Resolving sediment subduction and crustal contamination in the Lesser Antilles island Arc: A combined He-O-Sr isotope approach

We report an extensive helium isotope survey of basaltic to andesitic lavas from the Lesser Antilles island arc-an arc system with well-documented evidence of crustal contamination. Given the sensitivity of helium isotopes as a tracer of the effects of crustal additions, our aim is to evaluate the relationship of ³He/⁴He ratios to other indices of contamination processes such as oxygen and strontium isotopes. To this end, we have carried out 53 ³He/⁴He analyses on separated minerals (olivines and pyroxenes) from throughout the arc, which we compare with whole-rock strontium and phenocryst oxygen isotope measurements. We show that the three isotopic tracers show coherent patterns throughout the Lesser Antilles, indicating a regional control on crustal contamination. The southern section of the arc (Grenada to Martinique) shows clear evidence for major crustal contamination in all three isotopic systems with results for our samples in the range ³He/⁴He_(olivine) 3.6-7.6R_A7 δ¹⁸O_(olivine) 4.74-5.76%, and ⁸⁷Sr/⁸⁶Sr_(whole-rock) 0.703970-0.705463. We suggest that terrigenous sediments incorporated into the arc crust are the principal contaminant. In contrast, there is minimal contamination in the northern part of the arc [Martinique-(Dominica)-Guadeloupe to Saba] with results for the samples in the range ³He/⁴He_(olivine) 6.8-8.4R_A, δ¹⁸O_(olivine) 5.01-5.29‰, and ⁸⁷Sr/⁸⁶Sr _{(whole-rocks)} 0.703221-0.703843. In this part of the arc, only pyroxene ³He/ ⁴He values (1·4-6.8R_A) consistently record evidence of relatively minor, late-stage additions from the arc crust. Martinique appears to represent the transition between the two sections of the arc. The tracer with the greatest sensitivity appears to be helium isotopes, as contamination is seen first and most prominently in this system.