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Showing papers on "Phenocryst published in 2015"


Journal ArticleDOI
TL;DR: A suite of Miocene high-Mg diorite porphyries from Qulong in southern Tibet, the largest porphyry Cu-Mo deposit in China, has been investigated using laser ablation inductively coupled plasma mass spectrometry zircon U-Pb dating as mentioned in this paper.
Abstract: We have investigated a suite of Miocene high-Mg diorite porphyries from Qulong in southern Tibet, the largest porphyry Cu–Mo deposit in China. Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating shows that the high-Mg diorite porphyry was emplaced at 15.7±0.2 Ma, which is slightly younger than the Qulong adakite-like Rongmucuola pluton (c. 19.5–16.4 Ma). The Qulong high-Mg diorites have phenocrysts showing disequilibrium textures and include high-Mg# clinopyroxene (0.91–0.97). These porphyry rocks exhibit both ultrapotassic and adakite-like features, and are characterized by high contents of MgO (4.2–5.1 wt %), K2O (3.2–3.6 wt %), and compatible trace elements (e.g. Ni: 115–142 ppm; Cr: 214–291 ppm), as well as by high Sr/Y and La/Yb ratios. The rocks have highly radiogenic isotopic compositions with (87Sr/86Sr)i = 0.707004–0.707198, eNd(t)=-5.1 to -5.5, 207Pb/204Pb = 15.697–15.704, and 208Pb/204Pb = 39.082 – 39.116, as well as variable zircon eHf values of -3.0 to +5.9. Petrographic, elemental, and isotopic evidence suggests that the Qulong high-Mg diorites were formed by mixing between ultrapotassic and adakite-like melts, derived from metasomatized Tibetan lithospheric mantle and juvenile lower crust, respectively. In contrast, the Qulong pre-ore Rongmucuola pluton is characterized by high SiO2 (66.3–68.9 wt %) and Al2O3 (16.4–17.0 wt %) contents, high Sr/Y ratios (121–151), low compatible element contents (e.g. Ni = 16.0 – 17.4 ppm; Cr=14.5–20.2 ppm), low Mg# values (0.44–0.52), positive large-ion lithophile element (LILE) anomalies, marked negative high field strength element (HFSE) anomalies, positive eNd(t) values (+0.4 to +2.5), and low (87Sr/86Sr)i values (0.704847–0.705237). These features indicate that the Rongmucuola pluton was formed by partial melting of subduction-modified juvenile lower crust within the garnet stability field. The newly identified Qulong high-Mg diorite allows us to propose a mixing model for the origin of the Gangdese high-K, adakite-like rocks. In this model, the formation of these rocks occurred in two stages: (1) partial melting of highly metasomatized lithospheric mantle that generated ultrapotassic mafic melts; (2) underplating of such melts beneath thickened juvenile lower crust, which resulted in melting of the lower crust and the generation of adakite-like magmas. Mixing of the adakite-like melt with ultrapotassic magmas elevated the K2O, MgO, and other LILE (e.g. Rb and Ba) contents of the adakite-like melt. Exogenous water necessary for formation of the Gangdese porphyry Cu systems was mainly added during mixing of ultrapotassic magma with adakite-like melt at lower and/ or upper-crustal depths, reflecting the large decrease in the H2O solubility of the ultrapotassic mafic melt upon ascent and decompression. Upper-crustal fractionation of the Rongmucuola magma could also possibly increase the water content of mineralization-related, adakite-like porphyry intrusions at Qulong. Fluid exsolution from the ultrapotassic magma is likely to have been a key process in the generation of the Gangdese porphyry Cu deposits, as well as other porphyry Cu deposits in the Tibetan collisional orogens.

168 citations


Journal ArticleDOI
01 Nov 2015-Lithos
TL;DR: In this article, the authors show that chemical zoning is a near closed-system process and propose that it arises through melting of cognate cumulate mush beneath a crystal-poor body of melt due to heating by invading mafic or intermediate magma with little mass transfer to the eruptible magma.

107 citations


Journal ArticleDOI
TL;DR: In this article, the authors applied high-precision in situ Fe and Mg isotope analyses by femtosecond laser ablation (fs-LA) MC-ICP-MS on chemically zoned olivine xeno-and phenocrysts from intra-plate volcanic regions in order to investigate the magnitude of Fe and mg fractionation and its suitability to gain information on magma evolution.

