Genesis of Recent silicic magmatism in the Medicine Lake Highland,California: evidence from cognate inclusions found at Little Glass Mountain

We use granular inclusions and phenocrysts in the Little Glass Mountain rhyolite flows to estimate temperature, pressure and the fugacities of O₂, H₂ and H₂O. The compositions of magnetite-ilmenite are used to estimate temperature and oxygen fugacity. Fugacities of H₂ and H₂O are estimated from the compositions of associated biotite-sanidine-magnetite. P_Total depends on the compositions of magnetite -ferrosilite-silica. Lastly, hydrothermal experiments were conducted at the estimated T, P and f_O2 to establish the beginning of melting of the most evolved of the inclusions in CO₂-H₂O fluids.The data suggest that the most evolved inclusions formed at ～ 830°C, a total pressure of 5200 bars, f_O2 of 10^?13 and P_H2O ～ 1000 bars. Of these variables total pressure is most difficult to estimate accurately. The values of T, P etc., previously stated produce a maximum estimate of the depth of equilibration between host magma and the inclusions whereas, assuming P_H2O = P_Total yields a minimum estimate. The physical conditions together with texture suggest a plutonic origin at a minimum depth of 3.4 km but no deeper than 15–18 km beneath the Medicine Lake Highland.The composition and mineralogy suggest that the rhyolite was derived from the dacite by crystal fractionation. The relation between dacite and associated basaltic or andesitic rocks is uncertain. The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios (essentially 0.7040 for both inclusions and lavas) do not require involvement of crustal rocks. A source in the uppermost mantle or lower lithosphere is considered most probable for the parental liquid which gave rise to the dacite.     

Magmatic processes that generated the rhyolite of Glass Mountain, Medicine Lake volcano, N. California

Glass Mountain consists of a 1 km³, compositionally zoned rhyolite to dacite glass flow containing magmatic inclusions and xenoliths of underlying shallow crust. Mixing of magmas produced by fractional crystallization of andesite and crustal melting generated the rhyolite of Glass Mountain. Melting experiments were carried out on basaltic andesite and andesite magmatic inclusions at 100, 150 and 200 MPa, H₂O-saturated with oxygen fugacity controlled at the nickel-nickel oxide buffer to provide evidence of the role of fractional crystallization in the origin of the rhyolite of Glass Mountain. Isotopic evidence indicates that the crustal component assimilated at Glass Mountain constitutes at least 55 to 60% of the mass of erupted rhyolite. A large volume of mafic andesite (2 to 2.5 km³) periodically replenished the magma reservoir(s) beneath Glass Mountain, underwent extensive fractional crystallization and provided the heat necessary to melt the crust. The crystalline residues of fractionation as well as residual liquids expelled from the cumulate residues are preserved as magmatic inclusions and indicate that this fractionation process occurred at two distinct depths. The presence and composition of amphibole in magmatic inclusions preserve evidence for crystallization of the andesite at pressures of at least 200 MPa (6 km depth) under near H₂O-saturated conditions. Mineralogical evidence preserved in olivine-plagioclase and olivine-plagioclase-high-Ca clinopyroxene-bearing magmatic inclusions indicates that crystallization under near H₂O-saturated conditions also occurred at pressures of 100 MPa (3 km depth) or less. Petrologic, isotopic and geochemical evidence indicate that the andesite underwent fractional crystallization to form the differentiated melts but had no chemical interaction with the melted crustal component. Heat released by the fractionation process was responsible for heating and melting the crust. Received: 26 March 1996 / Accepted: 14 November 1996     

Evidence of hydrous differentiation and crystal accumulation in the low-MgO,high-Al2O3 Lake Basalt from Medicine Lake volcano,California

The late Pleistocene Lake Basalt of Medicine Lake volcano, California is comprised of variably porphyritic basalt and basaltic andesite flows and scoria. These eruptives are similar in composition and phenocryst abundance to the low-MgO, high-Al²O³ mafic magmas common in convergent margin settings. The petrogenesis of the magmas that produced the Lake Basalt has been inferred from field relations, melting experiments and subsequent major and trace element modeling. Their formation involved both hydrous differentiation and plagioclase accumulation and thus the Lake Basalt can be used to constrain the relative contributions of these processes to the production of high-Al²O³ arc basalt. Phenocryst-poor lavas of the Lake Basalt formed by hydrous differentiation; their compositions and observed phenocrysts were reproduced in 1 kbar, H²O-saturated melting experiments. Anorthite-rich plagioclase compositions of the lavas of the Lake Basalt necessitate crystallization from melts with between 4 and 6 wt% dissolved H²O. Phenocryst-rich lavas of the Lake Basalt, with 18 modal% phenocrysts and greater, formed by plagioclase accumulation in magmas similar to the phenocryst-poor lavas. This interpretation is supported by the depleted incompatible element abundances and enriched Sr/Zr ratio of the more porphyritic lavas relative to the phenocryst-poor lavas. We model the formation of the Lake Basalt as a two-stage process that combines a differentiation model and a plagioclase accumulation model. Stage one involved hydrous fractionation, granitic assimilation and mixing with undifferentiated parent magma. This process generated lavas with up to 19.2 wt% A1²O³ and 7 modal% phenocrysts. In stage two, plagioclase accumulated in these liquids and produced more aluminous and porphyritic lavas with up to 21.8 wt% A1²O³ and 33 modal% phenocrysts.     

