Effect of fine‐scale microstructures in titanohematite on the acquisition and stability of natural remanent magnetization in granulite facies metamorphic rocks, southwest Sweden: Implications for crustal magnetism

Abstract

Abstract. Mid-Proterozoic granulites in SW Sweden, having opaque minerals hematiteilmenite with minor magnetite, and occurring in an area with negative aeromagnetic anomalies, have strong and stable reversed natural remanent magnetization -9.2 A/m, with 100% remaining after demagnetization to 100 mT. Samples were characterized by optical microscopy, electron microprobe (EMP), transmission electron microscopy (TEM), and rockmagnetic measurements. Earliest oxide equilibrium was between grains of titanohematite and ferri-ilmenite at 650ø-600øC. Initial contacts were modified by many exsolution cycles.

Earth analog for Martian magnetic anomalies: remanence properties of hemo-ilmenite norites in the Bjerkreim-Sokndal intrusion, Rogaland, Norway

“…However, if the remanent vector had been closer to the inducing vector the response would be to produce a very large magnetic high and as a first approximation, would be attributed to induced magnetization of a magnetite-rich layer. This case study shows that induced magnetization is not the sole cause of large magnetic anomalies and reinforces the opinion that remanence can be an important contributor to deep crustal anomalies (McEnroe and Brown, 2000;McEnroe et al, 2001aMcEnroe et al, ,b, 2002aWorm, 1989). Clark (1999) has also concluded that remanence could be important in many mafic plutons.…”

“…The strong and extremely stable NRM is carried by highly exsolved hemo-ilmenite and ilmeno-hematite grains (McEnroe, 1995;McEnroe et al, 2001a). Instead of a regional magnetic high over the granulite terrane, there are subdued magnetic anomalies because of the strong influence of a negative remanent vector.…”

“…Recently, Proterozoic rocks,~1 billion years old, that have faithfully retained a large part of their magnetic memory and are associated with sizeable magnetic anomalies, have been studied in detail for their magnetic properties, mineralogy, chemistry, and magnetic responses. Mid-Proterozoic high-grade metamorphic rocks from the Adirondack Mts., USA (Balsley and Buddington, 1958;McEnroe and Brown, 2000), granulite-facies metamorphic rocks from Sweden (McEnroe, 1995;McEnroe et al, 2001a) and igneous rocks in southern Norway (McEnroe, 1997;McEnroe et al, 1996McEnroe et al, , 1998McEnroe et al, , 2001bMcEnroe et al, , 2002aRobinson et al, 2001Robinson et al, , 2002a and Quebec, Canada (Hargraves and Burt, 1967) are all from areas where the magnetic response is strongly influenced or dominated by an NRM vector that is reversed compared to the Earth's present-day magnetic field. For this type of response to occur, the ancient magnetic memory must contribute strongly or dominate over the present-day induced response of the rock body.…”

Pressure effects on martian crustal magnetization near large impact basins

“…Titanohematite and/or hematite occurs frequently in the oxidized crust of the Earth including the lower crustal rocks (Kletetschka and Stout 1998;McEnroe et al 2001aMcEnroe et al , 2001b. Crustal rocks can be also oxidized later as a result of hydrothermal activity producing alteration zones with gradual decomposition of primary magnetite to hematite (Just et al 2002) (martitization).…”

Inversion of magnetic measurements of the CHAMP satellite over the Pannonian Basin

“…Many of these anomalies have negative signs. For our geological interpretation we call upon analogous source regions such as the MidProterozoic granulites in southwestern Sweden (McEnroe et al, 2001); Proterozoic Åna Sira anorthosite in Rogaland, Norway (McEnroe et al, 2004Robinson et al, 2002) and the metamorphic complex in Modum district, Southern Norway (Fabian et al, 2008).…”