Silicic End-members Research Articles

Experimental Constraints on Magma Storage Conditions of Two Caldera‐Forming Eruptions at Towada Volcano, Japan

AbstractTowada volcano is an active volcano in northeast Japan, that caused two large caldera‐forming eruptions at 36 ka (episode N) and 15.5 ka (episode L). Petrological analyses and phase equilibrium experiments were performed on silicic endmember rhyolitic pumices from these two eruptions to constrain the pre‐eruptive magma storage conditions. A common mineral assemblage of plagioclase + orthopyroxene + clinopyroxene + magnetite + ilmenite was observed in both eruptions, whereas amphibole (hornblende) was observed only in the episode L pumice. Mineral thermometers presented temperatures of 832–883°C and 802–870°C for the episode N and L pumices, respectively, with an oxygen fugacity of ∼nickel–nickel oxide (NNO) + 1 in log units. The water‐saturation pressures evaluated using the plagioclase‐melt hygrometer were in the range of ∼100–200 MPa. Experiments at 100–350 MPa and 825–900°C under water‐saturated and NNO‐buffered conditions successfully reproduced the mineral assemblages in both pumices, except for magnetite. Further constraints by phase compositions and phase proportions resulted in the preferred storage conditions of 840–850°C and 150–170 MPa for both eruptions. The emergence of hornblende in the episode L magma was likely caused by the higher CaO content compared to the episode N magma and not by the difference in the storage conditions. The storage depths at ∼5–7 km coincide with the depth of the low seismic velocity beneath the present Towada volcano, suggesting possible magma accumulation at the same depths as in the past caldera‐forming eruptions.

Tracing the evolution of dissolved organic carbon (DOC) pool in the Ediacaran ocean by Germanium/silica (Ge/Si) ratios of diagenetic chert nodules from the Doushantuo Formation, South China

The Ediacaran Period (ca. 635–538 Ma) was a critical interval in Earth’s history, bridging the Proterozoic and the Phanerozoic Earth systems. It witnessed the diversification of eukaryotic phytoplanktons in the early stage, the evolution of macroscopic multicellular lives in the late stage, and frequent oscillations of carbonate carbon isotopes of large magnitudes, presumably reflecting large perturbations of the global carbon cycle. It has been proposed that a large dissolved organic carbon (DOC) pool, hundreds to thousands times larger than the present one, was developed in the Ediacaran deep ocean. The wax and wane of the marine DOC pool might be responsible for the secular variation of carbon isotopes of marine dissolved inorganic carbon (DIC) pool, which was recorded in the carbonate carbon isotope chemostratigraphy. However, neither the extent nor the evolutionary history of marine DOC pool in the Ediacaran ocean has been well constrained. In this study, we use Germanium/silica (Ge/Si) ratios of chert nodules from the Ediacaran Doushantuo Formation in the Yangtze Gorges area, South China, to trace the evolution of marine DOC pool. These chert nodules yield abundant microfossils, i.e. acanthomorphic acritarchs, and were precipitated in the early diagenetic replacement of calcareous silty/muddy sediments. Most chert nodules show positive correlations between Al contents and Ge/Si ratios, which can be resolved by a binary mixing between a silica (quartz) endmember and an Al-rich silicate (clay) endmember. On the one hand, the Ge/Si ratios of silica component ((Ge/Si)SiO2) are comparable to the modern seawater composition, suggesting the normal seawater rather than hydrothermal fluids was the major source of Si. On the other, the Ge/Si ratios of the clay component ((Ge/Si)clay) are one to two orders of magnitudes higher than that of normal marine clays. The Ge-enrichment of the clay component could be attributed to the extensive chelation of Ge by organic ligands and massive formation of organic-Ge-Clay complex at high DOC concentration. This observation supports the presence of a large DOC pool in the Ediacaran ocean. In addition, there is an increase of (Ge/Si)clay from the Member II to Member III of the Doushantuo Formation, suggesting an increase of DOC concentration. The expansion of marine DOC pool is coincident with the reduction of the sizes of the upper acanthomorphic acritarch assemblage in the Member III. We suggest that the increase of marine DOC pool might be attributed to the reduction of sizes of particulate organic matter, which favored DOC generation and accumulation in the Ediacaran seawater.

Mineralogical Evidence of Pre-caldera Magma Petrogenesis in the Jemez Mountains Volcanic Field, New Mexico, USA

