Pyroxene Crystals Research Articles

Smectites: The key to the cost overruns in the construction of the third set of locks of the Panama Canal

The third set of locks of the Panama Canal is one the most impressive civil infrastructures in the world. Its construction, however, was delayed by two years incurring cost overruns in billions that were mainly related to the use of the local basalt as the raw material for concrete production. The rapid degradation during the construction of an apparent sound basalt resulted in the huge loss of fines during the manufacturing of concrete. In the rock, olivine and glass are, respectively, altered to iddingsite and palagonite that are mainly composed of smectites. Smectites are also found around plagioclase and pyroxene crystals and filling the dense network of micro- and nano-cracks. Moreover, these smectites are spatially interconnected, which is the key to explaining the unexpected behavior and the rapid degradation of the basalt. Such a rapid degradation of basic igneous rocks – exposed to the weather and then utilized as a construction material – is not frequent, yet it is significant enough to call for the development of a new standard for the use of these materials in construction. • The rapid degradation during the industrial process of an apparently fresh basalt was due to the presence of smectites. • The dense network of connected micro- and nano-cracks filled by smectites, was responsible for the rapid degradation of the basaltic rock. • The weakening of that network during the processes of blasting, transport, storage, and crushing was responsible for the high loss of fines. • These results show the need for the development of a new standard for the use of these materials in construction.

Compositions and Classification of Fractionated Boninite Series Melts from the Izu–Bonin–Mariana Arc: A Machine Learning Approach

AbstractMuch of the boninite magmatism in the Izu–Bonin–Mariana arc is preserved as evolved boninite series compositions wherein extensive fractional crystallization of pyroxene and spinel have obscured the diagnostic geochemical indicators of boninite parentage, such as high Mg and low Ti at intermediate silica contents. As a result, the usual geochemical discriminants used for the classification of the broad range of parental boninites are inapplicable to such highly fractionated melts. These issues are compounded by the mixing of demonstrably different whole-rock and glass analyses in classification schemes and petrological interpretations based thereon. Whole-rock compositions are compromised by entrainment of variable proportions of crystalline phases resulting in inconsistent differences from corresponding in situ glass analyses, which arguably better reflect prior melt compositions. To circumvent such issues, we herein present a robust method for the classification of highly fractionated boninite series glasses. This new classification leverages the analysis of trace elements, which are much more sensitive to evolutionary processes than major elements, and benefits from the use of unsupervised machine learning as a classification tool. The results show that the most fractionated boninite series melts preserve geochemical indicators of their parentage, and highlight the pitfalls of interpreting whole-rock and glass analyses interchangeably.

Eruption history, petrogeochemistry, and geodynamic background of Tengchong volcanoes in Yunnan Province, SW China

Abstract The Tengchong Volcanic Field (TVF) is one of the youngest volcanic areas in China, and is located in the southwestern part of Yunnan Province, China, adjacent to the collision zone between the Indian and Eurasian plates. This paper summarizes the results of previous research on the eruption history, petrochemistry and geodynamic background of the TVF and presents a detailed analysis of the available data. Eruptions took place from the Pliocene to the Holocene and were divided into five stages. The composition of the magma went through two cycles: the N 2 − Q P 1 − Q P 2 volcanic rocks went through the evolutionary process from basalt to dacite and the Q P 3 − Qh magma evolved from trachybasalt to trachyte. The evolution of magma is mainly related to the fractional crystallization of pyroxene and ilmenite. The mantle source area of the TVF volcanic rocks was caused by mixing between the mid-ocean ridge basalt-source mantle and the eastern Indian continental margin sediments to different degrees. Geophysical data also showed that the Indian plate has been subducted under the TVF area. There are also magma chambers in the crust within a depth of 25 km in the TVF that are replenished by mantle magma.

Mineral chemistry and geothermobarometry of gabbroic rocks from the Gysel area, Alborz mountains, north Iran

The gabbroic rocks in the Gysel area of the Central Alborz Mountains in north Iran are intruded into the Eocene Volcano-sedimentary units. The main gabbroic rocks varieties include gabbro porphyry, olivine gabbro, olivine dolerite and olivine monzo-gabbro. The main minerals phases in the rocks are plagioclase and pyroxene and the chief textures are sub-hedral granular, trachytoidic, porphyritic, intergranular and poikilitic. Electron microprobe analyses on minerals in the rock samples shows that plagioclase composition ranges from labradorite to bytonite, with oscillatory and normal chemical zonings. Clinopyroxene is augite and orthopyroxene is hypersthene to ferro-hypersthene. Thermometry calculations indicate temperatures of 650˚C to 750˚C for plagioclase crystallization and 950˚C to 1130˚C for pyroxene crystallization. Clinopyroxene chemistry reveals sub-alkaline and calc-alkaline nature for the parental magma emplaced in a volcanic arc setting.

