Lithium isotopes may trace subducting slab signatures in Aleutian arc lavas and intrusions

Abstract

We report [Li] and δ7Li values for a well-characterized suite of 52 geographically (165–184°W), compositionally (SiO2 = 46–70 wt.%), and temporally (0–38 Ma) diverse lavas and intrusive samples. The δ7Li in these rocks range from −0.7‰ to +14.2‰, with 32 of the 35 lavas and 12 of the 17 intrusive samples falling within the depleted mantle range (δ7Li +1.6 to +5.6‰), as sampled by mid-ocean ridge basalts (MORB). The δ7Li values of Aleutian lavas do not exhibit the spatial trends observed in other slab component tracers, nor do δ7Li values correlate with any slab component indicators, such as radiogenic isotopes, oxygen isotopes, or trace element ratios such as Cs/La and Th/La. The δ7Li values in Aleutian intrusions also do not exhibit temporal trends, however, an overall positive relationship exists between δ7Li and Th/Nd. Mixing models for δ7Li and 143Nd/144Nd values suggest that Aleutian samples within or above the MORB δ7Li range can be explained by addition of <1–2% sediment-derived aqueous fluid and ≤3% sediment melt to depleted mantle; both are required to explain the range in δ7Li that is observed. Sediment-derived fluid exerts a stronger control on Aleutian samples having higher δ7Li values than the MORB range, while sediment melt skews the Li isotopic compositions of MORB-range samples to slightly lower values than if sediment fluid was the only slab influence. Our study demonstrates that a slab signature may be deciphered via modeling even in arcs where spatial trends in δ7Li values and correlations with slab component indicators are lacking.