Abstract The Jemez Mountains volcanic field (JMVF) is the site of the two voluminous, caldera-forming members of the Bandelier Tuff, erupted at 1·60 and 1·25 Ma, following a long and continuous pre-caldera volcanic history (∼10 Myr) in this region. Previous investigations utilizing whole-rock geochemistry identified complex magmatic processes in the two major pulses of pre-caldera magmatism including assimilation–fractional crystallization (AFC) and magma mixing. Here we extend the petrological investigation of the pre-caldera volcanic rocks into the micro-realm and use mineral chemistry and textural information to refine magma evolution models. The results show an increasing diversity of mineral populations as the volcanic field evolved. A range of plagioclase textures (e.g. sieved cores and rims) indicate disequilibrium conditions in almost all pre-caldera magmas ranging from andesite to rhyolite, reflecting plagioclase dissolution and regrowth. Coarsely sieved or dissolved plagioclase cores are explained by resorption via water-undersaturated decompression during upward migration from a deep melting, assimilation, storage and homogenization (MASH) zone. Plagioclase crystals with sieved rims are almost ubiquitous in dacite-dominated magmatism (La Grulla Plateau andesite and dacite erupted at ∼8–7 Ma, as well as Tschicoma Formation andesite, dacite and rhyolite at ∼5–2 Ma), reflecting heating induced by magma mixing. These plagioclase crystals often have An-poor cores that are chemically distinct from their An-rich rims. The existence of different plagioclase populations is consistent with two distinct amphibole groups that co-crystallized with plagioclase: a low-Al, low-temperature, high-fO2 group, and a high-Al, high-temperature, low-fO2 group. Calculation of melt Sr, Ba, La, and Ce concentrations from plagioclase core and rim compositions suggests that these chemical variations are largely produced by magma mixing. Multiple mafic endmembers were identified that may be connected by AFC processes in the MASH zone in the middle to lower crust. The silicic component in an early andesite-dominated magmatic system (Paliza Canyon andesite, dacite and rhyolite, 10–7 Ma) is represented by contemporaneous early rhyolite (Canovas Canyon Rhyolite). A silicic mush zone in the shallow crust is inferred as both the silicic endmember involved in the dacite-dominant magmatic systems and source of the late low-temperature rhyolite (Bearhead Rhyolite, 7–6 Ma). Recharging of the silicic mush by mafic melts can explain observed diversity in both mineral disequilibrium textures and compositions in the dacitic magmas. Overall, the pre-caldera JMVF magmatic system evolved towards cooler and more oxidized conditions with time, indicating gradual thermal maturation of local crust, building up to a transcrustal magmatic system, which culminated in ‘super-scale’ silicic volcanism. Such conditioning of crust with heat and mass by early magmatism might be common in other long-lived volcanic fields.

Evidence for a ‘third’ endmember of the Unzen 1991–1995 eruption from amphibole thermometry and crystal clots

Amphibole phenocrysts and crystal clots composed of amphibole, plagioclase, and interstitial glass were observed in dacitic products of the Unzen 1991–1995 eruption and analyzed in order to understand pre-eruptive magmatic processes. Amphibole phenocrysts in the samples erupted at different times are classified by composition as either magnesiohornblendes or tschermakites. Temperatures of 770–830 °C are estimated from magnesiohornblende phenocrysts and amphiboles in the crystal clots, which are consistent with the proposed temperature of the silicic endmember magma. The average approximate composition of the interstitial melt is 67–73 wt% SiO2, ~0.25 wt% TiO2, ~12 wt% Al2O3, ~1.1 wt% FeO, 0.14 wt% MgO, 0.80 wt% CaO, 3.0 wt% Na2O, and 4.0 wt% K2O. The SiO2 plus H2O content (~8.8 wt%) of the melt estimated from these amphiboles (SiO2melt = 71–72 wt%) is consistent with the compositions of interstitial melts observed in the crystal clots. The H2O saturation depth estimated for the interstitial melt and plagioclase compositions is consistent with that of the magma chamber where the silicic endmember magma was likely stored (11–15 km). These results suggest that interstitial melts in crystal clots represent the silicic endmember melt. Temperatures estimated from tschermakite phenocrysts are 870–950 °C, which is lower than the proposed temperature of the mafic endmember magma. In addition, a temperature gap is observed between the tschermakites and magnesiohornblendes. These results suggest the contribution of a previously unrecognized third endmember magma to the 1991–1995 magma, which we term the mid-temperature magma (MT magma).

Geochemistry and chronology of the granites in Alasituo, west Tianshan Orogen: Implications for a magma mixing origin

There are intense magmatic activities in the Alasituo integrated gold exploration area, west Tianshan Orogen. On the basis of field characteristics, geochemistry features, and chronology in Alasituo, the formation mechanism of complex granite rocks was explored. The intrusive rocks are divided into 3 parts: a silicic endmember, a mafic endmember, and intermediate rocks. Mafic microgranular enclaves, as well as spotted structure, silicic veins intruding mafic rocks and plagioclase xenocrysts, developed in rocks. Under the microscope, there are many unbalanced mineral combinations, needle‐like apatites, and poikilitic structures. Futhermore, there is an obvious linear trend between the mafic endmember samples, intermediate samples, and the silicic endmember samples in Harker diagrams. All samples were evolved from magma mixing in TFe2O3‐MgO diagram. The trace element concentrations of intermediate rocks have the transitional features of both silicic and mafic endmember rocks. The above characteristics show that rocks have experienced low degree and uneven physical and chemical magma mixing. The silicic endmember rocks have a high content of K2O + Na2O, Zr, Nb, Y, and REE; high Ga/Al ratios, and; low content of Sr, Eu, Ti, and P, which have the characteristics of A‐type granites with a crustal source. The high content of MgO, TFe2O3, and low K2O/Na2O in mafic endmember rocks indicate a mantle source. The LA‐ICP‐MS U–Pb zircon age of the granodiorites is 350.3 ± 3.0 Ma. Combined with regional geological background and geochemical characteristics, the intrusive rocks were formed when the thinning lithosphere triggered mafic magma underplating, heating upper crust granites, and causing them partial melt.