94 citations


Journal ArticleDOI
01 Nov 2015-Lithos
TL;DR: Ongonites were defined at their type locality at Ongon Khairkhan, central Mongolia, as pristine magmatic topaz-bearing albite-quartz-keratophyres with up to 4.5% F and containing phenocrysts of albite, K-feldspar, quartz and rare mica and topaz hosted in a groundmass composed of the same minerals as mentioned in this paper.

94 citations


Journal ArticleDOI
TL;DR: In this article, the authors explore the differentiation conditions that gave rise to the observed geochemical variation in lavas and plutonic (cumulate) xenoliths from St. Vincent, and show that initial H2O content has significant influence on differentiation paths of primary HMB magma, with different lava varieties generated under discrete, well-constrained P-T-H2O conditions.
Abstract: Equilibrium crystallization experiments have been performed on a primitive high-MgO basalt (HMB) from Soufriere, St. Vincent, with three initial H2O contents (0·6, 2·3 and 4·5 wt %), at pressures of 0·4, 0·7, 1·0 and 1·3 GPa and temperatures from 1350 to 950°C. Redox conditions, as determined by µXANES analysis of Fe3+ in experimental glasses, were 1–4 log units above the nickel–nickel oxide (NNO) buffer. The aim of the study was to explore the differentiation conditions that gave rise to the observed geochemical variation in lavas and plutonic (cumulate) xenoliths from St. Vincent. An experiment with 4·5 wt % initial H2O is multiply saturated close to its liquidus (1180°C and 1·3 GPa) with a spinel lherzolite assemblage, which is consistent with a primary origin for HMB in the mantle wedge. Multiple saturation of HMB with 2·3 wt % H2O was not observed, but is inferred to occur at pressures >1·3 GPa. The experimental results show that initial H2O content has significant influence on differentiation paths of primary HMB magma, with different lava varieties generated under discrete, well-constrained P–T–H2O conditions. Low-magnesian basalts (LMB) can be generated from HMB with 2·3–4·5 wt % H2O at pressures of 1·0–1·3 GPa, corresponding to Moho depths beneath St. Vincent. The CaO contents of LMB are sensitive to differentiation pressure: high-CaO LMB are produced at pressures >0·5 GPa. Basaltic andesites (BA) can be generated at 0·7–1·0 GPa from HMB with 0·6–2·3 wt % H2O. High-alumina basalts (HAB) are produced at mid- to upper-crustal conditions (≤0·4 GPa) by differentiation of HMB with high initial H2O (≥4 wt %) through delay of plagioclase crystallization and dominant fractionation of olivine, clinopyroxene and spinel. St. Vincent andesites could be produced from relatively dry (≤0·6 wt % H2O) HMB only at lower-crustal conditions. This is suggestive of a partial melting origin from precursor HMB that had solidified at depth to produce gabbros with ∼30% hornblende (i.e. ∼0·6 wt % structurally bound H2O). The experimentally determined differentiation conditions are consistent with polybaric differentiation within a hot zone that extends from the Moho and uppermost mantle to the mid- or upper crust. Within the hot zone differentiation occurs by a combination of crystallization of HMB with 2–5 wt % H2O and partial melting of ancestral HMB gabbros. Although the experimental melts provide an excellent match to erupted lava compositions, experimental crystal compositions do not match either phenocrysts or cumulate crystals, as preserved in xenoliths. The failure to reproduce natural crystal compositions suggests that these are formed as differentiated magmas ascend and attain their H2O-saturated liquidi at shallower pressures. Thus there is a disconnect between the high-pressure phase compositions and assemblages that generate liquid compositional diversity and the low-pressure composition and assemblages that occur as phenocrysts and in cumulate xenoliths. This finding lends support to the idea of cryptic fractionation in the generation of arc magmas.

93 citations


Journal ArticleDOI
01 Jan 2015-Lithos
TL;DR: In this article, the compositions of olivine, clinopyroxene, plagioclase and titanomagnetite in trachybasaltic lavas at Mt. Etna were compared with those obtained through thermodynamic simulations and experiments conducted under anhydrous, water-under saturated and water-saturated conditions.

64 citations


Journal ArticleDOI
TL;DR: In this paper, the authors focused on the search for mineral inclusions in olivine and for phenocrysts with a clear zoning in the kimberlite of the Udachnaya-east pipe.

