Phenocryst Minerals Research Articles

Progressive build-up of a trans-crustal system beneath an adakite-like volcanic complex (Chachimbiro, Ecuador): an example of an embryonic porphyry Cu system?

Abstract Arc magmas display global trends of increasing adakite-like indices (e.g., Sr/Y, La/Yb) with increasing crustal thickness, which are interpreted as the result of an increasingly deeper evolution of the magmas in a thick crust. Several volcanic edifices in continental arcs display a transition from normal to adakite-like magmas during their geologically short lifetimes and are precious examples to understand in detail how adakite-like signatures are acquired by magmas in thick continental arcs. Understanding the temporal transition from normal to adakite-like magmas has important implications on fundamental geological processes that are associated with adakite-like magmas, like the genesis of porphyry Cu deposits. The Quaternary Ecuadorian arc hosts numerous volcanic edifices featuring this transition during the last ~1 Ma, among which the Chachimbiro Volcanic Complex (CVC). The CVC records a history of effusive and explosive eruptions during the last ~400 ka that is characterized by progressively increasing adakite-like indices (e.g., Sr/Y, La/Yb), similar to that observed in magmatic systems associated with supergiant porphyry Cu deposits. It is therefore a suitable example to investigate the magmatic processes associated with these changes and their potential implications for the formation of porphyry Cu deposits. Here, we provide an extensive dataset on major and trace element geochemical compositions of the three main phenocryst minerals (pyroxene, amphibole, plagioclase) of the CVC. We retrieve thermobarometric data of amphiboles and pyroxenes and discuss the occurrence of different compositional clusters of the three phenocryst minerals in the frame of the ~400 ka temporal evolution of the CVC. Our data show that the oldest products of CVC, andesitic lava flows of the CH1 unit, were the result of staging of mantle-derived magmas in the lower crust and subsequent establishment of an upper crustal magma reservoir where plagioclase- and pyroxene-dominated fractionation occurred. After a magmatic lull of ~180 ka, volcanic activity resumed with effusive and explosive products of the CH2 and CH3 units characterized by more felsic compositions (high-SiO2 andesite to dacite). Thermobarometric data and contrasting REE patterns of amphiboles suggest sampling by magma coming from depth of an extensive mid- to upper crustal system at this time. The CH4 unit (~6 ka) consists of pyroclastic products which have the most evolved (rhyodacitic) composition of the whole CVC. Thermobarometric data and REE patterns of amphiboles suggest that also at this stage magmas ascending from depth sampled an extensive transcrustal system from mid- to upper crustal levels. For all evolutionary stages of the CVC, bulk rocks convey a signature that corresponds to a deeper-seated magmatic differentiation compared to magmas in equilibrium with phenocrystic minerals, which crystallized in mid- to upper crustal portions of the transcrustal system and were mechanically incorporated by magmas ascending from depth. Our study documents the progressive build-up of a transcrustal system over 400 ka during the transition to adakite-like magmatism favourable to porphyry Cu deposit mineralisation, which could represent an embryonic porphyry-related magmatic system.

Geology of the Kebasen Paleovolcano in Banyumas Regency, Central Java Province, Indonesia

Abstract The new interpretation on geology of Kebasen district in Central Java, Indonesia which is located in the middle parts of Sunda Arc indicates the existence of a paleovolcano. This conclusion is achieved through 1:25,000 geological mapping with a focus on volcanic facies, characterization of lithology types, mineralogy, and fossil analysis, to clarify its volcanic history and paleo-environment. Laboratory analysis works on representative rock samples are done using petrography and fossil identification on calcareous sedimentary members. Our research results conclude that Kebasen paleovolcano was a composite volcano with a diameter of about 8 km, elongated in NW-SE orientation, with Kebasen city located at the SW rim of the old volcanic edifice. The volcanic history and resulting volcanic rocks consisted of two contrasting volcanic stages. The first one was explosive caldera-forming eruptions that produced thick and partly welded pyroclastic rocks, dominated by pyroclastic breccia at the lower parts and finer tuff members at the upper parts. After caldera-forming eruptions, volcanic activity was followed by effusive eruptions of basaltic to andesitic lavas that covered the caldera floor. The lavas have abundant pillow structures indicating a submarine environment, intermediate to basaltic composition, and porphyritic texture, with plagioclase and pyroxene as the most abundant phenocryst minerals. Available rock geochemistry data demonstrate that lavas have mafic to intermediate compositions, with magma affinity at the transition of calc-alkaline to tholeiitic series. Results of geomorphological and lithology association analysis conclude that the research area was part of the central to distal facies of a paleovolcano complex, with the paleo depositional environment in the open marine environment. The results of fossil analysis on calcareous sedimentary rocks between volcanic layers indicate that the age of volcanism was Early Pliocene (N18) with the paleobathymetry transitioning from upper-middle bathyal to outer–middle neritic, which were parts of the continental margin. It is concluded that Kebasen paleovolcano is a newly identified submarine caldera-type Pliocene volcano within the middle parts of the Sunda Arc in Central Java, Indonesia.

