STABLE ISOTOPIC SIGNATURES OF AUTHIGENIC MINERALS IN A HOLOCENE OPHIOLITIC DEBRIS FLOW, SOUTHLAND, NEW-ZEALAND

Authigenic chrysotile, stevensite, calcite, aragonite and pectolite ha ve formed together in a Holocene ophiolitic debris flow in Southland, New Zealand. Mineral growth occurred about 4700-5700 years ago. The te mperature of formation of these minerals is estimated from climatologi cal data to be 5-10 degrees C. Surface water and groundwater delta(18) O is currently about -10 parts per thousand, and was estimated to be a bout -9.5 +/- 1 parts per thousand during mineralization. Coexisting c alcite (delta(18)O = +23 parts per thousand) and aragonite (delta(18)O = +24 parts per thousand) were in equilibrium with each other and wit h the groundwater at 5-10 degrees C. Stevensite delta(18)O is +14 to 16 parts per thousand, chrysotile has delta(18)O = +5.5 parts per thou sand, End authigenic pectolite has delta(18)O near +10 parts per thous and. Carbon isotope ratios for calcite and aragonite are strongly depl eted (delta(13)C = -13 to -18) which suggests that dissolved CO2 had d elta(13)C below -27. This isotopically light carbon probably resulted from a high organic component of carbon dissolved in the groundwater.