63 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present the first analysis of olivine-hosted melt inclusions in Cenozoic basalts from the North Hainan Island and report the discovery of both tholeiitic and alkalic melt-inclusions in a single sample.

57 citations


Journal ArticleDOI
TL;DR: In this paper, an extensive dataset of major element, volatile, and trace element abundances in quartz-hosted melt inclusions and pumice matrix glass with major element geochemistry from phenocrysts was combined to reconstruct the pre-eruptive conditions and the source and evolution of metals within the Hideaway Park tuff.
Abstract: The Hideaway Park tuff is the only preserved extrusive volcanic unit related to the Red Mountain intrusive complex, which produced the world-class Henderson porphyry Mo deposit. Located within the Colorado Mineral Belt, USA, Henderson is the second largest Climax-type Mo deposit in the world, and is therefore an excellent location to investigate magmatic processes leading to Climax-type Mo mineralization. We combine an extensive dataset of major element, volatile, and trace element abundances in quartz-hosted melt inclusions and pumice matrix glass with major element geochemistry from phenocrysts to reconstruct the pre-eruptive conditions and the source and evolution of metals within the magma. Melt inclusions are slightly peraluminous topaz rhyolitic in composition and are volatile-charged (� 6w t % H2O, � 600 ppm CO2, � 0� 3–1� 0w t % F, � 2300–3500 ppm Cl) and metal-rich (� 7–24 ppm Mo, � 4–14 ppm W, � 21–52 ppm Pb, � 28–2700 ppm Zn, <0� 1–29 ppm Cu, � 0� 3–1� 8 ppm Bi, � 40–760 ppb Ag, � 690–1400 ppm Mn). Melt inclusion and pumice matrix glass chemistry reveal that the Hideaway Park magma evolved by large degrees of fractional crystallization (� 60–70%) during quartz crystallization and melt inclusion entrapment at pressures of � 300 MPa (� 8 km depth), with little to no crystallization upon shallow ascent and eruption. Filter pressing, crystal settling, magma recharge and mixing of less evolved rhyolite melt, and volatile exsolution were important processes during magma evolution; the low estimated viscosities (� 10 5 –10 10 Pa s) of these H2O- and F-rich melts probably enhanced these processes. A noteworthy discrepancy between the metal contents in the pumice matrix glass and in the melt inclusions suggests that after quartz crystallization ceased upon shallow magma ascent and eruption, the Hideaway Park magma exsolved an aqueous fluid into which Mo, Bi, Ag, Zn, Mn, Cs, and Y strongly partitioned. Given that the Henderson deposit contains anomalous abundances of not only Mo, but also W, Pb, Zn, Cu, Bi, Ag, and Mn, we suggest that these metals were sourced from similar fluids exsolved from unerupted portions of the same magmatic system. Trace element ratios imply that Mo was sourced deep, from either the lower crust or metasomatized mantle. The origin of sulfur remains unresolved; however, given the extremely low S solubility of rhyolite melts in the shallow crust we favor the possibility that another source of S might supplement or account for that present in the ore deposit, probably the comagmatic, mantle-derived lamprophyres that occur in minor quantities with the voluminous topaz rhyolites in the area. To account for the 437 Mt of MoS2 (� 1� 0 � 10 6 t Mo) present in the Henderson ore deposit, a volume of � 45 km 3 of Hideaway Park rhyolite magma would have been necessary to supply the Mo (a cylindrical pluton measuring

55 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the relationship between chondrules and matrix in the Kakangari K-grouplet chondrite and found that the chondritic components are genetically related in the sense that material that formed matrix was one of the precursors of chond rules and some fraction of matrix experienced the same thermal processing event.

