Water and magmas; a mixing model

A model for the mixing of H₂O and silicate melts has been derived from the experimentally determined effects of H₂O on the viscosity (fluidity), volumes, electrical conductivities, and especially the thermodynamic properties of hydrous aluminosilicate melts. It involves primarily the reaction of H₂O with those O^?2 ions of the melt that are shared (bridging) between adjacent (Al, Si)O₄ tetrahedra to produce OH^? ions. However, in those melts that contain trivalent ions in tetrahedral coordination, such as the Al³⁺ ion in feldspathic melts, the model further involves exchange of a proton from H₂O with a non-tetrahedrally coordinated cation that must be present to balance the net charge on the AlO₄ group. This cation exchange reaction, which goes essentially to completion, results in dissociation of the H₂O and is limited only by the availability of H₂O and the number of exchangeable cations per mole of aluminosilicate.In the system NaAlSi₃O₈-H₂O, upon which this thermodynamic model is based, there is 1 mole of exchangeable cations (Na⁺) per mole (GFW) of NaAlSi₃O₈, consequently ion exchange occurs for H₂O contents up to a 1:1 mole ratio (X^m_w = mole fraction H₂O = 0.5). For mole fractions of H₂O greater than 0.5, no further exchange can occur and the reaction with additional bridging oxygens of the melt produces 2 moles of associated OH^? ions per mole of H₂O dissolved. These reactions lead to a linear dependence of the thermodynamic activity of H₂O (a^m_w) on the square of its mole fraction (X^m_w) for values of X^m_w, up to 0.5 and an exponential dependence on X^m_w at higher H₂O contents. Thus, for values of $\text{X}{}^{\mathrm{m}}{}_{\mathrm{w}}\text{? 0.5, a}{}^{\mathrm{m}}{}_{\mathrm{w}}\text{= k(X}{}^{\mathrm{m}}{}_{\mathrm{w}}\text{)}{}^2$, where k is a Henry's law constant for the dissociated solute.Extension of the thermodynamic model for NaAlSi₃O₈-H₂O to predict H₂O solubilities and other behavior of compositionally more complex aluminosilicate melts (magmas) requires placing these melts on an equimolal basis with NaAlSi₃O₈. This is readily accomplished using chemical analyses of quenched glasses by normalizing to the stoichiometric requirements of NaAlSi₃O₈, first in terms of equal numbers of exchangeable cations for mole fractions of H₂O up to 0.5 and secondly in terms of 8 moles of oxygen for higher H₂O contents. Chemical analyses of three igneous-rock glasses, ranging in composition from tholeiitic basalt to lithium-rich pegmatite, were thus recast and the experimental H₂O solubilities were computed on this equimolal basis. The resulting equimolal solubilities are all the same, within experimental error, as the solubility of H₂O in NaAlSi₃O₈ melt calculated from the thermodynamic relations.The equivalence of equimolal solubilities implies that the Henry's law constant (k), which is a function of temperature and pressure, is independent of aluminosilicate composition over a wide range. Moreover, as a consequence of the Gibbs-Duhem relation and the properties of exact differentials, it is clear that the silicate components of the melt, properly defined, mix ideally. Thus, a relatively simple mixing model for H₂O in silicate melts has led to a quantitative thermodynamic model for magmas that has far-reaching consequences in igneous petrogenesis.     

Origin of basaltic magmas in the Mojave Desert area,California

The basaltic lavas erupted throughout the Mojave Desert are basanites (SiO₂<46%, normative nepheline>5%, and K₂O>1.5%), alkali-olivine basalts (SiO₂=46–48%; ne=0–5%; and K₂O=1.0–1.5%), and low-alumina, sub-alkaline basalts (SiO₂=48–51%; ne=0; K₂O<1.0%). One volcano, Pisgah Crater, erupted five times, with lava from each successive phase containing more silica and less potash than the one proceeding it. This compositional trend is the reverse of that expected from differentiation of a single alkalic magma, and therefore, may represent a succession of magmas tapped from a zone of continuing partial melting in the mantle.These lava compositions suggest that first melting was under high water pressure and was followed by relatively dry partial melting of gamet-orthopyroxene-clinopyroxene-olivine assemblages. The successive increase in silica and alkali decrease also requires that the partial melting zone move to shallower levels.All lavas sampled in the Mojave Desert area have compositions that can best be explained by the extraction of magma from such a rising melting zone, analogous to the mantle diapirs suggested by Green and Ringwood.     

