Stable Titanium Isotopic Fractionation During Subduction Initiation of the Izu-Bonin-Mariana Arc

Abstract:

Subduction initiation is an enigmatic, yet fundamental process in the plate tectonic cycle. IODP Exp. 352 to the Bonin forearc, recovered the first in situ record of subduction initiation. The initiation process was short-lived, progressing from fore-arc basalts (FABs) to boninites over ~1 Ma [1]. Previous work suggests that P-T conditions during subduction initiation allow HFSEs such as Ti to be mobilised from the slab during boninite generation [2]. Yet it is unclear if mobilisation occurs via a melt or fluid phase. Further, the distinct Ti/V ratios of FABs, MORB, boninites, and back-arc basins have been explained by both changes in redox conditions and by variable source depletion [3]. Here we present the Ti stable isotopic composition (δ⁴⁹Ti) of the FAB-boninite magmatic sequence, to explore the changing mantle source(s), and input(s) from the nascent subducting slab during subduction initiation. Resolvable stable isotopic fractionation of Ti is well-documented at high temperatures, and mainly interpreted to reflect co-ordination differences between oxides and silicate melts/minerals [e.g. 4, 5, 6]. Subducting slabs have long been known to contain significant rutile, which dominates the Ti budget. In contrast, the mantle source of FABs and boninites is Ti-depleted, and most Ti is hosted by spinel. We find that FABs are isotopically lighter than boninites, but both are significantly heavier than the current BSE estimate and MORB average [4]. Furthermore, we observe a striking positive correlation in the boninites between δ⁴⁹Ti and Ba concentration that is unrelated to fractional crystallisation or alteration. In contrast, boninite δ⁴⁹Ti does no co-vary with REE. The correlation of fluid mobile elements and δ⁴⁹Ti suggests that Ti can be partially mobilised by a fluid phase during subduction initiation. The fractionation mechanism appears to release heavy Ti, likely leaving an isotopically light residue to subduct deeper into the mantle. The generation of such isotopic heterogeneity during subduction has important implications for the potential δ⁴⁹Ti heterogeneity of the mantle and the use of redox-controlled elements such as Ti to examine past redox conditions.

[1] Reagan et al. 2019

[2] Li et al. 2019

[3] Shervais 1981

[4] Millet et al. 2016

[5] Deng et al. 2019

[6] Hoare et al. 2020