53 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present the results of hydrothermal experiments on a dacite from Fish Canyon Tuff (FCT) in Colorado (USA), a classic example of a monotonous intermediate deposit, in order to characterize the variations in chemical and physical properties of hydrous Dacite magmas as a function of temperature.
Abstract: Large-volume, high-crystallinity, chemically homogeneous ignimbrites, dubbed ‘monotonous intermediates', provide a unique opportunity to investigate the evolution of crustal magmatic reservoirs. We present the results of hydrothermal experiments on a dacite from Fish Canyon Tuff (FCT) in Colorado (USA), a classic example of a monotonous intermediate deposit, in order to characterize the variations in chemical and physical properties of hydrous dacite magmas as a function of temperature. The experiments (200 MPa, 720–1100C) span the inferred pre-eruptive conditions of FCT magmas, and are shown to provide the best match to the chemical and physical properties of the erupted magmas at 790+-10C under conditions at or close to water-saturation. The results show the important effect of water content in controlling the chemical and physical evolution of magma, and the contrasted behaviour of water-saturated v. water-undersaturated magmas. In both cases, however, there is a broad interval of temperature (200C) over which crystal fraction changes little. By recasting this behaviour in terms of enthalpy, rather than temperature, as the independent variable we show that this interval corresponds to a minimum in the change in crystallinity per unit of energy added or subtracted from the system,such that small perturbations to the heat content of the system (e.g. by cooling or new magmainjections) results in very little change in magma properties. The crystal content in this intervalis 55–65 wt%, which is close to the phenocryst content (40–55 wt%) of monotonous intermediates. We propose that crystal-rich magmas tend to settle in this ‘petrological trap', changing little in physical and chemical properties over time as the system grows. Petrological trapping enables very large volumes of intermediate magma to accumulate in the shallow crust until suchtime as the net buoyancy force of these crystal-rich magma is sufficient to overcome the strength of the roof rocks, leading to a potentially very large eruption.

Journal ArticleDOI
TL;DR: In this article, the conditions of differentiation (pressure, temperature, volatile fugacities) and the parental magma relationship with the andesitic eruptive rocks are investigated in Santorini volcano.
Abstract: Santorini volcano in the Aegean region (Greece) is characterized by andesitic- to silicic-dominated explosive activity and caldera-forming eruptions, sourced from magmatic reservoirs located at various structural levels beneath the volcano. There is a good understanding of the silica-rich magmatism of the island whereas the andesite-dominated volcanism and the petrogenesis of the parental mafic magmas are still poorly understood. To fill this gap we have performed crystallization experiments on a representative basalt from Santorini with the aim of determining the conditions of differentiation (pressure, temperature, volatile fugacities) and the parental magma relationship with the andesitic eruptive rocks. Experiments were carried out between 975 and 1040 � C, in the pressure range 100‐400 MPa, fO2 from QFM to NNO þ 3� 5 (where QFM is quartz‐ fayalite‐magnetite and NNO is nickel‐nickel oxide), with H2Omelt contents varying from saturation to nominally dry conditions. The results show that basalt phenocrysts within the basalt crystallized at around 1040 � C in a magma storage reservoir located at a depth equivalent to 200‐400 MPa pressure, with 3‐5 wt % dissolved H2O, and fO2 around QFM. Comparison with the xenocryst and phenocryst assemblages of the Upper Scoria 1 andesite shows that andesitic liquids are produced by fractionation of a similar basalt at 1000 � C and 400 MPa, following 60‐80 wt % crystallization of an ol þ cpx þ plag þ Ti-mag þ opx 6 pig‐ilm assemblage, with melt water contents around 4‐6 wt %. At Santorini, the andesitic low-viscosity and water-rich residual liquids produced at these depths segregate from the parent basaltic mush and feed the shallow magma reservoirs, eventually erupting upon mixing with resident magma. Changes in prevailing oxygen fugacity may control the tholeiitic‐calc-alkaline character of Santorini magmas, explaining the compositional and mineralogical differences observed between the recent Thyra and old eruptive products from Akrotiri.

