Plagioclase Crystals Research Articles

Study of micro-textures and chemistry of feldspar minerals of East Sarbisheh volcanic complex (Eastern Iran), for evidence of magma chamber process

The Eocene-Oligocene Sarbisheh volcanic complex is a part of the Lut-Sistan Zone that outcrops in eastern Iran. In the east of this complex, three groups of volcanic rocks (i.e., andesite, dacite, and rhyolite) exist. Plagioclase as the main mineral of these rocks is found with varying micro-textures. Based on a changing trend in the concentration of anorthite, the developed micro-textures (coarse/fine-sieve, fine-scale oscillatory zoning, and resorption surfaces) are not affected by the chemical composition of the magma. Rather, such changes can occur by temperature variations during magma crystallization or H2O fugacity changes in the magmatic system. The recharge of basic magma leads to a temperature rise, partial melting of the central part of the crystal, and formation of sieve texture, and resorption surfaces. Consequently, the chemical changes of magma in the chamber cause the formation of An-enrichment in the outer layer of the plagioclase crystal and formation of oscillatory zoning. In addition, the morphological micro-textures (i.e., glomerocryst, synneusis, swallow-tailed, microlite, and broken crystals) are developed by the influence of dynamic behavior of the crystallizing magma and magmatic differentiation. The thermobarometry evaluation using pyroxene and biotite chemistry showed that the temperature ranges between 700 and 1150°C and the pressure were less than 2 kbar.

Late Cretaceous–Eocene magmatism induced by slab rollback and breakoff in the Tengchong terrane, SW China

ABSTRACT Late Cretaceous–Eocene granites are widespread in the Tengchong terrane and related to subduction and closure of the Neo–Tethyan Ocean. However, the reason of their compositional and spatial variations is still under explored. This study reports zircon U–Pb geochronological, whole–rock and Nd isotopic data for Guyong, Houqiao and Menglong granites in the Tengchong terrane. Our data show that the three plutons crystallized at 70.4 ± 0.3Ma, 64.4 ± 0.3Ma and 47.6 ± 0.2Ma. Guyong monzogranites show an I–type affinity and may have experienced fractional crystallization of plagioclase, biotite and sphene. Negative εNd(t) values (−9.19 to −11.01) with two-stage model ages (T2DM = 1.59–1.78 Ga) indicate that Guyong monzogranites are derived from a predominantly Paleoproterozoic crustal source with little involvement of mantle–derived materials in an extensional setting. Lower εNd(t) (−9.33 to −11.63) and negative correlations of HREE, LREE, Th, and Y vs. SiO2 indicate that Houqiao plutons are S–type granites derived from a pure crustal source during crustal thickening. Low Zr/Hf and Nb/Ta ratios, zircon saturation temperature (TZr = 750°C–786°C) and negative δEu (0.11–0.40) of Menglong biotite granites indicate that they have experienced fractional crystallization of plagioclase, biotite and sphene. Relatively high εNd(t) (−6.81 to −7.07) values indicate that they are products of felsic magma mixed with mafic magma, which is also supported by numerous coeval mafic–intermediate rocks (Eocene) such as metagabbros, diorites and MMEs (mafic microgranular enclaves) in the western Tengchong terrane. A process of slab rollback and slab breakoff may have triggered the late Cretaceous–Eocene magmatism and the compositional variation of associated granitoids in the Tengchong terrane.

Geochronology, geochemistry and Hf isotopic composition of Late Cretaceous Laojunshan granites in the western Cathaysia block of South China and their metallogenic and tectonic implications

