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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