Cluster Analysis of Mineral-chemical Data of Basaltic Rocks from Linga, Eastern Deccan Volcanic Province (EDVP) and its Petrogenetic Implications

Abstract Cluster analysis studies based on electron microprobe data of phenocryst mineral phases (namely olivine, clinopyroxene and plagioclase) of basaltic rocks around Linga (21° 58' N: 78° 56' E), eastern Deccan volcanic province have been carried out for the first time. Based on highly significant correlation coefficients obtained for relevant major oxides of phenocrystal mineral phases, it is suggested that mineralogical parameters such as Fo% in olivine, Ca/Mg# in clinopyroxene and An% of plagioclase play important role to decipher the evolution of the phase chemistry. Those highly significant major oxides were subjected to multivariate statistical analysis, involving hierarchical segmentation, cluster analysis and construction of dendrograms. For each of the minerals, the clusters decrease systematically. The ultimate cluster patterns may either correspond to liquid immiscibility-related intraflow chemical variation or to multiple staged magmatic emplacements.

The dynamics of the Sorol Trough magmatic system: Insights from bulk‐rock chemistry and mineral geochemistry of basaltic rocks

The Yap trench‐island arc system, where three tectonic plates (i.e., the Philippine Sea Plate, Pacific Plate, and Caroline Plate) interact, has undergone a relatively complicated geological evolution. East of the system, the Caroline Plateau are being subducted, and the plateau has been rifted into east and west Caroline ridges by the development of the Sorol Trough. However, until now, knowledge of the nature of the basement of the Sorol Trough is lacking, which limits our understanding of the complete tectonic evolution of the Yap island arc‐trench system and the Caroline Plateau. In this study, we performed petrological and mineral geochemical (in situ major element compositions and trace element compositions) analysis of newly acquired subducting basalt samples from the Sorol Trough segment, which is being subducted along the Yap Trench, and K‐Ar age of the Sorol basalts is 23.8 ± 0.7 Ma. The results show that phenocryst minerals of basalt from Sorol Trough are mainly olivine and plagioclase, with minor clinopyroxene microlites. The forsterite (Fo) values of the olivine phenocrysts in these rocks vary from 84.97 to 86.85, and plagioclase (Pl) phenocrysts are predominantly labradorite, with a few andesine plagioclase microlites. Bulk‐rock chemical compositions confirm that these rocks are intraplate alkaline basalts and show ocean island basalt (OIB)‐like geochemical characteristics. The mantle source of these basement basalts could be peridotite, and these Sorol Trough basalts may have been formed by 5–10% partial melting of mantle spinel lherzolite. The potential temperature of the mantle beneath the Sorol Trough was inferred to be 1,331–1,393°C, shows that there is no thermal anomaly in the mantle source. In addition, the crystallization temperatures of the clinopyroxenes (1,068–1,263°C) and plagioclase (702–1,216°C) and their compositional characteristics indicate that the magmatic processes have a conspicuously fast rate of magma upwelling. In conclusion, the basalts in the Sorol Trough were the products of low‐degree partial melting of a relatively enriched mantle source that passively upwelled as the Sorol Trough separated the Caroline Ridge into its western and eastern parts.

Apatite fingerprints on the magmatic-hydrothermal evolution of the Daheishan giant porphyry Mo deposit, NE China