Sn-, In- and W-rich granites are identified in Laojunshan area in the western Cathaysia block, South China, including coarse-grained two-mica monzogranite (CMG) and fine-grained two-mica monzogranite (FMG). Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb dating of CMG and FMG are from 85.6 ± 0.8 Ma to 90.37 ± 0.7 Ma and 89.9 ± 1.4 Ma, respectively. All samples have relatively high total alkalis contents (5.74 wt% to 9.73 wt%) and Al saturation indices (0.95 to 1.79), indicating that most of granites belong to the strongly peraluminous granites. The granites are enriched in P, Rb, Cs, Sn, Li, Be, and U, and are depleted in Ti, Mg, Co, Ni, Sr, Ba, Zr, and Hf. The fractional crystallization of plagioclase and K-feldspar was the principal process of magmatic differentiation that controlled Rb, Sr, Ba and Eu concentrations. While rare earth elements were fractionated by accessory minerals such as apatite, monazite or ilmenite. These geochemical features suggest that most of rocks are highly fractionated S-type granites. The εHf(t) values of the zircon samples from CMG and FMG range from −12.88 to −1.19, with the two-stage Hf model ages ranging from 1.2 Ga to 1.9 Ga. The geochemical and isotopic data show that the Laojunshan granites are derived from a crystalline basement that has metamorphosed pelitic rocks from the Mes-paleoproterozoic continental crust. The upwelling of the asthenosphere caused by a strong lithospheric extension led to the partial melting of the metamorphosed pelitic rocks to form widespread Late Cretaceous S-type granites in the Laojunshan area. Lithospheric extension and Cretaceous magma in SE Yunnan are probably related to the Palaeo-tethys ocean crustal delamination, which subducted beneath the Indochina block.

Cretaceous magmatic evolution in the Deylaman igneous complex, Alborz zone, Iran: change from extensional to compressional regime

The Deylaman igneous complex, as a part of the Late Cretaceous rock unit that lies behind the Paleogene Alborz magmatic arc, in the northern Alborz zone, is composed of basaltic sheet lavas alternating with the pelagic calcareous sediments, basaltic pillow lavas, felsic lavas and gabbroic-monzodioritic intrusions. The pelagic calcareous deposits contain microfossils representing the Santonian–Maastrichtian ages. Furthermore, petrographic textures such as the hyalomicrolitic texture and swallow-tail plagioclase crystals in the pillow lavas, and also segregation vesicles in the basaltic sheet lavas, imply high external (hydrostatic) pressures as the magma was extruded in a deep-water environment. The rock samples show both compositional bimodality and characteristic trends in the variation diagrams. Also, some geochemical characteristics imply that the basaltic lavas originated from the partial melting of an undepleted deep mantle source containing spinel lherzolite: the enrichment patterns of LREE/HREE ratios of the samples [(La/Yb)n = 3.93–4.16 for basaltic lavas and 10.92 for felsic lavas] lying between those characteristic of OIBs [(La/Yb)n = 12.92] and EMORBs [(La/Yb)n = 1.91]; similarities between the patterns of multi-element spider-diagrams; LILE bulges in the basaltic samples compared with those of OIBs. Moreover, the samples show influence from two geotectonic environments: supra-subduction zone (SSZ) settings and plume-type within-plate magmas. Therefore, because of the deep submarine environment inferred for the effusive volcanic eruptions in Santonian–Maastrichtian time, it seems that the Deylaman igneous complex evolved through two stages: first, a tensional regime in a supra-subduction zone (farther from the Mesozoic magmatic arc) and formation of an embryonic rift-related oceanic basin in the Late Jurassic–Early Cretaceous; secondly, a compressive regime in the Late Cretaceous–Early Paleocene and inland migration of the magmatic arc. Consequently, the Cretaceous magmatism can be interpreted as a prelude to the Eocene magmatic flare-up in the magmatic arcs of Iran.

Shallow marine basaltic volcanism of the Machadodorp Member (Silverton Formation, Pretoria Group), Transvaal Basin, South Africa — An example of Paleoproterozoic explosive intraplate volcanic activity in an epeiric embayment

The early Paleoproterozoic Machadodorp Member of the Silverton Formation (Pretoria Group, Transvaal Supergroup), South Africa, is an example of ancient submarine explosive volcanism in an intra-cratonic setting under incipient rifting conditions. The Silverton Formation consists of three different members, which include, in ascending order, the Boven Member, the Machadodorp Member and the Lydenburg Member. The formation is primarily characterized by the muddy sediments of the Boven and Lydenburg members, which formed in a relatively shallow submarine environment, predominantly as sub-storm wave-base pelagic deposits, within an epeiric embayment on the Kaapvaal Craton. The subaqueous Machadodorp Member volcanic activity is mostly distributed in the eastern part of the basin. So far, the volcanism of the Machadodorp Member has not been studied in detail. Therefore, a detailed facies analysis of its volcanism was initiated, allowing constraints on its volcanic evolution.The Machadodorp Member consists of a basal unit of basaltic sheet lava, followed by a fining-upward succession of volcaniclastic rocks, ranging from breccias, tuff-breccias and tuffs, rich in fluidal-shaped clasts, to laminated tuffs that are related to subaqueous mass flows, and a final sheet lava that characterized the end of volcanic activity in the section studied.The Machadodorp Member rocks are tholeiitic basalts in composition, with all samples plotting in the low-TiO2 suite. Fractional crystallization of clinopyroxene, olivine and plagioclase was the predominant evolutionary process as illustrated by mineral chemistry and major element geochemical modeling. AFC (Assimilation and Fractional Crystallization) processes could not account for all the trace element variations, nor for the arc-like signatures observed on the primitive mantle normalized plots. Variable ancient mantle enrichment beneath this area is a more plausible cause for the trace elements variations observed in the Machadodorp rocks. Trace element modeling indicates that the parental composition of these rocks was a variably enriched spinel lherzolite source. Such enrichment must have occurred before 3 Ga during episodes of arc collision and subduction.

