Origin of chromite nodules in podiform chromitite from the Kızıldağ ophiolite, southern Turkey

Highlights

• Combined study of major, trace element and EBSD analyses of Kızıldağ chromite nodule.

• Parental magma of the chromite is hydrous, but its H₂O content is far less than the H₂O solubility.

• The dynamic flow of the parental magma could be quasi-laminar due to its low Reynolds number.

• Two processes play role in the formation of chromite nodule: upward magma flow and convective circulation.

Abstract

Chromite nodule is unique in podiform chromitite of ophiolitic suites and there is no agreement on its petrogenesis. The nodular chromitites of the Kızıldağ ophiolite in southern Turkey contain typical chromite nodules, and here their geochemical compositions and crystallographic orientations have been analyzed to study the formation of nodular orebody and related geodynamic processes. The EBSD data reveal that chromite nodule is composed of a patchwork of chromite grains that have random crystallographic orientations, and all grains have low misorientation angles (<6°) without extensive subgrain rotation. These characteristics are inconsistent with the viewpoint that chromite nodule is emanating from a skeletal chromite core. In addition, chromite crystals in nodules have relatively homogeneous compositions, implying the limited importance of chaotic mixing between two distinct magmas in the formation of chromite nodules. Based on the H₂O contents of olivine and clinopyroxene in chromitite, we calculated that the parental magma of chromitite is hydrous (<~3.48%), but its water content is far less than the required H₂O solubility for exsolution of fluid and vapour phase. Moreover, the dynamic flow of the parental magma should be quasi-laminar due to its low Reynolds number, distinct from the turbulent flow that has long been recognized as a critical factor in forming chromite nodules. When an upward magma flow passes through the lenticular segment of a narrow conduit, some parts will be separated from the remaining forward flow to form a convective circulation in the enlarged area. This convective flow facilitates the melting of pyroxenes along the peridotite wall rock, forming extensive Si- and Cr-rich melt droplets which, in turn, mix with the primitive magma to crystallize chromite grains. Trajectories of these chromite crystals with different sizes highly gather in a quasi-steady area inside the boundary between the convective current and upward ascending magma flow, where allows numerous impacts and collisions between chromite grains, leading to the coalescence of clusters of chromite grains and eventually the formation of chromite nodules. This scenario is underpinned by the laminar flow pattern in the podiform chromitite from the Kızıldağ ophiolite, and may be also transferrable to other ophiolitic chromitite in general.

Introduction

Podiform chromite deposit is one of the special deposits in mantle peridotite of ophiolites, and fundamentally is lenticular, tabular, pencil-shaped, or irregular in form (Thayer, 1964). The podiform chromite bodies commonly occur in harzburgite and are accompanied by thin segregation of dunites (e.g., Zhou et al., 2001, Huang et al., 2004). These enveloped dunites are proposed to be the result of local melting of the pyroxene in the surrounding harzburgite (Zhou et al., 1996). In most podiform chromite deposits, besides the ordinary massive and disseminated chromite ores, nodular chromitites are also observed, which is a unique characteristic in podiform chromite deposits (e.g., Thayer, 1969, Ahmed, 1982, Prichard et al., 2015). These nodules commonly have ellipsoidal shapes, and range in size from 3 to 50 mm (~10–25 mm in most cases) (Pavlov et al., 1977). Generally, chromite nodules occur in groups and are in contact with each other (e.g., Ahmed, 1982, Paktunc, 1990). Many studies reported that the texture of nodular ores appears to be primary and can provide important clue to the mode of formation of podiform chromite deposit (e.g., Cassard et al., 1981, Ballhaus, 1998, Zhou et al., 2001, Gundewar and Sinha, 2013).

Although the nodular textures of chromitite have been emphasized in metallogenic theories of the economic podiform chromite deposits, the origin of chromite nodules is still controversial. Firstly, Thayer (1969) pointed that the nodular texture could be the product of aggregation of free-formed chromite grains prior to settling. Then, Dickey (1975) proposed the nodular textures may be generated by the snowballing of chromite crystals in a turbulent zone of magma segregation. Cassard et al. (1981) attributed the formation of nodules to pelletization driven by elutriation in parts of magmatic conduits where downward motion of chromite remains in dynamic equilibrium with ascending magma flow. As the research progresses, the chromite nodules were considered as the results of incomplete mixing between the Cr-rich melt and Mg-rich melt in a magma conduit (e.g., Pavlov et al., 1977, Ballhaus, 1998). Zhou et al. (2001) proposed that the nodular chromite grains are produced by primitive melt-mantle harzburgite interaction that followed by mixing of the new-formed melt droplets with the primitive magma in porous dunite conduits. Others raised that nodule is formed by crystallization and growth of the dendritic chromite from a magmatic system that is undercooled and/or supersaturated in Cr (e.g., Greenbaum, 1977, Leblanc, 1980, Prichard et al., 2015). There is no agreement on how these chromite nodules form up to now, which is mainly due to that chromite is isotropic and predominantly opaque, posing difficulties in recognizing the substructures during routine microscopic studies (Ghosh et al., 2017).

Analytical techniques-electron backscattered diffraction (EBSD) has become available for performing complex petrographic analyses, which provides a new approach to evaluate the hypotheses of chromite nodule origin. The EBSD technique is capable of determining the crystallographic orientation of grain lattices in the intergrowths (e.g., Xu et al., 2015) that are crucial to distinguish whether they are components of a single crystal or grew as a cluster of randomly oriented nuclei. Recently, the EBSD technique has been used to study the crystallographic relationships (e.g., Vukmanovic et al., 2013, Prichard et al., 2015, Ghosh et al., 2017, Yudovskaya et al., 2019) and deformation mechanism of chromite grains (e.g., Satsukawa et al., 2015). However, only one study has discussed the details of possible formation processes of chromite nodules (Prichard et al., 2015). On the basis of EBSD and geochemical observations of unusual skeletal chromite nodules from the Troodos ophiolite in Cyprus, Prichard et al. (2015) found that these nodules were emanated from a single skeletal chromite core. However, nodular texture without skeletal core is the more general phenomenon observed from worldwide ophiolite complexes, and the scenario underpinned by the skeletal chromite nodule may not be a representative case.

In this study, we carried out a detailed microtextural and geochemical study on a chromite nodule without skeletal cores from the Kızıldağ chromitite using electron backscattered diffraction (EBSD), electron microprobe analysis (EPMA) and laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) techniques, that provide new insight into the contentious question of how the chromite nodules form.