Amphibolites from 4.6 and 9.1 km depth in the Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland (KTB) borehole have developed extensive, interconnected networks of carbonaceous films on cracks and grain boundaries as determined by electron probe and timeâ€ofâ€flight secondary ion mass spectrometry. In the sample from 9.1 km depth, the carbon is dominantly to completely elemental. In contrast, the carbonaceous matter in the samples from 4.6 km depth comprises a mixture of elemental carbon and simple hydrocarbons such as alkanes, and possibly Câ€Oâ€H compounds. The microcracks within the 4.6 km sample also contain a retrograde microâ€assemblage consisting of ferriâ€oxyâ€hydroxide, calcite, and possibly clay minerals, suggesting that the carbonaceous matter and retrograde minerals formed together at relatively high crustal levels and at a time much later than the peak metamorphism. Because the carbon films likely influence the electrical conductivity of the rocks in situ, it is proposed that production of hydrocarbons during retrograde metamorphism of grain boundary and microcrack carbon tends to increase the resistivity of the rocks. This chemical destruction of the interconnectivity of electrical pathways may contribute to the observed diminished conductivity of the shallow crust relative in relation to thto the deep crust.
