Iron and magnesium isotope fractionation in oceanic lithosphere and sub-arc mantle: Perspectives from ophiolites

Abstract

words: 235 18 Abstract 25 We present high-precision Fe and Mg isotopic data for the Purang ophiolite, 26 southwestern Tibet, representing the first combined Fe and Mg isotopic study of the 27 oceanic lithosphere hitherto. The δ 56 Fe and δ 26 Mg values of the ophiolitic peridotite, 28 * Corresponding authors: Ben-Xun Su, subenxun@mail.igcas.ac.cn; Fang-Zhen Teng, fteng@u.washington.edu. Su et al. Page 2dunite and gabbro vary from -0.209 to 0.187‰ and from -0.28 to -0.14‰, respectively. 29The average δ 56 Fe of the peridotites is -0.030 ± 0.143‰ (2SD, n = 17), a value 30 indistinguishable from abyssal peridotites and chondrites, and lower than oceanic basalts. 31The average δ 26 Mg value of the peridotites is -0.20 ± 0.10‰, a value slightly higher than 32 both chondrites and oceanic basalts. Correlations between δ 56 Fe and indices of partial 33 melting indicate fractionation of 0.323‰ in δ 56 Fe between the oceanic lithospheric 34 mantle and the overlying mafic crust during an early episode of partial melting, 35presumably beneath a spreading centre. Subsequent metasomatism in a supra-subduction 36 zone caused elevated oxygen fugacity and heavy Fe isotopic compositions in the oceanic 37 lithospheric mantle. The dunite with high Ba/La, a proxy for oxygen fugacity, and high 38 δ 56 Fe values was likely formed during this process of sub-arc mantle-melt interaction. 39The negatively coupled Fe-Mg isotopic variations of the Purang ophiolite indicate that 40Mg isotope fractionation may also occur during high-temperature mantle processes. The 41 observed isotopic variations among different lithologies in the ophiolite may 42 satisfactorily account for the isotopic differences between arc lavas and mantle 43 peridotites with respect to oceanic basalts, thus providing implications for crust-mantle 44 differentiation. 45

Melt evolution of upper mantle peridotites and mafic dikes in the northern ophiolite belt of the western Yarlung Zangbo suture zone (southern Tibet)

“…These signatures together with the various Sr, Nd, and Pb isotopes suggest that magmas of the mafic dikes intruding the Cuobuzha and Baer peridotites were influenced by island arc-type fluids. This interpretation is consistent with the geochemical and Re-Os isotopic data of peridotites from the Cuobuzha, Dongbo, and Purang massifs, suggesting that their upper mantle peridotites had interacted with arc-related fluids and melts Niu et al, 2015;Su et al, 2015;Feng et al, 2017).…”

Subduction initiation for the formation of high-Cr chromitites in the Kop ophiolite, NE Turkey

“…7); they are similar to the spoon shaped patterns reported for some reactive dunites from the Tibetan ophiolites in China (Su et al, 2015;Zhou et al, 2005). Because REE are highly incompatible in both olivine and chromite (Nagasawa et al, 1980;Nielsen et al, 1992;Mckay, 1986;Spandler et al, 2007), continuous crystallization of both minerals cannot largely fractionate the REE patterns in residual magmas.…”

“…Different REE patterns have been reported for chromitite-associated dunite (Proenza et al, 1999;Su et al, 2015;Zhou et al, 2005), implying that the parental magmas of high-Cr chromitites also have diverse REE distributions. However, the LREE enriched and U-shaped REE patterns of peridotites in supra-subduction zones could be generated by metasomatism (Parkinson and Pearce, 1998;Saccani and Photiades, 2004;Ulrich et al, 2010), casting doubt on whether these REE patterns of dunites are primary or secondary.…”

Iron isotope fractionation in subduction-related high-pressure metabasites (Ile de Groix, France)

“…Craddock et al (2013) estimated the average Fe isotope composition of the terrestrial mantle to be 0.025 ± 0.025‰ (95% CL), based on analyses of abyssal peridotites. However, large Fe isotope variations from −0.54 to +0.16‰ have been identified in bulk mantle xenoliths (Beard and Johnson 2004;Weyer and Ionov 2007;Zhao et al 2010;Poitrasson et al 2013;Su et al 2015). In the following discussion, we will address how processes in subduction zones could induce Fe isotope mantle heterogeneities.…”

“…Thin dotted lines are the supposed trajectories followed by the rocks since the basaltic protolith emplacement to its embedment in the subduction zone and percolation in the fore-arc mantle wedge is expected to trigger mantle metasomatism and, thus, to generate Fe isotopic heterogeneities in the mantle (e.g. Beard and Johnson 2004;Weyer and Ionov 2007;Su et al 2015). Subsequent melting of the metasomatised heavy-Fe upper mantle may in turn be source of OIBs with elevated δ 56 Fe values (Konter et al 2016), supporting a deep mantle cycle.…”