The ratio of 3 He and 22 Ne varies throughout the mantle. This observation is surprising because 3 He and 22 Ne are not produced in the mantle, are highly incompatible during mantle melting, and are not recycled back into the mantle by subduction of oceanic sediment or basaltic crust. Our new compilation yields average 3 He/ 22 Ne ratios of 7.5±1.2 and 3.5±2.4 for mid-ocean ridge basalt (MORB) mantle and ocean island basalt (OIB) mantle sources respectively. The low 3 He/ 22 Ne of OIB mantle approaches planetary precursor 3 He/ 22 Ne values; ~1 for chondrites and ~1.5 for the solar nebula. The high 3 He/ 22 Ne of the MORB mantle is not similar to any planetary precursor, requiring a mechanism for fractionating He from Ne in the mantle and suggesting isolation of distinct mantle reservoirs throughout geologic time. New experimental results reported here demonstrate that He and Ne diffuse at rates differing by one or more orders of magnitude at relevant temperatures in mantle materials. We model the formation of a MORB mantle with an elevated 3 He/ 22 Ne ratio through kinetically modulated chemical exchange between dunite channel-hosted basaltic liquids and harzburgite wallrock beneath mid-ocean ridges. Over timescales relevant to mantle upwelling beneath spreading centers, He may diffuse tens to hundreds of meters into wallrock while Ne is effectively immobile, producing a mantle lithosphere regassed with respect to He and depleted with respect to Ne, with a net elevated 3 He/ 22 Ne. Subduction of high 3 He/ 22 Ne mantle lithosphere throughout geologic time would generate a MORB source with high 3 He/ 22 Ne. Mixing models suggest that to preserve a high 3 He/ 22 Ne reservoir, MORB mantle mixing timescales must be on the order of hundreds of millions of years or longer, that mantle convection has not been layered about the transition zone for most of geologic time, and that Earth's convecting mantle has lost at least 96% of its 3 primordial volatile elements. The most depleted, highest 3 He/ 22 Ne mantle may be best preserved in the lower mantle where relatively high viscosities impede mechanical mixing.

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Acoustic compressional and shear wave velocities (V P , V S) of anhydrous (AHRG) and hydrous rhyolitic glasses containing 3.28 wt% (HRG-3) and 5.90 wt% (HRG-6) total water concentration (H 2 O t) have been measured using Brillouin Light Scattering (BLS) spectroscopy up to 3 GPa in a diamond anvil cell at ambient temperature. In addition, Fourier-transform infrared (FTIR) spectroscopy was used to measure the speciation of H 2 O in the glasses up to 3 GPa. At ambient pressure, HRG-3 contains 1.58 (6) wt% hydroxyl groups (OH-) and 1.70 (7) wt% molecular water (H 2 O m) while HRG-6 contains 1.67 (10) wt% OHand 4.23 (17) wt% H 2 O m where the numbers in parentheses are ±1σ. With increasing pressure, very little H 2 O m , if any, converts to OHwithin uncertainties in hydrous rhyolitic glasses such that HRG-6 contains much more H 2 O m than HRG-3 at all experimental pressures. We observe a non-linear relationship between high-pressure sound velocities and H 2 O t , which is attributed to the distinct effects of each water species on acoustic velocities and elastic moduli of hydrous glasses. Near ambient pressure, depolymerization due to OHreduces V S and G more than

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