Geochemical characteristics of mud volcano fluids in the southern margin of the Junggar basin,NW China: implications for fluid origin and mud volcano formation mechanisms

Mud volcanoes can provide important information about the underlying strata, hydrocarbon accumulation, and recent neotectonic movements in an area. The fluids erupting from mud volcanoes provide important information about their formation and evolution. The ion concentration and the hydrogen and oxygen isotopes of the fluids that were erupted from the three mud volcano groups, Baiyanggou, Aiqigou, and Dushanzi, and nearby rivers in the southern margin of the Junggar basin, northwestern China, are studied. The concentrations of Na and Cl in mud volcano fluids are clearly elevated, displayed as the Na-Cl type. The δD and δ18O values of the fluids are similar between the Baiyanggou and Dushanzi mud volcanoes, which are mainly from ancient sedimentary pore water. However, the Aiqigou mud volcano is depleted in dissolved Cl and shows lower δ18O values with mixed sources, including deep pore and local meteoric water. Two types of mud volcanoes are proposed in this study. One type is low-energy mud volcanoes with a low volume of fluid of deep origin on the hillcrest, which display as mud pool/pie/hole. The other type is high-energy mud volcanoes having mixed fluid origin in the valley and formed in the shape of a mud cone (dome).     

Contrasting compositional trends of rocks and olivine-hosted melt inclusions from Cerro Negro volcano (Central America): implications for decompression-driven fractionation of hydrous magmas

Melt inclusions in olivine Fo_83–72 from tephras of 1867, 1971 and 1992 eruptions of Cerro Negro volcano represent a series of basaltic to andesitic melts of narrow range of MgO (5.6–8 wt %) formed by ~46 wt % fractional crystallization of olivine (~6 wt %), plagioclase (~27 wt %), pyroxene (~13 wt %) and magnetite (<1 wt %) from primitive basaltic melt (average SiO₂ = 49 wt %, MgO = 7.6 wt %, H₂O = 6 wt %) as it ascended to the surface from the depth of about 14 km. The crystallization occurred at increasing liquidus temperature from 1,050 to 1,090 °C in the pressure range from 400 to 50 MPa and was induced by release of mixed H₂O–CO₂ fluid from the melt at decreasing pressure. Matrix glass compositions fall at the high-Si end of the melt inclusion trend and represent the final stage of melt crystallization during and after eruption. The bulk compositions of erupted Cerro Negro magmas (tephras and lavas) range from high- to low-MgO (3–10 wt %) basalts, which form a compositional array crossing the trend of melt inclusions so that virtually no rock from Cerro Negro has composition akin to true melt represented by the inclusions. The variations of the bulk magma (rocks) and melt (melt inclusions) compositions can be generated in a dyke connecting a deep primitive magma reservoir with the Cerro Negro edifice. While the melt inclusions represent the compositional trend of instantaneous melts along the magma pathway at decreasing pressure and H₂O content, occurrence of low-Mg to high-Mg basalts reflects the process of phenocryst re-distribution in progressively evolving melt. The crystallization scenario is anticipated to operate everywhere in dykes feeding basaltic volcanoes and can explain the predominance of plagioclase-rich high-Al basalts in island arc as well as typical compositional variations of magmas during single eruptions.     

The evolution of young silicic lavas at Medicine Lake Volcano,California: Implications for the origin of compositional gaps in calc-alkaline series lavas

At Medicine Lake Volcano, California, the compositional gap between andesite (57–62 wt.% SiO₂) and rhyolite (73–74 wt.% SiO₂) has been generated by fractional crystallization. Assimilation of silicic crust has also occurred along with fractionation. Two varieties of inclusions found in Holocene rhyolite flows, hornblende gabbros and aphyric andesites, provide information on the crystallization path followed by lavas parental to the rhyolite. The hornblende gabbros are magmatic cumulate residues and their mineral assemblages are preserved evidence of the phases that crystallized from an andesitic precursor lava to generate the rhyolite lavas. The andesitic inclusions represent samples of a parental andesite and record the early part of the differentiation history. Olivine, plagioclase and augite crystallization begins the differentiation history, followed by the disappearance of olivine and augite through reaction with the liquid to form orthopyroxene and amphibole. Further crystallization of the assemblage plagioclase, amphibole, orthopyroxene, magnetite, and apatite from a high-SiO₂ andesite leads to rhyolite. This final crystallization process occurs on a cotectic that is nearly horizontal in temperature-composition space. Since a large amount of crystallization occurs over a limited temperature interval, a compositional gap develops between rhyolite and high SiO₂ andesite.Liquidus surfaces with shallow slopes in temperature-composition space are characteristic of several late-stage crystallization assemblages in the andesite to rhyolite compositional range. Experimentally produced plagioclase+ amphibole+orthopyroxene+magnetite and plagioclase+ augite+low-Ca pyroxene+magnetite cotectics have liquidus slopes that are nearly flat. At other calc-alkaline volcanic centers crystallization processes involving large compositional changes over small temperature intervals may also be important in the development of bimodal volcanism (i.e. the existence of a composition gap). At Mt. Mazama and Mt. St. Helens, USA and Aso Caldera and Shikotsu, Japan the amphibole-bearing assemblage was important. At Krakatau, Indonesia and Katmai, USA, an augite+orthopyroxene-bearing assemblage was important. In addition to its role in the production of a compositional gap between intermediate and rhyolitic lavas, the crystallization process increases the H₂O content of the residual liquid. This rapid increase in residual liquid volatile content which results from the precipitation of a large proportion of crystalline solids may be an important factor among several that lead to explosive silicic eruptions.     