Major-trace element and Sr-Nd isotope compositions of mafic dykes of the Singhbhum Craton: Insights into evolution of the lithospheric mantle

The Singhbhum Craton in eastern India is host to at least seven sets of mafic dyke swarms. Four previously dated swarms (studied here) include the NNE-trending Keshargaria (ca. 2.80 Ga) and Ghatgaon (ca. 2.76 - 2.75 Ga) swarms, the ENE-trending Kaptipada swarm (ca. 2.26 Ga), and the ESE-trending Pipilia swarm (ca.1.76 Ga). The dykes range in composition from basalt to andesite and have transitional tholeiitic to calc-alkaline affinities. They show intra- and inter-swarm geochemical and Sr-Nd isotopic heterogeneities and have SiO2 content ranging from 47 to 60 wt.%. The chondrite normalized REE patterns show enrichment in LREE and the Primitive-Mantle normalized multi-element patterns show elevated U, Th, Cs, Rb, K, and Pb; and depletion in Nb, Ta, and Ti. These characteristics indicate involvement of crustal component in the petrogenesis of these dykes. The dykes of different swarms have variable 87Sr/86Sri and εNd(i) values, which define a crust-like isotopic growth trajectory with time from a common chondritic to depleted source that was enriched contemporaneously with the formation of the crustal rocks of the Singhbhum Craton. The isotope data indicate involvement of older enriched crustal material in the petrogenesis of these dykes. Variable but mostly high (compared to similarly evolved magmas) Ni (40 - 590 ppm), Cr (40 - 1110 ppm), and V (120 - 434 ppm) contents particularly of the most primitive dykes indicate that parental melts were in equilibrium with mantle peridotite and experienced only minor fractional crystallization of olivine, pyroxene, and magnetite. The Sr-Nd isotope ratios do not show any correlation with differentiation indices which indicates that the melts were not modified significantly by crustal assimilation during ascent and emplacement. The crust-like secular trend of the Sr and Nd isotopic compositions suggests that the enriched crustal material was incubated in the mantle (i.e., metasomatized lithospheric mantle) for a long time and this source was periodically tapped leading to multiple dyke emplacement events over at least 1 Gyr. The recycled crustal material played a role in metasomatizing the subcontinental lithospheric mantle prior to ca. 2.80 Ga. Mantle plume activity triggered melting of the metasomatized lithospheric mantle many times, leading to the emplacement of mafic dykes of different generations across the craton.

Beyond crystal mushes: evidence for uptake of high-T pyroxene antecrysts from mid- to upper crustal andesites into tephras from the Central Plateau, New Zealand

ABSTRACT Pyroxene crystals from recent Central Plateau tephras are used to deduce their formation conditions through two-pyroxene thermobarometry. Crystals return pseudo-pressures and pseudo-temperatures that are artefacts of uptake of antecrysts formed at a range of crustal levels by isobaric cooling of previously intruded magmas. MELTS modelling of tephra glass compositions shows that pseudo-PT conditions are reproduced at oxygen fugacities above the nickel-nickel oxide buffer (NNO+1, NNO+2), mid- to upper crustal pressures (100–400 MPa), and temperatures between c. 900°C and >1100°C. Modelled crystals from the deep crust (800 MPa) are restricted to clinopyroxenes. However, these display chemical equilibrium with shallow orthopyroxenes at higher pseudo-PT conditions than observed in Central Plateau pyroxenes. The data indicate uptake of high-temperature pyroxenes at mid- to shallow crustal levels into ascending andesitic melts and thus preclude the presence of long-lived crustal mush zones (<1000°C) as a source for the crystal cargo of the Central Plateau tephras studied here. Further, the apparent absence of deep crustal pyroxene antecrysts does not preclude models of arc andesite genesis without a ‘deep crustal hot zone’ beneath the Central Plateau. Generation and ascent of primary andesites from a heterogeneous mantle wedge is therefore a possible scenario at the southern Hikurangi margin.