Abstract Porphyry deposits are the main source for global Cu and Mo production. The generation of hydrous silicate magmas and subsequent separation of volatile-rich magmatic fluids with hydrothermal alteration are significant processes leading to the formation of porphyry deposits. However, a specific understanding of these processes has been limited by a lack of direct mineralogical records in the evolving magmatic-hydrothermal system. In this paper, we present an integrated textural and geochemical investigation on apatite from the giant Daheishan porphyry Mo deposit in NE China, illustrating that apatite can be a potential recorder of the magmatic-hydrothermal evolution of porphyry systems. Apatite from the ore-forming porphyry displays distinctive core-rim textures, with melt inclusions in the resorption cores (Type-A1) and co-existing of melt and fluid inclusions in the euhedral rims (Type-A2), indicating a magmatic-hydrothermal origin of apatite. This is also supported by both chemical and isotopic compositions obtained by in situ analyses using laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and LA-multi collector-ICP-MS. The late Type-A2 apatite is relatively enriched in incompatible elements, such as rare earth elements (REE) and Th, but slightly depleted in fluid-mobile elements such as Na and S, compared to the early Type-A1 apatite. Relatively homogeneous (87Sr/86Sr)i ratios (0.70436–0.70504) of the Type-A1 and Type-A2 apatites indicate that they were formed in a relatively closed system without detectable contamination. Meanwhile, some apatite in the wall rock (biotite granodiorite) shows characteristics of secondary altered textures, resulting from the intensive alteration by hydrothermal fluids exsolved from the porphyry system. Apatite trapped in mineral phenocrysts of the wall rock is usually unaltered (Type-B1 apatite), with clear oscillatory growth zones in cathodoluminescence (CL) images. In contrast, the intergranular apatite is commonly altered (Type-B2 apatite), with chaotic zoning in CL images, abundant micro-fractures and secondary fluid inclusions. Compositionally, the Type-B2 apatite shows notable tetrad REE patterns, relatively lower light-REE and S contents, and elevated 147Sm/144Nd ratios compared to the Type-B1 apatite. LA-ICP-MS U-Pb dating yields a lower intercept age of 171.4 ± 2.3 Ma for Type-B2, which is consistent with the age of 171.5 ± 2.4 Ma for Type-A2, but is notably younger than the Type-B1 apatite (175.5 ± 1.3 Ma). It is suggested that the Type-B2 apatite has been significantly reset by hydrothermal fluids exsolved from the porphyry system. Therefore, we conclude that the textures and geochemistry of apatite in porphyry systems can be used as a potential proxy for recording fluid exsolution and hydrothermal alteration processes.

In-situ chemistry of plagioclase and amphibole phenocrysts of Mt. Lamington volcano in Papua New Guinea: Evidence for influence of Woodlark spreading ridge to Papuan arc

Mt. Lamington is a Quaternary volcano located on the Papuan Peninsula, SE Pacific. Texture studies and in-situ chemistry analyses on mineral phenocrysts are performed on andesite and its enclaves and pre-erupted shoshonite to better understand the petrogenesis and tectonic settings of the volcano. Both amphibole and plagioclase phenocrysts have variable zoning textures in the three major lithologies. Although the andesite and enclaves have similar bulk 87Sr/86Sr ratios (~0.7039), their plagioclase phenocrysts show varied in-situ87Sr/86Sr: nging from 0.70360 to 0.70519 in andesite (An26–58) and from 0.70373 to 0.70393 in enclaves (An31–61). The shoshonite shows a bulk 87Sr/86Sr ratio of ~0.7037, and plagioclase phenocrysts in it show in-situ87Sr/86Sr ratios ranging from 0.70386 to 0.70409 (An27–57). A synthetic analysis of in-situ plagioclase strontium isotopes of the volcano, as well as bulk strontium and neodymium isotopes data from adjacent areas indicates three components: one high-143Nd/144Nd and low-87Sr/86Sr depleted mantle source represented by the most depleted samples of the Woodlark oceanic crust, one low-143Nd/144Nd and medium-87Sr/86Sr enriched mantle source represented by arc-type volcanics, and a third relatively low-143Nd/144Nd and high-87Sr/86Sr source represented by melts from arc crustal precursors. A spatial negative correlation of 143Nd/144Nd ratios of volcanics from east (Woodlark Basin) to west (Mt. Lamington) indicates that the magma sources have changed from depleted mantle to recently enriched lithospheric mantle plus contamination of the Papuan arc crust. This relationship agrees with a hypothesis that Mt. Lamington volcano represents the west extension of the Woodlark spreading ridge onto the Papuan arc, which in turn was a result of subduction from north.

Evolution of the calc-alkaline magma feeding system of the Komakusadaira pyroclastics in Zao volcano, NE Japan

