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Glass-bearing clinopyroxenite and hornblendite cognate xenoliths entrained in alkali basalts from the West Eifel Volcanic Field provide a snapshot of a semisolidified mush that was rapidly quenched upon entrainment, preserving the melt distribution as interstitial glass. As such, they provide a unique opportunity to investigate the physical relationship between crystals and melt within a mush. At present, this relationship is poorly constrained and petrologists are largely reliant on using fully solidified plutonic rocks to interpret melt distribution and geometries in two-dimensions. Understanding where melt is distributed, and how magmatic processes act to influence this is integral in improving our understanding of subvolcanic systems, with wider applicability to constraining the mechanisms that govern how melt is extracted from the crystal mush and erupted at the surface. Quantitative textural analysis including crystal size distribution (CSD), spatial distribution pattern (SDP) and shape preferred orientation (SPO), as well as qualitative textural observations, are used in conjunction with supplementary geochemistry to constrain the petrogenetic history of four cognate xenoliths derived from the lower-crust to upper-mantle realm. Together, this data provides evidence for open system magmatic processes driving mineral dissolution and reactive crystallisation. Three-dimensional X-ray Computed Tomography is also utilised to visualise and quantify the geometry of the melt, now glass, which represents the porosity of the mush at the point of entrainment. This provides a valuable insight into how melt is stored within crystal mushes, without textural overprinting by secondary recrystallisation processes that are often pervasive in solidified plutons. Combining the two-dimensional textural data with the threedimensional data identifies significant melt heterogeneity at a micro-scale, related to textural changes within the crystalline fraction acting to enhance melt migration. This not only provides an insight into melt distribution, but permits analysis of how the crystals and melt interact relative to one another in an active mush system. This work will contribute towards improving our understanding of crystal mushes and their significance in controlling volcanic eruptions.

The Tethys Ocean, a geodynamically pivotal transcontinental oceanic system sandwiched between the Laurasian and Gondwanan supercontinents, underwent successive evolutionary phases (Proto-Tethys, Palaeo-Tethys, Neo-Tethys) sculpted by continental drift, convergence, and breakup [...]

The North China Block and the South China Block collided in the Middle Triassic, but there is still a lack of consensus regarding the end of collisional orogeny and the closure time of the Paleo-Tethys. In this paper, we report zircon U-Pb ages and geochemistry for the Shimen pluton in the northern margin of the West Qinling Orogenic Belt to investigate its genesis and tectonic environment. The new findings allow to constrain the end time of the Triassic orogeny in the Qinling Orogenic Belt and the closure time of the Paleo-Tethys. The weighted average [sup.206]Pb/[sup.238]U ages of the Shimen pluton are 218.6 ± 1.5 Ma and 221.0 ± 1.7 Ma. Thus, we suggest that the Shimen pluton crystallized at the 218.6 Ma and 221.0 Ma and was formed during the Late Triassic (Norian). The Shimen pluton is mainly syenogranite and has alkaline dark minerals aegirine-augite. It is composed of 73.45 to 77.80 wt.% SiO[sub.2], 8.28 to 9.76 wt.% alkali, and 11.35 to 13.58 wt.% Al[sub.2]O[sub.3], with A/CNK ranging from 0.91 to 1.02 and 10,000 Ga/Al ranging from 2.39 to 3.15. These findings indicate that the Shimen pluton is typical A-type granite. The plutons have low rare earth element contents, ranging from 73.92 to 203.58 ppm, with a moderate negative Eu anomaly. All the samples are enriched in large-ion lithophile elements, such as Rb, Nd, Th and U, and light rare earth elements, and are depleted in high field strength elements, such as Nb, P, Zr, Ba, and Sr. The depletion of Ba, Sr, and Zr may be related to the fractionation and evolution of the granite. According to the petrological and geochemical characteristics, the Shimen pluton is an A[sub.1]-type granite formed in an anorogenic extensional environment. Combined with its tectonic characteristics and petrogenesis, the Shimen pluton was probably formed by the partial melting of the crust under high temperature and low pressure in the intraplate environment after the subduction of the South China Block beneath the North China Block. This observation indicates that the Triassic orogeny in the Qinling Orogenic Belt had ended and the Paleo-Tethys-Mianlve Ocean had also closed by the Late Triassic (Norian).

