Stability of Detrital Heavy Minerals on the Norwegian Continental Shelf as a Function of Depth and Temperature

Abstract

Abstract Petrographic analysis of 1931 thin sections from sandstones of Pliocene to Devonian age on the Norwegian continental shelf indicate that the heavy minerals zircon, tourmaline, rutile, apatite, and spinel survive to depths exceeding 5000 m and to temperatures of around 200°C without showing signs of intrastratal dissolution. On the other hand, the maximum depth and temperature of occurrence for amphibole are 841 m and 40°C, around 1500 m and 55°C for sphene when 0.5 km of probable uplift is taken into account, 2231 m and 92°C for kyanite, 2559 m and 95°C for epidote, 2810 m and 109°C for staurolite, and 4589 m and 175°C for garnet. Initial signs of garnet dissolution occur at around 2000 m depth, and all garnets are partially dissolved at depths greater than 3500 m. Monazite is present down to 4858 m and 182°C and chloritoid to 3811 m and 149°C. It is, however, uncertain whether this is due to the low number of samples deeper than 4858 m and to the rare occurrence of chloritoid, or to intrastratal dissolution. The same sequence of heavy-mineral stability is found when volume of quartz cement is used as a proxy for time-temperature exposure. Quartz cement is not found in samples containing amphibole or detrital sphene. A maximum of a fraction of a percent quartz cement is found in samples containing kyanite, up to 1% together with epidote, up to 6% with staurolite, up to 24% with chloritoid, up to 25% with garnet, whereas maximum values of 25–35% are present together with zircon, tourmaline, rutile, patite, spinel, and monazite. The sequence of persistence of heavy minerals versus depth found in this study is very similar to results from several previous studies, although unstable heavy minerals survive to greater depths in very young and rapidly buried sandstones, and there are some differences regarding the stability of sphene and epidote. Heavy mineral dissolution has created secondary pores and possibly has contributed to precipitation of kaolinite, illite, quartz, and carbonates in the studied sandstones, but these reactions are of very limited volumetric importance.