The generation of sodic granite magmas, western Palmer Land, Antarctic Peninsula

Subduction-related Mesozoic to Cainozoic granites s.l. in western Palmer Land, Antarctic Peninsula, have similar chemical compositions to Archean tonalite-trondhjemite-granodiorite (TTG) suites, Phanerozoic slab-melts (adakites), and to experimental partial melts of basaltic material in equilibrium with amphibole ± pyroxene ± garnet. They are predominantly sodic, metaluminous and most have Al₂O₃ > 15 wt% and Y < 18 ppm. All are light rare earth element (LREE)-enriched (2 < La/Yb_n <30) and most have small Eu anomalies. They have a wide range of initial ɛNd_(t) (−6.8 to +4.5) and ɛSr_(t) (+293.4 to −3.7), but most Pb isotope compositions deviate by < 0.3% from their mean. The Pb isotope data indicate a crustal component to all the granites, which Sr and Nd isotope variations suggest is pre-Triassic–Triassic. The ²⁰⁷Pb/²⁰⁴Pb_(t) range from 15.602 to 15.666 and appear to preclude a significant Proterozoic, or older, crustal component. The granites have chemical and isotopic compositions that suggest they are not partial melts of subducted oceanic lithosphere, as has been suggested for some Archean and Phanerozoic TTG magmas. We conclude that they were produced by mixing between basaltic-andesitic arc magmas, partial melts of juvenile basaltic lower crust and pre-Triassic crust. The low H(heavy)REE+Y content of some of the granites requires that garnet was a residual phase in the crust during partial melting, indicating a crustal thickness of >36 km. Between Triassic and Tertiary times the initial ɛNd_(t) of the magmatism increased and ɛSr_(t) decreased, suggesting that new continental crust was produced during this period. Underplating by mafic magma was an important crustal growth mechanism in the arc: the generation of abnormally thick crust, and its subse quent fusion, is considered to be a consequence of ca. ≥ 180 Ma of subduction and associated magmatism in the region. An implication of the model is that dense garnet-amphibolite and eclogite residues from partial melting of the lower crust will accumulate. In theory, the setting was appropriate for such residues to detach from the base of the crust and to sink into the convecting mantle. Such a process would leave the rest of the crust enriched in large ion lithophile elements/LREE, but depleted in HREE+Y. Received: 2 October 1995 / Accepted: 5 January 1997     

Paleomagnetic study at a nuclear power plant site near Bakersfield, California

At the San Joaquin plant site near Bakersfield, California, the Brunhes/Matuyama reversal was identified 4 m below an ash probably correlative with the Bishop Tuff. Deposition of coarse detritus in a high-energy environment preceded the reversal, whereas a low-energy lake and back-swamp type sedimentation persisted through most of the interval, separating the reversal and the ash. Sediments below the reversal are strongly remagnetized in a normal field. Identification of the original polarity is possible only with some of the specimens, so that accurate positioning of the reversal plane required dense and multiple sampling. Similar precaution at all sites where reversals are used for time-stratigraphic correlations is recommended. Paleomagnetic investigation was particularly useful in proving the presence of unfaulted strata at least 0.5 my old, as required by safety regulations of the U.S. Nuclear Regulatory Commission (NRC). This is due to the fact that any thick layer of reversely magnetized sediment is with high probability older than 0.7 my.     