Eudialyte Group Minerals from the Lovozero Alkaline Massif, Russia: Occurrence, Chemical Composition, and Petrogenetic Significance

The Lovozero Alkaline Massif intruded through the Archean granite-gneiss and Devonian volcaniclastic rocks ca. 360 Ma ago and formed a large laccolith-type body. The lower part of the massif (the Layered complex) is composed of regularly repeating rhythms: melanocratic nepheline syenite (lujavrite, at the top), leucocratic nepheline syenite (foyaite), foidolite (urtite). The upper part of the massif (the Eudialyte complex) is indistinctly layered, and lujavrite enriched with eudialyte-group minerals (EGM) prevails there. In this article, we present the results of a study of the chemical composition and petrography of more than 400 samples of the EGM from the main types of rock of the Lovozero massif. In all types of rock, the EGM form at the late magmatic stage later than alkaline clinopyroxenes and amphiboles or simultaneously with it. When the crystallization of pyroxenes and EGM is simultaneous, the content of ferrous iron in the EGM composition increases. The Mn/Fe ratio in the EGM increases during fractional crystallization from lujavrite to foyaite and urtite. The same process leads to an increase in the modal content of EGM in the foyaite of the Layered complex and to the appearance of primary minerals of the lovozerite group in the foyaite of the Eudialyte complex.

Reconstruction of 16th–17th Century Lead Smelting Processes on the Basis of Slag Properties: A Case Study from Sławków, Poland

The study focuses on the reconstruction of the technological process in the 16th–17th century lead smelter in Sławków based on chemical and petrographic analyzes of slags. There are three main types of material at the landfill: glassy, crystalline, and weathered. Glassy slags are made of amorphous phase in which crystals of pyroxene, willemite, olivine, wüstite, and lead oxide appear. Crystalline slags are composed of wollastonite, rankinite, melilite, anorthite, quartz, and Fe oxides. Weathered slags have a composition similar to glassy slags, but they also contain secondary phases: anglesite and cerussite. Chemical analyzes confirmed that the smelter used sulphide ores, which were roasted, and the main addition to the charge was quartz sand. The smelting process took place in a brick-built furnace, under reducing conditions, with varied oxygen fugacity ranging from WM to MH buffer. The slag characteristics show a knowledge of the workers in the field of smelting methods. The addition of SiO2 allowed for the binding of elements that could contaminate the obtained lead, and at the same time, the low melting point of the material (1150 °C) and the melt viscosity (logη = 1.34 for 1150 °C) was maintained, enabling the effective separation of liquid lead.

Magma Mushes of the Fogo Island Batholith: a Study of Magmatic Processes at Multiple Scales

Abstract This field, petrographic, and geochemical study examines mingling of compositionally similar rocks at multiple scales. Evidence of complex magma interaction in a multi-component crystal mush reservoir is preserved within the Wild Unit, located along the NE shoreline of Fogo Island, Newfoundland and Labrador, Canada. The irregular contacts and lack of chilled margins between units, the back-intrusion of younger units by older units, the similar composition of units, and an overlap in U–Pb zircon ages suggest that all units interacted as viscous crystal mushes at similar temperatures in the shallow crust. Abundant rounded to ellipsoidal magmatic enclaves, of which there are at least three populations based on composition and crystallinity, appear to represent separate magmas that were entrained either as earlier mush material or crystal-poor intrusions that experienced break-up. Evidence of changes in liquid environment at deeper levels is preserved both in the field and at the mineral scale, where it is highlighted by abrupt compositional spikes in traverses across early forming plagioclase and pyroxene crystals. Heterogeneity in textures and composition of both major minerals (plagioclase and pyroxene) and an accessory mineral (zircon) point to processes such as crystal exchange and capture affecting tonalite crystal mushes, magmatic enclaves, and other intrusions in the study area earlier in their histories at deeper levels.

