Petrographical observations from a sandstone buried to between 3.2 and 5.7 km in the North Sea Central Graben suggest a significant change in bulk chemistry during burial. The sand contained 35% or more feldspar upon deposition, but due to extensive dissolution, only 5% remains in the deep sandstones. This has released much more Al than is currently contained as authigenic minerals: up to 1460 μ mol cm−3 of Al have been exported. This deep secondary porosity is economically important, but its recognition by petrographic textures is unreliable, as are bulk-chemical provenance studies of deeply buried sedimentary and metamorphic strata.

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