Crustal contamination processes traced by helium isotopes: Examples from the Sunda arc, Indonesia

Abstract

Helium isotope data have been obtained on well-characterised olivine and clinopyroxene phenocrysts and xenocrysts from thirteen volcanic centres located between central Sumatra and Sumbawa in the Sunda arc of Indonesia. Olivine crystals in mantle xenoliths (lherzolite) from Bukit Telor basalts are primitive (Mg# = 90), and their 3 He 4 He value ( R R A = 8.8 ) indicates that the Sumatran mantle wedge is MORB-like in helium isotope composition. All other samples have lower 3 He 4 He ratios ranging from 8.5 R A to 4.5 R A, with most (thirteen out of eighteen) following a trend of more radiogenic 3 He 4 He values with decreasing Mg#. The only exceptions to this trend are phenocrysts from Batur, Agung and Kerinci, which have MORB-like 3 He 4 He values but relatively low Mg# (Mg# = 70–71), and two highly inclusion-rich clinopyroxenes which have 3 He 4 He values lower than other samples of similar Mg#. The results indicate that crustal contamination unrelated to subduction in the Sunda arc is clearly recorded in the 3 He 4 He characteristics of mafic phenocrysts of subaerial volcanics, and that addition of radiogenic helium is related to low-pressure differentiation processes affecting the melts prior to eruption. These conclusions may have widespread applicability and indicate that helium isotope variations can act as an extremely sensitive tracer of upper crustal contamination.