Early Cretaceous bimodal volcanism in the Duolong Cu mining district, western Tibet: Record of slab breakoff that triggered ca. 108–113 Ma magmatism in the western Qiangtang terrane

We report new zircon U–Pb ages and Hf isotope compositions, and whole-rock major and trace element and Sr–Nd isotope data for the Meiriqiecuo Formation (MF) bimodal volcanic rocks collected from the Duolong Cu mining district (DCMD) in the western Qiangtang terrane (QT), western Tibet. These data provide important constraints on the petrogenetic evolution and geodynamic setting of Early Cretaceous magmatism in the DCMD. The MF bimodal volcanic rocks are mainly basaltic andesite and andesite, with subordinate rhyolite. Four mafic samples yielded zircon U–Pb ages of ca. 108.2–113.0Ma, and one silicic sample has an age of 109.3±2.2Ma, indicating that the mafic and silicic eruptions were contemporaneous. The MF bimodal volcanic rocks belong to the medium-K calc-alkaline to shoshonite series. The rocks show arc-type affinities characterized by significant enrichment in light rare earth (LaN/YbN=7.74–12.60) and large-ion lithophile elements (Rb, Cs, K, and Pb), but depletions in the high-field-strength elements (Nb, Ta, and Ti), which geochemically resemble Andean arc basalts. Therefore, the MF bimodal volcanic rocks were likely emplaced at an Andean-type active continental margin and represent an Early Cretaceous magmatic arc that was located at the western QT margin. Moreover, the mafic volcanic rocks have high initial Sr isotopic ratios (0.705269–0.705413) and negative εNd(t) values of −1.5 to −0.6 compared with the silicic volcanic rocks ((87Sr/86Sr)i=0.704770–0.704903; εNd(t)=+1.2 to +1.3). Zircons from silicic samples have significantly higher εHf(t) values (+11.6 to +15.5) and predominantly lower Paleoproterozoic Hf crustal model ages (TDMC=180–428Ma) than the mafic samples, which have variable εHf(t) values of +3.4 to +13.0 and TDMC ages of 346–952Ma. These results indicate that the mafic and silicic end-members of the MF bimodal suite were generated from mantle and crustal sources, respectively. The basaltic andesite and andesite may have been derived from mantle enriched by the metasomatism of subducted fluids, whereas the rhyolite could have been derived by partial melting of mafic juvenile crust that originated from an older and more depleted mantle. In light of the geochemical characteristics and field relationships, we propose that breakoff of the Bangong–Nujiang oceanic lithosphere was responsible for the generation and emplacement of the MF bimodal volcanic rocks. The fact that the MF bimodal volcanic arc magmatism was active at ca. 108–113Ma indicates that it was associated with closure of the Bangong–Nujiang Ocean via an arc-arc “soft” collision during the Early Cretaceous.

The behavior of magnesium isotopes in low-grade metamorphosed mudrocks

Magnesium isotopic compositions of mudrocks metamorphosed at sub-greenschist facies from three lower Paleozoic basins (northern Lake District, southern Lake District, and Southern Uplands) in the British Caledonides were measured in order to understand the behavior of Mg isotopes during diagenesis and low-grade metamorphism. Carbonate-free mudrocks from the northern Lake District have heavy δ26Mg values varying from −0.17 to +0.25. By contrast, Mg isotopic compositions of carbonate-bearing mudrocks from the southern Lake District and Southern Uplands vary more widely, with δ26Mg ranging from −0.74 to −0.08. Acid leaching experiments on the latter show that the leachates have higher Ca/Al and Ca/K ratios than the residues due to the dissolution of leachable carbonates. The δ26Mg values of leachates (−1.54 to −0.21) are always lower than the corresponding residues (δ26Mg=−0.39 to +0.09), consistent with isotopically light Mg in carbonates. A rough, negative correlation between δ26Mg and Mg/Al for the residual silicate fraction of mudrocks suggests that their Mg isotopic compositions are controlled by the relative proportion of illite/muscovite and chlorite. Global clastic sediments display highly variable Mg isotopic compositions that are negatively correlated with CaO/Al2O3 and CaO/TiO2, implying that carbonates introduce light Mg isotopes to sediments, although the silicate end member itself has a wide range of δ26Mg, depending on its mineralogy. Magnesium isotopic compositions of mudrocks, as well as their silicate and carbonate fractions, do not vary systemically as metamorphism proceeds from diagenesis to low-grade metamorphism, suggesting limited Mg isotope fractionation during low-temperature metamorphic dehydration (<300°C). The general decrease of Mg fraction (by mass) contributed by carbonate with increasing metamorphic grade suggests that dissolution or decomposition of carbonates during metamorphism expelled light Mg isotopes. Thus, the Mg isotopic compositions of the silicate fractions in clastic sediments more faithfully reflect their provenance signatures. Our study shows that Mg isotopes can be used to study sedimentary diagenesis, and Mg isotopes may prove a useful tracer of sediments recycled into the mantle given their heterogeneous δ26Mg values.