Distribution of elements in biotite-hornblende pairs and in an orthopyroxene-clinopyroxene pair from zoned plutons,northern Sierra Nevada,California

Distribution of major and minor elements has been determined for five hornblende-biotite pairs from hornblende-biotite quartz diorite and monzotonalite and for a clinopyroxene-orthopyroxene pair from pyroxene diorite collected from the border zones and centers of zoned plutons in the northern Sierra Nevada, California. The distribution coefficients K _d [Mg/Fe] for biotite/hornblende are of the same magnitude (0.61–0.67) for both the mafic border zone and the silicic center.For comparison, K _D [Mg/Fe] values for biotite/hornblende from plutonic rocks of the central Sierra Nevada and the southern California batholith were calculated from data published by others. Rocks of the oldest age group (ca. 150 m.y.) in the central Sierra Nevada have an average distribution coefficient, K _D , of 0.64, close to the average K _D in the study area, where K-Ar dates are 143 to 129 m.y. The intermediate age group has an average K _D =0.81, and the youngest group has K _D =0.77. K _D [Mg/Fe] for biotite/hornblende from the southern California batholith is 0.83, close to the average of the intermediate age group in the central Sierra Nevada. The calculated difference in pressure of crystallization between rocks of the Feather River area and the southern California batholith is 1 kb; the rocks of the Feather River area being crystallized at a higher pressure. This is in good agreement with the low-pressure contact metamorphism in the south (pyroxene hornfels facies), as compared with a medium-pressure metamorphism around the northern plutons, where andalusitesillimanite-cordierite and andalusite-staurolite subfacies of the amphibolite facies indicate pressures of about 4 kb.Trace elements Cr, V, Ni, Co, Ga are distributed equally between biotite and hornblende, whereas Ba and possibly Cu are concentrated in biotite and Sr and Sc and possibly Zr in hornblende.Publication authorized by the Director, U.S. Geological Survey.     

Liquid immiscibility in deep-seated magmas and the generation of carbonatite melts

This paper reviews the results of investigations of melt inclusions in minerals of carbonatites and spatially associated silicate rocks genetically related to various deep-seated undersaturated silicate magmas of alkaline ultrabasic, alkaline basic, lamproitic, and kimberlitic compositions. The analysis of this direct genetic information showed that all the deep magmas are inherently enriched in volatile components, the most abundant among which are carbon dioxide, alkalis, halides, sulfur, and phosphorus. The volatiles probably initially served as agents of mantle metasomatism and promoted melting in deep magma sources. The derived magmas became enriched in carbon dioxide, alkalis, and other volatile components owing to the crystallization and fractionation of early high-magnesium minerals and gradually acquired the characteristics of carbonated silicate liquids. When critical compositional parameters were reached, the accumulated volatiles catalyzed immiscibility, the magmas became heterogeneous, and two-phase carbonate-silicate liquid immiscibility occurred at temperatures of ≥1280–1250°C. The immiscibility was accompanied by the partitioning of elements: the major portion of fluid components partitioned together with Ca into the carbonate-salt fraction (parental carbonatite melt), and the silicate melt was correspondingly depleted in these components and became more silicic. After spatial separation, the silicate and carbonate-silicate melts evolved independently during slow cooling. Differentiation and fractionation were characteristic of silicate melts. The carbonatite melts became again heterogeneous within the temperature range from 1200 to 800–600°C and separated into immiscible carbonate-salt fractions of various compositions: alkali-sulfate, alkali-phosphate, alkali-fluoride, alkali-chloride, and Fe-Mg-Ca carbonate. In large scale systems, polyphase silicate-carbonate-salt liquid immiscibility is usually manifested during the slow cooling and prolonged evolution of deeply derived melts in the Earth’s crust. It may lead to the formation of various types of intrusive carbonatites: widespread calcite-dolomite and rare alkali-sulfate, alkali-phosphate, and alkali-halide rocks. The initial alkaline carbonatite melts can retain their compositions enriched in P, S, Cl, and F only at rapid eruption followed by instantaneous quenching.     

