Electrical conductivity, temperatures, and fluids in the lower crust

Abstract

We have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles. Regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models; we avoided those data where purely conductive heat flow suggested temperatures more than about 1000°C, substantially higher than solidus temperatures and outside the range of validity of heat flow models. The resulting plots of log conductivity σ versus 1/T demonstrate that even low‐conductivity layers (LCL) have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits. In addition, technically active regions show systematically higher conductivities than do shield areas. Because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions, these materials probably account for the relatively higher conductivities of rocks in situ in the crust; free water in amounts of 0.01–0.1% in fracture porosity could explain crustal conductivities. Other possibilities are graphite, hydrated minerals in rare instances, and sulfur in combination with other volatiles. As most of the temperatures are less than 700°C, partial melting seems likely only in regions of highest heat flow where the conductive temperature profiles are inappropriate. Another result is that at a given temperature, crustal high‐conductivity layers (HCL) are more conductive by another order of magnitude and show more scatter than do LCL's. Because the differences between HCL's and LCL's are independent of temperature, we must invoke more than temperature increases as a cause for large conductivity increases; increased fluid concentration in situ seems a probable cause for enhanced conductivities in HCL's. From the point of view of these observations, it does not matter whether the fluids are in communication with the surface or trapped at lithostatic pressures.

Geophysical images of the deep crust: the Iapetus suture

“…The pervasive nature of high electrical conductivity in the lower crust is now well-established (Shankland & Ander 1983), and strong arguments exist for relating such zones to the presence of free fluids. In the 'dry' granitic crust considered by Olhoeft (1981) a resistivity of 10 ohm m cannot be achieved at temperatures below 1000 °C; however the same resistivity is obtained for 'wet' granite with 1-2% water at a temperature of 250 °C.…”

An integrated magnetotelluric and aeromagnetic investigation of the Serra da Cangalha impact crater, Brazil

“…Zhang et al (1988) studied the Siljan impact crater using MT and detected an anomalous upper crust having a resistivity of 1000 m compared to resistivity of 10,000 m found in the adjacent rocks in the region. They invoke the presence of free fluids as suggested by Shankland and Ander (1983), and existence of faults in the lower crust as the most probable source through which this anomaly developed. Mareschal and Chouteau (1990) analyzed MT data over the Charlevoix crater and delineated the existence of a deep vertical conductor and fault beneath the crater.…”

Major results of geophysical investigations at Yucca Mountain and vicinity, southern Nevada

“…Pore fluids in crustal rocks have resistivity that is orders of magnitude less than the mineral-grain resistivity (Brace, 1977;Kariya and Shankland, 1983;Shankland and Ander, 1983). Therefore, resistivity in the upper crust, and perhaps throughout most of the crust, is largely dependent on the temperature, volume fraction, connectivity and ioniccomposition of fluid contained in the strata (Olhoeft, 1981;Shankland and Ander, 1983;Hyndman and Shearer, 1989;Hyndman and Klemperer, 1989).…”

“…There is as yet no completely satisfactory explanation for this lowresistivity zone, although the presence of a "wet" crust is often invoked as one probable explanation (Shankland and Ander, 1983). The DLRZ may be causally related to the transition from the brittle to ductile strain domain in the crust inasmuch as both electrical conductivity and ductile creep are enhanced by increased temperature and fluid composition (Wannamaker, 1980;Jiracek, 1983).…”

“…Therefore, resistivity in the upper crust, and perhaps throughout most of the crust, is largely dependent on the temperature, volume fraction, connectivity and ioniccomposition of fluid contained in the strata (Olhoeft, 1981;Shankland and Ander, 1983;Hyndman and Shearer, 1989;Hyndman and Klemperer, 1989). Moreover, there can be large variations in the resistivity of pore fluid as well as the volume of fluid, both of which are controlled by various geologic processes.…”