--> ABSTRACT: Ostracod Shells as Indicators of Thermal History, by Mervin Kontrovitz; #91042 (2010)

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Ostracod Shells as Indicators of Thermal History

Mervin Kontrovitz

Ostracod shells from 20 species were subjected to various temperatures and pressures (T-P) to mimic burial. For the first time with ostracods, mineral mixtures, organic matter, and saline water were used to simulate sediments and pore fluids. Features developed that were diagnostic of narrow T-P ranges and sediment types.

Alteration in quartz was used as a standard for comparison. The shell features were, at increments of 30°C and 100 atm, as follows: at 120°C, 400 atm, low knobs, small pits; at 150°C, 500 atm, pinnacles, 300% enlargement of pore canals; 180°C, 600 atm, "badlands topography"; at 210°C, 700 atm, boxwork of platelets, clinging quartz; at 240°C, 800 atm, boxwork of platelets each 1.5-2.5 µm across, clinging quartz; at 270°C, 900 atm, rosettes, clinging quartz; at 300°C, 1,000 atm, 50% quartz covered, interlocking platelets; at 330°C, 1,100 atm, interlocking platelets 20 µm across; at 360°C, 1,200 atm, 75% quartz-covered cubic crystals. All, except the rosettes, are seen on fossil ostracods, although rosettes are common in sedimen s, also.

For each 100 atm less, an additional 15°-30°C was needed to achieve the alteration listed for a given temperature. Seawater (35 ppt) retarded alteration and had to be 30°C warmer to produce features like those in distilled water. More saline water (70 ppt) enhanced alteration and had to be 60°C cooler to match results in pure water.

An additional 60°C was needed with one clay (61% silica) to equal the effect of quartz at a given T-P; in the other clay (51% silica) and in calcite, 90°C and 90°-120°C were needed, respectively. The alteration in sediment mixtures was predictable, and depended on the proportion of silica.

Temperature, sediment, and pore water were the major controls of alteration of ostracod shells. For interpretations, a resolution of ±15°C was possible, but smaller increments in experiments may give even better results.

AAPG Search and Discovery Article #91042©1987 GCAGS and GC-SEPM Section Meeting, San Antonio, Texas, October 28-31, 1987.