SYNTHESIS (continued)

Geothermal Gradients: Tectonics and Hydrocarbon

The downhole temperature measurements at Leg 184 sites revealed two distinct temperature gradients as a function of the water depth (Fig. 26). At Sites 1143 (2772 m), 1145 (3175 m), and 1148 (3294 m), the gradients were relatively high (83° to 90°C/km) and were quite similar between the sites. These sites are all deep and relatively close to the boundary between continental and ocean crust. Preliminary calculations suggest that temperature gradients at these sites are close to predictions based on the pure shear stretching model of continental lithosphere, the corresponding water depth of the drill sites, and the assumed age of initial rifting (32 Ma). The two sites on the mid-continental slope (Sites 1144, 2037 m; and 1146, 2092 m) have smaller gradients (24° and 59°C/km, respectively) and differ from the deeper sites and from each other. The gradient at Site 1144 is substantially smaller than the predicted value based on water depth and spreading age, and a local process must be inferred to explain the low temperatures at this high sedimentation rate site.

Significant hydrocarbon concentrations were detected at three sites during Leg 184 (Fig. 27). Abundant methane of biogenic origin was inferred only from Site 1144. High TOC abundance and complete sulphate reduction in the upper few meters of this site (and the absence of significantly heavier hydrocarbons downhole) are characteristic of methanogenesis in immature sediments.

In contrast, the downhole increase in hydrocarbon abundance at Sites 1146 and 1148 is characteristic of thermogenic generation with age and increasing temperatures with depth. These two deeper penetration sites on the continental margin have predicted bottom-hole temperatures of only 35°C at Site 1146 (607 mbsf) but 71°C at Site 1148 (853 mbsf). The temperatures in the lower interval of Site 1148 are thought to be high enough to begin the process of thermal generation of hydrocarbons from the low amounts of organic carbon (0.3% to 0.5%) and may well explain the observation of sparse heavy hydrocarbons (C2+) at the site. None of the Leg 184 sites revealed conditions for source rock production, and no gas hydrates were observed.

Synthesis-Interstitial Waters: Silica and Sulfate

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