Carbonate-silicate composition of diamond-forming media of fibrous diamonds from the Snap Lake area (Canada)

This study presents new data on the compositions of microinclusions in fibrous diamonds from the Snap Lake area in the eastern part of the Slave Craton (Canada). The compositional trends of diamond microinclusions are consistent with those of diamond-forming media ranging continuously between a highly carbonatitic endmember and a highly silicic endmember. The microinclusions exhibit general enrichment of most incompatible elements, which is probably indicative of their crystallization during partial melting of mantle peridotites and eclogites. Our results also suggest that the diamond analyzed in this study may have formed as a result of interaction between carbonate-silicate melts and peridotitic wall-rocks at the base of a thick lithospheric mantle at depths below 300 km. The trace element distributions in the studied diamond microinclusions show a general similarity to those previously found in the parental kimberlites and carbonatites. These data suggest that diamonds may have crystallized either directly from a kimberlitic/carbonatitic melt or from a proto-kimberlitic fluid/melt, which was derived from a source also common to kimberlites. This is supported by differences in the major element compositions of diamond-forming fluids/melts and kimberlites.

Densities and volumes of hydrous silicate melts: New measurements and predictions

The equilibrium molar volumes of four series of anhydrous and hydrous aluminosilicate glasses and liquids (0 to 11mol% H2O) were determined at 1bar between 300 and 1050K. The anhydrous compositions range from highly polymerized NaAlSi3O8 to depolymerized synthetic iron-free analogs of tephrite and foidite magma compositions (NBO/T=0.8 and 1.5, respectively). For each sample the volume was derived from the room-temperature density of the glass and the thermal expansivity of the glass and supercooled liquid from 300K to a temperature about 50K higher than the standard glass transition. The partial molar coefficient of thermal expansion of water in hydrous silicate glasses is about (6.2±3.5)×10−5K−1, and in the melts ranges from 11×10−5 to 36×10−5K−1. The present molar volumes of hydrous supercooled liquids are reproduced with the model of Ochs and Lange (1999) to within 1.1%, except for the hydrous foidite series. This agreement confirms that the partial molar volume of water (V¯H2O) near the glass transition cannot depend strongly on the chemical composition of the silicate end-member, nor on water speciation. In order to reproduce the molar volumes of the foidite series, a combined model (using Lange, 1997 and Courtial and Dingwell, 1999 models and values derived from the new data) is used where an excess volume term between SiO2 and CaO is introduced. Finally, our experimental data are better fit if V¯H2O=23.8±0.5cm3mol−1 at 1273K, and dV¯H2OdT=15.9±1.6cm3mol−1K−1. Contrasting trends are also observed for the configurational contributions to the expansivity with a positive slope of dViconfdT versus water for the most polymerized base compositions (NBO/T≤0.21) and a negative slope for the two most depolymerized base compositions with NBO/T of 0.86 and 1.51.

On discrimination between carbonate and silicate inputs to Himalayan rivers

We review new and published analyses of river waters, bedloads and their constituent minerals from the Dhauli Ganga and Alaknanda, headwaters of the Ganges in Garhwal, and the Marsyandi in Nepal and their tributaries. These data are used to discriminate between the inputs of major cations and Sr from silicate and carbonate sources. Methods of estimating the proportion of the carbonate and silicate inputs to river waters using mixing arrays in Sr-Ca-Mg-Na-K-87Sr/86Sr space are shown to suffer from systematic correlations between the magnitude of the precipitation of secondary calcite and the fraction of the silicate component. This results in factor-of-two overestimates of the fractions of silicate-derived Ca, Mg and Sr. To correct for this the magnitude of secondary calcite precipitated and relative fractions of silicate and carbonate-derived cations are instead calculated by modeling the displacement of water compositions from the compositions of the carbonate and silicate components of the bedload in subsets of Sr-Ca-Mg-Na-K-87Sr/86Sr space. The compositions of the carbonate and silicate end-members in the bedload are determined by sequential leaching. The results of this modeling are compared with modeling of the modal mineral inputs to waters where mineral compositions are derived from electron-microprobe analyses of the minerals in the bedload. In the upper Marsyandi catchment, which drains low-grade Tethyan Sedimentary Series formations, a set of mainstem samples collected over a two-year period define tight correlations in Sr-Ca-Mg-Na-K-87Sr/86Sr space. Modeling of the magnitude of secondary carbonate precipitation and fractions of silicate-derived Ca, Mg and Sr in Sr-Ca-Mg-87Sr/86Sr space gives self-consistent results that are compatible with both the calculations of mineral modes and published Mg-isotopic compositions, if the ratio of chlorite to biotite weathering is high or if there is another silicate source of Mg. These calculations imply that between 12 and 31 percent of the Sr and 44 and 72 percent of the Mg is derived from silicate minerals where the range reflects the seasonal change in the ratio of silicate-derived to carbonate-derived cations. Modeling in Sr-Ca-Na and/or K space is inconsistent with the Sr-isotopic and Mg-isotopic constraints and we conclude that in this catchment dissolution of Na and K are incongruent relative to Sr-Ca-Mg. Potassium is preferentially retained in micas whereas the controls on Na are unclear. Modeling of the catchments underlain by High Himalayan Crystalline and Lesser Himalayan Series in Garhwal is complicated by the presence of dolomite as well as calcite in the carbonate and the results imply that dolomite dissolves faster in the acetic acid leaches than in nature. Up to 60 percent of the Sr in the catchment on High Himalayan Crystalline Series and 20 to 30 percent of Sr in the catchments on Lesser Himalayan Series are estimated to be derived from silicates. However it should be noted that the element budgets are not all self-consistent and the use of bedrock-element ratios to model the sources of chemical inputs to river waters remains subject to uncertainties.

