Shallow Crustal Conditions Research Articles

Partial Melting under Shallow-Crustal Conditions: A Study of the Pleistocene Caldera Eruption of Mendeleev Volcano, Southern Kuril Island Arc

Abstract The southern part of the Kuril Island Arc is one of the world’s most active modern volcanic zones, with widespread felsic caldera volcanism, but it has been less well studied compared with other arcs. The Mendeleev caldera-forming eruption (40 ka) on Kunashir Island, southern Kurils, is one of the most explosive Pleistocene eruptions to have occurred in this region. This study aimed to establish the origin and storage conditions of magma preceding the Pleistocene caldera eruption of Mendeleev volcano. Mineralogical and melt inclusion analyses reveal that the primary melts had felsic compositions and that the early stage of magmatic evolution involved amphibole breakdown into a two-pyroxene, plagioclase, and Fe–Ti oxide assemblage under pressure–temperature conditions of 107–314 MPa and 807–932°C. The caldera-forming products are represented by dacitic pumice composed of plagioclase + augite + hypersthene+ quartz + Fe–Ti oxides and melt with uniform low-K rhyolite composition. Pre-eruptive magma was stored in a reservoir at 77–195 MPa (3.0–7.6 km depth) and 830–890°C under H2O-saturated conditions. The mechanism of magmatic evolution implies the following two-step scenario: (1) generation of magma by the partial melting of an amphibole-bearing substrate accompanied by the formation of early Mg-rich clino- and orthopyroxene, plagioclase, Fe–Ti oxides, and peritectic rhyolitic melt; and (2) crystallization of late plagioclase and quartz directly from these partial melts. Local or regional extension during the Pleistocene, accompanied by increasing heat flow in the supra-subduction mantle, generated an active mafic intrusion into the upper crust. This process was accompanied by abundant subaerial eruptions of basaltic volcanoes and could have caused intense heating and partial melting of upper-crustal rocks. Our results indicate that the partial melting of amphibole-bearing substrates in island arcs may serve as a universal mechanism for the generation of silicic magmas during powerful caldera eruptions in young island arcs.

Structural Evolution of the Central Kiruna Area, Northern Norrbotten, Sweden: Implications on the Geologic Setting Generating Iron Oxide-Apatite and Epigenetic Iron and Copper Sulfides

AbstractTo guide future exploration, this predominantly field based study has investigated the structural evolution of the central Kiruna area, the type locality for iron oxide-apatite deposits that stands for a significant amount of the European iron ore production. Using a combination of geologic mapping focusing on structures and stratigraphy, petrography with focus on microstructures, X-ray computed tomography imaging of sulfide-structure relationships, and structural 2D-forward modeling, a structural framework is provided including spatial-temporal relationships between iron oxide-apatite emplacement, subeconomic Fe and Cu sulfide mineralization, and deformation. These relationships are important to constrain as a guidance for exploration in iron oxide-apatite and iron oxide copper-gold prospective terrains and may help to understand the genesis of these deposit types. Results suggest that the iron oxide-apatite deposits were emplaced in an intracontinental back-arc basin, and they formed precrustal shortening under shallow crustal conditions. Subsequent east-west crustal shortening under greenschist facies metamorphism inverted the basin along steep to moderately steep E-dipping structures, often subparallel with bedding and lithological contacts, with reverse, oblique to dip-slip, east-block-up sense of shears. Fe and Cu sulfides associated with Fe oxides are hosted by structures formed during the basin inversion and are spatially related to the iron oxide-apatite deposits but formed in fundamentally different structural settings and are separated in time. The inverted basin was gently refolded and later affected by hydraulic fracturing, which represent the last recorded deformation-hydrothermal events affecting the crustal architecture of central Kiruna.

Decoding multiple zoning patterns in clinopyroxene phenocrysts at Vulcano Island: A record of dynamic crystallization through interconnected reservoirs

