Hydrogen and oxygen isotope ratios in the waters of the Ngawha hydrothermal area,North Auckland |
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| Authors: | D C McDonald |
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| Abstract: | The ratios of D/H and O18/O16 in natural waters from streams, boreholes, soda springs, hot pools, ponds and larger bodies of water in the Ngawha hydrothermal area were determined. The results are considered in relation to the isotopic changes known to occur in water subjected to evaporation. Where applicable chemical and other work was also considered. It is assumed that stream water isotope composition is the mean value for the isotopic composition of meteoric waters. Measurements on waters taken from boreholes drilled to 65 feet and 350 feet and from the other water sources mentioned, indicate that they were of meteoric origin as judged by stream isotope composition. The waters from the soda springs appeared to be isotopically the same as the stream water, a finding consistent with the absence of evaporative surface. These borehole waters were similar but slightly different in O18 due probably to exchange between rock and water. Heavy isotope enrichment of the ponds and larger bodies of water appeared to be due to non-equilibrium evaporation at ambient temperature. The hot pools in the Ngawha springs area proper were enriched in the heavier isotopes probably due to non-equilibrium evaporation at the usual hot pool temperature of about 40°C and also to exchange of O18 between water and rock. The water from a further borehole drilled to approximately 2,000 feet appeared also to be of meteoric origin but was changed in O18 content to an extent consistent with the assumption that oxygen isotope exchange with rock had taken place at approximately 230°C. The results are used to illustrate possibilities for the use of oxygen and hydrogen isotope measurements in hydrothermal investigations. |
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