Journal ArticleDOI
TL;DR: In this article, the origin of these magma types using O and Sr isotopes was investigated using phenocrysts to constrain crust/mantle proportions for the large volume ignimbrites and explore the relationship to the evolution of the Andean crust.
Abstract: Volcanism during the Neogene in the Central Volcanic Zone (CVZ) of the Andes produced (1) stratovolcanoes, (2) rhyodacitic to rhyolitic ignimbrites which reach volumes of generally less than 300 km3 and (3) large-volume monotonous dacitic ignimbrites of up to several thousand cubic kilometres. We present models for the origin of these magma types using O and Sr isotopes to constrain crust/mantle proportions for the large-volume ignimbrites and explore the relationship to the evolution of the Andean crust. Oxygen isotope ratios were measured on phenocrysts in order to avoid the effects of secondary alteration. Our results show a complete overlap in the Sr–O isotope compositions of lavas from stratovolcanoes and low-volume rhyolitic ignimbrites as well as older (>9 Ma) large-volume dacitic ignimbrites. This suggests that the mass balance of crustal and mantle components are largely similar. By contrast, younger ( 70 km3 Ma−1 km−1 (assuming plutonic/volcanic ratios of 1:5) which are additional to, but within the order of, the arc background magmatic flux. Comparing our results to average shortening rates observed in the Andes, we observe a “lag-time” with large-volume eruptions occurring after accelerated shortening. A similar delay exists between the ignimbrite pulses and the subduction of the Juan Fernandez ridge. This is consistent with the idea that large-volume ignimbrite eruptions occurred in the wake of the N–S passage of the ridge after slab steepening has allowed hot asthenospheric mantle to ascend into and cause the melting of the mantle wedge. In our model, the older large-volume dacitic ignimbrites in the northern part of the CVZ have lower (15–37 %) crustal contributions because they were produced at times when the Central Andean crust was thinner and colder, and large-scale melting in the middle crust could not be achieved. Younger ignimbrite flare-ups further south ( 22°S) formed with a significantly higher crustal contribution (22–68 %) because at that time the Andean crust was thicker and hotter and, therefore primed for more extensive crustal melting. The rhyolitic lower-volume ignimbrites are more equally distributed in the CVZ in time and space and are produced by mechanisms similar to those operating below large stratovolcanoes, but at times of higher melt fluxes from the mantle wedge.

Journal ArticleDOI
01 Oct 2015-Geology
TL;DR: The formation of platinum-group minerals (PGM) during magma differentiation has been suggested to be an important process in primitive magma evolution, but decisive textural evidence is difficult to obtain because PGM tend to be very small and very rare as discussed by the authors.
Abstract: The formation of platinum-group minerals (PGM) during magma differentiation has been suggested to be an important process in primitive magma evolution, but decisive textural evidence is difficult to obtain because PGM tend to be very small and very rare. We have investigated Cr-spinel phenocrysts from two oxidized magmas (Siberian meimechite and Vanuatu [Ambae Island] arc picrite) and one reduced magma (Uralian [Russia] ankaramite) for PGM inclusions and their platinum-group element (PGE) contents. We observed Os-Ir and Pt-Fe alloys entrapped as inclusions in Cr-spinel in all three suites of lava. The alloys may occur in association with PGE-bearing sulfides and co-trapped silicate melt. Cr-spinel crystals also contain measurable amounts of Os, Ir, Ru, and Rh, which are at concentrations 2×–100× higher than mantle values. Thermodynamic models indicate that the arc picrite and ankaramite melts were probably both saturated with the observed PGM phases, whereas the Os-Ir alloy grain observed in the meimechite is not in equilibrium with the "bulk" melt. Our results demonstrate that PGM (alloys and sulfides) occur as liquidus phases in primitive (unfractionated) melts at high temperature and at a variety of redox conditions, and that Cr-spinel is a significant host of PGE, either in the crystal structure or as PGM inclusions.

Journal ArticleDOI
TL;DR: In this article, the crack initiation threshold and peak strength are influenced by the ratio of the area of fine grain groundmass (grains smaller than 200μm) to amphibole and plagioclase phenocrysts.

