Podiform chromitites: how do they form and evolve in suprasubduction mantle?
Dpto.Mineralogía y Petrología and Instituto Andaluz de Ciencias de la Ti
Podiform chromitites constitute major resources of chromium on Earth and the only source of refractory-type chromite ores (min. 25 wt% Al203 ; min.60 wt% Cr203 +Al203 ; max. 15 wt% FeO).They are usually hosted within intensely tectonitized peridotite forming the mantle section of several ophiolite complexes but mainly in those composed of strongly depleted harzburgite and replacive dunite. Some key features of “ophiolitic chromitites” include: 1) variable ore tonnage (from less than 10,000t up to 300Mt); 2) irregular ore bodies with pod-like morphology, surrounded by variably thick dunite envelopes and arranged discordant, subconcordant or concordant with respect to the foliation of host peridotites; 3) variable chromite texture (massive, disseminated, nodular, antinodular, orbicular...); 4) rather constant chemical composition within single ore bodies but highly variable at the scale a given ophiolite massif or complex; 5) variable platinum-group element (PGE) contents (usually between 30 and 2,000ppb PGE) having platinum-group mineral (PGM) assemblages largely dominated by Os, Ir and Ru minerals (mainly members of the laurite-erlichmanite solid solution series (RuS2-OsS2) and Os-Ir alloys).
Podiform chromite ores are almost exclusively found in the subarc mantle of intraoceanic island arcs, including those beneath fore-arc, intra-arc and back-arc positions. This has led to many authors to interprent the genesis of the podiform chromitites as related with arc-type melts generated in the suprasubduction zone. The models proposed for the genesis of almost monomineralic chromitite bodies are varied, including (1) cotectic crystallization of chromite+olivine followed by mechanical separation of chromite (Lago et al., 1982), (2) changes in fO2 (Hill and Roeder, 1974; Melcher et al., 1997), (3) assimilation of mafic rocks (Bédar and Hébert, 1998) and (4) mixing of primitive, olivine-saturated basalts with more fractionated melts (Arai and Yurimoto, 1994; Arai and Abe, 1995). The latter mechanism has been recently adapted to a new scenario where partial melts migrate along an interconnected network of dunitic channels and mix at the intersection of channels (González-Jiménez et al., 2014). Such a network of channels provides an ideal framework capable to sustain a continuous supply of melts with a variegated provenance and/or SiO2 , which once mingled at the intersection of channels may evolve to hybrid-mix melts able to crystallize only monomineralic chromite. In this model, the size of the chromitite bodies and texture that may form is a direct function on melt/rock ratios and on the existence of a hot source that supplies melts to the system.
Recently, some researchers have identified the presence of a new suite of inclusions in the chromitite, which includes minerals typical of high-pressure such as diamonds, coesite and stishovite as well as other that only form in highly reduced enviroments like mosissanite, FeSi alloy, native Cr, native Al, native Fe… (Yang et al., 2014; McGowan et al., 2015). The identification of this extrange suite of mineral has opened new avenues of research and interpretations, leading to believe that some podiform chromitites might record different stages of a complex evolutionary story. This would start by their crystallization at low pressure conditions followed by subduction down to the transition zone and late upwelling/exhumation to shallow mantle domains again. Additionally, Re-Os model ages calculated from Re-Os isotopic data by in situ LA-MC-ICPMS on individual PGM inclusions within chomite and U-Pb ages of zircons recovered from chromitites frequently provide a rarther widespread age spectra, providing further support to such evolutionary story of chromitites in time and space (e.g. McGowan et al., 2015). Additional evidences for a different type of subduction-upwelling evolution of podiform chromitites in suprasubduction zone environments could come from some metamorphic features. The presence of vermicular intergrowths of Al-rich and Fe3+-rich chromian spinels formed by break down of high temperature (and probably high pressure) chromite unusually rich in Fe2O3 and other trace elements in chromitites from Sierras Pampeanas, Argentina (Colas et al., 2016), provide evidences of complex prograde-retrograde metamorphic paths.
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