Here we document how the different growth features and intracrystalline distributions of both major and trace cations in clinopyroxene phenocrysts are important recorders of the intricate magma dynamics at Vulcano Island (Aeolian Arc, Italy). The compositions of clinopyroxene phenocrysts from products erupted over the last ~54 ka cluster at different degrees of evolution, paralleling the polybaric-polythermal differentiation of mantle-derived mafic magmas into more evolved silicic melts. The hotter lower crust is the most favorable location for the storage of mafic magmas and the early crystallization of diopsidic (Mg#91) clinopyroxene (Pmax ≈ 750 MPa and Tmax ≈ 1220 °C). Diopsidic phenocrysts are depleted in both rare earth elements (REE) and high field strength elements (HFSE) but are enriched in transition elements (TE). The transfer and accumulation of primitive magmas in the colder upper crustal regions lead to the formation of an interconnected series of more differentiated magmatic reservoirs (P ≈ 100–450 MPa and T ≈ 1100–1180 °C) hosting discrete populations of clinopyroxene (Mg#84–85) with a broad spectrum of zonations and dissolution features. Recharge bands in clinopyroxene are markers of multiple inputs of primitive REE-HFSE-poor, TE-rich magmas from depth. Augitic phenocrysts (Mg#82) with strong negative Eu anomaly and REE + HFSE enrichments crystallizes from highly differentiated trachytic and rhyolitic melts stored at very shallow crustal conditions (P ≤ 50 MPa and T ≤ 1100 °C). These silicic reservoirs represent residual melts trapped-extracted from crystal-dominated mush regions in the uppermost part of the plumbing system. The residence time of clinopyroxene increases from ∼0.1 to ∼44 years from basalt to rhyolite, together with an increasing number of recharge bands. The mineral assemblage in more silicic and viscous mush melts is sufficiently resilient to record numerous mafic injections and high degrees of magma mixing, hybridization, and crystallization before eruption. Overall, the compositional zoning pattern of clinopyroxene presents a picture of plumbing system that extends through the crust and is characterized by distributions of melts and crystals which are progressively more evolved and heterogeneous in both space and time.

The pressure–temperature–time–deformation history of the Beni Mzala unit (Upper Sebtides, Rif belt, Morocco): Refining the Alpine tectono‐metamorphic evolution of the Alboran Domain of the western Mediterranean

AbstractThe structural and thermal relaxation overprint associated with the Neogene Alboran rifting have obscured the early Alpine tectono‐metamorphic evolution of the Alboran Domain, representing the metamorphic core of the Betic–Rif orogen of the western Mediterranean region. This study focuses on the Beni Mzala unit, forming the lower and deeper structural level of the Alpine metamorphic nappe stack (Upper Sebtides) in the Moroccan Rif. Meso‐ and micro‐scale structural investigations are carried out on high‐P aluminum silicate (Ky‐bearing)‐quartz segregations that occur as boudins within the main retrogressive syn‐greenschist foliation (S2/D2) and assumed to preserve the early M1 HP metamorphism associated with the Alpine orogenic construction in the Alboran Domain. These boudins host an early crenulated high‐P foliation (S1, D1/M1) made of quartz–kyanite–white mica–rutile. A large spread in white mica composition is documented, with the highest Si content per formula unit (up to 3.18 apfu) preserved along the S1 foliation and the lower Si content observed in the white mica marking the S2 retrogressive foliation (D2/M2) and the rim of S1 mica. Microtextural evidence documents post‐tectonic andalusite growth and static recrystallization of the quartz microlithons. Inverse (Zr‐in‐Rt thermometry) and forward modelling thermobarometry are integrated with Ar–Ar white mica geochronology to define the peak and exhumation pressure–temperature–time (P–T–t) path of the Beni Mzala unit. Minimum thermo‐baric estimates for the M1 event are ~1.4 GPa and 600°C, corresponding to a metamorphic gradient of ~11°/km, consistent with subduction zone metamorphism. Exhumation is constrained by re‐equilibration of the white mica composition (from high to low celadonite) between c. 29 and 22 Ma, during a nearly isothermal retrogressive path, with final equilibration at high‐T/low‐P conditions within the andalusite stability field (~0.2–0.3 GPa and 500°C). A minimum late Oligocene age is proposed for the Alpine D1 tectono‐metamorphic stage in the Rif, suggesting as feasible the previously proposed Eocene timing for the subduction‐zone metamorphism of the Alboran Domain. Conclusive evidence is provided to link the early Miocene tectono‐metamorphic event to a late thermal perturbation that affected the Alboran Domain at shallow crustal conditions, post‐dating the almost complete exhumation of the deep roots of the Alpine belt in the western Mediterranean.

Estimating the depth and evolution of intrusions at resurgent calderas: Los Humeros (Mexico)

Abstract. Resurgent calderas are excellent targets for geothermal exploration, as they are associated with the shallow emplacement of magma, resulting in widespread and long-lasting hydrothermal activity. Resurgence is classically attributed to the uplift of a block or dome resulting from the inflation of the collapse-forming magma chamber due to the intrusion of new magma. The Los Humeros volcanic complex (LHVC; Mexico) consists of two nested calderas: the outer and older Los Humeros formed at 164 ka and the inner Los Potreros formed at 69 ka. The latter is resurgent and currently the site of an active and exploited geothermal field (63 MWe installed). Here we aim to better define the characteristics of the resurgence in Los Potreros by integrating fieldwork with analogue models and evaluating the spatio-temporal evolution of the deformation as well as the depth and extent of the intrusions responsible for the resurgence, which may also represent the local heat source(s). Structural field analysis and geological mapping show that the floor of the Los Potreros caldera is characterized by several lava domes and cryptodomes (with normal faulting at the top) that suggest multiple deformation sources localized in narrow areas. Analogue experiments are used to define the possible source of intrusion responsible for the observed surface deformation. We apply a tested relationship between the surface deformation structures and depth of elliptical sources to our experiments with sub-circular sources. We found that this relationship is independent of the source and surface dome eccentricity, and we suggest that the magmatic sources inducing the deformation in Los Potreros are located at very shallow depths (hundreds of metres), which is in agreement with the well data and field observations. We propose that the recent deformation at LHVC is not a classical resurgence associated with the bulk inflation of a deep magma reservoir; rather, it is related to the ascent of multiple magma bodies at shallow crustal conditions (<1 km depth). A similar multiple source model of the subsurface structure has also been proposed for other calderas with an active geothermal system (Usu volcano, Japan), suggesting that the model proposed may have wider applicability.

