ELECTRON-MICROSCOPY OF IRON-OXIDES AND IMPLICATIONS FOR THE ORIGIN OFMAGNETIZATIONS AND ROCK MAGNETIC-PROPERTIES OF BANDED SERIES ROCKS OFTHE STILLWATER COMPLEX, MONTANA

The origins of multiple magnetizations of the Archean Stillwater Compl ex have been investigated through scanning electron microscopy and sca nning transmission electron microscopy observations of mineralogical r elations, using representative samples from nine sites in mafic Banded series rocks. On the basis of directional grouping and demagnetizatio n behavior, three magnetizations (here labeled A, B, and C) have been recognized. The natural remanent magnetizaton (NRM) is typically domin ated by only one of these magnetizations and multicomponent behavior i n individual specimens is rare. The A remanence resides in magnetic gr ains of high median destructive fields and high, discrete laboratory u nblocking temperatures and is inferred to be a primary thermoremanent magnetization, of circa 2.71 Ga age. The B and C magnetizations, of lo wer median destructive fields and more distributed unblocking temperat ures, are inferred to be secondary and related to alteration, includin g serpentinization, involving limited, moderate to low-temperature flu id interaction, perhaps in response to thermotectonic events (e.g., ma fic dike emplacement). Samples with NRM dominated by the A magnetizati on contain titanium-free magnetite needles (width < 1 mu m and maximum dimension between about 2 and 50 mu m) with preferred orientations in cumulus plagioclase crystals; these oxides are interpreted to have fo rmed during initial crystallization of cumulus phases. In samples cont aining only B or C magnetizations, such needles are absent in cumulus plagioclase, whereas secondary Fe-bearing silicates, particularly zois ite and chlorite, are conspicuous. The samples dominated by the B or C magnetizations contain anhedral low-chromian magnetite (FeCr2-xFexO4, 1.5 < x < 2.0) along fractures in both cumulus plagioclase and pyroxe ne and in zones of more pervasive alteration of pyroxenes. The paragen esis of these oxides is consistent with an origin related to hydrother mal alteration. Hysteresis data show that plagioclase crystals contain ing magnetite needles have higher remanent coercivities (H-cr = 60 to 100 mT), whereas plagioclase crystals with anhedral chromian magnetite have remanent coercivities less than 60 mT, consistent with their alt ernating field response. Because oxygen isotope data show no indicatio n of appreciable alteration of host plagioclase, we infer that the abs ence of magnetite needles and their associated A magnetization are cau sed by selective alteration of the magnetite needles to nonmagnetic ph ases in the remagnetized gabbros. In this case, rock magnetic and pale omagnetic data appear to be higher resolution indicators of fluid/rock interaction than the stable isotope data. Anhedral chromian magnetite precipitated during hydrothermal alteration and therefore carries sec ondary magnetizations.