El condrito carbonáceo Aguas Zarcas: bólido, caída y recuperación de un meteorito extraordinario del tipo CM2

On April 23rd, 2019 at 21:08 local time, a fireball was observed crossing the night sky of central Costa Rica with a general southeast to northwest direction, in an event registered by both witnesses and cameras. A subsequent explosive sound (fragmentation in the atmosphere) was reported near San Carlos, followed by a meteorite shower in the districts of Aguas Zarcas and La Palmera, of San Carlos. The first recovered fragment (1071 g) came from a house near La Caporal in Aguas Zarcas, where the impact bored a hole in the ceiling breaking the meteorite in smaller pieces adding up to a total weight of 1152 g. In the following days several other fragments were recovered towards the northwest of Aguas Zarcas, in the towns of La Cocaleca, Santa Rosa, and La Palmera, with weights ranging from <1g to 1875 g. The total recovered mass of the meteorite was estimated in roughly 30 kg. After proper revision by the Meteoritical Society, the official name “Aguas Zarcas Meteorite” was approved on May 30th, 2019. The main masses of the meteorite, in excess of ~50 g, show regmaglypts (some with iridescent surfaces). Other fragments display drop, platy, and elliptical forms, with aerodynamic patterns, and showing a frontal dome. The inner parts of the analyzed specimens show chondrules on varying areal percentages, within a dark-gray to black matrix. Calcium-Aluminum rich inclusions (CAI), sub-millimetric crystals of plagioclase, olivine, and sulphide and metallic flakes are also present in the matrix. A clear fusion crust is evident in some fragments showing cooling microfractures. It is slightly magnetic and expels a particular organic-like odor. Geochemical and isotopic analyses performed at the Universities of Costa Rica, Arizona and Nuevo Mexico, among others, and the observed textural characteristics of the meteorite, indicate that it belongs to the Carbonaceous Chondrite classification of rocky meteorites, type CM2 which entails the presence of hydrated minerals within a fine matrix composed of carbon, and nickel-rich sulphides components. CM2 meteorites also show general compositions and oxygen isotope compositions similar to the primitive solar nebula. Carbonaceous chondrites represent only ~4% of the total recovered meteorites worldwide and they preserve the geochemical, isotopic, and mineralogical composition of the first millions of years of the history and evolution of the Solar System, making the Aguas Zarcas Meteorite an extraordinary event, and sparking national and international scientific interest.

Continuous magma mixing and cumulate separation in the High Tatra Mountains open system granitoid intrusion, Western Carpathians (Poland/Slovakia): a textural and geochemical study

In this study the formation of the polygenetic High Tatra granitoid magma is discussed. Felsic and mafic magma mixing and mingling processes occurred in all magma batches composing the pluton and are documented by the typical textural assemblages, which include: mafic microgranular enclaves (MME), mafic clots, felsic clots, quartz-plagioclase-titanite ocelli, biotite-quartz ocelli, poikilitic plagioclase crystals, chemically zoned K-feldspar phenocrysts with inclusion zones and calcic spikes in zoned plagioclase. Geochemical modelling indicates the predominance of the felsic component in subsequent magma batches, however, the mantle origin of the admixed magma input is suggested on the basis of geochemical and Rb-Sr, Sm-Nd and Pb isotopic data. Magma mixing is considered to be a first-order magmatic process, causing the magma diversification. The cumulate formation and the squeezing of remnant melt by filter pressing points to fractional crystallization acting as a second-order magmatic process.