From hot to cold - The temperature dependence on rock deformation processes: An introduction

Understanding rock deformation processes in solid Earth materials, from the crystal to the tectonic plate scale, is essential for characterising the evolution of the lithosphere and for predicting how rocks behave in the subsurface. Temperature is a key parameter that determines what rock deformation processes are active and therefore how tectonic structures form. Moreover, it controls the migration of fluids and melt in the Earth's crust that lead to phase transformations and changes in rock rheology. This special issue gathers a collection of research papers following the 21st International Conference on Deformation Mechanisms, Rheology and Tectonics (DRT-2017), which was held in Inverness (Scotland) in April–May 2017, organised by the University of Aberdeen. These contributions provide a significant advance in the study of rock deformation and fluid/melt migration at multiple crustal levels, from deformation bands near the Earth's surface to shear zones in partially molten rocks in the lower crust. In this introductory article, we first provide an overview of how temperature controls deformation mechanisms and processes and then introduce the collection of research papers ordered from those analysing deformation processes occurring at high temperatures to contributions reporting deformation at shallow crustal conditions.

Rapid clay precipitation in explosion-induced fractures

Abstract Fractures within the earth control rock strength and fluid flow, but their dynamic nature is not well understood. As part of a series of underground chemical explosions in granite in Nevada, we collected and analyzed microfracture density data sets prior to, and following, individual explosions. Our work shows an ∼4-fold increase in both open and filled microfractures following the explosions. Based on the timing of core retrieval, filling of some new fractures occurs in as little as 6 wk after fracture opening under shallow (<100 m) crustal conditions. These results suggest that near-surface fractures may fill quite rapidly, potentially changing permeability on time scales relevant to oil, gas, and geothermal energy production; carbon sequestration; seismic cycles; and radionuclide migration from nuclear waste storage and underground nuclear explosions.

Multiphase magma intrusion, ore-enhancement and hydrothermal carbonatisation in the Siah-Kamar porphyry Mo deposit, Urumieh-Dokhtar magmatic zone, NW Iran

The Siah-Kamar Mo deposit (SKMD) is located at the northwestern termination of the Urumieh-Dokhtar magmatic zone and it is the only porphyry Mo ore reserve in Iran. The exploration program documented 39.2 Mt proved reserves @ 539 ppm Mo and 66.4 Mt probable reserves @ 266 ppm Mo. In this study, field and petrographic investigations, integrated with geochemical (fluid inclusion and quartz chemistry) and geochronological (U-Pb zircon, Re-Os molybdenite, and Rb-Sr multimineral isochron) studies are used to propose a metallogenic model for the Mo mineralisation in the SKMD. The geology of the SKMD is characterized by the emplacement of a multiphase Oligocene basic/intermediate (at ca. 33–30 Ma) to acidic (29–28 Ma) magmatic suite, which intruded the Eocene volcanic country rocks. The alteration zone, about 4 × 3 km in size and with a general NW-SE trend, is centered within the main basic porphyry stock, grading from an inner potassic-sodic zone to peripheral phyllic/propylitic halos. The late acidic magmatic products (stocks and dykes) intruded and post-dated the main alteration zone. Two-stage Mo mineralisation is recognised, including: (i) stage-1, disseminated molybdenite, coeval with the formation of potassic-sodic alteration and minor, microscale Fsp, Bt, Qz + Po veinlets; and (ii) stage-2, high-grade molybdenite + carbonate (±sericite), structurally-controlled stockwork veining. Fluid inclusion systematics combined with TitaniQ thermometry documents a mineralising fluid system compatible with a transition from high-temperature (up to ca. 600 °C) magmatic to epithermal (250 °C) conditions during progressive cooling, exhumation and mixing with meteoric sources at shallow crustal conditions (ca. 7–3 km). The Re-Os molybdenite dating constrains the high-grade Mo ore formation at ca. 29–28 Ma, attesting for the intimate linkage between the main Mo mineralisation and the acidic magmatic phase in the area. The Rb-Sr geochronology of the potassic-sodic alteration zones confirms the two-stage magmatic/mineralisation scenarios, overlapping within errors with the results obtained from the U-Pb zircon geochronology and constraining the formation of the potassic-sodic and phyllic alteration at ca. 33 and 28 Ma, respectively. Our results document an uncommon scenario of two-stage porphyry Mo mineralisation associated with intensive late stage carbonate precipitation and achieved during a long-lasting and multiphase magmatic pulses of Oligocene age. We highlight the dominant role of acidic fluid neutralisation for further ore enrichment during polyphase magma intrusion as the dominant factor controlling the Mo mineralisation in the SKMD. Comparison at a regional-scale indicates that parameters such as longevity of magma supply, progressive magma crystallization/differentiation, and the presence of a possible pre-enriched crustal material should be considered responsible for the Mo endowment in the UDMZ.

