Evidence from borehole samples for the role of accessory minerals in lower-crustal conductivity

• Published in: Nature (London) 1994-01, Vol.367 (6458), p.59-61
• Main Authors: Duba, A, Heikamp, S, Meurer, W, Mover, G, Will, G
• Format: Article
• Language: English
• Subjects: Boreholes; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geology; Geophysics: general, magnetic, electric and thermic methods and properties; Humanities and Social Sciences; Internal geophysics; letter; Minerals; multidisciplinary; Science; Science (multidisciplinary)
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/367059a0

THE high electrical conductivity of the lower continental crust, as inferred from geophysical measurements 1 , has defied a simple explanation. Both pore-saturating brines 2 and interconnected carbon films 3–5 have been proposed as conducting pathways, but their relative importance remains uncertain, and may vary from place to place 6 . So far, attempts to address this question in the laboratory have had to rely on samples of metamorphic rock collected from surface exposures 7–10 . Although these rocks almost certainly originated in the lower crust, chemical alteration during their transport to, and residence at, the surface is likely to have affected their conducting properties. Here we report conductivity data for pristine crustal rocks recovered from depths of up to 4.6 km in the KTB boreholes in southern Germany. Our results show that the electrical conductivity of accessory phases such as Fe–Ti oxides and sulphides can enhance, or even surpass, the high conductivity produced by saline fluids.