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

The ultradeep bore hole of the German Continental Deep Drilling Program (KTB) reached a depth of 9100 m and in situ temperatures of about 260 degrees C, offering an unique opportunity to study natural pyrrhotite. An integrative approach using optical methods, electron microprobe analysis, Xray diffraction, transmission electron microscopy (see Posfai et al. 2000), and temperature-dependent magnetic susceptibility measurements were used to characterize pyrrhotite types as a function of lithology and depth. We found a lithology-controlled distribution of pyrrhotite types to a depth of 8080 m, with ferrimagnetic, monoclinic 4C pyrrhotite (metal content 46.0 to 47.2 at%) as the dominant magnetic phase in gneisses and metabasic rocks. In the gneisses, a second pyrrhotite type with higher metal concentrations (46.9 to 48.2 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 8080 m, suggests that 4C is unstable above 230 degrees C in these rocks. Instead of 4C, a 5C type with a ferrimagnetic structure occurs below 8080 m. Thermomagnetic experiments indicate that the metal-poor Weiss-type pyrrhotite is stabilized by oxygen that causes the formation of magnetite during heating. From our observations on natural pyrrhotites we suggest that the magnetic lambda-transition is related to the growth of ordered nA pyrrhotite domains to single domain size.