Modeling the Crystallization and Emplacement Conditions of a Basaltic Trachyandesitic Sill at Mt. Etna Volcano

This study documents the compositional variations of phenocrysts from a basaltic trachyandesitic sill emplaced in the Valle del Bove at Mt. Etna volcano (Sicily, Italy). The physicochemical conditions driving the crystallization and emplacement of the sill magma have been reconstructed by barometers, oxygen barometers, thermometers and hygrometers based on clinopyroxene, feldspar (plagioclase + K-feldspar) and titanomagnetite. Clinopyroxene is the liquidus phase, recording decompression and cooling paths decreasing from 200 to 0.1 MPa and from 1050 to 940 °C, respectively. Plagioclase and K-feldspar cosaturate the melt in a lower temperature interval of ~1000–870 °C. Cation exchanges in clinopyroxene (Mg-Fe) and feldspar (Ca-Na) indicate that magma ascent is accompanied by progressive H2O exsolution (up to ~2.2 wt. %) under more oxidizing conditions (up to ΔNNO + 0.5). Geospeedometric constraints provided by Ti–Al–Mg cation redistributions in titanomagnetite indicate that the travel time (up to 23 h) and ascent velocity of magma (up to 0.78 m/s) are consistent with those inferred for other eruptions at Mt. Etna. These kinetic effects are ascribed to a degassing-induced undercooling path caused principally by H2O loss at shallow crustal conditions. Rare earth element (REE) modeling based on the lattice strain theory supports the hypothesis that the sill magma formed from primitive basaltic compositions after clinopyroxene (≤41%) and plagioclase (≤12%) fractionation. Early formation of clinopyroxene at depth is the main controlling factor for the REE signature, whereas subsequent degassing at low pressure conditions enlarges the stability field of plagioclase causing trace element enrichments during eruption towards the surface.

Late-stage anhydrite-gypsum-siderite-dolomite-calcite assemblages record the transition from a deep to a shallow hydrothermal system in the Schwarzwald mining district, SW Germany

The majority of hydrothermal vein systems of economic interest occur at relatively shallow crustal levels, although many of them formed at significantly greater depths. Their present position is a consequence of uplift and erosion. Although, many aspects of their formation are well constrained, the temporal chemical evolution of such systems during uplift and erosion is still poorly understood. These vein minerals comprise calcite, dolomite-ankerite, siderite-magnesite, anhydrite and gypsum forming the last gangue assemblages in Jurassic and Tertiary sulphide-fluorite-quartz-barite veins of the Schwarzwald mining district, SW Germany. Mineral textures of samples from nine localities reveal that in these sequences, mineral precipitation follows a recurring pattern: early calcite is followed by anhydrite or gypsum, siderite and/or dolomite. This succession may repeat up to three times.In-situ (LA-ICP-MS) U-Pb age dating of 15 carbonates from three subsequent generations of the late-stage vein assemblage yield robust ages between 20 and 0.6 Ma. Each mineral sequence forms in a distinctive period of about 2–5 Ma. These ages clearly relate these late-stage mineral phases to the youngest geological episode of the Schwarzwald, which is associated with the Cenozoic Rhine Graben rifting and basement uplift.Based on thermodynamic modelling, the formation of the observed mineral assemblages required an deeply sourced Mg-, Fe- and SO4-rich fluid (b), which was episodically mixed with a shallow crustal HCO3-rich fluid (a). As a consequence of fluid mixing, concentrations of Mg, Fe and SO4 temporarily increased and initiated the formation of the observed sulphate-carbonate mineral sequences.This discontinuous large-scale vertical fluid mixing was presumably directly related to episodes of active tectonics associated with the Cenozoic strike-slip regime of the Upper Rhine Graben. Analogously, episodic fluid mixing is a major key in the formation of older (Jurassic to early Tertiary) Pb-Zn-fluorite-quartz-barite assemblages in the same specific vein systems, albeit involving different fluid compositions.Late-stage hydrothermal (∼20–70 °C) vein assemblages reported in this study record the transition from deep (>2 km) to very shallow (0–1 km) crustal conditions. As a consequence of successive uplift, increasing proportions of shallower and cooler (∼50–70 °C) fluids could take part in such mixing processes. Associated changes in the fluid composition caused the vein mineralogy to change from sulphide-quartz-fluorite-barite to calcite-anhydrite/gypsum-siderite-dolomite, as the system passively ascended closer to the surface.