Evidence for multiple diamondite-forming events in the mantle

A collection of 35 diamondite samples (polycrystalline diamond aggregates, sometimes referred to as framesites), assumed to be from southern Africa, have been studied to investigate their infrared (IR) spectroscopic characteristics. Due to the abundance of sub-micrometer, interlocking diamonds (polycrystalline) with mineral and fluid inclusions within the diamond material affecting their transparency, only fragments from 10 of the samples provided high-quality data. The IR spectra showed a wide range of generally high-nitrogen concentrations (386-2677 ppm), with a full range of nitrogen aggregation states, from pure IaA to pure IaB. Platelet characteristics were interpreted as being regular (i.e., not having been affected by deformation and/or heating events), meaning the nitrogen aggregation data could be interpreted with confidence. Surprisingly, the platelet data showed a positive correlation between their intensity (integrated area) and peak position. The primary hydrogen band (at 3107 cm-1) and secondary band (at 1405 cm) are both often present in the samples’ spectra, but show no correlation with any other characteristic. There is also no correlation between the samples’ paragenesis (as defined by their garnet chemistry) and any of the IR characteristics. While we have no independent determination of the samples mantle residence age, nor the temperature they resided at, we infer that diamondite formation has occurred episodically over a large time frame in single and distinct growth events (as opposed to over a short time frame but over a large depth/temperature range). This idea is more in keeping with the theory that C-O-H diamond- (and diamondite-) forming fluids are the result of localized small volume processes. Interestingly, one sample contained fluid inclusions that exhibited a water:carbonate molar ratio (~0.8), similar to the saline and silicic end-members of the monocrystalline diamond-forming fluid chemical spectrum.

New estimates of silicate weathering rates and their uncertainties in global rivers

This study estimated the catchment- and global-scale weathering rates of silicate rocks from global rivers using global compilation datasets from the GEMS/Water and HYBAM. These datasets include both time-series of chemical concentrations of major elements and synchronous discharge. Using these datasets, we first examined the sources of uncertainties in catchment and global silicate weathering rates. Then, we proposed future sampling strategies and geochemical analyses to estimate accurate silicate weathering rates in global rivers and to reduce uncertainties in their estimates. For catchment silicate weathering rates, we considered uncertainties due to sampling frequency and variability in river discharge, concentration, and attribution of weathering to different chemical sources. Our results showed that uncertainties in catchment-scale silicate weathering rates were due mostly to the variations in discharge and cation fractions from silicate substrates. To calculate unbiased silicate weathering rates accounting for the variations from discharge and concentrations, we suggest that at least 10 and preferably ∼40 temporal chemical data points with synchronous discharge from each river are necessary. For the global silicate weathering rate, we examined uncertainties from infrequent sampling within an individual river, the extrapolation from limited rivers to a global flux, and the inverse model selections for source differentiation. For this weathering rate, we found that the main uncertainty came from the extrapolation to the global flux and the model configurations of source differentiation methods. This suggests that to reduce the uncertainties in the global silicate weathering rates, coverage of synchronous datasets of river chemistry and discharge to rivers from tectonically active regions and volcanic provinces must be extended, and catchment-specific silicate end-members for those rivers must be characterized. With current available synchronous datasets, we suggest that the global silicate weathering rate (Ca+Mg silicate weathering flux) was ∼2.17 (2 σ range of 1.59–2.75)×1012mol/yr, and the global CO2 consumption rates from silicate weathering was ∼7.85 (5.78–9.93)×1012mol/yr. Since current synchronous datasets are not available for some rivers in tectonically active regions and highly active volcanic rocks, our estimate should be considered as lower limit of estimates. Including the estimates from volcanic provinces from Dessert et al. (2003), the global CO2 consumption rates from silicates can be estimated as 11.93 (9.86–14.01)×1012mol/yr, which is similar to the previous estimates of Gaillardet et al. (1999).

On the origin of hot metasedimentary quartzites in the lower crust of continental arcs