Late Mesozoic oxidized magma for porphyry Ag mineralization: A comparative study from mineralized and barren granite porphyries in the Lengshuikeng Ag-(Pb-Zn) deposit, South China

The Lengshuikeng Ag-(Pb-Zn) deposit (LSKD) is one of the largest independent porphyry silver deposits in China. Its silver mineralization is related to the Late Mesozoic granite porphyry, but the oxidizing hydrous conditions of the associated magma are still unclear. This study focuses on the associated granite porphyry (GP) and barren K-feldspar granite porphyry (KFGP) to constrain their oxidizing hydrous conditions. New zircon U-Pb dating shows the GP emplaced at 155.8 ± 1.9 Ma and 158.5 ± 2.3 Ma for GP, and the KFGP emplaced at 136.6 ± 1.9 Ma. Both granites are peraluminous with A/CNK ratios of 1.09–3.01 (mean = 1.48), similar to that of typical S-type granites. They are enriched in light rare earth elements (LREE), with large (La/Yb)N (7.0–19.1 for the GP and 1.8–34.3 for the KFGP) and negative Eu anomalies (0.29–0.67 for the GP and 0.02–0.05 for the KFGP). The GP has εNd(t) values ranging from − 10.0 to − 9.7 with TDM2 ages of 1688–1915 Ma, while the KFGP has higher εNd(t) values ranging from − 7.5 to − 7.6 with younger TDM2 ages of 1538–1548 Ma. The initial Pb isotopic ratios for (206Pb/204Pb)i, (207Pb/204Pb)i and (208Pb/204Pb)i are 17.647–17.952, 15.555–15.604 and 37.851–38.541, respectively. Our new chemical and isotopic data suggest that both granites were dominantly derived from dehydration melting of the Proterozoic meta-sedimentary rocks, with more dehydration fluids for the GP and relatively more mantle contribution to the KFGP. Both granites underwent fractional crystallization of plagioclase, K-feldspar and ilmenite/magnetite followed by minor assimilation of the upper crust. Comparative study on the relative water content, oxidation state and melting temperature of magma have shown relatively moderate oxidation state and water content in the associated magma probably played more important roles in porphyry silver mineralization than other factors in porphyry Cu deposit. This study helps deepen the understanding of silver mineralization.

Crystallization Conditions and Mineral Chemistry in the East of Tafresh, Central Iran, with Insights into Magmatic Processes

The Tafresh granitoids are located at the central part of the Urumieh‐Dokhtar Magmatic Arc (UDMA) in Iran. These rocks, mainly consisting of diorite and granodiorite, were emplaced during the Early Miocene. They are composed of varying proportions of plagioclase + K‐feldspar + hornblende ± quartz ± biotite. Discrimination diagrams and chemical indices of amphibole phases reveal a calc‐alkaline affinity and fall clearly in the crust‐mantle mixed source field. The estimated pressure, derived from Al in amphibole barometry, is approximately 3 Kb. The granitoids are I‐type, metaluminous and belong to the calc‐alkaline series. They are all enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements and display geochemical features typical of subduction‐related calc‐alkaline arc magmas. Most crystal size distribution (CSD) line patterns from the granitoids show a non‐straight trend which points to the effect of physical processes during petrogenesis. The presence of numerous mafic enclaves, sieve texture and oscillatory zoning along with the CSD results show that magma mixing in the magma chamber had an important role in the petrogenesis of Tafresh granitoids. Moreover, the CSD analysis suggests that the plagioclase crystals were crystallized in a time span of less than 1000 years, which is indicative of shallow depth magma crystallization.

Partial melting, fractional crystallisation, liquid immiscibility and hydrothermal mobilisation – A ‘recipe’ for the formation of economic A-type granite-hosted HFSE deposits