Major Element and Halogen (F, Cl) Mineral–Melt–Fluid Partitioning in Hydrous Rhyodacitic Melts at Shallow Crustal Conditions

Twenty-four internally heated pressure vessel experiments were conducted at 810–860°C, 1·5–4·05 kbar, and oxygen fugacities (fO2) = NNO –0·5 to NNO +2 log units (where NNO is the nickel–nickel oxide buffer), using hydrous rhyodacitic starting glasses from Mt Usu, Japan. Aqueous solutions were added to experimental charges such that the volatile phase(s) coexisting with the crystalline phases (amphibole, plagioclase, and clinopyroxene) at run conditions buffered the F, Cl, S, and CO2 concentrations in the melt. The resultant phenocryst phases and glass chemistries were analysed by electron microprobe and Fourier transform infrared spectroscopy, and final fluid Cl– contents by chloridometer. All experiments produced homogeneous glasses and large euhedral phenocrysts with minimal compositional zonation. The results of the crystal–melt partitioning data are applicable to understanding the geochemical evolution of rhyodacitic melts at fluid-saturated shallow crustal levels, the composition of fluid phases later exsolved owing to second boiling, and the effects of halogens on amphibole crystal chemistry. The residual glasses from these experiments span the dacite–rhyolite compositional join, and are hydrous (>5 wt % H2O) with F concentrations from ≤100 ppm to 0·63 wt % and Cl from ∼130 ppm to 0·72 wt %. Measured final fluid Cl contents show that Cl strongly prefers the fluid phase over the melt phase in all experiments, with DClfluid/melt ranging from 3·5 to 22·7. Amphibole compositions are calcic, Mg-rich, and typical of those found in natural calc-alkaline arc magmas. They are particularly sensitive to changes in melt halogen chemistry, with maximum amphibole F contents of 2·59 wt % (maximum DFamph/melt values of ≥15) and maximum Cl contents of 0·12 wt % (maximum DClamph/melt values of 0·40). Integration of the amphibole data with other experimental data shows that Cl incorporation is a strong function of the Mg–Cl crystallographic avoidance principle, and that addition of F to the melt strongly decreases Cl partitioning at equivalent Mg# (DClamph/melt = 0·10 ± 0·02 in experiments that were not F-doped, compared with DClamph/melt = 0·05 ± 0·01 in F-doped experiments). Plagioclase compositions are relatively restricted, with anorthite contents An59–An39, and clinopyroxene is similarly calcic, containing a significant enstatite component (Wo49–Wo30). The behaviour of Fe in the glass and crystalline phases is most significantly affected by the fO2 of the experiment. Higher total Fe contents are found in amphibole and clinopyroxene from experiments with fO2 < NNO +1, but with little effect on the absolute DFeOmineral/melt. Plagioclase, however, shows relatively decreased DFeOplag/melt reflecting its preferential incorporation of Fe3+, and less Fe3+ expected in melts crystallized at lower fO2 conditions. Comparison of the data with recently formulated amphibole geothermometers, barometers, and plagioclase hygrometers shows calculated results that are consistent with the actual experimental conditions but discrepancies arise owing to halogen-induced major element variations in amphibole, indicating the importance of routinely measuring F and Cl in both natural and experimental amphibole.

Ultrafine spherical quartz formation during seismic fault slip: Natural and experimental evidence and its implications

In recent works on the determination of pseudotachylyte within the principal slip zone (PSZ) of the Chelungpu fault (Taiwan), we demonstrated that frictional melting occurred at shallow depths during the 1999 Mw 7.6 Chi-Chi earthquake. Thus, the characteristics of melts are of paramount importance to investigate processes controlling dynamic fault mechanics during seismic slips. We conducted rock friction experiments on siltstone recovered from the Taiwan Chelungpu fault Drilling Project (TCDP) at a slip rate of 1.3m/s and a normal stress of 1MPa. Here we not only target to characterize experimental pseudotachylyte and evaluate the associated frictional behavior, but also compare it with natural frictional melts of TCDP. Our results show that (1) initial shear stress drop was related to the generation of low viscosity melt patches, (2) the evolution of shear stress in the postmelting regime was congruent with frictional melt rheology, and (3) the slip strengthening presumably resulted from the extremely low content of water of the frictional melt. In particular, the state-of-art of in situ synchrotron analyses (X-ray diffraction and Transmission X-ray Microscope) determine the presence of ultrafine spherical quartz (USQ) grains (~10nm to 50nm) in the glassy matrices presumably produced at high temperature. Our observations confirm that the USQ grains formed in rock friction experiments do occur in natural faults. We surmise the USQ is the result of frictional melting on siltstone and represents the latest slip zones of the Chelungpu fault, and further infer that the viscous melts may terminate seismic slips at shallow crustal conditions.