Modeling the geochemical evolution of impact melts in terrestrial impact basins: Vredefort granophyre dikes and Sudbury offset dikes

AbstractThe Vredefort impact structure, South Africa, is comparable to the Sudbury impact structure, Canada, in size, age, and target rock composition. Both impact structures feature impact melt dikes. The melt sheet of the Sudbury impact (Sudbury Igneous Complex; SIC) is genetically linked to the Sudbury offset dikes in the underlying target rock. At Vredefort, the melt sheet was eroded so that only the granophyre dikes retain compositional melt sheet characteristics. XRF analyses of 43 samples from four granophyre dikes are similar to previous studies, but identify an anomalous mafic phase within one of the dikes. The results from the Vredefort granophyre dikes are compared to the Sudbury offset dikes and shown to follow similar geochemical trends, controlled by crystallization of feldspar and pyroxene. The mafic granophyre phase is compositionally remarkably similar to the offset dike compositions. The program Rhyolite‐MELTS was used to test possible melt sheet compositions. Modeling results are broadly consistent with the overall chemical and mineral composition of the dikes. Modeling is consistent with offset dikes being derived from the basal mafic layer of the SIC, and the granophyre dikes being derived from alkali‐depleted bulk continental crust. For all modeled compositions, crystallization primarily occurred at temperatures between 1150°C and 1000°C. The emplacement of the felsic granophyre dikes from a homogenized crustal melt suggests emplacement within tens of years after the impact event. The presence of the mafic phase in one of the granophyre dikes is explained by its emplacement following some differentiation of the Vredefort melt sheet.

Crystal Growth and Crystallization Time Scales of the Panzhihua Layered Intrusion: Constraint from Crystal Size Distribution

AbstractThe Panzhihua layered intrusions is generated closely related to the Emeishan LIPs. This paper analyzes the spatial distribution of plagioclase and pyroxene. The quantitative texture analysis of 2209 plagioclase shows that the characteristic length of plagioclase is 0.54 to 0.96 mm, the intercept variation range is large, from –0.67 to 0.96, and the slope is –1.85 to –1.04, the Aspect Ratio shows from 1.84 to 2.59 and fractal dimension D is 1.908–1.933. The quantitative texture analysis of 2342 pyroxene shows that the characteristic length of pyroxene is 0.38–0.64 mm, the intercept shows from 0.46 to 2.26, The slope ranges from –2.6 to –1.47, the Aspect Ratio value varies from 1.53 to 1.71, the fractal dimension D is 0.93 to 1.13. All the CSDs results of the Panzhihua intrusions indicate that plagioclase and pyroxene form in an open magma system and undergo four replenishment of magma injection. The plagioclase crystals do not grow as the lathlike shape, and the fractal growth leads to complex crystal surface. The plagioclase undergoes deformation compaction during the crystal process, and then is oriented. The pyroxene crystals grow along an approximately triaxial ratio and undergo texture adjustment and small crystal dissolution reabsorption. When all crystals in magma system grows up to 2 mm, the pyroxene undergoes cumulation in the Panzhihua layered intrusions. The plagioclase crystallization time scale is 171.23–304.41 years, representing that the crystallization is the more uniform in central part of the melt. The nucleation density continuously increases during the crystallization process of the magma system. The time scale to reach the final maximum crystal nucleation density is 15.28–58.98 years.

Late Silurian to Early Devonian volcanics in the East Kunlun orogen, northern Tibetan Plateau: Record of postcollisional magmatism related to the evolution of the Proto-Tethys Ocean

The late Early Paleozoic volcanic rocks in the East Kunlun orogen are crucial to determining collisional processes related to the evolution of the Proto-Tethys Ocean. We report new zircon U-Pb ages, whole-rock geochemistry and Sr-Nd isotopes from these volcanic rocks and constrain their tectonic setting and magmatic processes. This volcanic suite consists mainly of foliated metarhyolites and minor metabasalts. Zircon U-Pb ages indicate that the bimodal volcanic rocks formed from Late Silurian to Early Devonian (ca. 420−409 Ma). The basalts show tholeiitic geochemical signatures characterized by low contents of SiO2 (47.19–54.83 %), MgO (2.21–7.52 %), and K2O/Na2O ratios (0.32−0.77%) and high contents of TiO2 (1.80–2.91 %). Their chondrite-normalized REE patterns are characterized by enrichments in LREEs (LREE/HREE = 3.68–6.09) with slight Eu anomalies (δEu = 0.67–1.00); their patterns are similar to the reference line of oceanic island basalt (OIB). The primitive mantle-normalized trace element diagram also shows similarities with OIBs and features overall enrichments in large ion lithophile and high field strength elements, except for Nb and Ta. The εNd(t = 422 Ma) values range narrowly from −1.79 to +1.32. These features suggest that the basalts were most likely derived from an asthenospheric mantle that was contaminated by small volumes of subcontinental lithospheric mantle (SCLM)/crust. The variations in major and trace elements show that the basalts experienced fractional crystallization of olivine, pyroxene and plagioclase. In contrast, the metarhyolites have high SiO2 (65.85–70.83 %), Na2O + K2O (6.71–10.09 %), K2O/Na2O ratios (1.69–2.80 %) and low MgO, Ni, and Cr contents. Their trace element signatures show enrichments in LILEs and LREEs (e.g. Cs, Rb, Ba), depletion of HFSEs (e.g. Nb, Ti, Ta) and high negative Eu anomalies. These features suggest a crustal origin lacking interactions with mantle melts. Considering all the geochemical and tectonic events of this area, it is concluded that the Maoniushan Formation volcanic rocks in EKOB formed in a postcollisional extensional setting related to asthenospheric mantle upwelling and continental delamination processes during the Late Silurian to Early Devonian. These tectonic-magmatic events, together with previous data, suggest that the Proto-Tethys Ocean had closed and evolved to postcollisional collapse stage since the Late Silurian.

