Magnesium Isotopic Composition of Subducting Marine Sediments

Abstract:

Subducted marine sediments have recently been called upon to explain the heterogeneous Mg isotopic composition (δ²⁶Mg, ‰) found in mantle wehrlites (-0.39 to +0.09 [1]) in the context of a homogeneous mantle (-0.25 ± 0.07 [2]). However, no systematic measurements of δ²⁶Mg on marine sediments are currently available to provide direct support to this model. To characterize the Mg inputs to global subduction zones, we measured δ²⁶Mg data for a total of 90 marine sediments collected from 12 drill sites outboard of the world’s major subduction zones. These sediments span a 1.73‰ range in δ²⁶Mg. The detritus-dominated sediments have δ²⁶Mg (-0.59 to +0.53) comparable to those of weathered materials on continents (e.g. -0.52 to +0.92 [3]), while the calcareous oozes yield δ²⁶Mg (as light as -1.20) more similar to the seawater value (-0.83 [4]). The negative correlation between δ²⁶Mg and CaO/Al₂O₃ in these sediments indicates the primary control of mineralogy over the Mg isotopic distribution among different sediment types, as carbonates are enriched in light Mg isotopes (-5.10 to -0.40 [5]) whereas clay-rich weathering residues generally have heavier δ²⁶Mg (e.g. up to +0.65 in saprolite [6]). In addition, chemical weathering and grain-size sorting drive sediments to a heavier δ²⁶Mg, as indicated by the broad positive trends between δ²⁶Mg with CIA (Chemical Index of Alteration [7]) and Al₂O₃/SiO₂, respectively. Collectively, the arc systems sampled in this study represent ~30% of global arc length and the extrapolated global Mg flux of subducting marine sediments accounts for ~9% of the yearly Mg riverine input with a flux-weighted average δ²⁶Mg at -0.26. Subduction of these heterogeneous sediments may not cause significant mantle heterogeneity on a global scale, but the highly variable Mg fluxes and δ²⁶Mg of sediments delivered to different trenches are capable of producing local mantle variations. Volcanic rocks sourced from these mantle domains are thus likely to carry a distinct Mg isotopic signal. [1] Xiao et al., 2013; [2] Teng et al., 2010a; [3] Li et al., 2010; [4] Ling et al., 2011; [5] Huang et al., 2015; [6] Teng et al., 2010b; [7] Nesbitt & Young, 1982.