Influence of calcite decarbonation on the frictional behavior of carbonate-bearing gouge: Implications for the instability of volcanic flanks and fault slip

The impact of heat, readily provided by magma, circulating hot fluids, or rapid fault slip on carbonate substrata is an important factor in determining the flank stability of volcanoes and the mechanical behavior of faults in areas where carbonate lithologies are prevalent. The mineralogy and mechanical properties of carbonate rocks are demonstrably altered by thermally induced decarbonation. While previous studies have considered the role of decarbonation in bulk strength loss at subvolcanic conditions and dynamic weakening during coseismic fault slip, little is known about the effects of decarbonation on the frictional properties of carbonate rocks during both the nucleation and inter-seismic phases. Here, we present results from experiments performed on a portlandite-rich material, a typical hydrous product of the decarbonation reaction. To evaluate frictional strength, stability, and healing at shallow crustal conditions, we sheared gouge layers of this material under saturated conditions at room temperature and at velocities comparable to those involved in earthquake nucleation. Our data indicate that the reaction of calcite to portlandite results in a distinct change in the mechanical behavior of the gouge. Decarbonated shear zones are (1) frictionally weaker at higher normal stresses, (2) more frictionally unstable, and (3) likely to regain their frictional strength more quickly than unaltered calcite-rich zones. The occurrence of portlandite could be key for interpreting the stability of volcanic flanks that root into carbonate substrata and for seismogenic normal faults located within thick sedimentary sequences, and thus provide a better understanding of the hazards they pose.

Mesostructural analysis of S-C fabrics in a shallow shear zone of the Umbria–Marche Apennines (Central Italy)

Abstract Several examples of foliated fault zones characterized by penetrative S-C fabric are exposed in the Umbria–Marche region, often associated with major thrusts. These tectonites, dominated by pressure-solution processes, are developed at shallow depths (&lt;3 km) in sedimentary rocks. In this paper, we perform a detailed structural analysis of the Scheggia Thrust Zone (STZ), a significant example of S-C tectonite spectacularly exposed in the Umbria–Marche Apennines. We reconstruct both the architecture and internal structure of the shear zone, focusing on the factors controlling the genesis and development of the tectonic pattern such as: (a) anisotropy; (b) lithological contrast; and (c) distance from the fault core. The STZ shows an intensely cleaved fault core (S-C fabric) and a thick and asymmetric damage zone developed in the footwall rocks. By analysing the spacing of the S-surfaces, we observe that the magnitude of deformation decreases with distance from the fault core. However, far from the fault core the magnitude of deformation is strongly controlled by the lithology of the protolith: almost undeformed calcareous beds alternate with intensely sheared intervals, localized along weak, marly horizons, also characterized by flattened S-planes. Based on these data we have divided the STZ into three subzones where the deformation is mainly driven by: (1) distance from the fault core; and (2) the lithology of the protolith. Marly horizons into these subzones, with respect to the calcareous horizon, show that: (i) S-planes are more closely spaced; and (ii) the angle between C- and S-planes is smaller. Our study confirms that the rhythmic alternation of calcareous and marly rocks can favour the development of foliated fault zones, also producing an increase in pressure-solution cleavage at shallow crustal conditions.

Constraints on the mobilization of Zr in magmatic-hydrothermal processes in subduction zones from in situ fluid-melt partitioning experiments

The partitioning of Zr between high P-T aqueous fluids and melts has been investigated in situ in the haplogranite-H2O and haplogranite-(F)-H2O systems to assess the mobilization of high field strength elements (HFSE) in magmatic-hydrothermal processes in subduction zones. The partition coefficients Df/Zmr were determined from Zr concentrations measured in situ by synchrotron X-ray fluorescence (SXRF) in both aqueous fluids and F-free or F-bearing hydrous haplogranite melts equilibrated in diamond-anvil cells at 575 to 800 °C and 0.3 to 2.4 GPa. This experimental approach eliminates the need for internal or external calibrations of the SXRF signal and/or post-mortem analysis of the melt phase, hence decreasing the total uncertainties on Df/Zmr below 16%. Above 0.6 GPa, Zr partitions favorably into the hydrous silicate melt in both F-free and F-bearing systems, with Df/Zmr that range between 0.19 ± 0.02 and 0.38 ± 0.03. However, the relatively high Df/Zmr values indicate that alkali-silica rich aqueous fluids generated by metamorphic devolatilization may contribute significantly to the recycling of HFSE in subduction zones. The efficient uptake of Zr (and likely other HFSE) by subduction zone fluids, regardless of their nature (aqueous fluid, hydrous melt, or supercritical fluid), supports the idea that the typical HFSE depletion recorded in arc magmas does not result from their incompatibility in water-rich slab-derived fluids but most probably originates from complex fluid-melt-rock interactions occurring at the slab interface and within the mantle wedge. At shallow crustal pressure conditions (800 °C and 0.3 GPa), Zr partitions reversely into the aqueous fluid in the presence of fluorine (Df/Zmr = 1.40 ± 0.10) as observed for Nb at similar conditions by Webster et al. (1989). The enrichment of the aqueous phase in HFSE (Zr, Nb) at shallow crustal conditions is likely related to the enhanced peralkalinity of low pressure, F-bearing aqueous fluid with temperature, that provides the favorable conditions for their mobilization via the formation of HFSE-O-Si/Na clusters. This mechanism may control the enrichment in HFSE (and plausibly other rare metals such as REE) in early magmatic fluids exsolved from granitic melts, leading to the formation of HFSE-enriched aggregates in shallow magmatic-hydrothermal environments (e.g., Strange Lake and Thor Lake Nechalacho deposit, Canada; Galineiro complex, Spain).

