95 research outputs found
Dynamics of a laminar plume in a cavity: The influence of boundaries on the steady state stem structure
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96724/1/ggge20016.pd
Rheological control of oceanic crust separation in the transition zone
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95124/1/grl9376.pd
Methods for thermochemical convection in Earth's mantle with forceâbalanced plates
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94785/1/ggge1131.pd
Multiple volcanic episodes of flood basalts caused by thermochemical mantle plumes
The hypothesis that a single mushroom-like mantle plume head can generate a large igneous province within a few million years has been widely accepted(1). The Siberian Traps at the Permian Triassic boundary(2) and the Deccan Traps at the Cretaceous Tertiary boundary(3) were probably erupted within one million years. These large eruptions have been linked to mass extinctions. But recent geochronological data(4-11) reveal more than one pulse of major eruptions with diverse magma flux within several flood basalts extending over tens of million years. This observation indicates that the processes leading to large igneous provinces are more complicated than the purely thermal, single-stage plume model suggests. Here we present numerical experiments to demonstrate that the entrainment of a dense eclogite-derived material at the base of the mantle by thermal plumes can develop secondary instabilities due to the interaction between thermal and compositional buoyancy forces. The characteristic timescales of the development of the secondary instabilities and the variation of the plume strength are compatible with the observations. Such a process may contribute to multiple episodes of large igneous provinces.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62705/1/nature03697.pd
Deep storage of oceanic crust in a vigorously convecting mantle
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94626/1/jgrb15203.pd
Subduction metamorphism of serpentiniteâhosted carbonates beyond antigorite-serpentinite dehydration (NevadoâFilĂĄbride Complex, Spain)
I. MartĂnez Segura and M. J. RomĂĄn Alpiste are thanked for their kind assistance during sample preparation and SEM operation, and M. T. GĂłmezâPugnaire and A. Jabaloy for early work on Almirez ophicarbonates. We are grateful to the Sierra Nevada National Park for providing permits for fieldwork and sampling at the Almirez massif. We further acknowledge the editorial handling by D. Whitney and D. Robinson and the reviews of M. Galvez and T. Pettke, whose comments and constructive criticism helped to improve the manuscript.
We acknowledge funding from the European Union FP7 MarieâCurie Initial Training Network ABYSS under REA Grant Agreement no. 608001 in the framework of M.D.M.'s PhD project, the Spanish âAgencia Estatal de InvestigaciĂłnâ (AEI) grants no. CGL2016â75224âR to V.L.S.âV and CGL2016â81085âR to C.J.G and C.M and grant no. PCINâ2015â053 to C.J.G. The âJunta de AndalucĂaâ is also thanked for funding under grants no. RNMâ131, RNMâ374 and P12âRNMâ3141. C.M. thanks MINECO for financing a RamĂłn y Cajal fellowship no. RYCâ2012â11314 and K.H. for a Juan de la Cierva Fellowship no. FPDIâ2013â16253 and a research contract under grant no. CGL2016â81085âR. This work and the research infrastructure at the IACT have received (co)funding from the European Social Fund and the European Regional Development Fund.At subâarc depths, the release of carbon from subducting slab lithologies is mostly
controlled by fluid released by devolatilization reactions such as dehydration of antigorite (Atgâ) serpentinite to prograde peridotite. Here we investigate carbonateâsilicate rocks hosted in Atgâserpentinite and prograde chlorite (Chlâ) harzburgite in the
Milagrosa and Almirez ultramafic massifs of the palaeoâsubducted NevadoâFilĂĄbride
Complex (NFC, Betic Cordillera, S. Spain). These massifs provide a unique opportunity to study the stability of carbonate during subduction metamorphism at PâT
conditions before and after the dehydration of Atgâserpentinite in a warm subduction
setting. In the Milagrosa massif, carbonateâsilicate rocks occur as lenses of Tiâclinohumiteâdiopsideâcalcite marbles, diopsideâdolomite marbles and antigoriteâdiopsideâdolomite rocks hosted in clinopyroxeneâbearing Atgâserpentinite. In Almirez,
carbonateâsilicate rocks are hosted in Chlâharzburgite and show a highâgrade assemblage composed of olivine, Tiâclinohumite, diopside, chlorite, dolomite, calcite, Crâ
bearing magnetite, pentlandite and rare aragonite inclusions. These NFC
carbonateâsilicate rocks have variable CaO and CO2 contents at nearly constant Mg/
Si ratio and high Ni and Cr contents, indicating that their protoliths were variable
mixtures of serpentine and Caâcarbonate (i.e., ophicarbonates). Thermodynamic
modelling shows that the carbonateâsilicate rocks attained peak metamorphic conditions similar to those of their host serpentinite (Milagrosa massif; 550â600°C and
1.0â1.4 GPa) and Chlâharzburgite (Almirez massif; 1.7â1.9 GPa and 680°C).