We examined the evolution of the magma feeding system at Zao volcano (NE Japan) during seven episodes of Komakusadaira pyroclastic activity (ca. 33–12.9 ka). The rocks, which are calc-alkaline and medium-K basaltic andesite to andesite (54.9–59.5% SiO2), are herein suggested to have been derived from two main magmatic end-members: silicic andesite magma (59~60% SiO2, 940~970°C, cpx-opx-plg as phenocrysts) and basaltic magma (52% SiO2, ~ 1085°C, Fo78~80 olv-plg as phenocrysts). These two end-members retained their compositional integrities throughout all seven eruptive episodes. The presence of Mg-rich (Fo> 80) olivine phenocrysts suggests the basaltic end-member was derived from fairly primitive magma (> 1100°C), which differentiated during ascent. By examining temporal variations in the chemical compositions of the phenocrystic minerals, three evolutionary pathways that produced the erupted products are defined; these are reflected in seven sequential episodes: (1) Crystal-rich silicic andesite magma was repeatedly withdrawn from the reservoir by the forced injection of basaltic magma (episode 1). (2) Crystal-poorer intermediate magma was formed by effective mixing of injected basaltic magma. Subsequent eruption of the intermediate magma was triggered by a new injection of mafic magma. The percentage of intermediate magma involved in the eruptions decreased over time (episodes 2–4). (3) By repeated fairly primitive to basaltic magma injections, the percentage of the mobilized part of the silicic andesite magma involved in the erupted magma gradually increased over time (episodes 5–7).

Persistent gas emission originating from a deep basaltic magma reservoir of an active volcano: the case of Aso volcano, Japan

Volcanic gas emission is considered to reflect the degassing of magma beneath volcanoes. The combined observations of gas measurement and petrological study are expected to constrain the volatile concentrations and storage depths of the pre-eruptive and primitive magma. Aso volcano (Japan) is a constantly-monitored, persistently-degassing volcano, and an ideal site to acquire gas and petrologic data. We analyzed the melt inclusions and phenocryst minerals of Holocene basaltic eruption products, and reported their major and volatile element concentrations. The samples showed abundant evidence of magma mixing, such as reverse mineral zoning, and highly variable mineral and glass compositions. SiO2 measured in melt inclusions varied from 46.0–65.8 wt. %. High-volatile concentration, S up to 3750 ppm, was only found in mafic melt inclusions hosted by high-Fo olivine phenocrysts (~ Fo82). The pre-eruptive storage depths were determined from volatile concentrations: 2 and 4 km depth for Strombolian eruption and sub-Plinian eruption, respectively. The volatile-rich primitive magma, one end member of the mixed-magma, originated from a deeper level (> 10 km) than these magma reservoirs. Initial volatile concentrations of the primitive magma were determined using multiple constraints: > 4.68 wt. % H2O, 400—750 ppm CO2, 3750 ppm S, 716 ppm Cl, and 324 ppm F. The observed variation of volcanic gas composition was best explained by the mixing of the gas segregated from at least a depth of 10 km, with that from the shallow reservoirs. This study illustrated the method to identify the primitive mafic magma responsible for deep volatile flux in a mature volcano with complex magmatic processes.

Caleta el Cobre 022 Martian meteorite: Increasing nakhlite diversity

AbstractCaleta el Cobre (CeC) 022 is a Martian meteorite of the nakhlite group, showing an unbrecciated cumulate texture, composed mainly of clinopyroxene and olivine. Augite shows irregular core zoning, euhedral rims, and thin overgrowths enriched in Fe relative to the core. Low‐Ca pyroxene is found adjacent to olivine. Phenocrysts of Fe‐Ti oxides are titanomagnetite with exsolutions of ilmenite/ulvöspinel. Intercumulus material consists of both coarse plagioclase and fine‐grained mesostasis, comprising K‐feldspars, pyroxene, apatite, ilmenite, Fe‐Ti oxides, and silica. CeC 022 shows a high proportion of Martian aqueous alteration products (iddingsite) in olivine (45.1 vol% of olivine) and mesostasis. This meteorite is the youngest nakhlite with a distinct Sm/Nd crystallization age of 1.215 ± 0.067 Ga. Its ejection age of 11.8 ± 1.8 Ma is similar to other nakhlites. CeC 022 reveals contrasted cooling rates with similarities with faster cooled nakhlites, such as Northwest Africa (NWA) 817, NWA 5790, or Miller Range 03346 nakhlites: augite irregular cores, Fe‐rich overgrowths, fine‐grained K‐feldspars, quenched oxides, and high rare earth element content. CeC 022 also shares similarities with slower cooled nakhlites, including Nakhla and NWA 10153: pyroxene modal abundance, pyroxenes crystal size distribution, average pyroxene size, phenocryst mineral compositions, unzoned olivine, and abundant coarse plagioclase. Moreover, CeC 022 is the most magnetic nakhlite and represents an analog source lithology for the strong magnetization of the Martian crust. With its particular features, CeC 022 must originate from a previously unsampled sill or flow in the same volcanic system as the other nakhlites, increasing Martian sample diversity and our knowledge of nakhlites.