Experimental phase relations of a low MgO Aleutian basaltic andesite at XH2O = 0.7–1

We conducted melting experiments on a low MgO (3.29 wt.%) basaltic andesite (54.63 wt.% SiO2) from Westdahl volcano, Alaska, at XH2O = 0.7–1 and fO2 ~ Ni–NiO, at pressures = 0.1–180 MPa and temperatures = 900–1,200 °C. We examine the evolution of the melt along a liquid line of descent during equilibrium crystallization at high H2O and fO2 conditions, starting from a high FeOt/MgO, low MgO basaltic andesite. Ti-magnetite formed on the liquidus regardless of XH2O, followed by clinopyroxene, plagioclase, amphibole, and orthopyroxene. We observe slight but significant differences in the phase stability curves between the XH2O = 1 and 0.7 experiments. Early crystallization of Ti-magnetite and suppression of plagioclase at higher pressures and temperatures resulted in strongly decreasing melt FeOt/MgO with increasing SiO2, consistent with a “calc-alkaline” compositional trend, in agreement with prior phase equilibria studies on basalt at similar H2O and fO2. Our study helps quantify the impact of small amounts of CO2 and high fO2 on the evolution of melts formed during crystallization of a low MgO basaltic andesite magma stored at mid- to shallow crustal conditions. Like the prior studies, we conclude that H2O strongly influences melt evolution trends, through stabilization of Ti-magnetite on the liquidus and suppression of plagioclase at high P–T conditions.

Silica gel formation during fault slip: Evidence from the rock record

Dynamic reduction of fault strength is a key process during earthquake rupture. Many mechanisms for causing coseismic weakening have been proposed based on theory and laboratory experiments, including silica gel lubrication. However, few have been observed in nature. Here we report on the fi rst documented occurrence of a natural silica gel coating a fault surface. The Corona Heights fault slickenside in San Francisco, California, is covered by a shiny layer of translucent silica. Microstructures in this layer show fl ow banding, armored clasts, and extreme comminution compared to adjacent cataclasites. The layer is composed of ~100 nm to 1 µm grains of quartz, hydrous crystalline silica, and amorphous silica, with 10‐100 nm inclusions of Fe oxides and ellipsoidal silica colloids. Kinematic indicators and mixing with adjacent cataclasites suggest the shiny layer was fl uid during fault slip. The layer therefore represents a relict silica gel that formed during fault motion, and which could have resulted in frictional instability. These observations confi rm that the silica gels formed in rock friction experiments do occur in natural faults and therefore that silica gel formation can act as a dynamic weakening mechanism in faults at shallow crustal conditions.

Linking permeability to crack density evolution in thermally stressed rocks under cyclic loading

We explore the influence of incremental crack damage induced from cyclic loading on the mechanical and transport properties of thermally stressed crystalline rocks in shallow crustal conditions that are representative for volcanic edifices and high thermal gradient in the upper crust. The behavior of a lava flow basalt from Mt. Etna volcano, containing an high level of pre-existing crack damage induced from natural thermal cooling (Mt. Etna basalt) is contrasted with the response of crystalline rock (Westerly granite) with little initial crack damage. Selected samples of Westerly granite were thermally-treated at 500 and 800 C in order to study the role of induced thermal stressing on increasing crack damage. Permeability and water volume content were measured throughout the loading history together with deformation modulus. Permeability show a reduction at low stresses followed by an increase at intermediate stresses and ultimately a steady magnitude at failure. We use water volume content as a proxy for fluid storage and show that both permeability and storage evolve with damage and evolution of crack density. We use mechanistic models of crack evolution to make constrained predictions of the evolution in porosity and permeability via equivalent medium theories. These observations are used to address important questions regarding the coupled role of damage state and stressing in controlling the transport properties of rocks.