Journal ArticleDOI
TL;DR: In this paper, a series of phase equilibrium experiments were conducted under pure-H2O fluid-saturated conditions in a cold-seal pressure vessel between 30 and 300 MPa and 750 and 950 C. The results showed that the plagioclase population in each obsidian sample could have grown from their respective melts, with the exception of a single calcic core (An60-63) in one sample.
Abstract: Three crystal-poor obsidian samples (one dacite, 67 wt % SiO2; two rhyolites, 73 and 75 wt % SiO2), which erupted effusively from monogenetic vents, contain sparse (<2%) plagioclase phenocrysts that span a remarkably wide and continuous range in composition ( 30 mol % An). Many, but not all, of the plagioclase crystals display diffusion-limited growth textures (e.g. swallow-tails, skeletal, vermiform). Hypotheses to explain the paradox of a wide compositional range despite a low abundance of plagioclase include (1) incorporation of xenocrysts and/or magma mingling, (2) slow crystallization of plagioclase driven by slow cooling in a magma chamber, (3) slow crystallization of plagioclase followed by a resorption (e.g. heating) event, and (4) crystallization driven by rapid degassing (i.e. loss of melt H2O)6 rapid cooling during ascent. To test these hypotheses, a series of phase equilibrium experiments were conducted under pure-H2O fluid-saturated conditions in a cold-seal pressure vessel between 30 and 300 MPa and 750 and 950 C. The results show that the plagioclase population in each obsidian sample could have grown from their respective melts, with the exception of a single calcic core (An60–63) in one sample. The results additionally rule out slow cooling in a magma chamber, because this would lead to equilibrium abundances of plagioclase ( 20%), which are far higher than what is observed in the samples (<2%). Nor can resorption (i.e. heating) explain the low abundance of plagioclase, because this would eliminate the more sodic plagioclase crystals and hence the wide compositional range of plagioclase that is observed. The most viable hypothesis is that the sparse plagioclase phenocrysts grew relatively rapidly during magma ascent to the surface; this is consistent with the results of isothermal (850 C) continuous decompression experiments (2 9, 1 0, 0 8, and 0 1 MPa h), under pure-H2O fluid-saturated conditions, which were performed on one of the rhyolites (MLV-36; 73 wt % SiO2) and quenched at PH2O1⁄489, 58 and 40 MPa. The four decompression rates correspond to degassing rates of 1 6, 0 56, 0 45 and 0 06 wt % H2O per day. Decompressions 1 0 MPa(PH2O) h, initiated above the liquidus, quenched to 100% glass at all final PH2O. Decompressions at 0 8 MPa(PH2O) h, also initiated above the liquidus, led to plagioclase crystals nearly five times larger than those grown in runs decompressed at the same rate, but initiated just below the plagioclase-in curve. It is the kinetic hindrance to nucleation that permits crystal growth to be concentrated on relatively few crystals, leading to larger crystals. Plagioclase growth rates from these experiments show that the largest phenocrysts ( 1 mm) in the MLV-36 obsidian could have grown in <42 h. A cooling rate of 1 2 C h closely matches both the increase in melt viscosity with time and the effective undercooling with time that occurs during the 0 8 MPa(PH2O) h decompression over the first 50 h. The combined results point to crystallization of sparse plagioclase driven by relatively rapid rates of degassing6 cooling VC The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com 981 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2015, Vol. 56, No. 5, 981–1006 doi: 10.1093/petrology/egv025

Journal ArticleDOI
TL;DR: In this paper, the authors presented new geochemical (bulk rock), petrologic (phenocrysts and their hosted melt inclusions) and geochronological (Ar-Ar) data from Late Paleozoic to Cenozoic volcanic rocks from the Tienshan-Junggar region cropping out in the territories of Kyrgyzstan, Kazakhstan, and China.

Journal ArticleDOI
TL;DR: Petrography and bulk rock, mineral and glass analyses of eruptive products of the 2012-13 eruption of Tolbachik volcano, Central Kamchatka Depression, Russia are reported in this article.

Journal ArticleDOI
01 Apr 2015-Lithos
TL;DR: The Wajilitage nephelinite in the northwestern margin of the Tarim large igneous province (TLIP), Xinjiang, NW China has porphyritic textures with clinopyroxene, nepheline and olivine as major phenocryst phases, together with minor apatite, sodalite and alkali feldspar as discussed by the authors.

Journal ArticleDOI
TL;DR: The South Soufriere Hills Volcano on Montserrat is a type locality for such hybridization processes and yet, just 3?km south of the crater, voluminous basalts have erupted from the South Soufsriere Hill volcano within the same time period as the Soufsyre Hills Volcano was erupting hybrid andesites (131-128?ka).
Abstract: Mafic magmas are common in subduction zone settings, yet their high density restricts their ascent to the surface. Once stalled in the crust, these magmas may differentiate, and assimilate crust and other melts and crystal mushes to produce hybridized intermediate magmas. The Soufriere Hills Volcano on Montserrat is a ‘type locality’ for such hybridization processes and yet, just 3?km south of the crater, voluminous basalts have erupted from the South Soufriere Hills volcano within the same time period as the Soufriere Hills Volcano was erupting hybrid andesites (131–128?ka). Basaltic South Soufriere Hills magmas have 48–53?wt % SiO2 and 4–6?wt % MgO. They were hot (970–1160°C), volatile-rich (melt inclusions contain up to 6·2?wt % H2O) and were stored at 8–13?km depth prior to eruption (based on olivine- and pyroxene-hosted melt inclusion volatile geochemistry). Melt inclusions do not preserve basaltic liquids: they are andesitic to rhyolitic in composition, related to one another by a line of descent controlled by simple closed-system fractionation. Whole-rock compositions, however, are best described by a hybridization model involving ‘back-mixing’ of andesitic to rhyolitic melts with mafic crystal phases such as magnetite, olivine, orthopyroxene and clinopyroxene. Phenocryst zoning illustrates repeated mixing events between evolved melts and mafic phenocrysts; this feature, when coupled with the heterogeneity of crystal compositions, strongly suggests that although the bulk compositions are basaltic (containing Fo80 olivine), they were assembled from disparate ingredients, probably derived from mafic crystal mushes and more evolved melt lenses of variable composition. The mixing events occur days to weeks prior to eruption. We propose that the South Soufriere Hills basaltic magmas, with their higher bulk density relative to andesites from neighbouring volcanoes, ultimately may have been eruptible owing to both the transtensional tectonics imposed by offshore grabens (related to oblique subduction in the Lesser Antilles arc) and surface unloading caused by large-scale edifice collapse. Our observations support the idea that compositional changes in arcs might reflect not only changes in source compositions, but also effects caused by variations in crustal strain and tectonics.

