundary diffusion creep (GBDC) (i.e. pressure solution), both of which involve dissolution-precipitation as we define it, will occur simultaneously with mineral reactions involving dissolution-precipitation in the presence of a non-zero deviatoric stress. The exact relationships between reaction and strain are different depending on whether GBMDC or GBDC is controlling strain, but the mechanistic link exists in both cases. We present theoretical arguments which show that bulk strain by GBMDC or GBDC, which may additionally be accommodated by processes not involving dissolution-precipitation, such as dislocation glide and climb or grain boundary sliding, should in most cases be somewhat faster than the bulk reaction rates as observed. With few exceptions, for natural metamorphic systems undergoing plastic deformation, strain rates provide an upper limit for bulk reaction rates occurring simultaneously in the same rocks. The data suggest that mineral reaction rates may typically be within one order of magnitude of the strain rate.

The initial Sr isotopic compositions of glass and mineral separates from the 0.74 Ma Bishop Tuff ashflow in eastern California were determined to investigate the time scales of magmatic processes in a large silicic system. It was found that there is substantial isotopic heterogeneity, both between eruptive units and between glass and phenocryst phases of individual units. The frist-erupted, lower temperature units generally have higher initial 87Sr/86Sr than later crupted, higher temperature units. Within each unit, feldspar phenocrysts have the lowest 87Sr/86Sr, associated glass has higher 87Sr/86Sr, and biotite phenocrysts have the highest 87Sr/86Sr. These isotopic differences were produced by processes in the magma chamber and not by post-eruptive alteration. Two samples were similar Fe−Ti oxide temperatures but from widely separated localities have nearly identifical Sr isotopic characteristics, indicating the existence of compositionally uniform layers of substantial volume within the chamber. Trace element data indicate that the feldspars crystallized from a liquid represented by the associated glass, and that the feldspar-glass pairs are not accidental. The rhyolitic liquids of the Bishoptuff magma chamber apparently experienced increasing 87Sr/86Sr at a rate too fast for feldspar phenocrysts to remain in isotopic equilibrium. The increasing 87Sr/86Sr is caused primarily by radioactive decay of 87Rb in the high-Rb/Sr liquids and not primarily by assimilation of radiogenic wall-rock material. A self-consistent model can be constructed to account for all of the isotopic data except for those on biotite phenocrysts. The time scale for evolution of the system is bounded on the high side at about 500 ky by observations made on precaldera lavas, and on the low side at approximately 300 ky by the time necessary to establish homogeneous layers in an actively differentiating chamber. The deduced time scale is consistent with model Rb−Sr ages, which date the differentiation of low temperature liquids from higher temperature liquids, and is compatible with the observed isotopic disequilibrium between feldspars and glass because of the low diffusivity of Sr in fieldspars ( 3x105 years. The silicic-magma production rate within the Bishop Tuff magma chamber is estimated to be 10-3km3/y. The growth rate of alkali feldspar is estimated to be about 10-14 cm/s based on the Sr isotopic difference between sanidine and glass of the lower Bishop Tuff. The biotite population is inferred to be partially (>50 ppm) xenocrystic, the xenocrysts being introduced to the chamber less than one year prior to eruption.

Isotopic ratios of Nd and Sr have been measured in a suite of samples spanning most of the exposed stratigraphy of the Skaergaard intrusion in order to detect and quantify input (such as assimilated wallrock and fresh magma) into the magma chamber during crystallization. Unlike δ18O and δD, Nd and Sr isotope ratios do not appear to have been significantly affected by circulation of meteoric waters in the upper part of the intrusion. Variations in initial 87Sr/86Sr and eNd suggest that the Skaergaard magma chamber was affected during its crystallization by a small amount (2%–4%) of assimilation of Precambrian gneiss wallrock (high 87Sr/86Sr, low eNd) and possibly recharge of uncontaminated magma. Decreases in eNd and increases in 87Sr/86Sr during the early stages (0%–30%) of crystallization give way to approximately unchanging isotopic ratios through crystallization of the latest-deposited cumulates. Modelling of assimilation-fractional crystallization-recharge processes using these data as constraints shows that the assimilation rate must have been decreasing throughout crystallization. In addition, the isotope data allow replenishment by an amount of uncontaminated magma equal to 20%–30% of the total intrusion mass, occurring either continuously or in pulses over the first 75% of crystallization. Comparison of the recharge models with published Mg/(Mg+Fe2+) data from Skaergaard cumulates shows that the modelled replenishment rates are not inconsistent with available major element data, although significant recharge during the final ∼25% of crystallization can be ruled out. The isotope data show that the Skaergaard magma could have incorporated only a small amount of the gneiss that it displaced from the floor of the chamber; assimilation appears to have taken place primarily across a partially molten zone that formed at the roof from the wallrock that was dislodged during emplacement. In the latest stages of crystallization (>75% crystallized), the Skaergaard magma may have become stratified into two separately-convecting layers, effectively insulating Layered Series cumulates from further contamination.