A Pyroxenic View on Magma Hybridization and Crystallization at Popocatépetl Volcano, Mexico

The Popocatepetl Volcanic Complex (PVC) is an active arc volcano located in Central Mexico, 70 km southeast of Mexico City. Current models of the PVC’s plumbing system and magma petrogenesis are largely based on the study of isolated Plinian eruptions in the past 23.5 ka and present-day Vulcanian activity, while voluminous effusive summit and flank eruptions remain underrepresented. Here, we present a detailed petrological characterisation focussed on ortho- and clinopyroxene in six effusive flank eruptions and two Plinian eruptions of the PVC during the last ~14.1 ka. Texturally and compositionally defined pyroxene populations facilitate novel approaches to constraining magmatic temperatures and deconvolving crystallisation histories. At least two inter-connected magma reservoirs are present in the mid-to upper crust beneath the PVC: (1) a magnesian andesitic reservoir crystallising high-Mg orthopyroxene (Mg# 79–90) + clinopyroxene (Mg# 82–91) + Cr-spinel ± sulfide at temperatures of 1000–1115°C, and (2) a more evolved, shallower magma reservoir crystallising plagioclase (An50–28) + low-Mg orthopyroxene (Mg# 56–71) + clinopyroxene (Mg# 64–79) + magnetite + ilmenite + apatite ± sulfide at a long-term storage temperature of ~970°C. These mid- to upper crustal magma reservoirs are periodically recharged with a hydrous, high-Mg basaltic mantle melt that crystallised fosteritic olivine (Fo82–89) + Cr-spinel + low-Ca clinopyroxene with elevated Al2O3 (≤6 wt. %) in the lower- to mid-crust at 1080–1220°C. Short- and long-term melt and crystal hybridisation is the primary driver producing the diverse crystal cargo and restricted whole-rock compositions that characterise effusive and explosive eruptions of the PVC in the last ~14.1 ka. Injections of the magnesian andesite into the evolved reservoir produce transient melts crystallising hybrid rims around pre-existing pyroxene and plagioclase crystals. Crystals are subsequently stored in a heterogeneous mush comprising multiple generations of crystals dominated by injected pyroxene of mafic origin. The majority of mafic pyroxene crystals (≤78 %) were stored in the evolved reservoir for centuries to millennia, resulting in near-complete diffusive overprinting of the mafic compositions. Magma injections from depth frequently remobilise the crystal mush and may lead to effusive and explosive eruptions of hybridised PVC andesites and dacites.

Late Cenozoic Uguumur and Bod-Uul Volcanic Centers in Northern Mongolia: Mineralogy, Geochemistry, and Magma Sources