Formation of a third volcanic chain in Kamchatka: generation of unusual subduction-related magmas

The unusual development of three volcanic chains, all parallel to the trend of the subduction trench, is observed in Kamchatka at the northern edge of the Kurile arc. Elsewhere on the Earth volcanic arcs dominantly consist of only two such chains. In the Kurile arc, magmatism in the third volcanic chain, which is farthest from the trench, is also unusual in that lavas show concentrations of incompatible elements intermediate between those of the two trenchward chains. This observation can be explained by relatively shallow segregation of primary magmas and high degrees of partial melting of magmas in the third chain, compared to the conditions of magma separation expected from a simple application of the general acrossarc variation. Initial magmas in such an atypical third chain may be produced by melting of K-amphibolebearing peridotite in the down-dragged layer at the base of the mantle wedge under anomalously hightemperature conditions. Such an unusual melting event may be associated with the particular tectonic setting of the Kamchatka region, i.e. the presence of subductiontransform boundary. Such a mechanism is consistent with the across-arc variation in Rb/K ratios in the Kamchatka lavas: lowest in the third chain rocks and highest in the second chain rocks.     

Geochemistry of Mesozoic plutons, southern Death Valley region, California: Insights into the origin of Cordilleran interior magmatism

Mesozoic granitoid plutons in the southern Death Valley region of southeastern California reveal substantial compositional and isotopic diversity for Mesozoic magmatism in the southwestern US Cordillera. Jurassic plutons of the region are mainly calc-alkaline mafic granodiorites with )_Ndi of -5 to -16, ⁸⁷Sr/⁸⁶Sr_i of 0.707-0.726, and ²⁰⁶Pb/²⁰⁴Pb_i of 17.5-20.0. Cretaceous granitoids of the region are mainly monzogranites with )_Ndi of -6 to -19, ⁸⁷Sr/⁸⁶Sr_i of 0.707-0.723, and ²⁰⁶Pb/²⁰⁴Pb_i of 17.4-18.6. The granitoids were generated by mixing of mantle-derived mafic melts and pre-existing crust - some of the Cretaceous plutons represent melting of Paleoproterozoic crust that, in the southern Death Valley region, is exceptionally heterogeneous. A Cretaceous gabbro on the southern flank of the region has an unusually juvenile composition ()_Ndi -3.2, ⁸⁷Sr/⁸⁶Sr_i 0.7060). Geographic position of the Mesozoic plutons and comparison with Cordilleran plutonism in the Mojave Desert show that the Precambrian lithosphere (craton margin) in the eastern Mojave Desert region may consists of two crustal blocks separated by a more juvenile terrane.     

Evidence for crustal assimilation,mixing of magmas,and a87Sr-rich upper mantle

¹⁸O/¹⁶O,⁸⁷Sr/⁸⁶Sr and chemical analyses were made on 39 lavas and ignimbrites from M. Vulsini, the most northerly district of the K-rich Quaternary Roman Province of Italy. These rocks belong mainly to the undersaturated, leucite-bearing (High-K) series, but also included are samples from the less abundant, SiO₂-saturated, hypersthene-(quartz)-normative (Low-K) series. The effects of post-eruption alteration on the ¹⁸O of these lavas were taken into account by analyzing phenocrysts or by using the extrapolation procedure developed for the nearby Alban Hills center. Because of the high Sr contents (500–2400 ppm), the⁸⁷Sr/⁸⁶Sr ratios of these rocks were little affected by such alteration processes. The M. Vulsini volcanics have Sr- and O-isotopic ratios much less uniform, and on the average much higher, than at any of the other volcanic centers of the province:⁸⁷Sr/⁸⁶Sr=0.7097 to 0.7168; ¹⁸O=6.5 to 13.8. This is attributable to the fact that M. Vulsini is one of the sites of greatest crustal assimilation and hybridism between K-rich Roman magmas and SiO₂-rich Tuscan anatectic magmas. The High-K series parent magmas at M. Vulsini had a very high and uniform⁸⁷Sr/⁸⁶Sr=0.7102 to 0.7104, and a somewhat more variable ¹⁸O=+5.5 to +7.5; they must have come from an upper mantle source region previously metasomatically enriched in⁸⁷Sr and LIL elements. These¹⁸O/¹⁶O and⁸⁷Sr/⁸⁶Sr ratios are identical to the parent magma at the Alban Hills, 120 km to the south, where Low-K lavas are absent. Low-K series magmas at M. Vulsini originated from a lower-⁸⁷Sr source region than the High-K series (<0.7097); a similar relationship is observed in all of the other localities in Italy where the two magma series coexist.Contribution No. 4167, Division of Geological and Planetary Sciences, California Institute of Technology     

