CRUSTAL CONTAMINATION PROCESSES TRACED BY HELIUM-ISOTOPES - EXAMPLES FROM THE SUNDA ARC, INDONESIA

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