Upper Mantle Oxygen Fugacity and Its Relationship to Metasomatism

We have calculated fO_2's and temperatures of various mantle environments worldwide using published analyses of coexisting olivine, orthopyroxene, clinopyroxene, and Fe^(3+)-bearing spinel from 280 peridotites. Most calculated fO_2's fall within ± 2 log units of the Fayalite-Magnetite-Quartz (FMQ) buffer at 15 kbar. Our data set defines a general trend in fO_2-T space that is not related to FMQ or to other Fe-bearing buffers. Variations in major-element, trace-element, and oxygen isotopic composition of xenoliths correlate with variations in calculated fO_2. Rare "fertile" xenoliths record fO_2's close to WM (Wüstite-Magnetite) buffer at 15 kbar and 900°C. Xenoliths with both cryptic and/or modal metasomatic overprinting are generally oxidized relative to xenoliths without evidence of such open system processing. Based on trace element and oxygen isotopic data, the best candidate for the metasomatic agent is a CO_2-H_2O-rich fluid. We suggest that metasomatic fluids are derived from oxidized, hydrated material subducted at convergent margins and that this process may have led to progressive oxidation of the earth's upper mantle through much of geologic time. This is consistent with the observation that xenoliths from Hawaii and Tahiti record fO_2's higher than mantle array's average, as do some xenoliths from the circumpacific region

Lithosphere-atmosphere-ionosphere coupling after the 2003 explosive eruption of the Soufriere Hills Volcano, Montserrat

International audienceResonant coupling between the Earth and the atmosphere at frequencies where the solid Earthmodes overlap the fundamental modes of the atmosphere allows for the triggering of oscillatoryacoustic perturbations by ground excitation and vice versa. Here, we describe oscillatoryperturbations observed in the solid Earth (from volumetric borehole strainmeter data) and inthe atmosphere (from GPS-derived ionospheric total electron content) following the 2003 July13, Soufri`ere Hills Volcano explosion (Montserrat, Lesser Antilles). Spectral analysis showsan amplitude peak at 4 mHz for both data sets, with similar waveforms and signal duration.Using a normal mode summation technique, we show that both signals are explained by asingle explosive source in the atmosphere. Similarities in waveforms, in particular a doublewave train also reported after several other explosion-triggered atmospheric perturbations,result from the superposition of the dominant (fundamental) atmospheric modes that triggerresonant coupling with the solid Earth around 4 mHz

CARIB18: A Stable Geodetic Reference Frame for Geological Hazard Monitoring in the Caribbean Region

We have developed a Stable Caribbean Reference Frame 2018 (CARIB18) using long-term continuous observations from 18 continuously operating Global Positioning System (GPS) stations fixed on the margins of the stable portion of the Caribbean plate. The frame stability of CARIB18 is approximately 0.7 mm/year in the horizontal direction and 0.9 mm/year in the vertical direction. A method that employs a total of seven parameters for transforming positional time series from a global reference frame (IGS14) to a regional reference frame is introduced. The major products from this study include the seven parameters for realizing CARIB18 coordinates and three-component site velocities of 250 continuous GPS stations (>3 years) with respect to CARIB18. Geological hazard monitoring using GPS has traditionally been performed using the carrier-phase differential method that requires single or multiple reference stations to be simultaneously operated in the field. CARIB18 allows for precise geological hazard monitoring using stand-alone GPS, which substantially reduces field costs and simplifies logistics for long-term geological hazard monitoring. Applications of CARIB18 in plate motion, post-seismic, and volcano monitoring and research are demonstrated in this article. The regional reference frame will be periodically updated every few years with more reference stations and longer periods of observations to mitigate the degradation of the frame over time and will be synchronized with the updates of the International GNSS Service (IGS) IGS reference frame

GPS Measurement of Surface Deformation Around Soufriere Hills Volcano, Montserrat from October 1995 to July 1996

Global Positioning System geodesy was used to measure surface deformation on Soufriere Hills volcano, Montserrat from October 6, 1995 to July 1, 1997 during initial dome growth and gravitational collapse. Our data from this period show non-axially symmetric horizontal displacements, and decreasing subsidence as a function of radial distance from the former topographic high of the volcanic edifice. Forward modeling suggests that surface deformation is caused by a shallow vertical dike (\u3c 3 km), which expanded approximately 1 m, coupled with a deflating Mogi source at about 6 km depth. These inferred source parameters are in good agreement with independent observations of regional dike widths and preemption magma storage depth

Kinematics of the Nicaraguan Forearc from GPS Geodesy

Campaign GPS data from a network in the Nicaraguan forearc show a strong component of arc-parallel motion indicating northwest translation of a nearly rigid forearc sliver. Our measured mean velocity for forearc sites of 15.1 mm yr−1 agrees well with the arc-parallel sliver motion predicted previously by DeMets (2001) derived from closure constraints on oblique convergence between the Cocos and Caribbean plates. The lack of a northeasterly oriented arc-normal component of motion in forearc velocities indicates that there are complexities involved beyond a simple interpretation of sliver motion being driven by oblique convergence. The forearc is reasonably well-fit by rigid rotation about an Euler pole with a rms misfit of residual velocities of 4.9 ± 2.6 mm yr−1. Current motion of the forearc sliver relative to the stable Caribbean plate yields predominantly boundary parallel NW motion with boundary normal extension in the northwestern region averaging ∼5 mm yr−1

Long term surface deformation of Soufrière Hills Volcano, Montserrat from GPS geodesy: Inferences from simple elastic inverse models

Campaign and continuous GPS geodetic measurements on Soufrière Hills Volcano, Montserrat are reported from 1995 to 2009, spanning three dome growth and repose episodes. Uniform elastic half-space inversions were used to examine how crustal pressure sources evolved by inverting subsets of all available 3D site data for any given episode using a single Mogi-source. Changes in network topology were also examined. The average best-fitting single Mogi model yields X = 0.3 ± 0.5 km, Y = 0.8 ± 0.4 km, and a depth of Z = 10.4 ± 2.1 km (1-σ, Z positive down), relative to the center of the model domain. The mean Mogi depth for effusive (deflation) versus repose (inflation) is different and significant at >90% confidence, yielding Z = 11.4 ± 2.0 km and 9.3 ± 1.6 km, respectively. A vertical, prolate ellipsoid improves the fit at >95% confidence over a single Mogi source or stacked, two-source model for the 2003–2005 repose and 2005–2007 dome growth episodes