The NoMelt experiment imaged the mantle beneath 70 Ma Pacific seafloor with the aim of understanding the transition from the lithosphere to the underlying convecting asthenosphere. Seafloor magnetotelluric data from four stations were analyzed using 2-D regularized inverse modeling. The preferred electrical model for the region contains an 80 km thick resistive (>10 3 Xm) lithosphere with a less resistive (50 Xm) underlying asthenosphere. The preferred model is isotropic and lacks a highly conductive (10 Xm) layer under the resistive lithosphere that would be indicative of partial melt. We first examine temperature profiles that are consistent with the observed conductivity profile. Our profile is consistent with a mantle adiabat ranging from 0.3 to 0.5 C/km. A choice of the higher adiabatic gradient means that the observed conductivity can be explained solely by temperature. In contrast, a 0.3 C/km adiabat requires an additional mechanism to explain the observed conductivity profile. Of the plausible mechanisms, H 2 O, in the form of hydrogen dissolved in olivine, is the most likely explanation for this additional conductivity. Our profile is consistent with a mostly dry lithosphere to 80 km depth, with bulk H 2 O contents increasing to between 25 and 400 ppm by weight in the asthenosphere with specific values dependent on the choice of laboratory data set of hydrous olivine conductivity and the value of mantle oxygen fugacity. The estimated H 2 O contents support the theory that the rheological lithosphere is a result of dehydration during melting at a mid-ocean ridge with the asthenosphere remaining partially hydrated and weakened as a result.

Decompression of hot mantle rock upwelling beneath oceanic spreading centers causes it to exceed the melting point (solidus), producing magmas that ascend to form basaltic crust ~6 to 7 kilometers thick. The oceanic upper mantle contains ~50 to 200 micrograms per gram of water (HO) dissolved in nominally anhydrous minerals, which-relative to its low concentration-has a disproportionate effect on the solidus that has not been quantified experimentally. Here, we present results from an experimental determination of the peridotite solidus containing known amounts of dissolved hydrogen. Our data reveal that the HO-undersaturated peridotite solidus is hotter than previously thought. Reconciling geophysical observations of the melting regime beneath the East Pacific Rise with our experimental results requires that existing estimates for the oceanic upper mantle potential temperature be adjusted upward by about 60°C.

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Inner Solar System bodies are depleted in volatile elements relative to chondrite meteorites, yet the source(s) and mechanism(s) of volatile-element depletion and/or enrichment are poorly constrained. The timing, mechanisms and quantities of volatile elements present in the early inner Solar System have vast implications for diverse processes, from planetary differentiation to the emergence of life. We report major, trace and volatile-element contents of a glass bead derived from the D'Orbigny angrite, the hydrogen isotopic composition of this glass bead and that of coexisting olivine and silicophosphates, and the Pb-Pb age of the silicophosphates, 4568 ± 20 Ma. We use volatile saturation models to demonstrate that the angrite parent body must have been a major body in the early inner Solar System. We further show via mixing calculations that all inner Solar System bodies accreted volatile elements with carbonaceous chondrite H and N isotope signatures extremely early in Solar System history. Only a small portion (if any) of comets and gaseous nebular H species contributed to the volatile content of the inner Solar System bodies.This article is part of the themed issue 'The origin, history and role of water in the evolution of the inner Solar System'.

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A magnetotelluric survey in the Barotse Basin of western Zambia shows clear evidence for thinned lithosphere beneath an orogenic belt. The uppermost asthenosphere, at a depth of 60-70 km, is highly conductive, suggestive of the presence of a small amount of partial melt, despite the fact that there is no surface expression of volcanism in the region. Although the data support the presence of thicker cratonic lithosphere to the southeast of the basin, the lithospheric thickness is not well resolved and models show variations ranging from~80 to 150 km in this region. Similarly variable is the conductivity of the mantle beneath the basin and immediately beneath the cratonic lithosphere to the southeast, although the conductivity is required to be elevated compared to normal lithospheric mantle. In a general sense, two classes of model are compatible with the magnetotelluric data: one with a moderately conductive mantle and one with more elevated conductivities. This latter class would be consistent with the impingement of a stringer of plume-fed melt beneath the cratonic lithosphere, with the melt migrating upslope to thermally erode lithosphere beneath the orogenic belt that is overlain by the Barotse Basin. Such processes are potentially important for intraplate volcanism and also for development or propagation of rifting as lithosphere is thinned and weakened by melt. Both models show clear evidence for thinning of the lithosphere beneath the orogenic belt, consistent with elevated heat flow data in the region.

TXWater plays a fundamental role in planetary processes and is essential for the habitability of planets. Determining when and how the inner solar system received its water is critical in determining how planets evolved. The inner solar system planets are thought to have first accreted dry, then accreted wet material [1]. Recently, water has been found in eucrite phosphates [2], which crystallized at least by 8-15 million years after the start of the solar system [3], as defined by the age of CAI (4.567 Ga). Eucrites have an earth-like H, N, and C isotope signature [2], thus, probably accreted the same water source as Earth. The discovery of earth-like water in eucrites moves back the time of known water accretion to that of planetesimal formation in the inner solar system by more than 100 Myr. The oldest basaltic meteorites, angrites, can expand on recent work because some are older than eucrites and can help constrain the amount of early water accretion. Angrites are a small group of differentiated meteorites that can be classified as intrusive and extrusive. They are extremely depleted in volatile metals, but the depletion is not seen in all elements, e.g., noble gasses [4]. Hydrogen, carbon, fluorine, and chlorine, volatile elements, have not been previously measured in angrites, and could help to constrain the timing, flux, and origin of volatile elements in the inner solar system. SEM.We used the Hitachi TM3000 at Woods Hole Oceanographic Institution (WHOI) and the JEOL 7600F at NASA-Johnson Space Center to map all angrites prior to SIMS analysis. EPMA. We used the Cameca SX100 at NASA Johnson Space Center, Houston, TX. An accelerating voltage of 20 kV and a probe current of 40 nA was used to measure major and minor element concentrations in olivine and pyroxene SIMS. We used the Cameca IMS 6f at the Carnegie Institution of Washington to measure H, C, F, Cl and S in olivine and pyroxene. A 15 nA primary beam was rastered over a 20 x 20 μm 2 area and the central ~4 x 4 μm 2 of the secondary beam was collected using a field aperture.We measured major, minor, and volatile, element concentrations in olivine in the angrites D'Orbigny and Sahara 99555. Angrite olivine water contents range from 7-25 µg/g H2O and strongly correlate with major element, C, F, and Cl contents.To determine if post-crystallization diffusion occurred in angritic olivines, an empirical forward fractional crystallization model based on published mineral-melt equilibrium was constructed. The model reproduces the observed trends in major and volatile elements, which demonstrates that little to no post-crystallization modification occurred. We determined the volatile content of the primitive melt based on well-established olivine-melt partition coefficients. A calculation of the volatile content of the angrite mantle assuming 10-20 % partial melting, yields a mantle volatile content similar to the Earth's depleted upper mantle for H and C, but a depletion in F and Cl. Even though angrites are the most volatile element-depleted basalts, exhib...

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