Melt distribution in olivine rocks based on electrical conductivity measurements

Abstract

[1] Properties of partially molten rocks depend strongly on the grain-scale melt distribution. Experimental samples show a variety of microstructures, such as melt lenses, layers, and multigrain melt pools, which are not readily explained using the theory for melt distribution based on isotropic interface energies. These microstructures affect the melt distribution and the porosity-permeability relation. It is still unclear how the melt distribution changes with increasing melt fraction. In this study, electrical conductivity measurements and microstructural investigation with scanning electron microscopy and electron backscatter diffraction are combined to analyze the melt distribution in synthetic, partially molten, iron-free olivine rocks with 0.01-0.1 melt fraction. The electrical conductivity data are compared with the predictions of geometric models for melt distribution. Both the conductivity data and the microstructural data indicate that there is a gradual change in the melt distribution with melt fraction (X m ) between 0.01 and 0.1. At a melt fraction of 0.01, the melt is situated in a network of triple junction tubes, and almost all grain boundaries are free from melt layers. At 0.1, the melt is situated in a network of grain boundary melt layers, as well as occupying the triple junctions. Between melt fractions 0.01 and 0.1, the number of grain boundary melt layers increases gradually. The electrical conductivity of the partially molten samples is best described by Archie's law (s sample /s melt = CX m n ) with parameters C = 1.47 and n = 1.30.Citation: ten Grotenhuis, S. M., M. R. Drury, C.

The elusive lithosphere–asthenosphere boundary (LAB) beneath cratons

“…Competing observations have been reported about the degree of partial melt required for efficient interconnectivity (and thus a detectable reduction in electrical resistivity), from values as low as 0.02% (Drury and Fitz Gerald, 1996) to values of 2-3% (Faul, 1997;ten Grotenhuis et al, 2005). Drury and Fitz Gerald (1996) observed that very thin (1.0-1.5 nm) films can be found on grain boundaries that can only be observed using high-resolution electron microscopy and are not observable by light or SEM observations.…”

Resistivity distribution from mid-crustal conductor to near-surface across the 1200 km long Liquiñe-Ofqui Fault System, southern Chile

“…Cementation factor for melt is best represented by m = 1.3 (ten Grotenhuis et al 2005). Electrical conductivity of molten rock was estimated to range between 1 and 30 S m −1 (Lebedev & Khitarov 1964;Tyburczy & Waff 1983).…”

Electrical conductivity of synthetic iron-bearing olivine

“…In addition, a geotherm can be constructed based on these laboratory results which demonstrate a conductivity dependence on temperature and whether the mantle may be layered (Xu et al 1998). These data provide a background level from which additional parameters can be investigated such as the effect of partial melts on the conductivity in olivine (for melts from 0.01 to 0.1% there is an increase in conductivity by at least two orders of magnitude; ten Grotenhuis et al 2005).…”