The paper presents new data on mineralogy, geochemistry, and Sr-Nd-Pb isotope systematics of Late Cenozoic eruption products of Uguumur and Bod-Uul volcanoes in the Tesiingol field of Northern Mongolia, with implications for the magma generation conditions, magma sources, and geodynamic causes of volcanism. The lavas and pyroclastics of the two volcanic centers are composed of basanite, phonotephrite, basaltic trachyandesite, and trachyandesite, which enclose spinel and garnet peridotite and garnet-bearing pyroxenite xenoliths; megacrysts of Na-sanidine, Ca-Na pyroxene, ilmenite, and almandine-grossular-pyrope garnets; and carbonate phases. The rocks are enriched in LILE and HFSE, show strongly fractioned REE spectra, and are relatively depleted in U and Th. The low contents of U and Th in Late Cenozoic volcanics from Northern and Central Mongolia represent the composition of a magma source. The presence of carbonate phases in subliquidus minerals and mantle rocks indicates that carbon-bearing fluids were important agents in metasomatism of subcontinental lithospheric mantle. The silicate-carbonate melts were apparently released from eclogitizied slabs during the Paleo-Asian and Mongol-Okhotsk subduction. The parent alkali-basaltic magma may be derived as a result from partial melting of Grt-bearing pyroxenite or eclogite-like material or carobantized peridotite. The sources of alkali-basaltic magmas from the Northern and Central Mongolia plot different isotope trends corresponding to two different provinces. The isotope signatures of megacrysts are similar to those of studied volcanic centers rocks. The P-T conditions inferred for the crystallization of pyroxene and garnet megacrysts correspond to a depth range from the Grt-Sp phase transition to the lower crust. Late Cenozoic volcanism in Northern and Central Mongolia may be a response to stress propagation and gravity instability in the mantle associated with the India-Asia collision.

Magmatic systems beneath Ashikule volcanic cluster (Western Kunlun, China): insights from compositional and textural features of lavas

This study considered the petrology, chemical composition, texture, and magma temperature and pressure of the volcanic rocks in the Ashikule volcanic cluster (AVC), Western Kunlun, China, to investigate the magmatic systems prior to eruption. The whole rock geochemistry of AVC lavas revealed a wide compositional range that included phonotephrite, basaltic trachyandesite, trachyandesite, trachyte, and acid rhyolite but mainly trachyandesite. The K2O/Na2O ratios were greater than 1, falling into the shoshonite series in a K2O vs. SiO2 diagram. Oxide diagrams showed that intermediate and acidic volcanic rocks have similar magmatic evolutions, mainly related to the fractional crystallization of pyroxene, plagioclase, apatite, and ilmenite. Acidic magma was found as the product of magmatic crystallization in the final stage. Trace element contents indicated that the magma underwent mixing with crustal components in the process of crystallization before eruption. Xishan phonotephrite revealed a different evolutionary series compared with the intermediate and acidic volcanic rocks, highlighting the differences of their magmatic sources. The equilibrium temperature and pressure of phonotephrite were 1146–1316 °C and 0.38–1.70 GPa, respectively, which correspond to a depth of 14–62 km. The equivalent values for intermediate rocks were 1062–1215 °C and 0.22–1.18 GPa, respectively, corresponding to a depth of 8–43 km. Acidic rocks had lower magmatic temperatures (778–889 °C). Active tectonics led to multiple events of magma upwelling, and the magma in each event underwent its own evolution. There were many different components of magma beneath the study area contemporaneously, i.e., multiple magma capsules with different compositions.

