Low temperature magnetic behaviour near 35 K in unmetamorphosed claystones

There is growing evidence that the magnetic assemblage of claystones, illustrated by low-temperature magnetic transitions at ~120 K and ~35 K, may be representative of the peak burial temperature in the so-called oil-window (60-150°C). In previous studies, it was proposed that this magnetic assemblage is characterized by fine-grained pyrrhotite (Fe₇S₈) and magnetite (Fe₃O₄). However, evidence of pyrrhotite was not so obvious and the magnetic transition near 35 K of fine-grained pyrrhotite has similarities with those of siderite (FeCO₃) or rhodochrosite (MnCO₃). Here, we propose some diagnostic tests to distinct <50 K behaviours by studying claystones from Netherlands borehole and Borneo Prism that experienced different peak burial temperatures. We perform magnetic susceptibility, temperature dependency of SIRM (ZFC, RT-SIRM) and field cooled hysteresis loops. On cooling of RT-SIRM (300 K to 10 K), we applied a magnetic field of 5 μT to enhance Néel type magnetic transition. It is found that our samples can be classified in two categories based on the shape of the RT-SIRM curve: one displaying an abrupt break-in-slope in the remanence at ~30 K, called N-behaviour, and the other one characterized by a progressive increase of the remanent magnetization by 80 K, named P-behaviour. The first category contains essentially magnetite and Fe-Mn carbonates, the second one magnetite and probably iron sulphides.