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

초록

We report [Li] and delta Li-7 values for a well-characterized suite of 52 geographically (165-184 degrees W), compositionally (SiO2 = 46-70 wt.%), and temporally (0-38 Ma) diverse lavas and intrusive samples. The delta Li-7 in these rocks range from -0.7 parts per thousand to +14.2 parts per thousand, with 32 of the 35 lavas and 12 of the 17 intrusive samples falling within the depleted mantle range (delta Li-7 +1.6 to +5.6 parts per thousand), as sampled by mid-ocean ridge basalts (MORB). The delta Li-7 values of Aleutian lavas do not exhibit the spatial trends observed in other slab component tracers, nor do delta Li-7 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 delta Li-7 values in Aleutian intrusions also do not exhibit temporal trends, however, an overall positive relationship exists between delta Li-7 and Th/Nd. Mixing models for delta Li-7 and Nd-14(3)/Nd-144 values suggest that Aleutian samples within or above the MORB delta Li-7 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 delta Li-7 that is observed. Sediment-derived fluid exerts a stronger control on Aleutian samples having higher delta Li-7 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 delta Li-7 Li values and correlations with slab component indicators are lacking. (C) 2019 Elsevier Ltd. All rights reserved.

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