Petrographic features of oceanic peridotites as reflected by their magnetic characteristics

Abstract

Petrographic and petromagnetic studies of a representative collection of peridotites from various localities on the Mid-Atlantic Ridge have been carried out. It is shown that secondary magnetization of peridotites comes into being usually prior to the onset of serpentinization, in the course of the preceding medium-temperature metamorphic recrystallization of the rocks. The formative conditions of magnetite in peridotites can be inferred from its integrated grain size, quantified from petromagnetic characteristics. Grain size of magnetite that is formed during serpentinization of oceanic peridotites averages 3–4 µm. Increased mean grain size of magnetite (20 µm or more) is suggestive of its having crystallized in the course of medium-temperature metamorphism of peridotites. Based on our own and published data, an increased mean grain size of the magnetic phase is characteristic of most oceanic peridotites, which provides a further proof of extensive development of medium-temperature metamorphic recrystallization of these rocks both within transform faults and on rift valley slopes. The obtained data afford the conclusion that the thickness of the magnetic layer of the MOR lithosphere ranges from 6 to 15 km, its lower limit being set by the 580◦C isotherm, corresponding to the Curie point for magnetite.