INVESTIGATION OF DISSOLVED SULPHATE IN VARIOUS GEOTHERMAL FIELDS OF SUMATRA USING OXYGEN AND SULPHUR ISOTOPES
ABSTRACT: There are at least
30 high temperature systems; eleven active volcanoes, five degassing volcanoes
and one caldera volcano controlled by Sumatra Fault Zone over a length of 1700
km. To understand this geothermal field system, some information about geochemistry
including isotope composition in its fluid is needed. Sulphur-34 and oxygen-18
isotopes in dissolved sulphate pair have been used to determine the origin of
acidic fluid of sulphate and to evaluate the process involved. The fluids from
eight hot springs, two fumaroles, four deep wells and crater have been
collected in along Sumatra geothermal fields. Sulphur-34 ( 34 S(SO4) , 0 /00 CDT) and oxygen-18 ( 18
O(SO4) ,0 /00SMOW) in sulphate is analyzed according to Robinson-Kusakabe and
Rafter method, respectively. The 34 S(SO4)values
from Sibayak wells are more enriched of 16.80 /00 to 18.20 /0 that may indicate
the dissolution of anhydriteminerals or isotope partitioning in hydration of SO
2 . The 34 S(SO4) values from two fumaroles (Pusuk
Bukit – NorthSumatra and Rantau Dadap – South Sumatra) are at depleted value of
–0.15 0 / 00 and 1.8 0 / 00 , those are close to 34
Sfrom magmatic sulphur. In general, the 4 S(SO4)
of springs spread in a wide range of 5.250 /00 to14.20 /00 and show amixing
process between atmospheric sulphate and sulphate from deep wells. The 18 O(SO4) from wells exhibitsdepleted value around –3.6 0 / 00
suggesting that 87.5% of sulphate oxygen is derived from groundwater oxygen and
12.5% is derived from atmospheric molecular oxygen in sulphide oxidation
reaction. In the other hand, hot springs (except Semurup), crater and fumaroles
have enriched value of 18 O(SO4) . These enriched values
suggest that ahigher percentage of atmospherically derived oxygen compared to
those from the depth.
Author: E. Ristin Pujiindiyati,
and Zainal Abidin
Journal Code: jpkimiagg060025