We have characterized the distribution of 25 trace elements in magnetite (Mg, Al, Si, P, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Nb, Mo, Sn, Hf, Ta, W, and Pb), using laser ablation ICP-MS and electron microprobe, from a variety of magmatic and hydrothermal ore-forming environments and compared them with data from the literature. We propose a new multielement diagram, normalized to bulk continental crust, designed to emphasize the partitioning behavior of trace elements between magnetite, the melt/fluid, and co-crystallizing phases. The normalized pattern of magnetite reflects the composition of the melt/fluid, which in both magmatic and hydrothermal systems varies with temperature. Thus, it is possible to distinguish magnetite formed at different degrees of crystal fractionation in both silicate and sulfide melts. The crystallization of ilmenite or sulfide before magnetite is recorded as a marked depletion in Ti or Cu, respectively. The chemical signature of hydrothermal magnetite is distinct being depleted in elements that are relatively immobile during alteration and commonly enriched in elements that are highly incompatible into magnetite (e.g., Si and Ca). Magnetite formed from low-temperature fluids has the lowest overall abundance of trace elements due to their lower solubility. Chemical zonation of magnetite is rare but occurs in some hydrothermal deposits where laser mapping reveals oscillatory zoning, which records the changing conditions and composition of the fluid during magnetite growth. This new way of plotting all 25 trace elements on 1 diagram, normalized to bulk continental crust and elements in order of compatibility into magnetite, provides a tool to help understand the processes that control partitioning of a full suit of trace elements in magnetite and aid discrimination of magnetite formed in different environments. It has applications in both petrogenetic and provenance studies, such as in the exploration of ore deposits and in sedimentology.

For over 50 years, the use of high-pressure piston/cylinder apparatus combined with an increasing diversity of microbeam analytical techniques has enabled the study of mantle peridotite compositions and of magmas derived by melting in the upper mantle. The experimental studies have been guided by the petrology and geochemistry of peridotites from diverse settings and by the remarkable range of mantle-derived magma types. Recent experimental study using FTIR spectroscopy to monitor water content of minerals has shown that fertile lherzolite (MORB-source upper mantle) at ~1,000 °C can store ~200 ppm H 2 O in defect sites in nominally anhydrous minerals (olivine, pyroxenes, garnet and spinel). Water in excess of 200 ppm stabilizes amphibole (pargasite) at P  < 3 GPa up to the lherzolite solidus. However, at P  > 3 GPa, water in excess of 200 ppm appears as an aqueous vapour phase and this depresses the temperature of the upper mantle solidus. Provided the uppermost mantle (lithosphere) has H 2 O < 4,000 ppm, the mantle solidus has a distinctive P , T shape. The temperature of the vapour - undersaturated or dehydration solidus is approximately constant at 1,100 °C at pressures up to ~3 GPa and then decreases sharply to ~1,010 °C. The strongly negative d T /d P of the vapour-undersaturated solidus of fertile lherzolite from 2.8 to 3 GPa provides the basis for understanding the lithosphere/asthenosphere boundary. Through upward migration of near-solidus hydrous silicate melt, the asthenosphere becomes geochemically zoned with the ‘enriched’ intraplate basalt source (>500 ppm H 2 O) overlying the ‘depleted’ MORB source (~200 ppm H 2 O). From the study of primitive MOR picrites, the modern mantle potential temperature for MORB petrogenesis is ~1,430 °C. The intersection of the 1,430 °C adiabat with the vapour-saturated lherzolite solidus at ~230 km suggests that upwelling beneath mid-ocean ridges begins around this depth. In intraplate volcanism, diapiric upwelling begins from shallower depths and lower temperatures within the asthenosphere and the upwelling lherzolite is enriched in water, carbonate and incompatible elements. Magmas including olivine melilitites, olivine nephelinites, basanites, alkali picrites and tholeiitic picrites are consequences of increasing melt fraction and decreasing pressure at melt segregation. Major element, trace element and isotopic characteristics of island chain or ‘hot-spot’ magmas show that they sample geochemically distinct components in the upper mantle, differing from MORB sources. There is no evidence for higher-temperature ‘hot-spot’ magmas, relative to primitive MORB, but there is evidence for higher water, CO 2 and incompatible element contents. The distinctive geochemical signatures of ‘hot-spot’ magmas and their ‘fixed’ position and long-lived activity relative to plate movement are attributed to melt components derived from melting at interfaces between old, oxidised subducted slabs (suspended beneath or within the deeper asthenosphere) and ambient, reduced mantle. In convergent margin volcanism, the inverted temperature gradients inferred for the mantle wedge above the subducting lithosphere introduce further complexity which can be explored by overlaying the phase relations of appropriate mantle and crustal lithologies. Water and carbonate derived from the subducted slab play significant roles, magmas are relatively oxidised, and distinctive primary magmas such as boninites, adakites and island arc ankaramites provide evidence for fluxing of melting in refractory harzburgite to lherzolite by slab-derived hydrous adakitic melt and by wedge-derived carbonatite.