阿尔泰中蒙边界塔克什肯口岸后造山富碱侵入岩体的形成时代、成因及其地壳生长意义

塔克什肯口岸富碱侵入岩体是阿尔泰造山带典型的后造山岩体。本次锆石U-Pb定年给出~(206)Pb/~(238)U年龄286±1Ma(MSWD=0．05),代表其形成年龄。这为阿尔泰后造山岩浆作用提供了一个可靠的年代学证据。该岩体主要岩石类型为正长岩、石英二长岩、石英碱长正长岩、正长花岗岩,富钾、富钠、准铝,富集轻稀土、大离子亲石元素,亏损高场强元素,与区内相邻的碱性花岗岩(A型花岗岩)在岩石学、地球化学方面不同。该岩体Sr初始值变化于0．7038～0．7040,ε_(Nd)(t)值为正值(+6．2～+6．3),Nd模式年龄T_(DM)为542～546Ma,与中亚造山带典型的高(正)ε_(Nd)(t)值花岗岩类似。而且,其ε_(Nd)(t)值远高于区内同造山花岗岩,也高于造山带内部同时期的Ⅰ-A型后造山花岗岩。依据岩体构造特征、年代学、地球化学和区域地质背景综合分析,该岩体应为后造山岩体,在时空上可与蒙古南部碱性岩带对比,其形成可能与富集高场强元素的亏损地幔岩浆底侵,导致下地壳重熔,并发生岩浆混合有关。这说明,在阿尔泰造山带后造山阶段,除了可能的俯冲下埋的年轻洋壳或岛弧物质外,还有新的幔源物质加入到地壳。这为中亚造山带后造山阶段陆壳垂向生长提供了一个新证据。同时,也为东北北部-蒙古南部碱性岩带向西沿入阿尔泰造山带提供了证据。     

Petrogenesis of Sierra Nevada plutons inferred from the Sr, Nd, and O isotopic signatures of mafic igneous complexes in Yosemite Valley, California

Mafic complexes in the central Sierra Nevada batholith record valuable geochemical information regarding the role mafic magmas play in arc magmatism and the generation of continental crust. In the intrusive suite of Yosemite Valley, major and trace element compositions of the hornblende-bearing gabbroic rocks from the Rockslides mafic complex and of the mafic dikes in the North America Wall are compositionally similar to high-alumina basalt. Of these rocks, two samples have higher Ni and Cr abundances as well as higher ε_Nd values than previously recognized for the intrusive suite. Plagioclase crystals in rocks from the North America Wall and the Rockslides have prominent calcic cores and sharply defined sodic rims, a texture commonly associated with mixing of mafic and felsic magmas. In situ analyses of ⁸⁷Sr/⁸⁶Sr in plagioclase show no significant isotopic difference from the cores to the rims of these grains. We propose that the high ⁸⁷Sr/⁸⁶Sr (~0.7067) and low ε_Nd (~?3.4) of bulk rocks, the homogeneity of ⁸⁷Sr/⁸⁶Sr in plagioclase, and the high δ¹⁸O values of bulk rocks (6.6–7.3 ‰) and zircon (Lackey et al. in J Petrol 49:1397–1426, 2008) demonstrate that continental crust was assimilated into the sublithospheric mantle-derived basaltic precursors of the mafic rocks in Yosemite Valley. Contamination (20–40 %) likely occurred in the lower crust as the magma differentiated to high-alumina basalt prior to plagioclase (and zircon) crystallization. As a consequence, the isotopic signatures recorded by whole rocks, plagioclase, and zircon do not represent the composition of the underlying lithospheric mantle. We conclude that the mafic and associated felsic members of the intrusive suite of Yosemite Valley represent 60–80 % new additions to the crust and include significant quantities of recycled ancient crust.     