A number of A-type granites host resources of the high field strength elements (HFSE), including the rare earth elements (REE), that have been considered for economic exploitation. The best known of them is the Strange Lake pluton (Canada) with a resource of 20 Mt grading 1.44 wt% REE2O3, of which ~50% are heavy rare-earth oxides. In this paper, we examine the various processes that are likely to be responsible for the hyper-enrichment of the HFSE, starting with the generation of a parental magma through a very low degree of partial melting of a mantle plume or a metasomatically enriched mantle, in both cases, below a continental crust (continental rift). We trace the evolution of this magma (olivine nephelinite) through fractional crystallisation of olivine and clinopyroxene followed by plagioclase, K-rich alkali feldspar and arfvedsonite to the final residues of hyper-enriched HFSE granitic pegmatites. In so doing, we show that at least two additional steps are essential to the process of HFSE enrichment. The first of these is the abrupt termination of plagioclase crystallisation prior to the final consumption of Ca or assimilation of a Ca source and the resulting exsolution of an immiscible Ca-fluoride melt that sequesters the LREE and Y. The second of these steps is prolonged crystallisation of arfvedsonite, which delays saturation of the magma with an aqueous phase and ensures the development of an agpaitic assemblage of Na-bearing HFSE minerals (e.g., Na-zirconosilicates and Na-titanosilicates). Finally, we show that saturation of the magma with an aqueous‑carbonic fluid, which evolves to a very acidic composition, is an essential ingredient in the ‘recipe’ for the formation of an economic A-type granite-hosted HFSE deposit.

Petrogenesis of three East Fork Member rhyolites of the Jemez volcanic field, Valles caldera, New Mexico, USA

The most recent volcanism in the Valles caldera is represented, in stratigraphic order, by the El Cajete Pyroclastic Beds (ECPB), Battleship Rock Ignimbrite (BRI), the VC-1 rhyolite (not studied herein), and Banco Bonito Flow (BBF), which are collectively known as the East Fork Member (EFM) of the Valles Rhyolite. The EFM was erupted approximately 74 to 68 ka after an approximate 460 ka lull in ring-fracture volcanism.Crystal assemblages consist of plagioclase, biotite, clinopyroxene, orthopyroxene, amphibole, sanidine, quartz, and oxides, in order of abundance. Electron probe microanalysis and detailed petrography indicates that two distinct crystal populations are present in the ECPB, BRI, and BBF. Large (≥1 mm), typically resorbed or subhedral crystals represent one population, and small (≤0.5 mm), generally euhedral crystals represent the other. The large resorbed plagioclase crystals typically have oligoclase to andesine rim overgrowths. 40Ar/39Ar geochronology performed on euhedral biotite and groundmass glass from the BBF returned isochron ages of 478 ± 3 ka and 575 ± 2 ka and total gas ages of 125 ± 1 ka and 129 ± 1 ka, respectively. High molar Mg numbers of large euhedral biotite and 40Ar/39Ar ages older than the accepted age range indicate these crystals are xenocrystic. Radiogenic isotopes are consistent with mixing between the mantle and lower crustal reservoirs. General trends are evident between whole-rock major and trace elements with increasing SiO2 and Nb, respectively. In general, incompatible trace elements increase and compatible trace elements decrease with increasing Nb. For the major elements, MgO, P2O5, Al2O3, FeO* show decreasing trends while K2O and Na2O show increasing trends with increasing SiO2. Incompatible trace element ratios (Ta/Yb, Y/Yb, Th/Yb, Th/Nb) indicate the presence of a single magma batch.The heterogeneity in crystal morphology and chemistry can be explained by a model in which partial melting of mid- to deep continental crust occurred due to an intrusion of an intermediate composition magma. Magma mixing and an exchange of crystals took place between the partial melt and the intruding magma. The hybrid magma rose to the upper crust, where it underwent fractional crystallization prior to eruption. The geochemical and isotopic data from this study are best explained by a modified version of the rapid production and eruption model put forth by Huppert and Sparks (1988).

Nucleation and growth of bubbles on plagioclase crystals during experimental decompression degassing of andesitic melts

The nucleation, growth and attachment/detachment of gas bubbles on crystals in silicate melts is one of the key drivers of volcanic eruptions and can greatly influence their explosivity. Formerly, oxides were considered the best candidates for heterogeneous bubble nucleation, but recent studies showed that silicate crystals can also be bubble nucleation sites at 1atmosphere (atm). This study examines whether bubbles can nucleate on plagioclase crystal surfaces during decompression, which is a more common natural situation than 1atm degassing. Experimental samples were synthesised from andesitic rock powder, H2O and plagioclase seed crystals, and melted and decompressed in a piston-cylinder apparatus. The products were imaged with 3D X-ray computed tomography. Due to partial melting, the plagioclase crystals developed sieve rims and rough crystal-melt interfaces. All plagioclase crystals in all samples were covered with bubbles. Bubble size distribution comparisons between bubbles in contact with plagioclase and bubbles in the melt, show that bubbles belong to two populations, generated by two different events, one of which is heterogeneous nucleation on plagioclase crystal surfaces and the other is homogeneous nucleation within the melt, the latter potentially accompanied by heterogeneous nucleation on oxide microlite surfaces. The calculated attachment force between the bubbles and the plagioclase surface is stronger than the detachment force; hence, such bubbles would remain attached during crystal movements. In our experiments, the net buoyancy of the bubble-crystal aggregates indicates that they could sink in an andesitic melt. Our findings highlight the need to reconsider the role of plagioclase crystals in magmatic degassing and shows interesting possibilities for magma mixing-triggered degassing.

