Iron and magnesium isotopic constraints on the origin of chemical heterogeneity in podiform chromitite from the Luobusa ophiolite, Tibet

Abstract

We present high‐precision measurements of iron (Fe) and magnesium (Mg) isotopic compositions of olivine, orthopyroxene, and chromite separates from harzburgites, dunites, and chromitites in the mantle section of the Luobusa ophiolite, southern Tibet, to investigate the origins of podiform chromitite. Two harzburgites in the Zedong ophiolite, southern Tibet, are also reported for comparison. The olivine and orthopyroxene in the Luobusa and Zedong harzburgites have similar Fe and Mg isotopic compositions, with δ56Fe values ranging from 0‰ to +0.083‰ in olivine, from −0.034‰ to +0.081‰ in orthopyroxene and δ26Mg values ranging from −0.25‰ to −0.20‰ in olivine, from −0.29‰ to −0.26‰ in orthopyroxene, respectively. The olivines of two dunites from the Luobusa display small Fe and Mg isotopic variations, with δ56Fe values of +0.014‰ and +0.116‰ and δ26Mg values of −0.21‰ and −0.29‰. All chromites in the Luobusa chromitites have lighter Fe isotopic compositions than the coexisting olivines, with δ56Fe values ranging from −0.247‰ to +0.043‰ in chromite and from −0.146‰ to +0.215‰ in olivine (Δ56FeChr‐Ol = −0.294 to −0.101‰). The chromite δ26Mg values span a significant range from −0.41‰ to +0.14‰. Large disequilibrium Fe and Mg isotope fractionation between chromite and olivine, as well as positive correlation of chromite δ56Fe values with their MgO contents, could be attributed to Fe‐Mg exchange between chromite and olivine. In the disseminated chromitites, the higher modal abundances of olivine than chromite would result in a more extensive Fe‐Mg exchange, whereas chromite in the massive chromitite where olivine is rare could not be affected by this process.

Origin of Reverse Zoned Cr-Spinels from the Paleoproterozoic Yanmenguan Mafic–Ultramafic Complex in the North China Craton

“…There are limited variations in the contents of Ni, Mn, and Ti in Cr-spinel ( Figure 5). In this respect, much attention should be paid to these minor elements in their application to trace geologic processes (Figure 7c), since these elements are not affected by elemental exchange and are therefore more reliable and valid than the traditional index [64]. [5].…”

“…Elemental exchange with coexisting silicate minerals is another important factor modifying the composition of spinel [61][62][63][64][65], and is commonly induced by the variation of physical conditions [66][67][68][69]. Generally, diffusion-induced zonation is caused by the long-term exchange between crystals subjected to continuous, high subsolidus temperatures [23,[70][71][72].…”

Chromite-induced magnesium isotope fractionation during mafic magma differentiation

“…Kinetic fractionation induced by sub-solidus Fe-Mg interdiffusion between olivine and chromite has been well documented in studies of layered intrusions and ophiolites, during which Mg in chromite and Fe in olivine exchange with each other [11,28,38,39]. Since light isotopes diffuse faster than their heavy counterparts [40], this process will lead to higher Mg# and lower d 26 Mg in olivine with a corresponding increase in d 26 Mg of chromite, which agrees with the results of olivine-chromite pairs from Tibetan ophiolites [11]. This is opposite, however, to the positive correlation between Mg# and d 26 Mg observed in the olivine from the Xiadong dunite (Fig.…”

“…Magnesium isotopic variation has been increasingly reported in igneous rocks and was attributed to various mechanisms. Source heterogeneity is a main factor controlling Mg isotopic variations in volcanic rocks [1][2][3][4][5][6]; silicate-carbonatite liquid immiscibility and carbonatite magma differentiation can result in significant Mg isotope fractionation [7], whereas diffusive Mg-Fe exchange with melt or chromite produces large Mg isotope fractionation in olivine [8][9][10][11]. Although Mg isotope fractionation during the differentiation of granitic and basaltic magma is limited [3,[12][13][14][15], magma differentiation involving chromite can potentially produce large Mg isotope fractionation.…”

Timing of Secondary Hydrothermal Alteration of the Luobusa Chromitites Constrained by Ar/Ar Dating of Chrome Chlorites

“…These studies include: major and trace elemental and/or isotopic compositions of different textural chromitites and their host peridotites (e.g., [1][2][3][4][5][6][7][8]), and unusual ultrahigh-pressure (UHP), highly reduced and crustally derived minerals found either in situ or as separates in the chromitites and peridotites (e.g., [9][10][11][12][13][14][15][16][17][18][19][20][21]). [42]), and the distribution of magmatism on the Lhasa terrane (modified from Chung et al [43]).…”