Geochemistry of the Sweetwater Wash Pluton,California: Implications for “anomalous” trace element behavior during differentiation of felsic magmas

Field relations, mineralogy and major and trace element data for the very felsic, peraluminous Sweetwater Wash pluton establish a differentiation sequence dominantly controlled by fractional crystallization processes. Elements Ba and Sr show depletion by factors of 50–60X from the earliest granite unit analyzed to the late-stage pegmatites and aplites. The strong Ba depletion is largely due to the partitioning behavior of this element in K-feldspar, while the Sr depletion is due to the combined effects of the two feldspars. The 4-fold increase in Rb during crystallization is also predictable from mineral/ melt partition coefficients.Coefficients for the light rare-earth elements (LREE) in major mineral species predict that these elements should behave incompatibly during crystallization and increase with fractionation. In fact, the LREE abundances decrease by a factor of 10–20X during crystallization. This anomalous behavior is commonly observed in felsic plutonic and volcanic sequences. In the Sweetwater Wash pluton monazite occurs in minute quantities, but it is sufficiently abundant to govern the partitioning of LREE and Th during crystallization. Petrographic observations indicate that monazite was on the liquidus throughout most of the crystallization. Analyses of silicate mineral separates suggest that the monazite contains more than 75% of the LREE in the whole rocks.Fractionation of REE-rich accessories (in particular monazite) from felsic magmas may be the general cause of REE depletion during differentiation of these melts. Monazite can easily be mistaken for zircon and, because it typically contains 50% LREE, extremely minute and easily overlooked quantities of monazite can control LREE abundances.     

Plate subduction,and generation of earthquakes and magmas in Japan as inferred from seismic observations: An overview

A dense nationwide seismic network recently constructed in Japan has been yielding large volumes of high-quality data that have made it possible to investigate the seismic structure in the Japanese subduction zone with unprecedented resolution. In this article, recent studies on the subduction of the Philippine Sea and Pacific plates beneath the Japanese Islands and the mechanism of earthquake and magma generation associated with plate subduction are reviewed. Seismic tomographic studies have shown that the Philippine Sea plate subducting beneath southwest Japan is continuous throughout the entire region, from Kanto to Kyushu, without disruption or splitting even beneath the Izu Peninsula as suggested in the past. The contact of the Philippine Sea plate with the Pacific plate subducting below has been found to cause anomalously deep interplate and intraslab earthquake activity in Kanto. Detailed waveform inversion studies have revealed that the asperity model is applicable to interplate earthquakes. Analyses of dense seismic and GPS network data have confirmed the existence of episodic slow slip accompanied in many instances by low-frequency tremors/earthquakes on the plate interface, which are inferred to play an important role in stress loading at asperities. High-resolution studies of the spatial variation of intraslab seismicity and the seismic velocity structure of the slab crust strongly support the dehydration embrittlement hypothesis for the generation of intraslab earthquakes. Seismic tomography studies have shown that water released by dehydration of the slab and secondary convection in the mantle wedge, mechanically induced by slab subduction, are responsible for magma generation in the Japanese islands. Water of slab origin is also inferred to be responsible for large anelastic local deformation of the arc crust leading to inland crustal earthquakes that return the arc crust to a state of spatially uniform deformation.     

A model for mixing basaltic and dacitic magmas as deduced from experimental data

It has been demonstrated experimentally that basaltic and dacitic magmas can be easily mixed to form both banded dacite and homogeneous andesite in less than a few hours. The presence of phenocrysts larger than 0.5 mm increased considerably the efficiency of mixing. Flow patterns in the experimental system were visualized using Pt spheres, which indicated that convection occurs in basalt melt, but not in dacite melt. The Reynolds numbers of the basaltic and dacitic melts in the experimental system were calculated to be about 10^–3 and 10^–6, respectively. Mixing proceeds initially by mechanical mixing of the two magmas in a large scale, but later by coupling interfacial convection and mutual diffusion. Thus, depending on the depth where vesiculation and following disruption of the magma occurs, banded pumice, homogeneous pumice and homogeneous andesite lava are erupted. The observed textures of mixed rocks of Plinian type eruption and the limiting occurrence of banded pumice are satisfactorily accounted for on this model.     

