Abstract

Authigenic chrysotile, stevensite, calcite, aragonite and pecolite have formed together is a Holocene ophiolitic debris flow in Southland, New Zealand. Mineral growth occurred about 4700-5700 years ago. The temperature of formation of these minerals is estimated from climatological data to be 5-10 degrees C. Surface water and groundwater sigma 18 O is currently about -10 per thousand, and was estimated to be about -9.5 plus or minus 1 per thousand during mineralization. Coexisting calcite (sigma 18 O = +23 per thousand) and aragonite (sigma 18 O = +24 per thousand) were in equilibrium with each other and with the groundwater at 5-10 degrees C. Stevensite sigma 18 O is +14 to +16 per thousand, chrysotile has sigma 18 O = +5.5 per thousand, and authigenic pectolite has sigma 18 O near +10 per thousand. Carbon isotope ratios for calcite and aragonite are strongly depleted (sigma 13 C = -13 to -18) which suggests that dissolved CO 2 had sigma 13 C below -27. This isotopically light carbon probably resulted from a high organic component of carbon dissolved in the groundwater.

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