Search

The effect of variations in the fo, fa, en, fs, di, an and ab components in high-Mg basaltic melts on the topology of the spinel liquidus was quantified by conducting simulations with the new SPINMELT-2.0 model for the spinel–melt equilibrium. It has been established that enrichment of the melt in pyroxene components leads to an increase, and conversely, enrichment in plagioclase and olivine components, to a decrease in the solubility of chromite. This effect can be important at the gravitational compaction of cumulates accompanied by the extraction of intercumulus melt and its infiltration upward. In this case, it is reasonable to expect sequential reequilibration of the infiltrating melt with cumulate piles of different composition. This suggests that Cr-spinel can be transferred and concentrated again during the postcumulus solidification stage of layered intrusions. The concentrating process involves Cr-spinel extraction into the melt enriched in pyroxene components and the subsequent redeposition of the mineral during when this melt reacts with the feldspar-rich matrix of proto-anorthosite layers or horizons enriched in olivine. Some of these layers and horizons may be produced by additional injections of more primitive magma. The realism of the proposed mechanism is evident from the well-known spatial relations between the chromite layers with anorthosites and dunite–harzburgites of the Bushveld complex. [ABSTRACT FROM AUTHOR]

Several recent petrogenetic models propose that layers of stratiform chromitites in the Bushveld Complex are produced by mechanical separation of chromite from co-existing silicate minerals (e.g., olivine, orthopyroxene, plagioclase) within crystal-rich slurries. One feature in common for the slurry models is that they imply that interstitial liquid in stratiform chromitites solidifies underneath a crystal pile ranging from 20 to 100 m thick. This means that the interstitial liquid would not be able to chemically communicate with the resident melt overlying the crystal pile. Equilibrium crystallization of interstitial melt within chromitites will, therefore, produce interstitial plagioclase with an average Ca/Na ratio similar to that of the interstitial liquid itself, i.e., plagioclase must be evolved in composition (∼40–50% An-content). In contrast to this prediction, extensive microprobe data show that interstitial plagioclase in Bushveld chromitites has a rather primitive composition (∼60–80% An-content) that is close to that of cumulus plagioclase (∼70–80% An-content) from adjacent norites and anorthosites that are produced by fractional crystallization. This implies that interstitial plagioclase is not a product of equilibrium crystallization within a deeply buried chromite-rich pile. Modification of interstitial plagioclase composition by various additional processes (fractional crystallization, selective diffusion, reactive melt flow, etc.) are considered but shown to be inconsistent with field, textural or chemical observations. We conclude that either slurry models for the origin of stratiform chromitites are incorrect and should be abandoned or must be substantially modified to address the issue of the high An-content of interstitial plagioclase in stratiform chromitites. • Slurry models imply chromitite interstitial liquid solidifies at depth. • Crystallization of interstitial melt should produce plagioclase with An40-50 content. • Interstitial plagioclase in Bushveld chromitites shows up to 70-80% An content. • Interstitial plagioclase did not form in a deeply buried chromitite-rich pile. • Slurry models must be modified to explain the plagioclase composition. [ABSTRACT FROM AUTHOR]

No genetic link between the two main types of chromitite, stratiform and podiform chromitites, has ever been discussed. These two types of chromitite have very different geological contexts; the stratiform one is a member of layered intrusions, representing fossil magma chambers, in the crust, and the podiform one forms pod-like bodies, representing fossil magma conduits, in the upper mantle. Chromite grains contain peculiar polymineralic inclusions derived from Na-bearing hydrous melts, whose features are so similar between the two types that they may form in a similar fashion. The origin of the chromite-hosted inclusions in chromitites has been controversial but left unclear. The chromite-hosted inclusions also characterize the products of the peridotite–melt reaction or melt-assisted partial melting, such as dunites, troctolites and even mantle harzburgites. I propose a common origin for the inclusion-bearing chromites, i.e., a reaction between the mantle peridotite and magma. Some of the chromite grains in the stratiform chromitite originally formed in the mantle through the peridotite–magma reaction, possibly as loose-packed young podiform chromitites, and were subsequently disintegrated and transported to a crustal magma chamber as suspended grains. It is noted, however, that the podiform chromitites left in the mantle beneath the layered intrusions are different from most of the podiform chromitites now exposed in the ophiolites. [ABSTRACT FROM AUTHOR]