Chromitites in ophiolites: How, where, when, why? Part II. The crystallization of chromitites
A review of previous work relevant to the formation of concentrations of chromite in peridotites from ophiolitic (s.l.) sequences highlights some of the key problems in understanding the complex processes involved. This review forms the basis for chromitite-genesis models that integrate new geochemical data with petrologic, field and microstructural observations, and for a re-interpretation of previous data and concepts. The geochemical data include major- and trace-element contents of chromite and coexisting phases and especially the nature and Os-isotope compositions of platinum-group minerals (PGM) and base-metal sulfides (BMS); the PGM data in particular provide new insights into chromitite formation.
Differences in the morphology, structural relationships, and geochemical signatures of chromitites allow the recognition of three distinct types. Type I is the most abundant and is distinguished by bulk-rock enrichment in Os, Ir and Ru relative to Rh, Pt, and Pd; it shows no consistent spatial location within the ophiolite “stratigraphy”. The second type (Type IIA) is generally confined to the shallower zones of the oceanic lithosphere (mainly as concordant layers, bands and seams, but also as discordant pods or irregular bodies), and is significantly enriched in the incompatible platinum-group elements (PGE) with generally higher total PGE contents than Type I. The third type (Type IIB) shows the same spatial distributions and PGE patterns as Type IIA but has a more limited range of Cr# and a wider range of Mg# that overlap with the compositional range of chromites from layered mafic intrusions.
Reaction of melts with peridotite wall-rocks results in the extraction of pyroxene into the melts, forming anastomosing dunitic melt channels in the mantle sections of ophiolites. The Os-isotope heterogeneity in PGMs within single chromitite samples, as described in Part I, provides clear evidence that melt mingling take place on very small scales. This suggests that ophiolitic chromitites are generated through the disequilibrium precipitation of chromite, forced by small-scale mingling of melts that had different SiO2 contents, reflecting derivation from different source rocks, different degrees of partial melting and/or wall-rock reaction. Progressive reaction, crystallization and mixing of melts within the channel system assures the presence of a spectrum of melts at any one time, making the system self-sustaining; each new injection of mafic melt would find more evolved melts with which to react, producing more chromite. Chromite is carried to its final deposition by migration of the chromite-bearing melts, or fluids derived from them. This explains the general association of chromitite with the dunitic portions of ophiolitic mantle; dunite margins around chromite segregations represent the original host rock intruded by chromitite-forming fluids.
Keywords: Ophiolitic chromite; Chromitite origin; Platinum group elements in ophiolites; Mafic magma mixing; Chromite trace elements; Mantle sulfide Re–Os isotopes
To Cite this article: González-Jiménez, J.M., Griffin, W.L., Proenza, J.A., Gervilla, F., O'Reilly, S.Y., Akbulute, M., Pearson, N.J., Arai, S. (2014): Chromitites in ophiolites: How, where, when, why? Part II. The crystallization of chromitites. Lithos, 189, 140-158.