Petrogenesis of volcanic rocks from Eastern Manus Basin: indications in mineralogy and geochemistry

To understand the petrogenesis and magma evolution history in the Eastern Manus Basin (EMB), geochemistry of pyroxene and plagioclase mineral phenocrysts in basaltic andesites and dacites were reported. The plagioclase-melt thermometry showed that, plagioclase in dacites crystalized in 1 027.2–1 028.5°C under 3.37–5.08 kbar, whereas in basaltic andesite was 1 181.5–1 187.0°C under 1.79–4.46 kbar. Pyroxene compositions and invariable La/Sm vs La values of whole rock powders indicate that lavas were erupted in rapid cooling rate and mainly controlled by fractional crystallization (FC). In addition, oscillatory zonings in plagioclase and pyroxene phenocrysts indicated small local perturbations and degassing episodes in the magma chamber. Some high Mg# clinopyroxene antecrysts were found in EMB lavas. The highest Mg# of parental EMB melts is 69, which falls into the range of initial partial melts from upper mantle peridotite source (68–75). In terms of isotopic compositions, the EMB lavas most likely originated from Indian-type MORB mantle which was influenced by subduction components. In details, the subduction components are mainly derived from the dehydration of a subducted altered oceanic crust, and the contribution of sediment influence is minor. The Pb isotopic compositions and end member modeling further suggest that the source of subduction components is more likely from the Pacific Plate instead of the Solomon Plate.

An Experimental Investigation of Interaction between Andesite and Hyperacidic Volcanic Lake Water

Alteration in magmatic-hydrothermal systems leads to distinct changes in rock texture and mineralogy, and a strong redistribution of elements between fluid and rock. Here, we experimentally interacted andesite scoria with hyperacidic, high-sulfidation style fluids from Kawah Ijen volcano (Indonesia) at 25 and 100 °C, seeking to reproduce the textures observed in natural samples from this volcano, and to understand the element fluxes that accompany alteration. The susceptibility to alteration in the experiments is Cu–Fe-sulphide > calcic plagioclase > pyroxene > titano-magnetite > sodic plagioclase, with complete preservation of glass. Silicate minerals alter to opaline silica, and gypsum, barite and a Zr-phase precipitate. The selective alteration of the phenocryst minerals results in a preferential release of compatible elements, as the glass is the main incompatible element host. The experiments reproduce the alteration textures of the natural samples, including the preservation of glass, but the predicted compatible over incompatible element enrichment in the alteration element flux is not observed in the natural setting. This suggests that alteration at Kawah Ijen is dominated by lithologies that lack abundant glass, in particular lava flows where the glass has devitrified, despite these lava flows having a lower surface area compared to scoria.

Geochronology and petrogenesis of Jurassic intraplate alkali basalts in the Junggar terrane, NW China: Implication for low-volume basaltic volcanism

High-volume volcanic fields which are observed at continental intraplate setting are generally known as the large igneous provinces (LIPs), fed by hot and active plumes upwelling from the deep mantle. However, the source and mechanism of low-volume effusive volcanism within continental interiors remains poorly unknown. Here, we present a combined study of 40Ar/39Ar geochronology, mineral chemistry, whole-rock major and trace elements as well as Sr-Nd isotopes of Jurassic basalts from the Karamay area, Junggar terrane (NW China), aiming to determine their formation ages, constrain the petrogenesis and reveal their tectonic implications.New whole-rock 40Ar/39Ar dating yields consistent ages of 189.4–193.3 Ma for magma emplacement. The basalts exhibit a columnar joint structure and porphyritic texture with phenocryst minerals of olivine (Fo63–79), clinopyroxene (Wo39–46En38–47Fs12–22) and plagioclase (An25–54). They have SiO2 contents ranging from 45.0–51.8 wt% and belong to alkaline series (δ2.7–13.3, average of 5.0). The basalts are characterized by oceanic island basalt (OIB)-like trace element distribution patterns with enrichment in L-MREE, HFSE (e.g., Nb and Ta) and LILE (e.g., K, Sr and Ba), and slight depletion in HREE, relative to normal mid-ocean ridge basalt (N-MORB). Positive εNd(t) (+3.0), low to moderate 87Sr/86Sri (0.7048–0.7049) isotopic compositions, and trace element ratios (e.g., high Nb/U > 49.0 and Ce/Pb > 13.9) suggest that crust contamination was insignificant in the formation process. P–T estimates of major phenocryst minerals in these basalts reflect high crystallization temperatures (>1100 °C) but low pressures (<9.8 kbar). Geochemical features and REE modeling indicate that the basaltic magmas were likely derived by low degrees of partial melting (~5% – 10%) of an asthenospheric mantle source in the garnet stability field. Calculated lithospheric thickness and initial melting depths for the studied basalts are about 120 km and 180–200 km, respectively. Based on the available data and regional secular geological evolution, we suggest that decompression melting of upwelling asthenosphere triggered by small-scale removal of thickened lithospheric root could be a possible mechanism to explain Jurassic low-volume intraplate volcanism in the Junggar terrane.

