PEDONE, VICKI A., CSU Northridge, Northridge, CA; J. A. D. DICKSON, Cambridge University, Cambridge. UK; and P.C. SMALLEY, BP Research Centre, Sunbury, UK

ABSTRACT: Laser-Ablation Stable-Isotope Data from Zoned Calcite Cement, Devonian, Western Australia

Concentrically zoned, non-ferroan calcite cement fills vuggy porosity formed during karstification of a Devonian carbonate platform in the Late Carboniferous. The alternately zoned non-luminescent and yellow-luminescent cement formed from meteoric water in the upper portion of the phreatic zone near the unconformity surface. Individual zones range in thickness from 2 to 600 micrometers and can be correlated between crystals. Twenty analyses of oxygen and carbon isotopic composition were measured in situ in time-sequential and time-equivalent portions of three crystals, using a single-spot, laser-ablation sample extraction technique. Circular ablation diameters ranged from 40 to 80 micrometers and penetrated the entire sample thickness of 60 micrometers.

Time-sequential analyses from core to rim show irregular, inter-zonal oscillations, up to 9 o/oo in (isotope){18}O and 6 o/oo in (isotope){13}C superposed on an overall, few-permil depletion in (isotope){18}O and (isotope){13}C. (isotope){18}O values range from +1.4 to -12.3 o/oo, (x=-6.7 o/oo), and (isotope){13}C values range from -3.6 to -10.4 o/oo (x=-7.4 o/oo). Values measured in time-equivalent portions of the same crystal are similar, whereas analyses of the same zone in different crystals often differ significantly. The overall depletion trend reflects gradual change in isotopic composition of surface precipitation owing to cooling climate. Small-scale oscillations indicate episodic changes in the composition of meteoric porewater owing to short-term fluctuations in isotopic co position of surface precipitation, variable fluid-interaction with soil CO2 and clay minerals, and variable extents of evaporation. Intercrystalline variations indicate that groundwater in the upper portion of the phreatic zone was poorly mixed. High-spatial resolution laser-ablation analyses provide insight into fluid composition and groundwater flow that is not possible from standard acid-digestion analyses.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.