We report on laser-heated diamond anvil cell (LHDAC) experiments in the FeO-MgO-SiO2-CO2 (FMSC) and CaO-MgO-SiO2-CO2 (CMSC) systems at lower mantle pressures designed to test for decarbonation and diamond forming reactions. Sub-solidus phase relations based on synthesis experiments are reported in the pressure range of similar to 35 to 90 GPa at temperatures of similar to 1600 to 2200 K. Ternary bulk compositions comprised of mixtures of carbonate and silica are constructed such that decarbonation reactions produce non-ternary phases (e.g. bridgmanite, Ca-perovskite, diamond, CO2-V), and synchrotron X-ray diffraction and micro-Raman spectroscopy are used to identify the appearance of reaction products. We find that carbonate phases in these two systems react with silica to form bridgmanite +/- Ca-perovskite + CO2 at pressures in the range of similar to 40 to 70 GPa and 1600 to 1900 K in decarbonation reactions with negative Clapeyron slopes. Our results show that decarbonation reactions form an impenetrable barrier to subduction of carbonate in oceanic crust to depths in the mantle greater than similar to 1500 km. We also identify carbonate and CO2-V dissociation reactions that form diamond plus oxygen. On the basis of the observed decarbonation reactions we predict that the ultimate fate of carbonate in oceanic crust subducted into the deep lower mantle is in the form of refractory diamond in the deepest lower mantle along a slab geotherm and throughout the lower mantle along a mantle geotherm. Diamond produced in oceanic crust by subsolidus decarbonation is refractory and immobile and can be stored at the base of the mantle over long timescales, potentially returning to the surface in OIB magmas associated with deep mantle plumes. (C) 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license.

Kilauea Volcano (Hawai'i, USA) is underlain by a complex, laterally extensive magmatic plumbing system. Although in recent decades it has mainly erupted through vents along the middle East Rift Zone and summit caldera, eruptions can occur anywhere along its laterally extensive rift zones, as demonstrated by the dramatic eruptive activity of 2018. Forecasting eruptive activity requires an understanding of whether an episode of volcano-seismic unrest at Kilauea and similar volcanoes is caused directly at the edges of an active intrusion or reservoir, or in a volume of wall rock at a distance from the intrusion. Seismic unrest in Kilauea's upper East Rift Zone (UERZ) has to date been interpreted as the result either of magma intrusion in this region of the volcano or of stresses due to seaward flank migration. However, recent observations suggest that UERZ seismicity may result from variable pressurization of Kilauea's summit magma system. We analyze seismic and deformation (multi-temporal interferometric synthetic aperture radar [InSAR] and GPS) data during a period of variable summit deformation and UERZ seismicity in mid- to late 2007 and calculate Coulomb stress changes on UERZ faults due to modeled summit inflation or deflation. UERZ seismicity during our study period can be explained entirely by stresses arising from pressure changes within Kilauea's summit reservoirs. Furthermore, a comparison of UERZ fault plane solutions (FPSs) calculated for this study to published UERZ FPSs for previous periods suggests that the UERZ, has undergone a transition from a mechanically strong, discontinuous, and immature magma transport system to a mature, mechanically weak, and fully connected transport system over the course of the 1983-2018 eruption.

Many of Earth's volcanoes experience well-defined states of "quiescence" and "unrest," with unrest occasionally culminating in eruption. Some volcanoes, however, experience an unusually protracted (i.e., decades-long) period of noneruptive unrest and are thus categorized as "persistently restless volcanoes" (PRVs). The processes that drive persistently restless volcanism are poorly understood, as our knowledge of PRVs is currently based on a small number of case studies. Here we examine multidisciplinary observations of the 2015 eruptive episode at Telica Volcano, Nicaragua, in the context of its long-term behavior. We suggest that the latter phases of the 2015 eruption were ultimately driven by destabilization of its shallow magma reservoir. Based on previous geodetic-seismic studies of Telica (Geirsson et al., 2014, https://doi.org/10.1016/j.jvolgeores.2013.11.009; Rodgers et al., 2013, https://doi.org/10.1016/j.jvolgeores.2013.08.010 and 2015, https://doi.org/10.1016/j.jvolgeores.2014.11.012) and on multiparameter observations at Telica over a 7-year period, we propose that three distinct states of unrest occur at Telica over decadal timescales: a stable open state involving steady conduit convection and two distinct "unstable" states that may lead to eruptions. In the "weak sealing" state, phreatic explosions result from steady conduit convection underlying a weak seal. In the "destabilized" state, destabilization of the top of the convecting magma in the conduit leads to rapid accumulation of high pressures leading to strong/impulsive phreatomagmatic explosions. Our observations and interpretations suggest that continuous seismic, ground-based deformation, gas emission, and thermal monitoring and interpretation of these data within a paradigm of sustained conduit convection modulated by episodes of sealing and destabilization of shallow magma reservoirs may allow robust forecasting of eruption potential, energy, and duration at Telica and similar PRVs worldwide.

