Mineralogy and magnetic behavior of pyrrhotite from a 260 degrees C section at the KTB drilling site, Germany

The ultradeep bore hole of the German Continental Deep Drilling Program (KT B) reached a depth of 9100 m and in situ temperatures of about 260 degrees C, offering an unique opportunity to study natural pyrrhotite. An integrati ve approach using optical methods, electron microprobe analysis, Xray diffr action, transmission electron microscopy (see Posfai et al. 2000), and temp erature-dependent magnetic susceptibility measurements were used to charact erize pyrrhotite types as a function of lithology and depth. We found a lit hology-controlled distribution of pyrrhotite types to a depth of 8080 m, wi th ferrimagnetic, monoclinic 4C pyrrhotite (metal content 46.0 to 47.2 at%) as the dominant magnetic phase in gneisses and metabasic rocks. In the gne isses, a second pyrrhotite type with higher metal concentrations (46.9 to 4 8.2 at%) and antiferromagnetic behavior also occurs. At depths greater than 8080 m (in situ temperature > 230 degrees C) antiferromagnetic pyrrhotite, predominates in all lithologies. That 4C pyrrhotite does not occur below 8 080 m, suggests that 4C is unstable above 230 degrees C in these rocks. Ins tead of 4C, a 5C type with a ferrimagnetic structure occurs below 8080 m. T hermomagnetic experiments indicate that the metal-poor Weiss-type pyrrhotit e is stabilized by oxygen that causes the formation of magnetite during hea ting. From our observations on natural pyrrhotites we suggest that the magn etic lambda-transition is related to the growth of ordered nA pyrrhotite do mains to single domain size.