Final Report for "Experimental Petrology and Chemistry of Volatile-Bearing Silicate Melts"

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The goal of Part 1 was the definitive determination of the dependence of the diffusion coefficient for water (DH2O, defined as the diffusion coefficient of total water) in various compositions of silicate melts with respect to water content (CH2O). We measured profiles of CH2O in hydration and diffusion couple experiments by Fourier transform infrared spectroscopy and secondary ion mass spectrometry. DH2O values were determined from the profiles using both direct calculations (Boltzmann-Matano methods) and models assuming specific relationships between DH2O and CH2O (including constant, proportional, and exponential relationships, and a simple speciation model assuming that water molecules are mobile (with ... continued below

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Stolper, Edward M. March 29, 2013.

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The goal of Part 1 was the definitive determination of the dependence of the diffusion coefficient for water (DH2O, defined as the diffusion coefficient of total water) in various compositions of silicate melts with respect to water content (CH2O). We measured profiles of CH2O in hydration and diffusion couple experiments by Fourier transform infrared spectroscopy and secondary ion mass spectrometry. DH2O values were determined from the profiles using both direct calculations (Boltzmann-Matano methods) and models assuming specific relationships between DH2O and CH2O (including constant, proportional, and exponential relationships, and a simple speciation model assuming that water molecules are mobile (with constant diffusivity) and hydroxyl groups are immobile). As expected, the constant DH2O model was never the best fit to our diffusion profile data. In order to distinguish among the models with varying diffusion coefficients, all of which require increasing DH2O with increasing CH2O, we ran a series of experiments with small ranges of CH2O, so that we could assume that DH2O was constant. If either the proportional or speciation model holds, then DH2O = 0 at CH2O = 0, whereas the exponential model predicts a finite value for DH2O at CH2O = 0. Results for haplobasalt and haploandesite compositions are consistent with DH2O = exp(DH2O). We have confirmed this conclusion by looking at experiments with very low CH2O, in which we found a finite DH2O= 2-4E-10 m2/s. Using our data and results in the literature over a range of composition (including major elements and water), temperature, and pressure conditions, we defined a general relationship between water diffusivity and viscosity (η) of silicate liquids. This is an important result, since it allows the calculation of diffusivity for compositions for which there are no measurements. Part 2 is the study of the zonation of P and other elements in olivines: (1) Complex zoning patterns in P in olivines from terrestrial komatiites, basalts, andesites, and dacites and from an SNC (Martian) meteorite are decoupled from zoning in divalent cations. The P zoning patterns can be: (i) P-rich crystal cores with skeletal, hopper, or euhedral shapes; (ii) thin, widely spaced, concentric P-rich zones, especially near crystal rims, and other types of oscillatory zoning; (iii) structures suggesting resorption of P-rich zones and replacement by P-poor olivine; and (iv) sector zoning. Crystallization experiments on a Hawaiian basalt at constant cooling rates produced olivine with many comparable zoning features, demonstrating that they can form by crystal growth during simple cooling histories. Al and Cr zoning can be correlated with P zoning in experiments and in natural crystals. The development of oscillatory zoning in olivine from isothermal experiments or at a constant cooling rate from the liquidus indicates that such zoning in natural samples cannot necessarily be ascribed to changing magmatic conditions. (2) For Sc-doped bulk compositions, most Sc in the experimental olivines substitutes independent of P but up to half participates in a coupled substitution with P in a roughly 5:3 ratio. However results from our 1-atm experiments are consistent with a 1:1 relationship between trivalent cations and P, suggesting that the simple substitution P + M3+ + ☐ = Si + 2M2+ (where M2+ denotes Mg, Fe2+, Mn, Ca, and Ni; M3+ denotes Sc, Al, and Cr; and ☐ denotes a vacancy) is probably the dominant substitution mechanism in Sc- and P-bearing bulk compositions. In the absence of Sc, both molar Al-P and Cr-P plots yield much higher slopes (>~5), suggesting a different mechanism. (3) Based on (1) above, the absence of elemental zoning (P, Al, or Cr) with hopper or chain morphologies in the spinifex olivines from komatiites from Gorgona Island suggests that these grains did not initially grow with these morphologies followed by subsequent growth to “fill in” the voids. We suspect that this inference also holds for Archean komatiites. (4) One-atm cooling rate experiments in thermal gradients produced large, non-skeletal olivine grains with morphologies similar to those displayed by olivines in the spinifex zones of komatiite flows. The experimental olivines show linear Sc-enriched bands (Sc used as a proxy for Cr) running parallel with the long dimension of the crystals. These enriched bands are morphologically similar to the enriched regions in spinifex olivines from Gorgona komatiites supporting the model of Faure et al. (2006) that a thermal gradient is necessary for the crystallization of spinifex textures. (4) Spatially correlated zoning in Ti, Cr, Al, and P on <20 μm scales in olivines from the Kilauea Iki lava lake display different degrees of relaxation (i.e., Ti, Cr, and Al bands are broader than those defined by P) suggesting that the relative diffusion rates of these elements in olivine are: P > Ti >> Cr > Al.

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  • Report No.: DOE/ER15773-1
  • Grant Number: FG02-06ER15773
  • DOI: 10.2172/1072053 | External Link
  • Office of Scientific & Technical Information Report Number: 1072053
  • Archival Resource Key: ark:/67531/metadc844386

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  • March 29, 2013

Added to The UNT Digital Library

  • May 19, 2016, 9:45 a.m.

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  • June 20, 2016, 12:36 p.m.

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Stolper, Edward M. Final Report for "Experimental Petrology and Chemistry of Volatile-Bearing Silicate Melts", report, March 29, 2013; United States. (digital.library.unt.edu/ark:/67531/metadc844386/: accessed June 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.