Convergent margin volcanism originates with partial melting, primarily of the upper mantle, into which the subducting slab descends. Melting of this material can occur in one of two ways. The flow induced in the mantle by the slab can result in upwelling and melting through adiabatic decompression. Alternatively, fluids released from the descending slab through dehydration reactions can migrate into the hot mantle wedge, inducing melting by lowering the solidus temperature. The two mechanisms are not mutually exclusive. In either case, the buoyant melts make their way towards the surface to reside in the crust or to be extruded as lava. Here we use magnetotelluric data collected across the central state of Washington, USA, to image the complete pathway for the fluid-melt phase. By incorporating constraints from a collocated seismic study into the magnetotelluric inversion process, we obtain superior constraints on the fluids and melt in a subduction setting. Specifically, we are able to identify and connect fluid release at or near the top of the slab, migration of fluids into the overlying mantle wedge, melting in the wedge, and transport of the melt/fluid phase to a reservoir in the crust beneath Mt Rainier.

New VLA images of NH 3 (1,1), (2,2), and (3,3) emission in the central 10 parsecs of the Galaxy trace filamentary streams of gas, several of which appear to feed the circumnuclear disk (CND). The NH 3 images have a spatial resolution of 16.5 ′′ × 14.5 ′′ and have better spatial sampling than previous NH 3 observations. The images show the "southern streamer," "50 km s −1 cloud," and new features including a "western streamer", 6 parsecs in length, and a "northern ridge" which connects to the CND. NH 3 (3,3) emission is very similar to 1.2 mm dust emission indicating that NH 3 traces column density well. Ratios of the NH 3 (2,2) to (1,1) line intensities give an estimate of the temperature of the gas and indicate high temperatures close to the nucleus and CND. The new data cover a velocity range of 270 km s −1 , including all velocities observed in the CND, with a resolution of 9.8 km s −1 . Previous NH 3 observations with higher resolution did not cover the entire range of velocities seen in the CND. The large-scale kinematics of the CND do not resemble a coherent ring or disk. We see evidence for a high velocity cloud within a projected distance of 50 ′′ (2 pc) which is only seen in NH 3 (3,3) and is likely to be hot. Comparison to 6 cm continuum emission reveals that much of the NH 3 emission traces the outer edges of Sgr A East and was probably pushed outward by this expanding shell. The connection between the northern ridge (which appears to be swept up by Sgr A East) and the CND indicates that Sgr A East and the CND are in close proximity to each other. Kinematic evidence for these connections is presented in this paper, while the full kinematic analysis of the central 10 pc will be presented in Paper II.

Five magnetotelluric (MT) profiles have been acquired across the Cascadia subduction system and transformed using 2-D and 3-D nonlinear inversion to yield electrical resistivity cross sections to depths of 200 km. Distinct changes in plate coupling, subduction fluid evolution, and modes of arc magmatism along the length of Cascadia are clearly expressed in the resistivity structure. Relatively high resistivities under the coasts of northern and southern Cascadia correlate with elevated degrees of inferred plate locking, and suggest fluid-and sediment-deficient conditions. In contrast, the north-central Oregon coastal structure is quite conductive from the plate interface to shallow depths offshore, correlating with poor plate locking and the possible presence of subducted sediments. Low-resistivity fluidized zones develop at slab depths of 35-40 km starting 100 km west of the arc on all profiles, and are interpreted to represent prograde metamorphic fluid release from the subducting slab. The fluids rise to forearc Moho levels, and sometimes shallower, as the arc is approached. The zones begin close to clusters of low-frequency earthquakes, suggesting fluid controls on the transition to steady sliding. Under the northern and southern Cascadia arc segments, low upper mantle resistivities are consistent with flux melting above the slab plus possible deep convective backarc upwelling toward the arc. In central Cascadia, extensional deformation is interpreted to segregate upper mantle melts leading to underplating and low resistivities at Moho to lower crustal levels below the arc and nearby backarc. The low-to high-temperature mantle wedge transition lies slightly trenchward of the arc.

We have used the BIMA array to image the Galactic Center with a 19-pointing mosaic in HCN(1-0), HCO + (1-0), and H 42α emission with 5 km s −1 velocity resolution and 13 ′′ × 4 ′′ angular resolution. The 5 ′ field includes the circumnuclear ring (CND) and parts of the 20 and 50 km s −1 clouds. HCN(1-0) and HCO + trace the CND and nearby giant molecular clouds while the H 42α emission traces the ionized gas in Sgr A West. We find that the CND has a definite outer edge in HCN and HCO + emission at ∼ 45 ′′ radius and appears to be composed of two or three distinct streams of molecular gas rotating around the nucleus.Outside the CND, HCN and HCO + trace dense clumps of high-velocity gas in addition to optically thick emission from the 20 and 50 km s −1 clouds. A molecular ridge of compressed gas and dust, traced in NH 3 emission and self-absorbed HCN and HCO + , wraps around the eastern edge of Sgr A East. Just inside this ridge are several arcs of gas which have been accelerated by the impact of Sgr A East with the 50 km s −1 cloud.HCN and HCO + emission trace the extension of the northern arm of Sgr A West which appears to be an independent stream of neutral and ionized gas and dust originating outside the CND. Broad line widths and OH maser emission mark the intersection of the northern arm and the CND.Comparison to previous NH 3 and 1.2mm dust observations shows that HCN and HCO + preferentially trace the CND and are weaker tracers of the GMCs than NH 3 and dust. We discuss possible scenarios for the emission mechanisms and environment at the Galactic center which could explain the differences in these images.