Subduction is a component of plate tectonics, which is widely accepted as having operated in a manner similar to the present-day back through the Phanerozoic Eon. However, whether Earth always had plate tectonics or, if not, when and how a globally linked network of narrow plate boundaries emerged are matters of ongoing debate. Earth's mantle may have been as much as 200-300 °C warmer in the Mesoarchean compared to the present day, which potentially required an alternative tectonic regime during part or all of the Archean Eon. Here we use a data set of the pressure (P), temperature (T), and age of metamorphic rocks from 564 localities that vary in age from the Paleoarchean to the Cenozoic to evaluate the petrogenesis and secular change of metamorphic rocks associated with subduction and collisional orogenesis at convergent plate boundaries. Based on the thermobaric ratio (T/P), metamorphic rocks are classified into three natural groups: high T/P type (T/P > 775 °C/GPa, mean T/P ∼1105 °C/GPa), intermediate T/P type (T/P between 775 and 375 °C/GPa, mean T/P ∼575 °C/GPa), and low T/P type (T/P < 375 °C/GPa, mean T/P ∼255 °C/GPa). With reference to published thermal models of active subduction, we show that low T/P oceanic metamorphic rocks preserving peak pressures >2.5 GPa equilibrated at P-T conditions similar to those modeled for the uppermost oceanic crust in a wide range of active subduction environments. By contrast, those that have peak pressures <2.2 GPa may require exhumation under relatively warm conditions, which may indicate subduction of young oceanic lithosphere or exhumation during the initial stages of subduction. However, low T/P oceanic metamorphic rocks with peak pressures of 2.5-2.2 GPa were exhumed from depths where, in models of active subduction, the slab and overriding plate change from being decoupled (at lower P) to coupled (at higher P), possibly suggesting a causal relationship. In relation to secular change, the widespread appearance of low T/P metamorphism in the Neoproterozoic represents a 'modern' style of cold collision and deep slab breakoff, whereas rare occurrences of low T/P metamorphism in the Paleoproterozoic may reveal atypical localized regions of cold collision. Low T/P metamorphism is not known from the Archean geological record, but the absence of blueschists in particular is unlikely to reflect secular change in the composition of the oceanic crust. In addition, the premise that the formation of lawsonite requires abnormally low thermal gradients and the postulate that oceanic subduction-related rocks register significantly lower maximum pressures than do continental subduction-related rocks, and imply different mechanisms of exhumation, are not supported. The widespread appearance of intermediate T/P and high T/P metamorphism at the beginning of the Neoarchean, and the subsequent development of a clear bimodality in tectono-thermal environments are interpreted to be evidence of the stabilization of subduction during a transition to a globally linked network of narrow plate boundaries and the emergence of plate tectonics.

The production of hydrogen gas from the Precambrian continental lithosphere has been underestimated; taking into account hydrogen from serpentinization and radiolysis may double estimates previously based on marine systems alone. Subsurface hydrogen source revealed Molecular hydrogen, produced by interactions between water and rocks in the Earth's subsurface and at deep ocean vents, can sustain microbial ecosystems. It was thought that global H 2 production originates mainly from the marine lithosphere, with a negligible contribution from Precambrian rocks. But here the authors present a global compilation of H 2 concentration data that identifies the fracture waters in the Precambrian continental subsurface as a domain of the terrestrial hydrosphere that produces H 2 at a rate comparable to estimates from marine systems. This finding revises our understanding of the production of reduced gases in the Earth's subsurface — and may aid in the exploration of life in deep habitat locations. Microbial ecosystems can be sustained by hydrogen gas (H 2 )-producing water–rock interactions in the Earth’s subsurface and at deep ocean vents 1 , 2 , 3 , 4 . Current estimates of global H 2 production from the marine lithosphere by water–rock reactions (hydration) are in the range of 10 11  moles per year 5 , 6 , 7 , 8 , 9 . Recent explorations of saline fracture waters in the Precambrian continental subsurface have identified environments as rich in H 2 as hydrothermal vents and seafloor-spreading centres 1 , 2 and have suggested a link between dissolved H 2 and the radiolytic dissociation of water 10 , 11 . However, extrapolation of a regional H 2 flux based on the deep gold mines of the Witwatersrand basin in South Africa 11 yields a contribution of the Precambrian lithosphere to global H 2 production that was thought to be negligible (0.009 × 10 11  moles per year) 6 . Here we present a global compilation of published and new H 2 concentration data obtained from Precambrian rocks and find that the H 2 production potential of the Precambrian continental lithosphere has been underestimated. We suggest that this can be explained by a lack of consideration of additional H 2 -producing reactions, such as serpentinization, and the absence of appropriate scaling of H 2 measurements from these environments to account for the fact that Precambrian crust represents over 70 per cent of global continental crust surface area 12 . If H 2 production via both radiolysis and hydration reactions is taken into account, our estimate of H 2 production rates from the Precambrian continental lithosphere of 0.36–2.27 × 10 11  moles per year is comparable to estimates from marine systems.