Water-in-olivine magma ascent chronometry: Every crystal is a clock

The syneruptive decompression rate of basaltic magma in volcanic conduits is thought to be a critical control on eruptive vigor. Recent efforts have constrained decompression rates using models of diffusive water loss from melt embayments, olivine-hosted melt inclusions and clinopyroxene phenocrysts; however, these techniques are difficult to apply because of the rarity of pyroxene crystals or melt embayments suitable for analysis and the complexities associated with modeling water loss from melt inclusions. We have developed a new magma ascent chronometer based on syneruptive diffusive water loss from olivine phenocrysts.We have found water zonation in every olivine phenocryst we have measured, from explosive eruptions of Seguam, Fuego, Kilauea and Cerro Negro volcanoes. The majority of the olivine phenocrysts were polished to expose a central plane normal to the crystallographic ‘b’ axis and volatile concentration profiles were measured along ‘a’ and ‘c’ axes by secondary ion mass spectrometry (SIMS). Profiles are compared to 1D and 3D finite-element models of diffusive water loss from olivine, whose boundaries are in equilibrium with a melt undergoing closed-system degassing.Least-squares fitting of measured water concentration gradients in olivine to a 1D Monte Carlo model produces constraints on magma decompression rates that are in good agreement with independent constraints from melt embayment studies and modeling of water loss from olivine-hosted melt inclusions at Fuego, Seguam, Kilauea, and Cerro Negro. Such agreement confirms the accuracy and sensitivity of the water-in-olivine chronometer over a range of decompression rates (dP/dt) spanning ~2 orders of magnitude (from 0.007 to 0.45 MPa/s). We find that the assumption of a zero-water boundary condition (in which the water concentration at the edges of the olivine phenocrysts is fixed at 0 ppm throughout their ascent) leads to an overestimation of the decompression rate by an order of magnitude compared to the closed-system degassing boundary condition assumed in our model, thereby highlighting the sensitivity of the water-in-olivine chronometer to the host magma degassing path.At Seguam, a wide range of best-fit values of dP/dt is obtained both from the water-in-olivine chronometer (0.04–0.23 MPa/s) and from melt embayments (0.02–0.13 MPa/s). We find systematically higher dP/dt values in the melt embayments that appear to have stalled or crystallized at the shallowest depths. Together, these observations are suggestive of magma acceleration during ascent.A strength of the water-in-olivine chronometer is the prevalence of olivine in mafic to intermediate magmas. This new technique yields many values of dP/dt from a single eruption, providing insight into the diversity of ascent records carried by the crystal cargo and possibly defining changes in dP/dt through time and space. This data density offers a more detailed window into syneruptive conduit processes than has been possible using other techniques for constraining dP/dt. In theory, each crystal is a clock, with the potential to record variable ascent in the conduit, over the course of an eruption, between eruptions, and among volcanic systems.

The brecciated texture of polymict eucrites: Petrographic investigations of unequilibrated meteorites from the Antarctic Yamato collection

AbstractWe report on the petrography and mineralogy of five Yamato polymict eucrites to better constrain the formation and alteration of crustal material on differentiated asteroids. Each sample consists of different lithic clasts that altogether form four dominant textures and therefore appear to originate from closely related petrological areas within Vesta′s crust. The textures range from subophitic to brecciated, porphyritic, and quench‐textured, that differ from section to section. Comparison with literature data for these samples is therefore difficult, which stresses that polymict eucrites are extremely complex in their petrography and investigation of only one thick section may not be representative for the host rock. We also show that sample Y‐793548 consists of more than one lithic unit and must therefore be classified as polymict instead of monomict. The variety and nature of lithic textures in the investigated Yamato meteorites indicate shock events, intense post‐magmatic thermal annealing, and secondary alteration. These postmagmatic features occur in different intensities, varying from clast to clast or among coexisting mineral fragments on a small, local scale. Several clasts within the eucrites studied have been modified by late‐stage alteration processes that caused deposition of Fe‐rich olivine and Fe enrichment along cracks crosscutting pyroxene crystals. However, formation of these secondary phases seems to be independent of the degree of thermal metamorphism observed within every type of clast, which would support a late‐stage metasomatism model for their formation.

Viscosity behaviour of silicate melts during cooling under variable shear rates

The viscosity of a crystal-bearing shoshonite from Vulcanello (Aeolian Archipelago, Italy) was measured using the concentric cylinder method. Experiments were performed in the temperature interval from superliquidus (1573 K) to 1373 K with the aim to investigate the rheological response of the magma during variable cooling rate (i.e. 1000, 100 and 10 K/h) and shear rate (0.01, 0.1 and 1 s−1). In one experiment, with a cooling rate of 10 K/h and shear rate of 1 s−1, clinopyroxenes (ca. 11 vol.%) with hopper texture crystallized. One further experiment was performed for a longer time with a cooling rate of 100 K/h and shear rate of 1 s−1 to the final temperature of 1323 K. In this case, the final product showed skeletal crystals of pyroxenes (ca. 17 vol.%) and an apparent viscosity of 1.10 × 104 Pa s, which was very close to the experiment performed at 10 K/h and shear rate of 1 s−1 to 1373 K (1.18 × 104 Pa s). The comparison between experiments performed at cooling rate of 100 K/h, and finally quenched at to 1373 and 1323 K, respectively, showed that a temperature decrease of 50 K produces an increase in the apparent viscosity of ca. 1 log unit due to the transition from crystal-free to crystal-bearing melts.This study provides new data on apparent viscosity variation as cooling and shear rates vary in a crystallizing magmatic system. Results show that, on average, the viscosity of a shoshonitic system can increase by about two orders of magnitude due to temperature decrease from 1573 to 1373 K and the incipient crystallization occurring at the lowest temperatures. Finally, the results confirm that crystal nucleation and growth kinetics, as well as magma viscosity, are controlled by the dynamic state of the system.