Volcanic arcs associated with subduction zones are thought to be the primary building blocks of continents. The composition of the magmas, particularly in continental arcs, is the product of mixing between differentiation of juvenile magmas and pre-existing crustal wallrock, the former being typically mafic and the latter more silicic. Because the upper continental crust is on average thought to be more silicic than the mafic lower crust, mixing with silicic endmembers should occur primarily in the upper crust. However, we show here that the lower crust of continental arcs contains silicic metasediments. We examine garnet-bearing, granulite-facies sedimentary quartzite xenoliths from the Sierra Nevada batholith in California, a Cretaceous continental arc. The quartzites have equigranular textures and contain quartz (>50%), plagioclase (<30%), garnet (10%), and small amounts (<1%) of rutile, aluminosilicate, biotite, monazite, zircon, graphite and trace orthopyroxene. Cathodoluminescent images show zircons with rounded detrital cores mantled by metamorphic overgrowths. Hf isotopic model ages and U–Pb upper intercept ages, for a given zircon, are similar, but the zircon population shows variable protolith ages ranging from Proterozoic to Archean. In contrast, all zircons share similar lower intercept U–Pb ages (103±10Ma), which coincide with the peak of arc magmatism in the Sierra Nevada. The Precambrian protolith ages are similar to North American cratonal basement, and together with the abundance of quartz and detrital zircons, suggest that these quartzites represent ancient, passive margin sediments instead of juvenile active margin sediments in the oceanic trench and accretionary prism. Importantly, these quartzites record peak metamorphic temperatures and pressures of 700–800°C using Ti-in-quartz thermometry and 0.7–1.1GPa using garnet–aluminosilicate–plagioclase thermobarometry, indicating that these xenoliths experienced significant heating and possible partial melting in the lower crust, most likely related to arc magmatism as suggested by similarities between the lower intercept U–Pb ages and the ages of plutonism in the Sierra Nevada. Possible mechanisms by which these sediments were transported into the lower crust include continental underthrusting beneath the continental arc, underplating by buoyant slab-derived sedimentary diapirs, or viscous downflow of country rock in response to diapiric ascent of plutons. Continental underthrusting has been independently documented during the Sevier orogeny, coinciding with the peak of arc magmatism. We thus speculate that supracrustal rocks may have been underthrusted into deep crustal magmatic zones. Regardless of how these metasediments arrived in the lower crust, our observations indicate that silicic metasediments occur in the lower crust of volcanic arcs, not just in the upper crust as is commonly thought. Transport of metasediments into deep crustal magmatic zones should influence the composition of arc magmas and continental crust in general.

The Crustal Magma Storage System of Volcán Quizapu, Chile, and the Effects of Magma Mixing on Magma Diversity

Crystal zoning as well as temperature and pressure estimates from phenocryst phase equilibria are used to constrain the architecture of the intermediate-sized magmatic system (some tens of km 3 ) of Volcan Quizapu, Chile, and to document the textural and compositional effects of magma mixing. In contrast to most arc magma systems, where multiple episodes of open-system behavior obscure the evidence of major magma chamber events (e.g. melt extraction, magma mixing), the Quizapu magma system shows limited petrographic complexity in two large historical eruptions (1846–1847 and 1932) that have contrasting eruptive styles. Quizapu magmas and peripheral mafic magmas exhibit a simple binary mixing relationship. At the mafic end, basaltic andesite to andesite recharge magmas complement the record from peripheral cones and show the same limited range of compositions. The silicic end-member composition is almost identical in both eruptions of Quizapu. The effusive 1846–1847 eruption records significant mixing between the mafic and silicic end-members, resulting in hybridized andesites and mingled dacites. These two compositionally simple eruptions at Volcan Quizapu present a rare opportunity to isolate particular aspects of magma evolution—formation of homogeneous dacite magma and late-stage magma mixing—from other magma chamber processes. Crystal zoning, trace element compositions, and crystal-size distributions provide evidence for spatial separation of the mafic and silicic magmas. Dacite-derived plagioclase phenocrysts (i.e. An 25 – 40 ) show a narrow range in composition and limited zonation, suggesting growth from a compositionally restricted melt. Dacite-derived amphibole phenocrysts show similar restricted compositions and furthermore constrain, together with more mafic amphibole phenocrysts, the architecture of the magmatic system at Volcan Quizapu to be compositionally and thermally zoned, in which an andesitic mush is overlain by a homogeneous dacitic magma that is the source for most of the 1846–1847 and 1932 erupted magmas. Dacite formation is best explained by mineral–melt separation (crystal fractionation) from an andesitic mush, which is inferred to have thermally and compositionally buffered the dacite magma thereby keeping it at relatively low crystallinity (<30 vol. %). The dominant cause of compositional diversity is melt separation. Back-mixing of mush (i.e. crystals with signatures of growth both in the andesitic mush and in the dacite magma) into the overlying dacite magma is rarely observed. Recharge events that increase crystal and magma diversity in the dacite magma are limited to an episode of mafic recharge and mixing just prior to the 1846–1847 eruption, where evidence for magma mixing is present on all scales. Chamber-wide mixing was incomplete (mixing efficiency of ∼0·53–0·85) as flow lobes vary significantly in composition along the proposed mixing array. Estimates of viscosity variations during the course of magma mixing suggest that mixing dynamics and the degree of magma interaction on all scales were established at the beginning of the recharge event.

Impacts of glacial/interglacial cycles on continental rock weathering inferred using Sr/Ca and 87Sr/86Sr ratios in Michigan watersheds

