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Element partitioning in magnetite under low-grade metamorphic conditions – A case study from the Proterozoic Belt Supergroup, USA

December 22, 2017

The distribution and partitioning of elements in igneous rocks is well established for various melt –(fluid) –solid pairs and provides important insights into the petrogenesis of these rocks. Studies of the partitioning behavior of elements under metamorphic conditions are scarce and commonly focus on high-grade metamorphic facies. Little is known about the partitioning behavior of elements under low-grade metamorphic conditions. Greenschist-facies metasedimentary rocks of the North American Belt Supergroup host magnetite that displays equilibrium features with co-existing mineral phases such as quartz and carbonate. Magnetite is an ideal target for geochemical investigations because it can incorporate a large number of cations and is sensitive to changes in temperature, oxygen fugacity, pressure, whole-rock composition, and cooling trends. Whole-rock major and trace element analyses have been undertaken on representative samples from Belt Supergroup metasedimentary rocks using X-ray luorescence. Electron microprobe and laser ablation ICP-MS were used to obtain major and trace element concentrations for magnetite hosted in these rocks. Stable-isotope geothermometry of magnetite–quartz and magnetite–carbonate pairs constrain metamorphic temperatures to ca. 390°C. Partition coefficients (D) for magnetite–matrix pairs presumably reflect equilibrium at these low-grade metamorphic conditions. Except for Mn and Ni, which show comparable partition coefficients, the calculated values are one to two orders of magnitude lower than those for igneous magnetite. Aluminum displays the lowest calculated partition coefficient with a value of 0.006 and Ni and Fe the highest values with 6.3 and 20.9, respectively. Of the elements that commonly occur in spinel-group minerals, two groups can be distinguished: (1) elements that preferentially partition into the host rock (D<1): Al, Mg, Pb, and Ti and (2) elements that show a preference to partition into magnetite (D>1): Zn, Mn, Cr, V, Ni, and Fe.

Publication Year 2017
Title Element partitioning in magnetite under low-grade metamorphic conditions – A case study from the Proterozoic Belt Supergroup, USA
DOI 10.1127/ejm/2017/0029-2657
Authors Patrick Nadoll, Jeffrey L. Mauk, Timothy Hayes, Alan Koenig, Stephen E. Box
Publication Type Article
Publication Subtype Journal Article
Series Title European Journal of Mineralogy
Index ID 70226960
Record Source USGS Publications Warehouse
USGS Organization Central Mineral and Environmental Resources Science Center; Geology, Minerals, Energy, and Geophysics Science Center