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12Magnetic Mineralogy of a Complete Oceanic Crustal Section (IODP Hole

Summary: 12Magnetic Mineralogy of a Complete
Oceanic Crustal Section (IODP Hole
David Krása, Emilio Herrero-Bervera, Gary Acton,
and Sedelia Rodriguez
Oceanic crust is the carrier of the marine magnetic anomalies and is therefore a valu-
able archive of geomagnetic information. ODP/IODP Hole 1256D was the first to
sample an entire sequence of oceanic crust down to the gabbro. We studied the vertical
variation of magnetic remanence carriers by means of scanning electron microscopy,
microanalysis and rock magnetic measurements. The extrusive layer contains den-
dritic, low-temperature oxidized titanomagnetites (TMs), i.e. titanomaghemite, with
initial compositions close to values previously reported for mid-ocean ridge basalts
(MORB). The degree of low-temperature oxidation (maghemitisation) remains fairly
constant across the extrusives. We explain the observed increase in Curie temperature
with depth by submicron inversion of titanomaghemite to intergrowths of titanomag-
netite and nonmagnetic phases, where the Ti-content of titanomagnetite is decreasing
with depth. In the underlying sheeted dikes, TMs are again the primary magnetic
mineral. Due to slower cooling, they are in most cases oxy-exsolved into lamel-
lar intergrowths of Ti-poor TMs and ilmenite. The magnetominerals are altered to


Source: Acton, Gary - Department of Geology, University of California, Davis


Collections: Geosciences