The distribution of water in the upper mantle is believed to have strong influence upon global dynamics by influencing mantle rheology, modal mineralogy, melting systematics and chemical differentiation. The principal input of water is the process of subduction which is estimated to have introduced several ocean volumes to the mantle over Earth history. In principal, a large proportion of this water may be dissolved in the nominally anhydrous silicate minerals (NAMs), but quantifying this has been challenging. Xenoliths only sample the upper 200 km, less mostly, and suffer concern about alteration on the way to emplacement. Recent laboratory data show that seismic velocity is not sensitive to intracrystalline hydration. However, electrical conductivity is strongly sensitive and could provide estimates of mineral water content with suitable constraints. An 1300 km E-W transect of ~400 magnetotelluric (MT) soundings spanning the period range 0.01 to 17,480 s has been acquired from the northern California coast over the Gorda plate, across the Great Basin of Nevada and western Utah, and spanning most of the Colorado Plateau of eastern Utah. Regularized 2D inversion reveals an upper mantle whose resistivity below the broad Great Basin falls progressively with depth from values of ~100 ohm-m near 50 km to <10 ohm-m by 400 km depth. We test the hypothesis that the vertical resistivity profile is consistent with the maximal hydration degree allowed by ambient T-P short of triggering H2O-undersaturated melting (cf. Ardia, 2012, EPSL). An obvious possible source of hydration would be the Gorda, and to some extent the prior Farallon, subducting plates. Assuming standard and enhanced adiabats, deep resistivity profiles predicted using lab data of Novella (2017, Sci Rpts) suggest only resistivities in the near ‘hanging wall’ of the Gorda subduction zone under northwestern Nevada are low enough to represent full NAMs hydration. Under the central (eastern Nevada) and eastern (western Utah) Great Basin, large-scale resistivities are 2-3x too high, nominally. However, channelization of fluid upward from the plate could mean a mixed saturated-unsaturated peridotite upper mantle. Support has been from U.S. Dept of Energy contract DE-0006732 and National Science Foundation grants EAR-0838043 and OPP-1443532, and numerous prior.

At regional scales, electrical resistivity illuminates Earth processes involving fluid evolution and transport, temperature contrasts, and fault characteristics and behavior. It also clarifies continental terrane assembly and event sequencing through electronic mineral markers. Magnetotellurics (MT) is sensitive over such scales, but faces high property contrasts, small signals, 3-D complexity, discontinuous fields, and ill-posed inversion. So-called wideband (~0.003 – 500 s wave period) MT recording constrains crustal structure, and high fidelity through its central dead band is routinely achieved now via distant remote referencing, continuous streaming, and outlier removal. To resolve across the upper mantle, long period data must be of high quality through 10,000 s. Electronics modifications now permit good quality MT data over polar ice-covered regions, and non plane wave outliers appear largely avoidable. Regularized 3D non-linear inversion using simulation equations that recognize a spatially discontinuous electric field has become common practice and lends essential credibility to interpretations. However, resistivity model non-uniqueness is seldom tested enough, and assuming isotropic resistivity can lead to artifacts. Fluids interpreted to cause low resistivity in ductile deep crust should be at lithostatic pressures and have compositions compatible with ambient temperature and metamorphic grade. Vertical current channeling enhances resolution of large-scale fault zones connecting deep and shallow structures. Stabilized terranes can exhibit strong, quasi-linear conductors marking belts of graphite or sulfides deposited in sediment-starved foredeeps or rift margin basins, with a particular concentration in the Proterozoic corresponding to atmospheric oxygenation events. An exciting recent avenue is estimating H2O content of nominally anhydrous minerals (NAMs) in the upper mantle, which strongly affects electrical conductivity but not seismic velocity. The large bandwidth of MT data affords a broad-scale, unified view of Earth processes from mantle level sources through crustal storage and evolution to near-surface deposition. Support has been from U.S. Dept of Energy contract DE-0006732 and National Science Foundation grant OPP-1443532, and numerous prior.

Large-scale electrical resistivity investigations of the Antarctic crust and upper mantle utilizing the magnetotelluric method (MT) are limited in number compared to temperate regions, but provide physical insights difficult to obtain with other techniques. Key to the method’s success in polar environments are instrumentation advances that allow micro-volt level measurements of the MT electric field in the face of mega-ohm contact resistances. In this presentation, we analyze MT data from four campaigns over the Antarctic interior using modern 3D non-linear inversion analysis and offer additional geophysical conclusions and context beyond the original studies. A profile of MT soundings over transitional Ellsworth-Whitmore block in central West Antarctica implies near-cratonic lithospheric geothermal conditions with interpreted graphite-sulphide horizons deformed along margins of high-grade silicate lithologic blocks. Data across South Pole soundings confirms large-scale low resistivity spanning Moho depths that is consistent with limited seismic tomography and elevated crustal thermal regime inferences. Upper mantle under a presumed adiabatic thermal gradient below the Ross Ice Shelf region of West Antarctica appears to be of a moderately hydrated state but not sufficient so as to induce melting. The degree of hydration there is comparable to that below the north-central Great Basin extensional province of the western U.S. Comprehensive 3D coverage of Mount Erebus and Ross Island reveals unprecedented views of the magmatic plumbing feeding the phonolitic volcanic system. This includes a lower middle crustal staging area for episodic magmatic replenishment of the upper middle crustal chamber hosting phonolitic differentiation, which in turn is fed from an upper mantle source region of the parental basanite.