The effect of cooling rate on immiscible silicate liquid microstructure: an example from the Palaeogene dykes of Northeast England

AbstractThe migration and accumulation of immiscible silicate liquids may play a significant role in the differentiation of crustal magma bodies and the formation of some economic mineral deposits. However, our understanding of the processes that control the segregation of these liquids is currently limited by the short timescales of petrological experiments. Detailed microstructural investigations of Palaeogene basaltic dykes from Northeast England, coupled with simple 1D thermal models, constrain the effects of cooling rate on the microstructure of unmixed immiscible silicate liquids under natural conditions. The size of unmixed Fe-rich droplets within a continuous silicic phase is related to the cooling rate by a power law, with droplet diameter increasing with decreasing cooling rate, accompanied by an increase in the number of droplets. Fe-rich droplet coarsening is a result of diffusion-controlled growth. The average apparent aspect ratio and grain size of matrix plagioclase crystals indicate that nucleation and growth of these grains probably occurred in a static (or only weakly convecting) fluid dynamical regime.

Variation of Iron Species in Plagioclase Crystals by X‐ray Absorption Fine Structure Analysis

AbstractSpeciation analysis of Fe in single plagioclase crystals separated from two different gabbros was performed to understand the crystallization mechanisms of magnetite exsolution. Iron species in single crystals were measured using Fe K‐ and LIII‐edge X‐ray absorption fine structure (XAFS) analysis. The K‐edge pre‐edge analysis showed variation in the averaged valence state of Fe in plagioclase crystals even if they had been separated from the same gabbro that was further confirmed by the LIII‐edge analysis. The K‐edge pre‐dedge analysis also suggests the various degrees of contribution from tetrahedral Fe. The mixing of tetrahedral and octahedral Fe leads to an underestimation of the averaged valence state of Fe for the K‐edge pre‐edge analysis; thus, we adopted the LIII‐edge result for the valence state of Fe in plagioclase crystals. Iron K‐edge extended XAFS (EXAFS) analysis of two plagioclase crystals separated from the same gabbro clearly showed different coordination environments. A weakening of EXAFS oscillation was recognized in one sample, because two Fe–O bonds (Fe3+–O1 and Fe2+–O2) cancelled out the oscillations of each other. The EXAFS spectrum of the other plagioclase crystal suggested a homogeneous distribution of Fe. The content of exsolved magnetite in these crystals is nearly identical, indicating that the exsolution of magnetite in plagioclase crystal had been completed before the temperature decrease that stopped the ordering of Fe ions in tetrahedral sites.

New Constraints on the Main Mineralization Event Inferred from the Latest Discoveries in the Bor Metallogenetic Zone (BMZ, East Serbia)

This study aims at better constraining the link between magmatism and metallogeny in the south-easternmost sector of the Bor Metallogenetic Zone (BMZ), where the world-class copper and gold deposit of Čukaru Peki was recently discovered. The obtained U/Pb zircon ages confirm the earlier knowledge that the major Cu–Au porphyry and epithermal mineralization in the BMZ is genetically related to the first volcanic phase (‘Timok andesite’; 85–90 Ma). However, the data also suggest that during this phase, two subgroups of andesite porphyry were formed; they are named volcanic phase 1A (V1A) and volcanic phase 1B (V1B). The V1A andesite (89–90 Ma) is plagioclase-hornblende phyric, holocrystalline and ubiquitously hydrothermally altered and/or mineralized, whereas the V1B (85–86 Ma) is hornblende-plagioclase phyric, holo- to hypocrystalline, fresh, and non-mineralized. According to our simplified model, the contrasting productivity of the V1A and V1B is explained by fluctuations during AFC (assimilation-fractional crystallization) processes of water-rich parental magma, which have controlled the order of crystallization of hornblende and plagioclase in the V1A and V1B andesite.