Primary melt from Sannome-gata volcano,NE Japan arc: constraints on generation conditions of rear-arc magmas

The conditions under which rear-arc magmas are generated were estimated using primary basalts from the Sannome-gata volcano, located in the rear of the NE Japan arc. Scoriae from the volcano occur with abundant crustal and mantle xenoliths, suggesting that the magma ascended rapidly from the upper mantle. The scoriae show significant variations in their whole-rock compositions (7.9–11.1 wt% MgO). High-MgO scoriae (MgO > ~9.5 wt%) have mostly homogeneous ⁸⁷Sr/⁸⁶Sr ratios (0.70318–0.70320), whereas low-MgO scoriae (MgO < ~9 wt%) have higher ⁸⁷Sr/⁸⁶Sr ratios (>0.70327); ratios tend to increase with decreasing MgO content. The high-MgO scoriae are aphyric, containing ~5 vol% olivine microphenocrysts with Mg# [100 × Mg/(Mg + Fe²⁺)] of up to 90. In contrast, the low-MgO scoriae have crustal xenocrysts of plagioclase, alkali feldspar, and quartz, and the mineralogic modes correlate negatively with whole-rock MgO content. On the basis of these observations, it is inferred that the high-MgO scoriae represent primary or near-primary melts, while the low-MgO scoriae underwent considerable interaction with the crust. Using thermodynamic analysis of the observed petrological features of the high-MgO scoriae, the eruption temperature of the magmas was constrained to 1,160–1,220 °C. Given that the source mantle was depleted MORB-source mantle, the primary magma was plausibly generated by ~7 % melting of a garnet-bearing spinel peridotite; taking this into consideration, and considering the constraints of multi-component thermodynamics, we estimated that the primary Sannome-gata magma was generated in the source mantle with 0.5–0.6 wt% H₂O at 1,220–1,230 °C and at ~1.8 GPa, and that the H₂O content of the primary magma was 6–7 wt%. The rear-arc Sannome-gata magma was generated by a lower degree of melting of the mantle at greater depths and lower temperatures than the frontal-arc magma from the Iwate volcano, which was also estimated to be generated by ~15 % melting of the source mantle with 0.6–0.7 wt% H₂O at ~1,250 °C and at ~1.3 GPa.     

A regular solution model for met-aluminous silicate liquids: Applications to geothermometry,immiscibility, and the source regions of basic magmas

Adopting a set of multioxide components and using published compositional data on olivineand plagioclase-liquid equilibria we have developed a 17 component regular solution model for met-aluminous silicate liquids. The partial molar excess free energies predicted from this model can be used together with phenocryst compositions as an effective geothermometer, with an approximate error of 20 °C (30 °C for olivine, 12 °C for plagioclase). The regular solution formulation is also successful in predicting liquid immiscibility at (1) high mole fractions of silica commonly observed in phase diagrams, and at (2) lower temperatures in lunar basalts and intermediate lavas. The model yields activities of silica which are consistent with those obtained from solid-liquid silica buffers in rocks which contain olivine and enstatite or quartz. From predicted activities of KAlSi₃O₈ in liquids coexisting with plagioclase a value is obtained for the limiting Henry's law activity coefficient of KAlSi₃O₈ in the solid. This coefficient agrees well with that inferred from plagioclase-sanidine equilibrium phenocryst assemblages in rhyolites. The activities of silica obtained from this model are used to place constraints on the pressure-temperature regions where various types of basic magmas are generated. In conjunction with plagioclase geothermometry an application is given where the pressure, temperature, and water content of an olivine andesite is predicted from the activity of silica.     

Geochemistry of silicic magmas in the Macolod Corridor, SW Luzon, Philippines: evidence of distinct, mantle-derived, crustal sources for silicic magmas