Occurrence of calcite in Sanbagawa pelitic schists: implications for the formation of garnet, rutile, oligoclase, biotite and hornblende

The frequency of occurrence of minerals in 1876 samples of Sanbagawa pelitic schist in central Shikoku is summarized on the basis of microscopic observation accompanied, in part, by use of an electron microprobe. All samples contain quartz, plagioclase, phengite, chlorite and graphite. More than 90% of samples contain clinozoisite, titanite and apatite. Garnet is present in 95% of samples from the garnet zone, and biotite is present in 64% of samples from the albite‐biotite zone. Calcite is found in about 40% of samples of the pelitic schist collected from outcrop, but occurs in 95% of the pelitic schist from drill cores. Calcite was apparently ubiquitous in the pelitic schist during the Sanbagawa metamorphism, but must have been dissolved recently by the action of surface or ground water. The mineral assemblages of the Sanbagawa pelitic schist have to be analyzed in the system with excess calcite, quartz, albite (or oligoclase), clinozoisite, graphite and fluid that is composed mainly of H₂O, CO₂ and CH₄. In the presence of calcite, reactions that produce garnet, rutile, oligoclase, biotite and hornblende, some of which define isograds of the metamorphic belt, should be written as mixed volatile equilibria that tend to take place at lower temperature than the dehydration reactions that have been proposed. The presence of calcite in pelitic schist suggests that fluid composition is a variable as important in determining mineral assemblages as pressure and temperature. Thus Ca‐bearing phases must be taken into account to analyze the phase relations of calcite‐bearing pelitic schist, even if CaO content of Sanbagawa pelitic schist is low. As calcite is a common phase, the mineral assemblages of the biotite zone pelitic schist may contravene the mineralogical phase rule and warrant further study.     

A rare composite xenolith from Salt Lake Crater,Oahu: high-pressure fractionation and implications for kimberlitic melts in the Hawaiian mantle

A composite xenolith of olivine-bearing garnet clinopyroxenite wall rock intruded by two spinels + garnet veins is described. Vein minerals exhibit textural evidence of a reaction relationship with the mineral phases in the wall rock. Wall rock clinopyroxene contains exsolved blebby garnet and very fine lamellar exsolution of orthopyroxene, indicating that this xenolith had undergone considerable subsolidus cooling. Garnet-clinopyroxene thermometry suggests that the xenolith last equilibrated in the mantle at a temperature of about 1,060 (ᆭ °C). The spinels in the veins are of two kinds: pleonaste (that occurs with vein garnet) and a high-Mg, high-Al titanomagnetite (MAT spinels). Intriguingly, the MAT spinels are chemically very similar to the spinels found as groundmass in kimberlites, are moderately subhedral to euhedral, have a weakly developed cumulate texture, and, at places, show a reaction relation with the pleonaste + garnet (cumulate?) assemblage in the vein. Based on petrographic, chemical, and phase equilibrium considerations, we propose the following evolutionary history of this composite xenolith. (1) In the first stage the olivine-bearing garnet clinopyroxenite formed as crystal extracts (cumulates) as a result of high pressure fractionation of an alkaline melt in the deepest levels of Hawaiian lithosphere/uppermost asthenosphere (100-110 km). (2) In the second stage, igneous veining (the melt composition of this vein is not precisely known but could be kimberlitic) occurs in the already existing wall rock resulting in the precipitation of pleonaste + garnet. A reaction relation between the igneous veins and the wall rock also characterizes this stage. (3) The last igneous episode in this xenolith is recorded by MAT spinels in the wall rock and their precipitation close to the previous pleonaste + garnet veins. The last igneous stage could well be due again to high pressure fractionation of a kimberlitic melt (the residual melt after precipitation of pleonaste + garnet). The time relationship between exsolution and the later igneous veining stages is not known. The MAT spinels are not a result of sub-solidus solvus processes as partial reaction (melt present) between the pleonaste + garnet (from the second igneous stage) and MAT spinel exists, pointing to the igneous nature of the MAT spinel. Based on striking similarity between the MAT spinels in our xenolith and those found as groundmass in kimberlites, we propose that the veining stages could well have been kimberlitic. Thus, even though kimberlitic melts are not seen on the Koolau shield, this particular xenolith clearly shows the existence of such melts at great depths beneath Hawaii. We also propose that the initial wall rock, which represents crystal extracts (even though it does not exhibit definitive cumulate texture) as a result of high-pressure fractionation of an alkaline melt and subsequent veining episodes, are of pre-Koolau age. This implies that the Koolau shield volcano may have had a pre-shield alkalic stage.     