Michigan soils have developed on thick glacial-drift deposits that include different proportions of ground granite and gneiss from the Canadian Shield region (with radiogenic Sr) and carbonate sedimentary rocks from within the Michigan Basin (with non-radiogenic Sr). This study of the Cheboygan, Huron and Kalamazoo watersheds shows how Sr-isotope and Sr/Ca ratios in soil waters, ground waters, and soils reflect relative weathering intensities of the dominant minerals in Michigan soils, including carbonates from the Michigan Basin, and amphibole, plagioclase and K-feldspar derived from the Canadian Shield.Soil water 87Sr/86Sr ratios evolve quickly to the carbonate weathering end-member (0.709–0.711) once a calcite and dolomite layer is reached at depth (~100–200cm) in the Huron and the Kalamazoo watersheds. Dissolution of plagioclase and amphibole controls shallow soil water 87Sr/86Sr ratios (0.711–0.713), with minor contributions from K-feldspar weathering. In contrast, soils in the previously studied Cheboygan watershed are completely depleted in carbonate minerals and contain little plagioclase and amphibole in the top 300cm of the profile. As a result, soil waters in this watershed are ionically dilute with high 87Sr/86Sr ratios (0.72 and 0.74), dominantly contributed by K-feldspar dissolution. Subsequent dissolution of plagioclase and amphibole at greater depths sharply increases soil water and ground water Mg2+, Ca2+, and Sr2+ concentrations, and lowers the Sr-isotopic ratios to ~0.709 for the Cheboygan watershed.Similarly, along hydrologic flow paths, soil water Sr/Ca ratios move from the silicate end-member (defined by amphibole and plagioclase) towards the carbonate end-member. The Sr-isotopic compositions and Sr/Ca ratios of soil waters thus reveal the types, directions and extent of chemical weathering processes in Michigan soils, augmenting information from previous soil water chemistry and soil mineralogy studies.This work also highlights the two-fold impacts of glacial/interglacial cycles on the riverine and oceanic Sr isotopes: due to the great extent of continental glaciation, Paleozoic carbonate minerals from the Michigan Basin were redistributed widely within the interior of the North American continent, leading to elevated Sr fluxes with lower Sr-isotopic ratios in natural waters after glacial retreat. The glacial ice also ground up the ancient Precambrian Canadian Shield, accelerating mineral weathering rates and releasing highly radiogenic Sr from K-feldspar.

Topological Principles of Borosilicate Glass Chemistry

Borosilicate glasses display a rich complexity of chemical behavior depending on the details of their composition and thermal history. Noted for their high chemical durability and thermal shock resistance, borosilicate glasses have found a variety of important uses from common household and laboratory glassware to high-tech applications such as liquid crystal displays. In this paper, we investigate the topological principles of borosilicate glass chemistry covering the extremes from pure borate to pure silicate end members. Based on NMR measurements, we present a two-state statistical mechanical model of boron speciation in which addition of network modifiers leads to a competition between the formation of nonbridging oxygen and the conversion of boron from trigonal to tetrahedral configuration. Using this model, we derive a detailed topological representation of alkali-alkaline earth-borosilicate glasses that enables the accurate prediction of properties such as glass transition temperature, liquid fragility, and hardness. The modeling approach enables an understanding of the microscopic mechanisms governing macroscopic properties. The implications of the glass topology are discussed in terms of both the temperature and thermal history dependence of the atomic bond constraints and the influence on relaxation behavior. We also observe a nonlinear evolution of the jump in isobaric heat capacity at the glass transition when substituting SiO(2) for B(2)O(3), which can be accurately predicted using a combined topological and thermodynamic modeling approach.

Origin of Cl-bearing silica-rich melt inclusions in diamonds: Experimental evidence for an eclogite connection

Research Article| December 01, 2010 Origin of Cl-bearing silica-rich melt inclusions in diamonds: Experimental evidence for an eclogite connection Konstantin D. Litasov; Konstantin D. Litasov 1Department of Earth and Planetary Material Science, Faculty of Science, Tohoku University, Sendai 980-8578, Japan2V.S. Sobolev Institute of Geology and Mineralogy, SB RAS (Siberian Branch, Russian Academy of Sciences), Novosibirsk 630090, Russia Search for other works by this author on: GSW Google Scholar Oleg G. Safonov; Oleg G. Safonov 3Institute of Experimental Mineralogy RAS (Russian Academy of Sciences), Chernogolovka 142432, Russia Search for other works by this author on: GSW Google Scholar Eiji Ohtani Eiji Ohtani 1Department of Earth and Planetary Material Science, Faculty of Science, Tohoku University, Sendai 980-8578, Japan Search for other works by this author on: GSW Google Scholar Geology (2010) 38 (12): 1131–1134. https://doi.org/10.1130/G31325.1 Article history received: 22 Apr 2010 rev-recd: 19 Jul 2010 accepted: 22 Jul 2010 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Konstantin D. Litasov, Oleg G. Safonov, Eiji Ohtani; Origin of Cl-bearing silica-rich melt inclusions in diamonds: Experimental evidence for an eclogite connection. Geology 2010;; 38 (12): 1131–1134. doi: https://doi.org/10.1130/G31325.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Melting phase relations of a model chloride- and carbonate-bearing eclogite have been studied at 7.0–10.5 GPa and 1200–1675 °C. The mineral assemblage coexisting with partial melts is garnet, omphacite, kyanite, and coesite or stishovite. At temperatures of 1200–1400 °C, the partial melt has an SiO2-poor, Cl-bearing carbonatite composition. With increasing temperature, it becomes progressively SiO2 rich, and at temperatures of 1500–1700 °C contains as much as 53 wt% SiO2. The compositions of the melts are comparable with those of silicic end members of inclusions in fibrous and/or cloudy diamonds worldwide, implying that they may be produced via chemical reactions of alkalic chloride-carbonate liquids with mantle eclogites. Our experiments reproduced the trends of the compositional variations of the fluid or melt inclusions within eclogitic diamonds, and thus suggest a reliable model for their origin. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geochemistry of surface waters associated with an undisturbed Zn–Pb massive sulfide deposit: Water–rock reactions, solute sources and the role of trace carbonate