Silicic volcanic deposits (>65 wt% SiO₂), which occur as domes, lavas and pyroclastic deposits, are relatively abundant in the Macolod Corridor, SW Luzon, Philippines. At Makiling stratovolcano, silicic domes occur along the margins of the volcano and are chemically similar to the silicic lavas that comprise part of the volcano. Pyroclastic flows are associated with the Laguna de Bay Caldera and these are chemically distinct from the domes and lavas at Makiling stratovolcano. As a whole, samples from the Laguna de Bay Caldera contain lower concentrations of MgO and higher concentrations of Fe₂O_3(t) than the samples from domes and lavas. The Laguna de Bay samples are more enriched in incompatible trace elements. The silicic rocks from the domes, Makiling Volcano and Laguna de Bay Caldera all contain high alkalis and high K₂O/Na₂O ratios. Melting experiments of primitive basalts and andesites demonstrate that it is difficult to produce high K₂O/Na₂O silicic magmas by fractional crystallization or partial melting of a low K₂O/Na₂O source. However, recent melting experiments (Sisson et al., Contrib Mineral Petrol 148:635–661, 2005) demonstrate that extreme fractional crystallization or partial melting of K-rich basalts can produce these silicic magmas. Our model for the generation of the silicic magmas in the Macolod Corridor requires partial melting of mantle-derived, evolved, moderate to K-rich, crystallized calc-alkaline magmas that ponded and crystallized in the mid-crust. Major and trace element variations, along with oxygen isotopes and ages of the deposits, are consistent with this model. Electronic Supplementary Material Supplementary material is available for this article at     

Lead and strontium isotopic evidence for crustal interaction and compositional zonation in the source regions of Pleistocene basaltic and rhyolitic magmas of the Coso volcanic field,California

The isotopic compositions of Pb and Sr in Pleistocene basalt, high-silica rhyolite, and andesitic inclusions in rhyolite of the Coso volcanic field indicate that these rocks were derived from different levels of compositionally zoned magmatic systems. The 2 earliest rhyolites probably were tapped from short-lived silicic reservoirs, in contrast to the other 36 rhyolite domes and lava flows which the isotopic data suggest may have been leaked from the top of a single, long-lived magmatic system. Most Coso basalts show isotopic, geochemical, and mineralogic evidence of interaction with crustal rocks, but one analyzed flow has isotopic ratios that may represent mantle values (⁸⁷Sr/⁸⁶Sr=0.7036,²⁰⁶Pb/²⁰⁴Pb=19.05,²⁰⁷Pb/²⁰⁴Pb=15.62,²⁰⁸Pb/²⁰⁴Pb= 38.63). The (initial) isotopic composition of typical rhyolite (⁸⁷Sr/⁸⁶Sr=0.7053,²⁰⁶Pb/²⁰⁴Pb=19.29,²⁰⁷Pb/²⁰⁴Pb= 15.68,²⁰⁸Pb/²⁰⁴Pb=39.00) is representative of the middle or upper crust. Andesitic inclusions in the rhyolites are evidently samples of hybrid magmas from the silicic/mafic interface in vertically zoned magma reservoirs. Silicic end-member compositions inferred for these mixed magmas, however, are not those of erupted rhyolite but reflect the zonation within the silicic part of the magma reservoir. The compositional contrast at the interface between mafic and silicic parts of these systems apparently was greater for the earlier, smaller reservoirs.     

A simple thermodynamic model for estimating the solubility of H2O in magmas

A symmetrical, strictly regular solution model is used to estimate H₂O solubilities in silicate melts. The standard state chemical potential of dissolved H₂O and the adjustable parameter in the activity coefficient are determined by least squares analyses of data on H₂O solubility in silicate melts. The adjustable parameter in the expression for the activity coefficient (In) is a function only of the anhydrous melt composition and eleven values are provided for melts ranging in composition from picrite to rhyolite. At the 95% confidence level, the model should estimate H₂O contents to within 4.8% of the amount present if the amount present is less than 10 wt.%. This compares to the reproducibility of 2.25% of the amount present for experimental determinations. To apply the model to rocks and magmas estimates ofT, P, and the fugacity of H₂O are required.Variation of the H₂O content of the melt changes the activity of other components. Knowledge of this variation removes the requirement that the fugacity of H₂O be estimated. Application of the properties of exact differentials to the Gibbs function for the hydrous melt provides an expression relating the chemical potential of a feldspar component to the H₂O content of the melt. This expression contains a second adjustable parameter which depends on the anhydrous melt composition. Using this second expression, the H₂O content can be estimated ifT, P, and feldspar composition are known. Data are too meagre to evaluate the quantitative success of the second method.