Chemical composition,volatile components,and trace elements in melts of the Karymskii volcanic center,Kamchatka, and Golovnina volcano,Kunashir Island: Evidence from inclusions in minerals

Melt inclusions were examined in phenocrysts in basalt, andesite, dacite, and rhyodacite from the Karymskii volcanic center in Kamchatka and dacite form Golovnina volcano in Kunashir Island, Kuriles. The inclusions were examined by homogenization and by analyzing glasses in more than 80 inclusions on an electron microscope and ion microprobe. The SiO₂ concentrations in the melt inclusions in plagioclase phenocrysts from basalts from the Karymskii volcanic center vary from 47.4 to 57.1 wt %, these values for inclusions in plagioclase phenocrysts from andesites are 55.7–67.1 wt %, in plagioclase phenocrysts from the dacites and rhyodacites are 65.9–73.1 wt %, and those in quartz in the rhyodacites are 72.2–75.7 wt %. The SiO₂ concentrations in melt inclusions in quartz from dacites from Golovnina volcano range from 70.2 to 77.0 wt %. The basaltic melts are characterized by usual concentrations of major components (wt %): TiO₂ = 0.7–1.3, FeO = 6.8–11.4, MgO = 2.3–6.1, CaO = 6.7–10.8, and K₂O = 0.4–1.7; but these rocks are notably enriched in Na₂O (2.9–7.4 wt % at an average of 5.1 wt %, with the highest Na₂O concentration detected in the most basic melts: SiO₂ = 47.4–52.0 wt %. The concentrations of volatiles in the basic melts are 1.6 wt % for H₂O, 0.14 wt % for S, 0.09 wt % for Cl, and 50 ppm for F. The andesite melts are characterized by high concentrations (wt %) of FeO (6.5 on average), CaO (5.2), and Cl (0.26) at usual concentrations of Na₂O (4.5), K₂O (2.1), and S (0.07). High water concentrations were determined in the dacite and rhyodacite melts: from 0.9 to 7.3 wt % (average of 15 analyses equals 4.5 wt %). The Cl concentration in these melts is 0.15 wt %, and those of F and S are 0.06 and 0.01 wt %, respectively. Melt inclusions in quartz from the dacites of Golovnina volcano are also rich in water: they contain from 5.0 to 6.7 wt % (average 5.6 wt %). The comparison of melt compositions from the Karymskii volcanic center and previously studied melts from Bezymyannyi and Shiveluch volcanoes revealed their significant differences. The former are more basic, are enriched in Ti, Fe, Mg, Ca, Na, and P but significantly depleted in K. The melts of the Karymskii volcanic center are most probably less differentiated than the melts of Bezymyannyi and Shiveluch volcanoes. The concentrations of water and 20 trace elements were measured in the glasses of 22 melt inclusions in plagioclase and quartz from our samples. Unusually high values were obtained for Li concentrations (along with high Na concentrations) in the basaltic melts from the Karymskii volcanic center: from 118 to 1750 ppm, whereas the dacite and rhyolite melts contain 25 ppm Li on average. The rhyolite melts of Golovnina volcano are much poorer in Li: 1.4 ppm on average. The melts of the Karymskii volcanic center are characterized by relative minima at Nb and Ti and maxima at B and K, as is typical of arc magmas.     

Chlorine variations in the magma of Soufrière Hills Volcano, Montserrat: Insights from Cl in hornblende and melt inclusions

The Soufrière Hills Volcano in Montserrat erupts a Cl-rich, porphyritic andesite. HCl degassing accompanies eruption and is dependent on the growth rate of the lava dome. The magma contains hornblende phenocrysts that show repetitive zoning in most elements, including Cl. On the basis of the zoning data, (Cl/OH) ratios in the melt, calculated from partitioning data, increase rimward through each zone, indicating that the phenocrysts formed under conditions of varying (Cl/OH)_m. An empirical relationship between A-site occupancy in the hornblende and temperature implies that crystallisation of each zone is also accompanied by increasing temperature. Each zone ends at a resorption horizon, and crystallisation recommences at lower temperature and (Cl/OH)_m. Melt inclusion H₂O and Cl contents for the 8th January 2007 explosive eruption can be explained by closed-system degassing with D_Cl^fl-m between 5 and 30, or by open-system degassing accompanied by a small amount of crystallisation. However, neither simple closed-system degassing nor convective circulation of magma can explain the positive correlation of (Cl/OH)_m with temperature. We suggest that the zoning can be caused by accumulation of CO₂-rich vapour in the andesite, probably as a result of mafic magma injection into the chamber. Decreasing H₂O fugacity and/or increasing Cl_m result in increasing (Cl/OH)_m while heat transferred with the volatiles causes the rise in temperature. Intermittently, the accumulated fluid is lost to the surface, possibly as a result of renewed eruptive activity. This model requires the CO₂-rich fluid to be decoupled from the magma, consistent with previous observations of continuous CO₂ emissions at the surface.     

Genesis and mechanisms of formation of rare-metal peralkaline granites of the Khaldzan Buregtey massif,Mongolia: Evidence from melt inclusions