Eruption age and duration of the ∼9 km3 Burney Mountain dacite dome complex, northern California, USA

Abstract At ∼9 km3, the six dacite domes (db1–db6) of Burney Mountain, northern California, USA, constitute the most voluminous Quaternary dome complex in the Cascades volcanic arc. Whole-rock geochemistry, electron microprobe, and petrographic data indicate that the domes are magmatically related, which when integrated with geomorphology and stratigraphic superposition, indicate early (db1, db2, and db3) and late (db4, db5, and db6) erupted groups. We present 40Ar/39Ar ages of 271.9 ± 4.6 ka (db1), 280.8 ± 8.2 and 281.7 ± 6.8 ka (db2), and 290.2 ± 6.0 ka (db3) along with a previous age of 280 ± 12 ka (db1). These ages scatter over 20 k.y., whereas remanent magnetic directions are similar between 53.3–59.0° inclination and 352.7–355.9° declination. The latter data set indicates that the dacite domes were emplaced over a geologically brief time interval, not thousands of years. Crystal-size distribution patterns of plagioclase were used to calculate residence times, which we use to infer the duration over which the eruptions likely occurred. Three slopes represent three populations of plagioclase crystals (fine-grained groundmass, coarse-grained groundmass, and phenocrysts). A commonly used growth rate for plagioclase in dacitic magmas (10−10 mm/s) yields 9–10 yr of growth for the coarse-grained groundmass (early erupted domes of db1, db2, and db3), whereas plagioclase in the fine-grained groundmass (late erupted domes of db4, db5, and db6) grew over 4–5 yr. All plagioclase phenocrysts have apparent residence times of 26–36 yr; however, they contain high anorthite (An)&amp;gt;70 resorbed cores with sieve textures, which have euhedral, lower An&amp;lt;65 overgrowth rims. Similarities in chemistry between groundmass plagioclase and phenocryst overgrowth rims indicate that they grew concurrently, and we therefore propose that both have similar residence times. Thus, the Burney Mountain dacite dome complex was emplaced during a single eruptive episode over the course of years to decades at 281.1 ± 4.8 ka (weighted mean age).

Petrogenesis of subduction-related lavas from the southern Tonga arc

We present new whole-rock geochemical data for volcanic rocks dredged from active submarine volcanoes TA15, TA16, and TA19 that mark the southern part of the Tonga Arc near Ata island. The lava samples show a wide range of compositions from basalt to dacite within the low-K tholeiitic series, and geochemical data indicate fractional crystallization of olivine, orthopyroxene, clinopyroxene, and plagioclase. The origin of magmas in the southern Tonga Arc can be explained by the mixing of three components: a depleted mantle wedge, a melt derived from the partial melting of subducted pelagic sediments (PS) and sediments of the subducted Louisville Seamount Chain (LSC), and a fluid produced during subduction by the dehydration of altered oceanic crust (AOC). Our geochemical modeling suggests that mafic magmas in the study area can be generated by 10%–25% partial melting of a depleted mantle that had been metasomatized by ~1% AOC fluids as well as ~1% melts derived from a 40:60 mixture of LSC and PS sedimentary components. On the other hand, volcanoes V, U, and Monowai, located farther south near the present LSC–Tonga Trench intersection, can be generated by 15%–25% partial melting of a depleted mantle that had been metasomatized by ~0.5% AOC fluids as well as ~1.5% sediment-derived melts (a 90:10 mixture of LSC and PS). These results are consistent with previous proposals that the influence of the LSC-derived components was stronger in the southerly volcanoes V, U, and Monowai than in the volcanoes near Ata.

