Low temperature magnetic behaviour near 35 K in unmetamorphosed claystones

SUMMARY 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 (Fe 7 S 8 ) and magnetite (Fe 3 O 4 ). 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 (FeCO 3 ) or rhodochrosite (MnCO 3 ). 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.