首页|Pressure and concentration effects on intermineral calcium isotope fractionation involving garnet

Pressure and concentration effects on intermineral calcium isotope fractionation involving garnet

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  • NSTL
  • Elsevier
To better understand the behaviors of calcium (Ca) isotopes during igneous and metamorphic processes that differentiate the Earth and other rocky planets, we investigated the effects of pressure and Ca/Fe concentration on the average Ca-O bond length and the reduced partition function ratio of Ca-44/Ca-40 (10(3)ln(44/40Ca)beta) in garnet using first-principles calculations. Our calculations show that (1) the average Ca-O bond length increases and 10(3)ln(44/40Ca)beta decreasing (approximately 0.41 parts per thousand at 1000 K from a Ca/(Fe + Ca + Mg) of 1/24 to 12/12) with increasing Ca concentrations. In contrast to orthopyroxene and forsterite, whose Ca-O bond lengths and 10(3)ln(44/40Ca)beta are sensitive to Ca concentrations only in narrow range, the garnet Ca-O bond length and 10(3)ln(44/40Ca)beta vary with Ca concentrations in wide ranges. (2) The average Ca-O bond length increases and 10(3)ln(44/40Ca)beta decreases with increasing Fe concentrations (by 0.12 parts per thousand at 1000 K, FeO content of 12.4 wt%). (3) The average Ca-O bond length decreases by similar to 0.014 angstrom, and 10(3)ln(44/40Ca)beta increases by 0.1 parts per thousand-0.2 parts per thousand at 1000 K when the pressure increases from 0 to 3 GPa. (4) However, the pressure effect on 10(3)ln(44/40Ca)alpha(garnet-clinopyroxene) is insignificant. Using our new results of the effects of pressure and Ca/Fe concentration on 10(3)ln(44/40Ca)alpha(garnet-clinopyroxene), we evaluated the observed difference in Ca isotope compositions between garnet and clinopyroxene in natural samples. Most have reached equilibrium, whereas some samples exceed our prediction, which might reflect the majorite effect or kinetic fractionation.

Equilibrium Ca isotope fractionationFirst-principles calculationPyropeGrossularConcentration effectsGarnetSolid solution seriesPressure effectCRYSTAL IR SPECTROSCOPY1ST-PRINCIPLES CALCULATIONSEQUILIBRIUM FRACTIONATIONORTHO-PYROXENECA ISOTOPESPYROPECLINOPYROXENESYSTEMATICSINSIGHTSSILICON

Li, Yonghui、Wu, Zhongqing、Huang, Shichun、Wang, Wenzhong

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Natl Supercomp Ctr Chengdu

Univ Sci & Technol China

Univ Nevada

2022

10.1016/j.chemgeo.2022.120722

Chemical geology

Chemical geology

EISCI
ISSN：0009-2541
年,卷(期)：2022.591
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