Velocity and density jumps across the 410-km seismic discontinuity generally indicate olivine contents of similar to 30 to 50 vol.% on the basis of the elastic properties of anhydrous olivine and wadsleyite, which is considerably less than the similar to 60% olivine in the widely accepted pyrolite model for the upper mantle. A possible explanation for this discrepancy is that water dissolved in olivine and wadsleyite affects their elastic properties in ways that can reconcile the pyrolitic model with seismic observations. In order to more fully constrain the olivine content of the upper mantle near the 410-km discontinuity, and to place constraints on the mantle water content at this depth, we determined the full elasticity of hydrous wadsleyite at the P-T conditions of the discontinuity based on density functional theory calculations. Together with previous determinations for the effect of water on olivine elasticity, we simultaneously modeled the density and seismic velocity jumps (Delta(rho), Delta V-p, Delta V-S) across the olivine-wadsleyite transition. Our models allow for several scenarios that can well reproduce the density and seismic velocity jumps across the 410-km discontinuity when compared to globally averaged seismic models. When the water content of olivine and wadsleyite is assumed to be equal as in a simple binary system, our modeling indicates a best fit for low water contents (<0.1 wt.%) with an olivine proportion of similar to 50%, suggesting a relatively dry, non-pyrolitic mantle at depths of the 410-km discontinuity. However, our modeling can be reconciled with a pyrolitic mantle if the water content in wadsleyite is similar to 0.9 wt.% and that in olivine is at its storage capacity of similar to 500-1500 ppm. The result would be consistent with a hydrous melt phase produced at depths just above the phase transition. (C) 2019 Elsevier B.V. All rights reserved.

The distribution and transportation of water in Earth's interior depends on the stability of water-bearing phases. The transition zone in Earth's mantle is generally accepted as an important potential water reservoir because its main constituents, wadsleyite and ringwoodite, can incorporate weight percent levels of H2O in their structures at mantle temperatures. The extent to which water can be transported beyond the transition zone deeper into the mantle depends on the water carrying capacity of minerals stable in sub-ducted lithosphere. Stishovite is one of the major mineral components in subducting oceanic crust, yet the capacity of stishovite to incorporate water beyond at lower mantle conditions remains speculative. In this study, we combine in situ laser heating with synchrotron X-ray diffraction to show that the unit cell volume of stishovite synthesized under hydrous conditions is similar to 2.3 to 5.0% greater than that of anhydrous stishovite at pressures of similar to 27 to 58 GPa and temperatures of 1,240 to 1,835 K. Our results indicate that stishovite, even at temperatures along a mantle geotherm, can potentially incorporate weight percent levels of H2O in its crystal structure and has the potential to be a key phase for transporting and storing water in the lower mantle.

Diamonds are unrivalled in their ability to record the mantle carbon cycle and mantle fO(2) over a vast portion of Earth's history. Diamonds' inertness and antiquity means their carbon isotopic characteristics directly reflect their growth environment within the mantle as far back as similar to 3.5 Ga. This paper reports the results of a thorough secondary ion mass spectrometry (SIMS) carbon isotope and nitrogen concentration study, carried out on fragments of 144 diamond samples from various locations, from similar to 3.5 to 1.4 Ga for P [peridotitic]-type diamonds and 3.0 to 1.0 Ga for E [eclogitic]-type diamonds. The majority of the studied samples were from diamonds used to establish formation ages and thus provide a direct connection between the carbon isotope values, nitrogen contents and the formation ages. In total, 908 carbon isotope and nitrogen concentration measurements were obtained. The total delta C-13 data range from -17.1 to -1.9 parts per thousand (P = -8.4 to -1.9 parts per thousand; E = -17.1 to -2.1 parts per thousand) and N contents range from 0 to 3073 at. ppm (P 0 to 3073 at. ppm; E = 1 to 2661 at. ppm). In general, there is no systematic variation with time in the mantle carbon isotope record since > 3 Ga. The mode in delta C-13 of peridotitic diamonds has been at similar to 5 (+/- 2) parts per thousand since the earliest diamond growth similar to 3.5 Ga, and this mode is also observed in the eclogitic diamond record since similar to 3 Ga. The skewness of eclogitic diamonds' delta C-13 distributions to more negative values, which the data establishes began around 3 Ga, is also consistent through time, with no global trends apparent.

A new diamond-anvil cell apparatus for in situ synchrotron X-ray diffraction measurements of liquids and glasses, at pressures from ambient to 5 GPa and temperatures from ambient to 1300 K, is reported. This portable setup enables in situ monitoring of the melting of complex compounds and the determination of the structure and properties of melts under moderately high pressure and high temperature conditions relevant to industrial processes and magmatic processes in the Earth's crust and shallow mantle. The device was constructed according to a modified Bassett-type hydrothermal diamond-anvil cell design with a large angular opening (theta = 95 degrees). This paper reports the successful application of this device to record in situ synchrotron X-ray diffraction of liquid Ga and synthetic PbSiO3 glass to 1100 K and 3 GPa.