We report results of a detailed geochemical study of surface waters from several streams in a small catchment in northern New Brunswick, Canada. Rocks in the catchment represent metamorphosed (greenschist facies) felsic volcanic rocks, metavolcaniclastic sediments, and mafic volcanic rocks; no massive carbonate or evaporite lithologies are present, thus providing an excellent opportunity to investigate the relative influence of silicate weathering compared to trace carbonate (vein and disseminated) dissolution and the influence of volcanogenic massive sulfide (VMS) mineralization on surface water chemistry. Surface waters, catchment lithologies and stream sediments were analyzed for a full suite of major and trace elements. Surface waters are dilute (typically < 60 mg/L total dissolved solids), and are of dominantly Ca–HCO 3-type. Most waters have Ca/Na molar > 1. The major ion chemistry of the waters is consistent with binary mixing between silicate weathering and dissolution of trace calcite; Si/Ca molar relationships suggest that trace calcic silicates are insignificant compared to calcite as a Ca source, and PO 4 concentrations of waters are too low for apatite to be a major source of Ca. Host rocks have highly variable Ca/Na, Mg/Na and K/Na values complicating the assignment of a silicate end-member. Geochemical modeling indicates that surface waters range from essentially 100% of the Na and Ca being derived by silicate weathering, to being dominantly controlled by trace calcite, also consistent with mass balance calculations. The Cl/Ca ratios are consistent with the host felsic and mafic metavolcanic rocks as being an important source of Cl in addition to precipitation. The host lithologies have much larger variations in Mg/Na, Ca/Na, K/Na, and Sr/Na than the stream sediments and waters and the sediments are shifted to higher Mg/Na and K/Na and lower Ca/Na than the waters. It is remarkable that the waters have such small variations in major ion ratios relative to the host rocks, indicating that the controls on solute loads of these streams is more a function of relative elemental solubility (Ca > Mg > Na), secondary mineral formation (e.g. Mg- and K-rich clays), incongruent dissolution, and water–rock reactions than end-member rock compositions. Oxidation of massive and disseminated sulfide mineralization accounts for on average 60% of the dissolved sulfate. Calculations indicate that sulfide oxidation (sulfuric acid) weathering accounts for around 20% of the cation flux in the watershed. On average 12% of the Ca and 72% of the Mg are derived from silicate weathering, although cationic silicate denudation rates are only 1.88 tonnes/km 2/year compared to 4.16 tonnes/km 2/year for trace carbonate dissolution. The total cation denudation rate and CO 2 consumption rate are similar to watersheds draining volcanic rocks in the Western Canadian Cordillera.

Simple mixing as the major control of the evolution of volcanic suites in the Ecuadorian Andes

Examination of an extensive major and trace element database for about 700 whole rocks from the Ecuadorian Andes reveals series of local trends typified by three volcanoes: Iliniza and Pichincha from the Western Cordillera and Tungurahua from the Eastern Cordillera. These local trends are included in a more scattered global trend that reflects typical across-arc chemical variations. The scatter of the global trend is attributed to greater crustal contributions or decreasing melt fractions. Trace element modelling shows that the local trends are consistent with mixing, and not with any fractional crystallization or progressive melting dominated processes. These local trends are extendable to include samples from other Ecuadorian volcanoes, suggesting that mixing processes are dominant throughout the region. Mixing model using trace and major element analyses identifies two end-members: low-silica, basaltic and high-silica, dacitic magmas. It also shows that mixing occurred between magmas after their segregation, rather than earlier mixing between the solid sources prior to melting. As a consequence, there must exist efficient magma-mixing processes that can overcome the obstacles to mixing magmas with contrasting physical properties, and can produce series of hybrid liquids over regional-scale. Model calculations show that estimated silicic end-members are primary magmas and are not co-magmatic derivatives of the corresponding mafic end-members. Lavas of Ecuadorian volcanoes are likely originated from magmas of contrasting origins, such as basaltic magmas generated by fluxed melting of peridotites in the mantle wedge and dacitic, adakite-type magmas originating from the slab or the mafic lower crust.

The Mg isotopic systematics of granitoids in continental arcs and implications for the role of chemical weathering in crust formation

Continental crust is too Si-rich and Mg-poor to derive directly from mantle melting, which generates basaltic rather than felsic magmas. Converting basalt to more felsic compositions requires a second step involving Mg loss, which is thought to be dominated by internal igneous differentiation. However, igneous differentiation alone may not be able to generate granites, the most silicic endmember making up the upper continental crust. Here, we show that granites from the eastern Peninsular Ranges Batholith (PRB) in southern California are isotopically heavy in Mg compared with PRB granodiorites and canonical mantle. Specifically, Mg isotopes correlate positively with Si content and O, Sr, and Pb isotopes and negatively with Mg content. The elevated Sr and Pb isotopes require that a component in the source of the granitic magmas to be ancient preexisting crust making up the prebatholithic crustal basement, but the accompanying O and Mg isotope fractionations suggest that this prebatholithic crust preserved a signature of low-temperature alteration. The protolith of this basement rock may have been the residue of chemical weathering, which progressively leached Mg from the residue, leaving the remaining Mg highly fractionated in terms of its isotopic signature. Our observations indicate that ancient continental crust preserves the isotopic signature of compositional modification by chemical weathering.