Melt inclusions were studied by various methods, including electron and ion microprobe analysis, to determine the compositions of melts and mechanisms of formation of rare-metal peralkaline granites of the Khaldzan Buregtey massif in Mongolia. Primary crystalline and coexisting melt inclusions were found in quartz from the rare-metal granites of intrusive phase V. Among the crystalline inclusions, we identified potassium feldspar, albite, tuhualite, titanite, fluorite, and diverse rare-metal phases, including minerals of zirconium (zircon and gittinsite), niobium (pyrochlore), and rare earth elements (parisite). The observed crystalline inclusions reproduce almost the whole suite of major and accessory minerals of the rare-metal granites, which supports the possibility of their crystallization from a magmatic melt. Melt inclusions in quartz from these rocks are completely crystallized. Their daughter mineral assemblage includes quartz, microcline, aegirine, arfvedsonite, polylithionite, a zirconosilicate, pyrochlore, and a rare-earth fluorocarbonate. The melt inclusions were homogenized in an internally heated gas vessel at a temperature of 850°C and a pressure of 3 kbar. After the experiments, many inclusions were homogeneous and consisted of silicate glass. In addition to silicate glass, some inclusions contained tiny quench zircon crystals confined to the boundary of inclusions, which indicates that the melts were saturated in zircon. In a few inclusions, glass coexisted with a CO₂ phase. This allowed us to estimate the content of CO₂ in the inclusion as 1.5 wt %. The composition of glasses from the homogeneous melt inclusions is similar to the composition of the rare-metal granites, in particular, with respect to SiO₂ (68–74 wt %), TiO₂ (0.5–0.9 wt %), FeO (2.2–4.6 wt %), MgO (0.02 wt %), and Na₂O + K₂O (up to 8.5 wt %). On the other hand, the glasses of melt inclusions appeared to be strongly depleted compared with the rocks in CaO (0.22 and 4 wt %, respectively) and Al₂O₃ (5.5–7.0 and 9.6 wt %, respectively). The agpaitic index is 1.1–1.7. The melts contain up to 3 wt % H₂O and 2–4 wt % F. The trace element analysis of glasses from homogenized melt inclusions in quartz showed that the rare-metal granites were formed from extensively evolved rare-metal alkaline melts with high contents of Zr, Nb, Th, U, Ta, Hf, Rb, Pb, Y, and REE, which reflects the metallogenic signature of the Khaldzan Buregtey deposit. The development of unique rare metal Zr–Nb–REE mineralization in these rocks is related to the prolonged crystallization differentiation of melts and assimilation of enclosing carbonate rocks.     

Mineral inclusions in pyrope crystals from Garnet Ridge, Arizona, USA: implications for processes in the upper mantle

Mineral inclusions in pyrope crystals from Garnet Ridge in the Navajo Volcanic Field on the Colorado Plateau are investigated in this study with emphasis on the oxide minerals. Each pyrope crystal is roughly uniform in composition except for diffusion halos surrounding some inclusions. The pyrope crystals have near constant Ca:Fe:Mg ratios, 0.3 to 5.7 wt% Cr₂O_3, and 20 to 220 ppm H₂O. Thermobarometric calculations show that pyrope crystals with different Cr contents formed at different depths ranging from 50 km (where T ≈ 600 °C and P = 15 kbar) to 95 km (where T ≈ 800 °C and P = 30 kbar) along the local geotherm. In addition to previously reported inclusions of rutile, spinel and ilmenite, we discovered crichtonite series minerals (AM₂₁O₃₈, where A = Sr, Ca, Ba and LREE, and M mainly includes Ti, Cr, Fe and Zr), srilankite (ZrTi₂O₆), and a new oxide mineral, carmichaelite (MO_2−x(OH)_x, where M = Ti, Cr, Fe, Al and Mg). Relatively large rutile inclusions contain a significant Nb (up to 2.7 wt% Nb₂O₅), Cr (up to ∼6 wt% Cr₂O₃), and OH (up to ∼0.9 wt% H₂O). The Cr and OH contents of rutile inclusions are positively related to those of pyrope hosts, respectively. Needle- and blade-like oxide inclusions are commonly preferentially oriented. Composite inclusions consisting mainly of carbonate, amphibole, phlogopite, chlorapatite, spinel and rutile are interpreted to have crystallized from trapped fluid/melt. These minerals in composite inclusions commonly occur at the boundaries between garnet host and large silicate inclusions of peridotitic origin, such as olivine, enstatite and diopside. The Ti-rich oxide minerals may constitute a potential repository for high field strength elements (HFSE), large ion lithophile elements and light rare earth elements (LREE) in the upper mantle. The composite and exotic oxide inclusions strongly suggest an episode of metasomatism in the depleted upper mantle beneath the Colorado Plateau, contemporaneous with the formation of pyrope crystals. Our observations show that mantle metasomatism may deplete HFSE in metasomatic fluids/melts. Such fluids/melts may subsequently contribute substantial trace elements to island arc basalts, providing a possible mechanism for HFSE depletion in these rocks. Received: 20 December 1997 / Accepted: 15 October 1998     

Multiphase inclusions in plagioclase from anorthosites in the Stillwater Complex,Montana: implications for the origin of the anorthosites