Chronology of Magmatic Activity and Petrologic–Mineralogical Characteristics of Lavas of Kazbek Quaternary Volcano, Greater Caucasus

The paper presents detailed isotope-geochronological, geological, and petrologic–mineralogical data on lavas of one of the greatest Quaternary magmatic area in the Greater Caucasus, the Kazbek neovolcanic center, including polygenetic Kazbek stratovolcano and a number of subordinate volcanic cones in its vicinities. The research was conducted based on a representative collection of more than 150 geological samples that characterize most of the volcanic cones and lava flows of different age, some of which were known previously, and other were discovered by the authors. The high-precision K–Ar data obtained on these materials make it possible to reproduce the evolutionary history of youngest magmatism at the Kazbek center and evaluate the total duration of this evolution at ~450 ka. The magmatic activity was subdivided into four phases (at 460–380, 310–200, 130–90, and <50 ka) with long-lasting interludes in between. Because the latest eruptions occurred in the Kazbek vicinity in the Holocene, this volcano is regarded as potentially active. The volcanic rocks of the Kazbek center make up a continuous compositional succession of basaltic (trachy)andesite–(trachy)andesite–dacite and mostly belong to the calc–alkaline series. The principal petrographic characteristics of the rocks and the composition of their phenocryst minerals are determined, mineral assemblages of these minerals are distinguished in the lavas of different type, and the temperature of the magmatic melts is evaluated. A principally important role in the petrogenesis of the Kazbek youngest magmas is proved to have been played by fractional crystallization and replenishment of mafic melts in the magmatic chambers beneath the volcano, which resulted in their mixing and mingling with the residual dacite melt and the origin of high-temperature hybrid andesite lavas. The comprehensive geological studies, involving interpretation of high-resolution satellite images, allowed the authors to compile the first detailed (1: 25 000) volcanologic map of the Kazbek center and a geochronologic chart supplemented with a stratigraphic column, which illustrate the origin sequence of the volcanic vents and their lava flows, geological relations between them, as seen in reference geological sections, and variations in the composition of the magmatic products with time.

Petrological and experimental evidence for differentiation of water-rich magmas beneath St. Kitts, Lesser Antilles

St. Kitts lies in the northern Lesser Antilles, a subduction-related intraoceanic volcanic arc known for its magmatic diversity and unusually abundant cognate xenoliths. We combine the geochemistry of xenoliths, melt inclusions and lavas with high pressure–temperature experiments to explore magma differentiation processes beneath St. Kitts. Lavas range from basalt to rhyolite, with predominant andesites and basaltic andesites. Xenoliths, dominated by calcic plagioclase and amphibole, typically in reaction relationship with pyroxenes and olivine, can be divided into plutonic and cumulate varieties based on mineral textures and compositions. Cumulate varieties, formed primarily by the accumulation of liquidus phases, comprise ensembles that represent instantaneous solid compositions from one or more magma batches; plutonic varieties have mineralogy and textures consistent with protracted solidification of magmatic mush. Mineral chemistry in lavas and xenoliths is subtly different. For example, plagioclase with unusually high anorthite content (An≤100) occurs in some plutonic xenoliths, whereas the most calcic plagioclase in cumulate xenoliths and lavas are An97 and An95, respectively. Fluid-saturated, equilibrium crystallisation experiments were performed on a St. Kitts basaltic andesite, with three different fluid compositions (XH2O = 1.0, 0.66 and 0.33) at 2.4 kbar, 950–1025 °C, and fO2 = NNO − 0.6 to NNO + 1.2 log units. Experiments reproduce lava liquid lines of descent and many xenolith assemblages, but fail to match xenolith and lava phenocryst mineral compositions, notably the very An-rich plagioclase. The strong positive correlation between experimentally determined plagioclase-melt KdCa–Na and dissolved H2O in the melt, together with the occurrence of Al-rich mafic lavas, suggests that parental magmas were water-rich (> 9 wt% H2O) basaltic andesites that crystallised over a wide pressure range (1.5–6 kbar). Comparison of experimental and natural (lava, xenolith) mafic mineral composition reveals that whereas olivine in lavas is predominantly primocrysts precipitated at low-pressure, pyroxenes and spinel are predominantly xenocrysts formed by disaggregation of plutonic mushes. Overall, St. Kitts xenoliths and lavas testify to mid-crustal differentiation of low-MgO basalt and basaltic andesite magmas within a trans-crustal, magmatic mush system. Lower crustal ultramafic cumulates that relate parental low-MgO basalts to primary, mantle -derived melts are absent on St. Kitts.

