Russian Journal of Earth Sciences
Vol. 4, No. 3, June 2002
Petrographic features of oceanic peridotites
as reflected by their magnetic characteristics
B. A. Bazylev, K. V. Popov, and V. P. Shcherbakov
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
m m. Increased mean grain size of magnetite (20
m m 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 580o C isotherm,
corresponding to
the Curie point for magnetite.