Multiphase inclusions, consisting of clinopyroxene+ilmenite+apatite, occur within cumulus plagioclase grains from anorthosites in the Stillwater Complex, Montana, and in other rocks from the Middle Banded series of the intrusion. The textures and constant modal mineralogy of the inclusions indicate that they were incorporated in the plagioclase as liquid droplets that later crystallized rather than as solid aggregates. Their unusual assemblage, including a distinctive manganiferous ilmenite and the presence of baddeleyite (ZrO₂), indicates formation from an unusual liquid. A process involving silicater liquid immiscibility is proposed, whereby small globules of a liquid enriched in Mg, Fe, Ca, Ti, P, REE, Zr and Mn exsolved from the main liquid that gave rise to the anorthosites, became trapped in the plagioclase, and later crystallized to form the inclusions. The immiscibility could have occurred locally within compositional boundaries around crystallizing plagioclase grains or it could have occurred pervasively throughout the liquid. It is proposed that the two immiscible liquids were analogous, n terms of their melt structures, to immiscible liquid pairs reported in the literature both in experiments and in natural basalts. For the previously reported pairs, immiscibility is between a highly polymerized liquid, typically granitic in composition, and a depolymerized liquid, typically ferrobasaltic in composition. In the case of the anorthosites, the depolymerized liquid is represented by the inclusions, and the other liquid was a highly polymerized aluminosilicate melt with a high normative plagioclase content from which the bulk of the anorthosites crystallized. Crystallization of the anorthosites from this highly polymerized liquid accounts for various distinctive textural and chemical features of the anorthosites compared to other rocks in the Stillwater Complex. A lack of correlation between P contents and chondrite-normalized rare earth element (REE) ratios of plagioclase separates indicates that the amount of apatite in the inclusions is too low to affect the REE signature of the plagioclase separates. Nevertheless, workers should use caution when attempting REE modelling studies of cumulates having low REE contents, because apatite-bearing inclusions can potentially cause problems.     

Jadeite, albite and nepheline as inclusions in spinel of chromitite from Hess Deep, equatorial Pacific: their genesis and implications for serpentinite diapir formation

Polymineralic inclusions which consist of a few grains of diopside, enstatite, jadeite, nepheline, albite, pargasite, phlogopites and olivine were found in chromian spinel in a chromitite pod and in troctolite from Hess Deep, equatorial Pacific. The inclusion mineral suite in chromitite is characterized by Na-Al silicates, such as jadeite, nepheline and albite. Jadeite and nepheline commonly coexist with enstatite, and tend to occur as interstitial grains between subhedral enstatite (or other minerals) and host spinel. Albite, diopside and enstatite occur as equant inclusions. The mafic minerals in the inclusions have similar chemistry to those found in the troctolite and dunite. The modes of occurrence and mineral chemistry of the inclusions are controlled by magmatic precipitation, and subsequent reequilibration due to decrease of temperature in the uppermost mantle. The mafic minerals in spinel inclusions were crystallized from a melt enriched in Cr and some incompatible components formed by melt-mantle interaction process mixed to various extent with subsequently supplied more primary melt. Albite and nepheline could also be formed at the magmatic stage. Jadeite was formed by a subsolidus reaction of albite and nepheline at low temperatures (250–300 °C) at slightly less than 3 kbar. This requires a remarkable temperature decrease, at least locally, of the uppermost mantle and crust. The Hess Deep rocks were formed in the uppermost mantle beneath a spreading-ridge axis at more than 1000 °C, and were transposed outwards from the axis by corner flow. At the off-ridge conditions, the rocks were cooled and serpentinized by circulation of sea water at the mantle depth to form jadeite in chromitite. The serpentinized portion could have risen as a kind of serpentinite diapir through the thin crust up to the ocean floor. Received: 24 January 1997 / Accepted: 6 November 1997     

Multiple-mineral inclusions in diamonds from the Snap Lake/King Lake kimberlite dike, Slave craton, Canada: a trace-element perspective

Multiple inclusions of minerals in diamonds from the Snap Lake/King Lake kimberlites of the southeastern Slave craton in Canada have been analyzed for trace elements to elucidate the petrogenetic history of these inclusions, and of their host diamonds. As observed worldwide, the harzburgitic-garnet diamond inclusions (DIs) possess sinusoidal REE patterns that indicate an early depletion event, followed by metasomatism by LREE-enriched, HREE-depleted fluids. Furthermore, these fluids appear to contain appreciable concentrations of LILE and HFSE, based on the increasing abundances of these elements in the olivine inclusion that occurs at the outer portion of a diamond compared to that near the core. The compositions of these fluids are probably a mixture of hydrous-silicic melt, carbonatitic melt, and brine, similar to the compositions of micro-inclusions in diamonds reported by Navon et al. (2003). Comparison between the compositions of majoritic and normal harzburgitic garnets shows that the former are more depleted in terms of major/minor elements (higher Cr#) but significantly more enriched in the REE (up to 10×). This characteristic may indicate the higher susceptibility for metasomatic enrichment of previously more depleted garnets. Garnets of eclogitic paragenesis show strong LREE-depleted patterns, whereas the coexisting omphacite inclusion has relatively flat light- and middle-REE but depleted HREE. Whole-rock reconstruction from coexisting garnet and omphacite inclusions indicates that the protolith of these inclusions was probably the extrusive section of an oceanic crust, subducted beneath the Slave craton.     