P08 阿蘇火山、Aso-4火砕流堆積物中の弁利スコリア流堆積物の層序的特徴と、斑晶鉱物および石基ガラスの化学組成について(ポスターセッション,日本火山学会2007年秋季大会)

P08 阿蘇火山、Aso-4火砕流堆積物中の弁利スコリア流堆積物の層序的特徴と、斑晶鉱物および石基ガラスの化学組成について(ポスターセッション,日本火山学会2007年秋季大会)

A21 蔵王火山、駒草平火砕岩の斑晶組織・化学組成とマグマ供給系の進化(火山の物質科学(2),日本火山学会2008年秋季大会)

A21 蔵王火山、駒草平火砕岩の斑晶組織・化学組成とマグマ供給系の進化(火山の物質科学(2),日本火山学会2008年秋季大会)

Mineral Chemistry in Volcanic Rocks of Ore Deposit Nochun and Sarcheshmeh Copper Mine, Iran

This paper studies on clinopyroxene, plagioclase of andesitic to dacite rocks of Nochun area to understand the geotectonic and geothermobarometry conditions. Eocene calc-alkaline volcanic rocks are exposed in Nochun that are near the Sarcheshmeh cooper mine. Volcanic rocks consist of andesitic to dacite rocks. Minerals in the volcanic rocks exhibit degrees of disequilibrium features. Plagioclase as dominant mineral in these rocks generally displays oscillatory zoning; sieves or dusty, cores are usually Ca-rich. Petrographic evidence and varying anorthite content (10% to 90%) of plagioclase and temperature estimates of clinopyroxene indicated fractional crystallization condition later than hydrothermal alteration and partial metasomatism occurred. The varying content indicates that the parent’s magma was generated from subalkaline composition and then hydrothermal alteration affected phenocryst minerals and matrix of andesitic to dacite rocks. The equilibration temperatures of clinopyroxene showed (800°C - 1200°C). Thus, in mafic minerals development of sieve textures in plagioclase, reaction rims, zoning and heterogeneity of plagioclase phenocrysts, the resorbed and embayed phenocrysts, unique composition of clinopyroxene phenocrysts; support fractional crystallization at deep reservoir and high pressure condition as an important process in the evolution of these rocks.

Transition from ultra-enriched to ultra-depleted primary MORB melts in a single volcanic suite (Macquarie Island, SW Pacific): Implications for mantle source, melting process and plumbing system

Compositional diversity of basalts forming the oceanic floor is attributed to a variety of factors such as mantle heterogeneities, melting conditions, mixing of individual melt batches, as well as fractionation and assimilation processes during magma ascent and emplacement. In this study the compositional range and origin of mid-ocean ridge basalts (MORB) is approached by petrological, mineralogical and geochemical studies of the Miocene Macquarie Island ophiolite, an uplifted part of the Macquarie Ridge at the boundary between the Australian and Pacific plates. In this study, earlier results on the enriched to ultra-enriched (La/Sm 1.4–7.9), isotopically homogeneous basaltic glasses are complemented by the compositions of olivine-phyric rocks, principal phenocrystic minerals and Cr-spinel hosted melt inclusions. Studied olivine, clinopyroxene and Cr-spinel phenocrysts are among the most primitive known for MORB (85–91mol% forsterite in olivine, 81–91 Mg# in clinopyroxene, and 66–77 Mg# and 34–60 Cr# in spinel) and represent primary and near-primary compositions of their parental melts. Geochemical characteristics of the liquids parental to clinopyroxene (La/Sm 0.8–6.3) and Cr-spinel (La/Sm 0.4–5) partly overlap with those of the basaltic glasses, but also strongly advocate the role of depleted to ultra-depleted primary melts in the origin of the Macquarie Island porphyritic rocks. The trace element composition of olivine phenocrysts and the systematics of rare-earth elements in glasses, melt inclusions, and clinopyroxene provide evidence for a peridotitic composition of the source mantle. Our data supports the mechanism of fractional “dynamic” melting of a single mantle peridotite producing individual partial melt batches with continuously changing compositions from ultra-enriched towards ultra-depleted. The incipient enriched melt batches, represented by basaltic glasses in this study, may erupt without significant modification, whereas consecutively derived less enriched and depleted melt fractions are affected by mixing and crystal fractionation. Continuous melt generation, extraction, replenishment of the plumbing system, mixing and eruption of “integrated” melts lead to obliteration of the initially enriched geochemical characteristics and ultimately result in dominantly “normal”, depleted MORB compositions.