Magmatic–hydrothermal evolution of the Anjerd Cu skarn deposit, NW Iran: Perspectives on mineral chemistry, fluid inclusions and stable isotopes

The Anjerd Cu skarn deposit is located 40 km north of Ahar, NW Iran, which comprises three different stages of skarn–type mineralization and metasomatic alteration including (i) prograde, (ii) early retrograde and (iii) late retrograde. Prograde stage is identified by garnet as andradite–grossularite and diopsidic pyroxene crystallization, whereas retrograde stage overprinted the prograde skarn and is predominantly composed of tremolite, actinolite, quartz, epidote and chlorite. Metasomatic alteration started with generation and evolution of the mineralizing fluids in the Shayvar batholith, followed by the fluids ascending into fractures of the skarn in the metamorphic zone. Subsequently, retrograde alteration continued to the end of the skarnification process. Based on mineral chemistry studies, garnet is dominated by andradite (27–78 mol. %) and grossularite (20–67 mol. %) compositions and pyroxenes by diopside (65–98 mol. %) and hedenbergite (2–34 mol. %) compositions. Based on detailed petrography studies, three types of fluid inclusions were recognized including two–phase liquid–rich (LV), vapor–rich (VL), and three–phase (LVS) inclusions. The characteristic of the mineralizing fluids during the skarn evolution was changed significantly from early retrograde stage with (i) high temperature (up to 460 °C) and high salinity (up to 39 wt% NaCl equiv.), through late retrograde stage with (ii) moderate to high temperature (334 to 446 °C) and low salinity (0.3 to 0.8 wt% NaCl equiv.), (iii) low to moderate temperature (158 to 362 °C) and moderate salinity (1.7 to 12.3 wt% NaCl equiv.), suggesting that high–temperature and moderately saline fluids are indicative of a magmatic source within the minerals belonging to the prograde skarn, while the fluids with lower temperature and low to moderate salinity indicating a meteoric source for the retrograde skarn assemblage. The δ18O values for prograde garnet and magnetite vary from ∑0 to 8.2‰, whereas δ18O and δD values for retrograde calc–silicates in equilibrium with fluid including epidote range from 6.7 to 9.2‰ and −10.2 to −31.4 and for chlorite range from 1.9 to 2.7‰ and −37.5 to −48.1‰, respectively, pointing out that mixing of magmatic fluids with meteoric water in the retrograde alteration. Moreover, the δ34S values of pyrite and chalcopyrite (−3.6 and +4.9‰), suggesting a magmatic source for sulfur which was largely responsible for the mineralization in the Anjerd deposit.

El Escondido tuff cone (38 ka): a hidden history of monogenetic eruptions in the northernmost volcanic chain in the Colombian Andes

El Escondido is a dacitic monogenetic volcano situated in the Samana monogenetic volcanic field, within the Central Cordillera of Colombia. The tuff cone was emplaced in a deeply incised and rainy mountainous zone, ca. 38 ky ago by an explosive eruption that affected not only the metamorphic and igneous basement but also the remnants of the ~ 154 ka Pela Huevos volcano. The El Escondido volcaniclastic deposits are composed of juvenile pumice and lithic fragments including dense volcanic rocks from the Pela Huevos volcano, as well as metamorphic and igneous rocks from the basement. The pumice shows tubes and spongy textures. The volcanic lithics are dominantly angular and fresh, and exhibit different mineralogy and whole-rock geochemistry in comparison to the pumice. Plagioclase and amphibole are ubiquitous; however, biotite and quartz crystals occur only in the pumice fragments (~ 70 wt% SiO2 volatile-free), whereas olivine and pyroxene crystals are only found in the volcanic lithics (~ 65 wt% SiO2 volatile-free). The El Escondido tuff cone is strongly eroded and Pela Huevos is a dome-like remnant in the SE sector. Because of this, along with the highly vegetated tropical zone where the volcanoes are emplaced as well as difficult political issues in the region, the edifices were not recognized until recently; this is why the younger cone was named “El Escondido” (which means “The Hidden”). These eruptions evidence that recent volcanism has occurred in a zone of the Central Cordillera that has been considered as non-volcanogenic in recent studies.