The isotopic compositions of Nd and Sr and concentrations of major and trace elements were measured in flows and tuffs of the Woods Mountains volcanic center of eastern California to assess the relative roles of mantle versus crustal magma sources and of fractional crystallization in the evolution of silicic magmatic systems. This site was chosen because the contrast in isotopic composition between Precambrian-to-Mesozoic country rocks and the underlying mantle make the isotope ratios sensitive indicators of the proportions of crustal- and mantle-derived magma. The major eruptive unit is the Wild Horse Mesa tuff (15.8 m.y. old), a compositionally zoned rhyolite ignimbrite. Trachyte pumice fragments in the ash-flow deposits provide information on intermediate composition magma types. Crustal xenoliths and younger flows of basalt and andesite (10 m.y. old) provide opportunities to confirm the isotopic compositions of potential mantle and crustal magma sources inferred from regional patterns. The trachyte and rhyolite have ɛNd values of -6.2 to -7.5 and initial 87Sr/86Sr ratios mostly between 0.7086 and 0.7113. These magmas cannot have been melted directly from the continental basement because the ɛNd values are too high. They also cannot have formed by closed system fractional crystallization of basalt because the 87Sr/86Sr ratios are higher than likely values for parental basalt. Both major and trace element variations indicate that crystal fractionation was an important process. These results require that the silicic magmas are end products of the evolution of mantle-derived basalt that underwent extensive fractional crystallization accompanied by assimilation of crustal rock. The mass fraction of crustal components in the trachyte and rhyolite is estimated to be between 10% and 40%, with the lower end of the range considered more likely. The generation of magmas with SiO2 contents greater than 60% appears to be dominated by crystal fractionation with minimal assimilation of upper crustal rocks.

Thermodynamic calculations, modified after Nicholls et al. (1971), which relate the activity of silica in a lava to the temperature and pressure conditions at which the lava could be in equilibrium with a mantle mineral assemblage, have been extended to H2O-bearing magmas by using published experimental data to derive the dependence of \(a_{{\text{SiO}}_{\text{2}} }\) on the weight fraction of H2O dissolved in a magma. A petrogenetic grid has been calculated which gives the P-T conditions under which a magma with a given \(a_{{\text{SiO}}_{\text{2}} }\) at its liquidus at 1 atm could equilibrate with a mantle mineral assemblage containing olivine (ol) and orthopyroxene (opx) for different amounts of H2O in the magma at its source. This grid is in good agreement with the results of experimental studies as summarized by Green (1971) and Brey and Green (1975). The results show that the pressure at which a given magma composition can equilibrate with ol + opx increases for increasing amounts of H2O dissolved in the magma at depth.

A selected suite of fresh volcanic rocks from the New Britain island arc has been analyzed for 143Nd/144Nd, 87Sr/86Sr, major and trace elements to investigate relationships between isotopes, trace elements and petrology, and depth to the underlying Benioff zone. From these relationships inferences about magma generation are made utilizing Nd and Sr isotope systematics in possible source materials. Lavas ranging in composition from basalt to rhyolite show minimal variation of 143Nd/144Nd. Small variations in 87Sr/86Sr do not correlate with depth to the Benioff zone, but are related to magma type. Nd-Sr isotopes suggest that island arc lavas in general are derived from a mixture of suboceanic mantle and hydrothermally altered mid-ocean ridge-type basalt, but the New Britain magma source appears homogeneous with little indication of either the involvement of oceanic crust or mantle inhomogeneity. Trace element patterns in New Britain lavas are not consistent with Nd isotope data for currently accepted petrologic and trace element models of magma genesis. Mafic lavas from New Britain and other island arcs have anomalously high Sr/Nd, possibly due to components derived from subducted oceanic crust.

Rhyolite flows and tuffs from the Long Valley area of California, which were erupted over a two-million- year time period, exhibit systematic trends in Nd, Hf, and Pb isotopes, trace element composition, erupted volume, and inferred magma residence time that provide evidence for a new model for the production of large volumes of silica-rich magma. Key constraints come from geochro- nology of zircon crystal populations combined with a refined eruption chronology from Ar-Ar geochronology; together these data give better estimates of magma resi- dence time that can be evaluated in the context of changing magma compositions. Here, we report Hf, Nd, and Sr isotopes, major and trace element compositions, 40 Ar/ 39 Ar ages, and U-Pb zircon ages that combined with existing data suggest that the chronology and geochemistry of Long Valley rhyolites can be explained by a dynamic interaction of crustal and mantle-derived magma. The large volume Bishop Tuff represents the culmination of a period of increased mantle-derived magma input to the Long Valley volcanic system; the effect of this input continued into earliest postcaldera time. As the postcaldera evolution of the system continued, new and less primitive crustal- derived magmas dominated the system. A mixture of varying amounts of more mafic mantle-derived and felsic crustal-derived magmas with recently crystallized granitic plutonic materials offers the best explanation for the observed chronology, secular shifts in Hf and Nd isotopes, and the apparently low zircon crystallization and saturation temperatures as compared to Fe-Ti oxide eruption tem- peratures. This scenario in which transient crustal magma bodies remained molten for varying time periods, fed eruptions before solidification, and were then remelted by fresh recharge provides a realistic conceptual framework that can explain the isotopic and geochemical evidence. General relationships between crustal residence times and magma sources are that: (1) precaldera rhyolites had long crustal magma residence times and high crustal affinity, (2) the caldera-related Bishop Tuff and early postcaldera