Comendites and pantellerites of Nemrut volcano, eastern Turkey: Genesis and relations between the trachyte-comenditic, comenditic, and pantelleritic melts

The paper reports data on the mineralogy, geochemistry, phase composition of comendites and pantellerites from Nemrut volcano, eastern Turkey; estimates of the crystallization conditions of minerals, composition of matrix glasses and melt inclusions in anorthoclase, fayalite, hedenbergite phenocrysts. LA-ICP-MS was applied to analyze the matrix glasses and phenocryst minerals. The distribution coefficients between phases and glass were calculated for P, B, Li, Rb, Cs, Ba, Sr, Zr, Hf, Ta, Nb, Sc, V, Cr, Ni, Cu, Pb, Th, U, Y, REE. Mass balance simulations of the comenditic and pantelleritic compositions, experimental data, data on melt inclusions are utilized to analyze the processes responsible for the derivation of the magmas, accumulation of components in them and to elucidate genetic links between the trachyte-comenditic, comenditic and pantelleritic melts. The origin of the residual comenditic and pantelleritic melts is explained by variations in the crystallization conditions of anorthoclase (dominant phase), hedenbergite, fayalite, Fe and Ti oxides in the parental trachyte-comenditic magma depending on the pressure and concentration of water dissolved in the melts. The accessory phases (REEand Sr-bearing fluorapatite and zircon) were likely involved in the fractionation of the melts. The following crystallization parameters were obtained by QUILF calculations for the hedenbergite, fayalite, and ilmenite phenocrysts (minimum values for quartz-free melts): 3 kbar, 763°C, ΔFMQ = −1.27 for the Fe-comendites; 3.3–3.8 kbar, 715°C, ΔFMQ = −1.8 for the pantellerites; 2.3 kbar, 748°C, ΔFMQ = −1.16 for the low-Fe comendites. The equilibrium crystallization of anorthoclase phenocrysts in the comenditic melts proceeded at temperature ∼750°C. Data on glasses of melt inclusions indicate that the comenditic and pantelleritic melts contained 1–3 wt % H2O. Analysis of literature data and estimates of the conditions under which the Nemrut magmas were derived suggest that the local chambers with H2O-undersaturated comenditic and pantelleritic melts could occur at centers of alkaline volcanism at depths within the range of 5 to 10–15 km (lithostatic pressure of 1–4 kbar), at temperatures <750°C and oxygen fugacity below the FMQ buffer.

Chrome spinel in normal MORB-type greenstones from the Paleozoic-Mesozoic Mino terrane, East Takayama area, central Japan: Crystallization course with a U-turn

Phenocrystic chrome spinel crystallized in normal MORB-type greenstones in the East Takayama area. Associated phenocryst minerals show a crystallization sequence that was olivine first, followed by plagioclase, and finally clinopyroxene. Chrome spinel ranges from 0.54 to 0.77 in Mg/(Mg+Fe2+) and 0.21 to 0.53 in Cr/(Cr+Al); the Fe3+ content varies from 0.07 to 0.22 p.f.u. (O = 4). Significant compositional differences of spinel were observed among the phenocryst mineral assemblages. Chrome spinel in the olivine–spinel assemblage shows a wide range in Cr/(Cr+Al), and is depleted in Fe2+ and Fe3+. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage is Fe2+- and Fe3+-rich at relatively high Cr/(Cr+Al) ratios. Basalt with the olivine–plagioclase–spinel assemblage contains both aluminous spinel and Fe2+- and Fe3+-rich spinel. The assumed olivine–spinel equilibrium suggests that chrome spinel in the olivine–spinel assemblage changed in composition from Cr- and Fe2+-rich to Al- and Mg-rich with the progress of fractional crystallization. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage, on the other hand, exhibits the reversed variations in Mg/(Mg+Fe2+) and in Cr/(Cr+Al) ratios that decrease and increase with the fractional crystallization, respectively. The entire crystallization course of chrome spinel, projected onto the Mg/(Mg+Fe2+)–Cr/(Cr+Al) diagram, exhibits a U-turn, and appears to be set on a double-lane route. The U-turn point lies in the compositional field of chrome spinel in the olivine–plagioclase–spinel assemblage, and may be explained by plagioclase fractionation that began during the formation of the olivine–plagioclase–spinel assemblage.