Journal ArticleDOI
TL;DR: In this paper, the authors used phenocryst compositions and mineral-liquid thermobarometers to determine the P-T conditions of the magma reservoirs where crystallization occurred, and then use these data to reconstruct the MAGMA plumbing system of the Emeishan large igneous province of SW China.
Abstract: Magmatic activity of the Emeishan large igneous province (ELIP) of SW China is one of the most significant geological events of the late Paleozoic. The large volume flood basalts plus rare picrites were erupted in Late Permian. Previous studies indicate that the basalts are the derivatives of primary mantle-derived magma by fractional crystallization, but the depths at which this process took place remain unknown. To answer this question, we use phenocryst compositions and mineral-liquid thermobarometers to determine the P-T conditions of the magma reservoirs where crystallization occurred, then use these data to reconstruct the magma plumbing system of the igneous province. Thermobarometric calculations show that most picrite-hosted clinopyroxene phenocrysts crystallized at ~25 km and 1200–1280 °C, whereas most basalt-hosted clinopyroxene phenocrysts crystallized at depths <20 km and temperatures <1200 °C. Some picrites containing primitive olivine with Fo up to Fo92 likely formed by eruption of the most primitive magma with composition similar to the primary magma from the deepest reservoir possibly at the Moho. Parental magmas yield mantle potential temperatures of 1740–1810 °C, which are the highest such temperatures yet recorded for terrestrial magmas of any age. Less primitive picrites containing both olivine and clinopyroxene phenocrysts formed by eruption of moderately fractionated magma from a reservoir in the middle crust. Basalts and basaltic andesites formed by eruption of the most fractionated magmas from the reservoirs in the upper crust, coinciding with the depths of coeval sulfide ore-bearing and Fe-Ti-V oxide ore-bearing mafic-ultramafic intrusions. The reason that the Emeishan volcanic sequence is dominated by basalts is because most of the mantle-derived magma was trapped in the middle and upper crusts, undergoing variable degrees of crystal fractionation plus crustal contamination before eruption. Primitive picrites are rare because their eruption requires a trans-lithosphere conduit, which is difficult to create and maintain due to increasing lithospheric pressure with depth. The results from this study reveal that magma reservoirs at the crustal levels play a critical role in magma differentiation in a continental setting.