拉萨地块南缘晚白垩世角闪辉长岩、花岗斑岩副矿物微量元素特征对成岩成矿的指示

拉萨地块南缘记录了新特提斯洋俯冲到印度-欧亚大陆碰撞及碰撞后的岩浆作用,其中晚白垩世的岩浆作用对研究印度-欧亚大陆碰撞前成岩成矿作用具有重要的意义。本文以拉萨地块南缘100Ma的角闪辉长岩和68Ma花岗斑岩的锆石、磷灰石、榍石为研究对象,利用背散射、阴极发光(CL)、电子探针(EPMA)和LA-ICP-MS原位微区分析等方法,查明锆石、磷灰石、榍石的主、微量元素特征,进一步反演岩石源区性质、结晶历史及结晶条件,并对岩体含矿性进行评价,有助于探讨冈底斯成矿带晚白垩世岩浆成因机制和成矿潜力。研究结果表明,角闪辉长岩锆石初始饱和温度为598～626℃,锆石Ti结晶温度为645～758℃,磷灰石饱和温度为690～819℃,榍石Zr温度为602～778℃;磷灰石具有中等-弱的负铕异常(δEu=0. 67)、富集LREE,早期高温阶段锆石的结晶主要受到磷灰石结晶影响,随着温度降低,受到少量榍石结晶的影响;角闪辉长岩中的锆石在低的Hf、温度较高时却具有较高的Th、U含量显示岩浆源区受到更多俯冲板片出溶流体的影响,磷灰石具有较高的(La/Sm)N值以及Sr含量低于主岩,显示岩浆源区均一、熔体富Cl特征。花岗斑岩的锆石初始饱和温度为704～736℃,锆石Ti温度为630～799℃,磷灰石饱和温度为846～891℃,结合锆石的Ce/Sm、Yb/Gd以及磷灰石较大的负铕异常(δEu=0. 29),显示花岗斑岩中的锆石从高温到低温阶段都受到磷灰石和榍石的共同结晶影响,磷灰石的结晶受到斜长石影响;花岗斑岩中大部分锆石Ti结晶温度高于其初始饱和温度,磷灰石Sr-Ap/Sr-WR为0. 78～1. 45,具有较高的F/Cl(32. 87～67. 60)、低的(La/Sm)N,指示花岗斑岩岩浆源区不均一,受到多期岩浆熔体的脉冲式灌入并加入了更多镁铁质的岩浆熔体,其熔体具有低的Cl。此外,花岗斑岩锆石具有较高的Ce~(4+)/Ce~(3+)、磷灰石具有较高的SO_3、熔体中更富S,指示花岗斑岩具有高的氧逸度和成矿潜力。本文研究结果表明结合锆石、磷灰石和榍石微量元素特征可有效指示岩浆岩的源区组成、结晶历史、结晶条件以及成矿潜力,为岩石学的研究提供了一个新的思路。     

Revisiting the compositions and volatile contents of olivine-hosted melt inclusions from the Mount Shasta region: implications for the formation of high-Mg andesites

Primitive chemical characteristics of high-Mg andesites (HMA) suggest equilibration with mantle wedge peridotite, and they may form through either shallow, wet partial melting of the mantle or re-equilibration of slab melts migrating through the wedge. We have re-examined a well-studied example of HMA from near Mt. Shasta, CA, because petrographic evidence for magma mixing has stimulated a recent debate over whether HMA magmas have a mantle origin. We examined naturally quenched, glassy, olivine-hosted (Fo_87–94) melt inclusions from this locality and analyzed the samples by FTIR, LA-ICPMS, and electron probe. Compositions (uncorrected for post-entrapment modification) are highly variable and can be divided into high-CaO (>10 wt%) melts only found in Fo > 91 olivines and low-CaO (<10 wt%) melts in Fo 87–94 olivine hosts. There is evidence for extensive post-entrapment modification in many inclusions. High-CaO inclusions experienced 1.4–3.5 wt% FeO^T loss through diffusive re-equilibration with the host olivine and 13–28 wt% post-entrapment olivine crystallization. Low-CaO inclusions experienced 1–16 wt% olivine crystallization with <2 wt% FeO^T loss experienced by inclusions in Fo > 90 olivines. Restored low-CaO melt inclusions are HMAs (57–61 wt% SiO₂; 4.9–10.9 wt% MgO), whereas high-CaO inclusions are primitive basaltic andesites (PBA) (51–56 wt% SiO₂; 9.8–15.1 wt% MgO). HMA and PBA inclusions have distinct trace element characteristics. Importantly, both types of inclusions are volatile-rich, with maximum values in HMA and PBA melt inclusions of 3.5 and 5.6 wt% H₂O, 830 and 2,900 ppm S, 1,590 and 2,580 ppm Cl, and 500 and 820 ppm CO₂, respectively. PBA melts are comparable to experimental hydrous melts in equilibrium with harzburgite. Two-component mixing between PBA and dacitic magma (59:41) is able to produce a primitive HMA composition, but the predicted mixture shows some small but significant major and trace element discrepancies from published whole-rock analyses from the Shasta locality. An alternative model that involves incorporation of xenocrysts (high-Mg olivine from PBA and pyroxenes from dacite) into a primary (mantle-derived) HMA magma can explain the phenocryst and melt inclusion compositions but is difficult to evaluate quantitatively because of the complex crystal populations. Our results suggest that a spectrum of mantle-derived melts, including both PBA and HMA, may be produced beneath the Shasta region. Compositional similarities between Shasta parental melts and boninites imply similar magma generation processes related to the presence of refractory harzburgite in the shallow mantle.     

