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Journal of Petrology | Volume 44 | Number 3 | Pages 491-515 | 2003
© Oxford University Press 2003
Rates and Processes of Potassic Magma Evolution beneath Sangeang Api Volcano, East Sunda Arc, Indonesia
1 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF BRISTOL, WILLS MEMORIAL BUILDING, BRISTOL BS8 1RJ, UK
2 DEPARTMENT OF GEOLOGY AND GEOPHYSICS, THE UNIVERSITY OF ADELAIDE, ADELAIDE, SA 5005, AUSTRALIA
3 LABORATORIES OF THE GOVERNMENT GEOCHEMIST, QUEENS ROAD, TEDDINGTON PW11 0LY, UK
4 SCHOOL OF EARTH SCIENCES, UNIVERSITY OF TASMANIA, HOBART, TAS 7001, AUSTRALIA
5 DEPARTMENT OF EARTH SCIENCES, MEMORIAL UNIVERSITY OF NEWFOUNDLAND, ST. JOHNS, NEWFOUNDLAND A1B 3X5, CANADA
Telephone: (44) 0117 9545440. Fax: (44) 0117 9253385. E-mail: simon.turner{at}bristol.ac.uk
U-series isotopes can provide unique insights into the physical processes of magma evolution by constraining the time scales over which they operate. This, however, requires rock suites that provide a clear and complete record of the liquid line of descent. Sangeang Api volcano, in the east Sunda arc, provides such an opportunity because it erupts potassic lavas (SiO2 
47–55%), which contain a spectrum of xenoliths interpreted to represent the cumulates complementary to the lavas. Combined, the cumulates and lavas span a large compositional range, which major and trace element modelling suggests reflects 70% polybaric crystallization that began at sub-Moho depths and continued into the upper crust. The parental magmas can be successfully modelled by 3% dynamic partial melting, in the presence of 0–4% residual garnet, of a mid-ocean ridge basalt (MORB) source enriched by 3% subducted Sunda sediment in addition to a contribution of fluid-mobile elements from the subducting slab. The effects of fluid addition and partial melting on U–Th disequilibria appear to be competing processes. A moderate range in Sr, Nd and Pb isotopes is interpreted to reflect a combination of source heterogeneity and <15% assimilation of Indian MORB crust. Neither interaction with metasomatized arc lithosphere nor the presence of enriched, plume-type mantle in the mantle wedge is required by our data. The cumulates and lavas have largely indistinguishable (230Th/232Th) over a wide range of U/Th ratios and thus any age differences are minimal relative to the half-life of 230Th. All whole rocks and minerals are characterized by 226Ra excesses and modelling of the 226Ra–230Th–Ba data suggests that magmatic evolution beneath this arc volcano occurs on time scales of 2000 years. Combining this with simple numerical calculations we estimate that the Sangeang Api magma chamber is 6–10 km3 in size, cooling rates are 0·05°C/yr and minimum crystal growth rates are (2–7) x 10-13 cm/s. It is possible that a significant proportion of the crystal growth and differentiation occurred during isobaric decompression of magmas ascending through conduits. An implication is that the net magmatic flux across the Moho is of magmas that are already significantly evolved from primary magmas and this may be significant for why average continental crust has an andesitic bulk composition even though the flux out of the mantle wedge is basaltic.
KEY WORDS: time scales; lava; xenolith; Sunda arc; potassic magma
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