194 research outputs found
Water in cratonic lithosphere : calibrating laboratory-determined models of electrical conductivity of mantle minerals using geophysical and petrological observations
Author Posting. © American Geophysical Union, 2012. 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 13 (2012): Q06010, doi:10.1029/2012GC004055.Measurements of electrical conductivity of “slightly damp” mantle minerals from different laboratories are inconsistent, requiring geophysicists to make choices between them when interpreting their electrical observations. These choices lead to dramatically different conclusions about the amount of water in the mantle, resulting in conflicting conclusions regarding rheological conditions; this impacts on our understanding of mantle convection, among other processes. To attempt to reconcile these differences, we test the laboratory-derived proton conduction models by choosing the simplest petrological scenario possible – cratonic lithosphere – from two locations in southern Africa where we have the most complete knowledge. We compare and contrast the models with field observations of electrical conductivity and of the amount of water in olivine and show that none of the models for proton conduction in olivine proposed by three laboratories are consistent with the field observations. We derive statistically model parameters of the general proton conduction equation that satisfy the observations. The pre-exponent dry proton conduction term (σ0) and the activation enthalpy (ΔHwet) are derived with tight bounds, and are both within the broader 2σ errors of the different laboratory measurements. The two other terms used by the experimentalists, one to describe proton hopping (exponent r on pre-exponent water content Cw) and the other to describe H2O concentration-dependent activation enthalpy (term αCw1/3 added to the activation energy), are less well defined and further field geophysical and petrological observations are required, especially in regions of higher temperature and higher water content.The SAMTEX data were acquired through funding
provided by the Continental Dynamics program of the U.S.
National Science Foundation (grant EAR0455242 to RLE),
the South African Department of Science and Technology
(grant to South African Council for Geoscience), and Science
Foundation Ireland (grant 05/RGP/GEO001 to AGJ) plus financial
and/or logistical support provided by all members of
the SAMTEX consortium. JF was initially supported by an IRCSET grant to AGJ for
the TopoMed project (TopoMed: Plate reorganization in the
western Mediterranean: Lithospheric causes and topographic
consequences) within the European Science Foundation’s TOPOEUROPE
EUROCORES (http://www.esf.org/activities/eurocores/
running-programmes/topo-europe.html), and subsequently by
an SFI PI grant (10/IN.1/I3022) to AGJ for IRETHERM
(www.iretherm.ie).2012-12-1
Velocity–conductivity relationships for mantle mineral assemblages in Archean cratonic lithosphere based on a review of laboratory data and Hashin–Shtrikman extremal bounds
Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Lithos 109 (2009): 131-143, doi:10.1016/j.lithos.2008.10.014.Can mineral physics and mixing theories explain field observations of seismic velocity
and electrical conductivity, and is there an advantage to combining seismological and
electromagnetic techniques? These two questions are at the heart of this paper. Using
phenomologically-derived state equations for individual minerals coupled with multi-phase,
Hashin-Shtrikman extremal-bound theory we derive the likely shear and compressional
velocities and electrical conductivity at three depths, 100 km, 150 km and 200 km, beneath
the central part of the Slave craton and beneath the Kimberley region of the Kaapvaal craton
based on known petrologically-observed mineral abundances and magnesium numbers,
combined with estimates of temperatures and pressures. We demonstrate that there are
measurable differences between the physical properties of the two lithospheres for the upper
depths, primarily due to the different ambient temperature, but that differences in velocity are
negligibly small at 200 km. We also show that there is an advantage to combining seismic and
electromagnetic data, given that conductivity is exponentially dependent on temperature
whereas the shear and bulk moduli have only a linear dependence in cratonic lithospheric
rocks.
Focussing on a known discontinuity between harzburgite-dominated and lherzolitic
mantle in the Slave craton at a depth of about 160 km, we demonstrate that the amplitude of
compressional (P) wave to shear (S) wave conversions would be very weak, and so
explanations for the seismological (receiver function) observations must either appeal to
effects we have not considered (perhaps anisotropy), or imply that the laboratory data require
further refinement
Do mantle xenoliths preserve deep OH signatures? Evidences from field and experiments
International audienc
Microscopie electronique en transmission de geomateriaux; volcanisme : produits pyroclastiques des Antilles; metamorphisme : coesite dans un quartzite alpin
SIGLECNRS T 58556 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc
Results of experimental tests of OH preservation in mantle xenoliths: Example from Allègre volcano samples (Massif Central, France)
International audienc
Hydrogen in Ca-rich garnets (diffusion and stability of the OH-defects)
TOULOUSE3-BU Santé-Centrale (315552105) / SudocSudocFranceF
Low-temperature infrared spectrum and atomic-scale structure of hydrous defects in diopside
Hydrous defects in diopside (CaMgSi2O6) play an important role in
the water budget of the Earth's mantle. Related OH-stretching modes lead to a
variety of infrared absorption bands observed in natural or experimental
samples. In the present study, we report new low-temperature infrared
spectra of reference natural diopside samples in the OH-stretching range. In
parallel, the structure and vibrational properties of a series of OH-bearing
defects in diopside are theoretically determined at the density functional
theory level. The infrared spectra make it possible to resolve additional
bands in the region above 3600 cm−1 and reveal that their anharmonic
behavior differs from that of the bands at lower frequency. A comparison of
theoretical results with experimental data makes it possible to propose
atomic-scale geometries corresponding to observed OH-stretching bands. It
confirms that the bands observed at 3620–3651 cm−1 are related to
M3+ ions substituted for Si in tetrahedral sites, while the 3420
cm−1 band is associated with the Na+ for Ca2+ substitution. In
both cases, H+ incorporation compensates the charge deficit due to the
heterovalent substitution. The other major mechanism of water incorporation
in diopside relates to the charge compensation of cationic vacancies, among
which Ca vacancies play a central role. The 3357 cm−1 band corresponds
to doubly protonated Ca vacancies in pure diopside. In experimental diopside-bearing trivalent cations, the bands at 3432–3460 cm−1 correspond to
singly protonated Ca vacancies with a nearby octahedral M3+ ion, while
the 3310 cm−1 band likely involves a more remote charge compensation by
M3+ ions. More complex defects associating Ca vacancies with
tetrahedral M3+ and octahedral Ti4+ ions are proposed for the
bands observed between 3500 and 3600 cm−1 in natural diopside. The
Fe2+ for Mg2+ and Fe2+ for Ca2+ substitutions are also
found to affect nearby OH-bearing defects, causing a shift and broadening of
OH stretching bands in chemically more complex diopside samples.</p
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