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Abstract

Fluid inclusion (FI) homogenization temperatures in diagenetic quartz are often used as crucial evidence to infer abnormally hot paleotemperatures and precipitation of pore-filling quartz cements following advection of large volumes of hot waters from depth. However, we consider that FI temperatures do not record their original paleotemperatures.

North Sea and Norwegian Jurassic oil field sandstones show volumes of diagenetic quartz cement which today increase monotonically with depth from 1.8 km to at least 4.2 km. Minimum, modal, and maximum FI temperatures within this diagenetic quartz all show a progressive temperature increase with depth. If subsidence of an oil field has been slow during the past 5 m.y., FI modal temperatures coincide exactly with rock temperature, but if subsidence has been rapid (>10 m/m.y.), then they systematically lag beneath rock temperatures. Additional geological evidence favoring resetting includes: high temperature quartz FIs in texturally and isotopically shallow veins; and high temperature quartz FIs inside and outside shallow-formed carbonate cement. Laboratory experiments also show some resetting of quartz FIs within one year of overheating. The resetting mechanism is unlikely to be by fracturing and leakage, but more likely to be via stretching and local shape change. Inclusion compositions are probably unaltered. An FI temperature distribution from any single depth records the pauses in subsidence and maximum burial temperature, but does not simply record the temperature of quartz growth. Resetting of FIs negates temperature evidence for massive circulation of hot basinal fluids transporting quartz. We conclude that diagenetic quartz in these rift basin sandstones may be supplied by local diffusion, so that cement volumes are inherently predictable.

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