In situ major and trace element compositions of apatites from Luanchuan orecluster: Implications for porphyry Mo mineralization

The effects of volatile concentrations and oxidation state of magma on the Mo mineralization potential have been explored through petrographic studies and in situ electron probe micro-analyzer (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of apatites from coeval economically Mo- mineralized granite porphyry and quartz diorite, and sub-economically mineralized granite porphyry at Luanchuan orecluster. The occurrence of abundant K-feldspar xenocrysts, possibly entrained from granitic magma, in quartz diorite implying a magma mingling process. Studied apatites are fluorapatites or fluor-hydroxyapatites with decreasing OH concentrations from economically mineralized quartz diorite (0.30–0.49 apfu) through mineralized granite porphyry (0.14–0.41 apfu) to sub-economically mineralized granite porphyry (0.01–0.11 apfu). Chondrite-normalized REE patterns of all the apatites are characterized by LREE enrichment relative to MREE and HREE, and negative Eu anomaly, similar to their host magmas, indicating a melt composition control. Strontium concentration of studied apatites are probably controlled by the crystallization of plagioclase or alkali feldspar. Mineralized quartz diorite formed at a relatively reduced environment compared to other rocks, but mineralized granite porphyry and sub-economically mineralized granite porphyry formed at a similar oxidation state, based on the SO3, Eu, Ce, and Mn content variation of apatites. The OH-contents of apatites, combined with published log (H2O/HF) ratios of melt in equilibrated with biotites, reflect that economically mineralized intrusions have higher water contents than the sub-economically mineralized intrusions. In a porphyry Mo system, a high magma water content is probably a key parameter controlling Mo mineralization. Mafic magma may supply water (and heat) to ore-forming granitic magmas, enhancing their Mo mineralization potential.

Deep open storage and shallow closed transport system for a continental flood basalt sequence revealed with Magma Chamber Simulator

The Magma Chamber Simulator (MCS) quantitatively models the phase equilibria, mineral chemistry, major and trace elements, and radiogenic isotopes in a multicomponent–multiphase magma + wallrock + recharge system by minimization or maximization of the appropriate thermodynamic potential for the given process. In this study, we utilize MCS to decipher the differentiation history of a continental flood basalt sequence from the Antarctic portion of the ~ 180 Ma Karoo large igneous province. Typical of many flood basalts, this suite exhibits geochemical evidence (e.g., negative initial εNd) of interaction with crustal materials. We show that isobaric assimilation-fractional crystallization models fail to produce the observed lava compositions. Instead, we propose two main stages of differentiation: (1) the primitive magmas assimilated Archean crust at depths of ~ 10‒30 km (pressures of 300–700 MPa), while crystallizing olivine and orthopyroxene; (2) subsequent fractional crystallization of olivine, clinopyroxene, and plagioclase took place at lower pressures in upper crustal feeder systems without significant additional assimilation. Such a scenario is corroborated with additional thermophysical considerations of magma transport via a crack network. The proposed two-stage model may be widely applicable to flood basalt plumbing systems: assimilation is more probable in magmas pooled in hotter crust at depth where the formation of wallrock partial melts is more likely compared to rapid passage of magma through shallower fractures next to colder wallrock.

Ge/Si Partitioning in Igneous Systems: Constraints From Laser Ablation ICP‐MS Measurements on Natural Samples

AbstractMineral/melt and intermineral Ge/Si exchange coefficients for nine common rock‐forming silicate minerals were determined by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICP‐MS). Ge/Si mineral/melt exchange coefficients were found to vary by up to a factor of 10. In mafic and ultramafic systems, Ge/Si mineral/melt exchange coefficients are less than 1 for plagioclase (0.48) and olivine (0.72), close to 1 for clinopyroxene (1.17) and orthopyroxene (1.07), and greater than 1 for garnet (2.69). In felsic and silicic systems, the Ge/Si mineral/melt exchange coefficient is less than 1 for quartz (0.23), plagioclase (0.67), and potassium feldspar (0.67) but much greater than 1 for biotite (4.80) and hornblende (3.95). We show that early, olivine‐dominated fractionation of primitive basalts does not fractionate Ge/Si significantly, but subsequent cotectic crystallization of plagioclase and pyroxene can increase the Ge/Si ratio from 6 × 10−6 to 7 × 10−6. We show that the only way to decrease Ge/Si during magmatic differentiation is by crystallization of hornblende or biotite (though biotite is typically a late crystallizing phase), consistent with hornblende being a major fractionating phase in hydrous intermediate magmas. The high compatibility of Ge in hornblende makes this element, in conjunction with Si, a potentially useful approach for distinguishing between hornblende and garnet in the source regions of intermediate magmas. The high compatibility of Ge in micas suggests that Ge/Si systematics may also be useful in understanding the origin of ultrapotassic magmas, which are often thought to derive from phlogopite‐rich sources.