MAGNETIC-PROPERTIES OF HEMATITE IN LAVA FLOWS FROM ICELAND - RESPONSE TO HYDROTHERMAL ALTERATION

The present study investigates changes in the magnetic properties of hematite, more precisely low-Ti titanohematite, in Icelandic subaerial basalts during low-temperature metamorphism (<300-degrees-C). The results have implications for detection of secondary magnetization directions in basalts and perhaps for seafloor magnetic anomaly modeling. Samples were collected both from outcrops and from the Iceland Research Drilling Project (IRDP) core and represent 3.6 km of crust. The natural remanent magnetization (NRM) unblocked below 590-degrees-C during thermal demagnetization was assigned to magnetite and that unblocked above 610-degrees-C to hematite. Unblocking temperatures of hematite gradually increased in the laumontite zeolite zone from the original low range of 610-degrees-650-degrees-C to 650-degrees-680-degrees-C. We attribute this increase to the migration of a small amount of initial Ti out of the hematite lattice. Titanium migration is consistent with microprobe analyses. The intensity of the hematite magnetization correlated with that of magnetite until the top of the laumontite zeolite zone (1.8 km depth). Below this depth, the intensity of the magnetite magnetization decreased and correlation became poor due to alteration of magnetite to nonmagnetic minerals. Both magnetite and hematite appeared to retain their primary directions of magnetization to the bottom of the laumontite zeolite zone even though Ti had migrated out of the lattices. If field-controlled chemical remanent magnetization is not produced during Ti migration, then seafloor basalts undergoing the same processes may also faithfully retain primary directions of remanence. Below 3.1 km depth in the epidote metamorphic zone, large secondary components in some samples could not be completely removed by thermal demagnetization to 550-degrees-C and were attributable to a thermo-viscous remanent magnetization (thermo-VRM) in magnetite. The durability of the hematite magnetization suggests that the identification of hematite components in subaerial basalts may help reveal subtle secondary components contained in magnetite directions of subaerial basalts.