Highly explosive 2010 Merapi eruption: Evidence for shallow-level crustal assimilation and hybrid fluid

The processes responsible for the highly explosive events at Merapi, Central Java, Indonesia have been investigated through a petrological, mineralogical and geochemical study of the first-stage tephra and pyroclastic flows sampled in October and November 2010, and second-stage ash sampled shortly after the 5–6th November 2010 paroxysmal subplinian eruption. Several chemical and physical parameters suggest that the magma assimilated calc-silicate xenoliths derived from the surrounding carbonate-bearing crust (Javanese limestone). The bulk volcanic samples have highly radiogenic 87Sr/86Sr (0.70571–0.70598) ratios that approach the compositional field of material similar to the calc-silicate xenoliths. The 2010 plagioclase phenocrysts from the pyroclastic flow and tephra reveal anorthite cores (up to An94–97) with low FeO contents (≤0.8wt.%), and 18O enrichment (6.5‰ δ18O). The major and trace elements of the silicic glasses and phenocrysts (plagioclase, low-Al augite and titanomagnetite), the Sr-isotopic compositions of the bulk samples and plagioclases erupted in 2010 can be explained by complete digestion of the 1998 and 2006 calc-silicate xenoliths. The bulk assimilation proceeded through binary mixing between a calcic melt (representing Crustal Assimilant, CaO up to 10.5wt.% and CaO/Al2O3 up to 1.2) and the deep source hydrous K-rich melt. Similarly to the 1998 and 2006 calc-silicate xenolith composition, the 2010 Crustal Assimilant is enriched in Mn (MnO up to 0.5wt.%), Zn, V, and Sc contents. In contrast, the hydrous K-rich melt is enriched in volatiles (Cl up to 0.37wt.% and bulk H2O+CO2 up to 5±1wt.%), Al2O3, TiO2 and REE contents, consistent with its derivation from deep source. This hydrous K-rich melt may have been saturated with an aqueous Cl-rich fluid at about 200MPa, a pressure consistent with the level of the crustal assimilation. We estimated that the pre-eruptive basaltic andesite magma assimilated from 15 to 40wt.% of the calc-silicate crustal material, corresponding to introduction of additional 0.19 to 2.1Mt of CO2 to the magma. Experimental leaching of the ash samples documents the release of an aqueous fluid enriched in Cl, Na, Ca, Cd, Sb and Zn during the paroxysmal subplinian eruption. The paroxysmal eruption may have been produced by saturation of the pre-eruptive basaltic andesite magma with hybrid aqueous carbonic NaCl–HCl-rich fluid due to bulk assimilation creating elevated partial pressure of CO2 at shallow crustal conditions of about 200MPa. In contrast, mildly explosive block-and-ash flows (typical Merapi-type) may result from selective assimilation of the carbonate-bearing xenoliths and lower CO2 partial pressure that may not lead to explosive degassing.

Remelting in caldera and rift environments and the genesis of hot, “recycled” rhyolites

Large and small volume rhyolites are generated in calderas and rift zones, inheriting older and isotopically diverse crystal populations from their volcanic predecessors. Low-δ18O values in many of these rhyolites suggest that they were derived from the remelting of solid, hydrothermally altered by meteoric water protoliths that were once close to the surface, but become buried by caldera collapse or rifting. These rhyolites persist for millions of years in these environments with little evidence of coeval basalts. We present a series of numerical experiments on convective melting of roof-rocks by the underplated by near liquidus to superheated silicic melts, generated at the base of the chamber by basaltic intrusions in shallow crustal conditions. We used a range of temperatures and compositions, an appropriate phase diagram with a defined extended eutectic zone appropriate for these environments, varied sill thickness, viscosity of the boundary layer, and considered hydrothermal and lower boundary heat losses. The goal was to estimate melting rates and mechanisms, define conditions that are required for efficient and rapid remelting in the upper crust, quantitatively describe novel details of the dynamics of convecting melting, and compare it to the earlier parametric and numerical treatments of roof melting by underplating. Resolution of numerical experiments allowed us to track mixed thermal and two-phase plume-like convection in silicic magma with a bulk viscosity of 104.5–105.5Pas. The following results were obtained: (1) remarkably fast melting/magma generation rates of many meters per year, (2) intrinsic inhomogeneities in the roof accelerates convection and melting rates via rapid gravitational settling of refractory blocks and exposing detachment scars to the melting front, (3) due to rapid melting, hydrothermal heat loss through the roof, and conductive heat dissipation through the bottom are less important on melting timescales. (4) Convective melting is capable of digesting cold roof-rocks, with high assimilation degrees, which are primarily controlled by sill thickness and roof-rock temperature: thin 10m sills are able to digest 40% of the initially hot roof-rock T=650°C roof-rock, but>100m sills achieve the same level of bulk digestion with T=400°C roof-rocks. The proposed model can explain the origin of hot (above 800–850°C), crystal-poor, “recycled” rhyolites in calderas and rift zones. It can also explain the generation of large, supervolcanic rhyolite volumes through remelting of their erupted and subvolcanic predecessors on rapid timescales, dictated by their zoned and disequilibrium crystalline cargo.