Microstructures, mineral chemistry and phase relations indicate that the hybrid carbonateâsilicate bulk rock compositions formed before prograde metamorphism,
likely during seawater hydrothermal alteration, and subsequently underwent subduction metamorphism. In the CaOâMgOâSiO2 ternary, these processes resulted in a
compositional variability of NFC serpentiniteâhosted carbonateâsilicate rocks along
the serpentineâcalcite mixing trend, similar to that observed in serpentiniteâhosted
carbonateârocks in other palaeoâsubducted metamorphic terranes. Thermodynamic modelling using classical models of binary H2OâCO2 fluids shows that the compositional variability along this binary determines the temperature of the main devolatilization reactions, the fluid composition and the mineral assemblages of reaction
products during prograde subduction metamorphism. Thermodynamic modelling
considering electrolytic fluids reveals that H2O and molecular CO2 are the main fluid
species and charged carbonâbearing species occur only in minor amounts in equilibrium with carbonateâsilicate rocks in warm subduction settings. Consequently, accounting for electrolytic fluids at these conditions slightly increases the solubility of
carbon in the fluids compared with predictions by classical binary H2OâCO2 fluids,
but does not affect the topology of phase relations in serpentiniteâhosted carbonateâ
rocks. Phase relations, mineral composition and assemblages of Milagrosa and
Almirez (meta)âserpentiniteâhosted carbonateâsilicate rocks are consistent with local
equilibrium between an infiltrating fluid and the bulk rock composition and indicate
a limited role of infiltrationâdriven decarbonation. Our study shows natural evidence
for the preservation of carbonates in serpentiniteâhosted carbonateâsilicate rocks beyond the Atgâserpentinite breakdown at subâarc depths, demonstrating that carbon
can be recycled into the deep mantle.Funding from the European Union FP7 MarieâCurie Initial Training Network ABYSS under REA Grant Agreement no. 608001Spanish âAgencia Estatal de InvestigaciĂłnâ (AEI) grants no. CGL2016â75224âR to V.L.S.âV and CGL2016â81085âR to C.J.G and C.M and grant no. PCINâ2015â053 to C.J.GJunta de AndalucĂa Funding under grants no. RNMâ131, RNMâ374 and P12âRNMâ3141MINECO for financing a RamĂłn y Cajal fellowship no. RYCâ2012â11314 and K.H. for a Juan de la Cierva Fellowship no. FPDIâ2013â16253 and a research contract under grant no. CGL2016â81085â
Seismicity of the incoming plate and forearc near the Mariana Trench recorded by ocean bottom seismographs
Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 21(4), (2020): e2020GC008953, doi:10.1029/2020GC008953.Earthquakes near oceanic trenches are important for studying incoming plate bending and updip thrust zone seismogenesis, yet are poorly constrained using seismographs on land. We use an ocean bottom seismograph (OBS) deployment spanning both the incoming Pacific Plate and the forearc to study seismicity near the Mariana Trench. The yearlong deployment in 2012â2013 consisted of 20 broadband OBSs and 5 suspended hydrophones, with an additional 59 short period OBSs and hydrophones recording for 1 month. We locate 1,692 earthquakes using a nonlinear method with a 3D velocity model constructed from active source profiles and surface wave tomography results. Events occurring seaward of the trench occur to depths of ~35 km below the seafloor, and focal mechanisms of the larger events indicate normal faulting corresponding to plate bending. Significant seismicity emerges about 70 km seaward from the trench, and the seismicity rate increases continuously towards the trench, indicating that the largest bending deformation occurs near the trench axis. These plateâbending earthquakes occur along faults that facilitate the hydration of the subducting plate, and the lateral and depth distribution of earthquakes is consistent with lowâvelocity regions imaged in previous studies. The forearc is marked by a heterogeneous distribution of low magnitude (<5 Mw) thrust zone seismicity, possibly due to the rough incoming plate topography and/or serpentinization of the forearc. A sequence of thrust earthquakes occurs at depths ~10 km below seafloor and within 20 km of the trench axis, demonstrating that the megathrust is seismically active nearly to the trench.We thank the captains, crew, and science teams on the R/V Thompson, Langseth and Melville, Dr. Patrick Shore for providing data management and technical support, and Ivan Komarov and Zhengyang Zhou for assistance with data analysis. We thank Ingo Grevemeyer and an anonymous reviewer for their comments to improve the manuscript. Instrumentation and technical support was provided by the PASSCAL program of the Incorporated Research Institutions in Seismology (IRIS) and the Woods Hole, LamontâDoherty, and Scripps facilities of the Ocean Bottom Seismograph Instrumentation Pool (OBSIP). Funding was provided by the MARGINS/GeoPRISMS program through NSF grant OCEâ0841074 (D.A.W.) and the Spencer T. and Ann W. Olin Fellowship program at Washington University in Saint Louis. Raw seismic data used in this study are available through the Data Management Center of the Incorporated Research Institutions for Seismology (http://www.iris.edu/dms/nodes/dmc) under network IDs XF and MI.2020-10-0
Effects of pressure on diffusion and vacancy formation in MgO from non-empirical free-energy integrations
The free energies of vacancy pair formation and migration in MgO were
computed via molecular dynamics using free-energy integrations and a
non-empirical ionic model with no adjustable parameters. The intrinsic
diffusion constant for MgO was obtained at pressures from 0 to 140 GPa and
temperatures from 1000 to 5000 K. Excellent agreement was found with the zero
pressure diffusion data within experimental error. The homologous temperature
model which relates diffusion to the melting curve describes well our high
pressure results within our theoretical framework.Comment: 4 pages, latex, 1 figure, revtex, submitted to PR
Stress, strain, and Bâtype olivine fabric in the foreâarc mantle: Sensitivity tests using highâresolution steadyâstate subduction zone models
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94925/1/jgrb15022.pd
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