Chambefort, I.; Bignall, G. 2013 Preliminary stable isotope study on the Lahendong geothermal system, Indonesia. Lower Hutt, N.Z.: GNS Science. GNS Science report 2013/14 5 p.
Abstract: Five core samples from two wells (LHD1 and LHD5) from Lahendong geothermal system were selected for stable isotope analyses. Oxygen, hydrogen and carbon isotopes were analysed on quartz, clay and calcite. [delta]18OVSMOW of quartz is 10.32, 2.78 and 11.51‰. Calcite [delta]18OVPDP ranges from -20.30 to 15.76‰, with [delta] 13CVPDP from -3.33 to -0.8‰. Clay (mainly chlorite) have a [delta] 18OVSMOW = 6.1‰ and [delta] DVSMOW = -74.9‰. Assuming temperature of precipitation between 250°C and 270°C, fluid in equilibrium with hydrothermal minerals can be recalculated. Oxygen value of the fluid that precipitated quartz, calcite and clay is generally enriched in oxygen, with isotopic value around 3‰ for pure quartz, 4.8‰ for clay and between 2 and 7‰ for calcite. The range of [delta]18Ofluid points to a mixed origin between depleted meteoric, neutral chloride thermal water, and magmatic oxygen. Based on these preliminary results, it is challenging to differentiate between a direct magmatic input with a high [delta]18Ofluid, or a low water-rock ratio interaction where [delta]18Ofluid is derived from isotopic exchange with 18O-enriched magmatic rocks. The value of [delta]13CCO2 in isotopic equilibrium with calcite is relatively light with [delta]13CCO2 varying between -2.03 and 0.47‰. Such CO2 values are above the assumed isotopic value of magmatic-derived CO2 and indicate that the carbon in the system is also influenced by calcite from near-by marine sediments. (auth)