Journal ArticleDOI
TL;DR: In this article, an inverse calculation based on the water content of clinopyroxene (cpx) phenocrysts, the ivAlcpx-dependent water partitioning coefficient Dwatercpx/melt, and secondary ion mass spectrometer was performed on the Fuyanyshan (FYS) volcano in Shandong, eastern China.
Abstract: The magma water contents and cpx δ18O values in alkali basalts from the Fuyanyshan (FYS) volcano in Shandong, eastern China, were investigated by an inverse calculation based on the water content of clinopyroxene (cpx) phenocrysts, the ivAlcpx-dependent water partitioning coefficient Dwatercpx/melt, and secondary ion mass spectrometer, respectively. The calculated water content (H2O wt.) of magma ranges from 0.58% to 3.89%. It positively correlates with heavy rare earth element concentrations and bulk rock 87Sr/86Sr ratios, and it negatively correlates with Nb/U ratios. However, it is not correlated with bulk Mg# (Mg# = 100 × Mg / (Mg + Fe)) and (La/Yb)n (n represents primitive mantle normalization). Combined with the rather homogenous distribution of water content within cpx grains, these correlations indicate that the water variations among different samples represent the original magma signature, rather than results of a shallow process, such as degassing and diffusion. The δ18O of cpx phenocrysts varies from 3.6‰ to 6.3‰ (±0.5‰, 2SD), which may be best explained by the involvement of components from the lower and upper oceanic crust with marine sediments within the mantle source. The H2O/Ce ratios of the calculated melts range from 113 to 696 and form a positive trend with bulk rock 87Sr/86Sr, which cannot be explained by the recycled Sulu eclogite or by the metasomatized lithospheric mantle. Our modeling calculation shows that the decoupling of eHf and eNd could be caused by the involvement of marine sediments. Combing the high Ba/Th ratios, positive Sr spikes, and low Ce/Pb ratios for the Fuyanshan basalts, we suggest that the hydrous nature of the FYS basalts was derived from the hydrous mantle transition zone with ancient sediments.

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TL;DR: In this article, an Electron microprobe and LA-ICP-MS compositions were used to study the magmatic and hydrothermal processes and the role of F in Sn mineralization.

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TL;DR: In this paper, high-resolution investigation of mineral textures, zoning and chemistry as well as whole-rock geochemical analyses were performed on stratigraphically controlled samples in order to characterize the magmas represented by the distinct eruptive units and to reveal the evolution of the deep magmatic system.

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TL;DR: In this paper, textural features, compositional parameters, and pre- and syn-eruptive P,T conditions between basaltic lavas of the Caburgua-Huelemolle Small Eruptive Centers (CHSEC) and the 1971 Basaltic andesite lava of the Villarrica Volcano located 10 km south of the CHSEC were compared.

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TL;DR: In this paper, the No. 431 dike is characterized in detail by electron microprobe and wet-chemical analysis, which provides a detailed framework for future studies that employ state-of-the-art analytical techniques.

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15 Jun 2015-Lithos
TL;DR: The Xiaoling lavas were derived from a pyroxenitic mantle source, which may have been formed by the solid-state reaction between recycled crustal materials and their surrounding peridotites as mentioned in this paper.

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TL;DR: The Mahoor Cu-Zn-bearing porphyritic granitoid rocks belong to the Lut Block volcanic-plutonic belt (central Eastern Iran) as discussed by the authors.

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TL;DR: In this paper, the authors performed in-situ oxygen isotope analysis on olivine grains in these ultrapotassic rocks using secondary ionization mass spectrometry (SIMS).

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TL;DR: The plutonic lithic clasts from the 0.7 ka Kaharoa eruption at the Tarawera volcano (Okataina Volcanic Complex, Taupo VOLCANIC Zone, New Zealand) provide insight into the processes of rhyolite formation.
Abstract: Granitoid lithic clasts from the 0.7 ka Kaharoa eruption at the Tarawera volcano (Okataina Volcanic Complex, Taupo Volcanic Zone, New Zealand) provide insight into the processes of rhyolite formation. The plutonic lithic clasts of the Kaharoa eruption consist of (1) quartz phenocrysts, which are often grouped into clusters of two to eight quartz grains, (2) plagioclase phenocrysts (mostly ~An 40 with up to An 60 cores), and (3) interstitial alkali feldspar. Quartz orientations obtained through electron backscatter diffraction (EBSD) methods show that 78% of the 82 analyzed clusters have at least one pair of quartz grains with the dominant dipyramidal faces matched. Variations in cathodoluminescence (CL) zoning patterns of the quartz suggest that quartz clusters came together after initial crystal growth and that many quartz crystals were subject to one or more resorption events. The process of quartz crystals with different magmatic histories coming together into common relative orientations to form clusters is indicative of oriented quartz synneusis and suggests a history of crystal accumulation. The quartz clusters are interpreted to have formed as part of a crystal cumulate mush within a shallow magma chamber where quartz crystals rotated into contact along their dominant dipyramidal faces during hindered settling and/or compaction. The preservation of oriented quartz clusters from the Kaharoa plutonic lithics thus provides evidence for synchronous, shallow pluton formation from a cumulate mush during active silicic volcanism. This result is consistent with models whereby melt-rich, high-silica rhyolite formation occurs via interstitial melt extraction from a low-silica rhyolite mush in the shallow crust.