Important role of hornblende fractionation in generating the adakitic magmas in Tongling,Eastern China: evidence from amphibole megacryst and cumulate xenoliths and host gabbros

ABSTRACT

Tongling, in eastern China, is an area well-known for intra-plate adakites. Here, we present the mineral chemistry and zircon U–Pb ages for amphibole cumulate xenoliths, the mineral chemistry of amphibole megacrysts, and the whole–rock chemistry, zircon U–Pb age and Sr–Nd isotopic compositions of host gabbros from Tongling. Zircon U–Pb dating yields a crystallization age of 120.6 ± 1.2 Ma (MSWD = 4.2) for the host gabbros, which are characteristically depleted in high field strength elements (Nb, Ta, and Ti) and enriched in large ion lithophile elements (Ba and Sr), with εNd (t) of ?3.00 to ?4.52 and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7068–0.7072, suggesting an enriched mantle source. Parental melts, as estimated from average amphibole megacryst and cumulate compositions, have Mg# values of 26–33, are enriched in Ba, Th, U, and Nd, and depleted in Nb, Ta, Zr, Hf, and Ti, similar to 136 Ma mafic magmas in Tongling. Zircon U–Pb dating yields a crystallization age of 135.4 ± 1.0 Ma (MSWD = 1.6) for the amphibole cumulates. It is concluded that the Tongling adakitic rocks were formed by polybaric crystallization involving early high-pressure intracrustal fractional crystallization of cumulates comprising hornblende and clinopyroxene, and late low-pressure fractional crystallization of hornblende and plagioclase phenocrysts. The flat subduction of Pacific plate and its subsequent foundering during the Cretaceous may have triggered the generation of extensive adakitic magmas and lithospheric thinning in the Lower Yangtze Region.     

An AEM study of garnet clinopyroxenite from the Sulu ultrahigh-pressure terrane: formation mechanisms of oriented ilmenite,spinel, magnetite,amphibole and garnet inclusions in clinopyroxene

Optical microscopy, secondary electron microscopy and analytical electron microscopy were used to characterize crystallographic orientation relationships between oriented mineral inclusions and clinopyroxene (Cpx) host from the Hujialing garnet clinopyroxenite within the Sulu ultrahigh-pressure (UHP) terrane, eastern China. One garnet clinopyroxenite sample (2HJ-2C) and one megacrystic garnet-bearing garnet clinopyroxenite (RZ-11D) were studied. Porphyroblastic clinopyroxene from sample 2HJ-2C contains oriented inclusions of ilmenite (Ilm), spinel (Spl), magnetite and garnet, whereas clinopyroxene inclusions within megacrystic garnet from sample RZ-11D contain oriented inclusions of ilmenite and amphibole. Specific crystallographic relationships were observed between ilmenite/spinel plates and host clinopyroxene in sample 2HJ-2C and between ilmenite plates and host clinopyroxene in sample RZ-11D, i.e. [1[`1]00 1\bar{1}00 ]<sub>Ilm</sub>//[0[`1]0 0\bar{1}0 ]_Cpx (0001)_Ilm//(100)_Cpx; and [110]_Spl//[0[`1]0 0\bar{1}0 ]<sub>Cpx</sub> ([`1]11 \bar{1}11 )_Spl//(100)_Cpx. These inclusions are suggested to be primary precipitates via solid-state exsolutions. Most of the needle-like magnetite/spinel inclusions generally occur at the rims or along fractures of clinopyroxene within sample 2HJ-2C. Despite the epitaxial relation with host clinopyroxene, these magnetite/spinel needles would have resulted from fluid/melt infiltrations. Non-epitaxial garnet lamellae in clinopyroxene of sample 2HJ-2C were formed via fluid infiltration-deposition primarily along (010) and subordinately along (100) partings. Epitaxial amphibole plates (with a thickness <1 μm) and lamellae (with a thickness = 1–10 μm) in host clinopyroxene of sample RZ-11D were probably results of hydration processes, although amphibole plates could otherwise be interpreted as exsolution products. Temporal relations between mineral inclusions in each sample can be established, and a semi-quantitative P–T path for this garnet clinopyroxenite body was derived accordingly. The present results show that the Hujialing garnet clinopyroxenite may not have subducted to mantle depths as deep as 250 km during UHP metamorphism as suggested by previous studies. This study demonstrates that the crystallographic and temporal/spatial relationships between aligned inclusions and host minerals are essential to a correct genetic interpretation of metamorphic rocks.     

Paragenesis and composition of laurite from chromitites of Othrys (Greece): implications for Os-Ru fractionation in ophiolitic upper mantle of the Balkan peninsula

Os-rich laurite and erlichmanite are the dominant PGM inclusions in chromitites of the Othrys ophiolite complex. Close association of the PGM sulfides with enstatite, Na-rich pargasite, clinopyroxene, phlogopite, and Cu-Ni sulfides indicates crystallization at high temperature, in the presence of an alkali-rich fluid phase, under relatively-high sulfur fugacity. Because of the predominance of Ru-Os-Ir phases, the PGM assemblage of Othrys is similar to that of chromitites located in the mantle unit of other ophiolite complexes of the Balkan peninsula (Vourinos, Skyros Island, Rhodope) although it is distinguished because of the Os-rich composition of laurite and the presence of erlichmanite. Comparison among literature data for these complexes indicates that the Os/Ru ratios vary consistently in laurite and bulk chromitite with respect to the chondritic Os/Ru value. This suggests that fractionation between the two elements occurred, being apparently registered in the composition of laurite inclusions. Among other factors, fluctuation of sulfur fugacity during fractionation, as well as variation of the Os/Ru ratio in the parent melts of the chromitites might be invoked to explain the Ru-Os decoupling. Received: 5 June 1998 / Accepted: 16 July 1998