Abstract:
Structure, primary composition, alteration and significant physical properties of lower oceanic crust were addressed during ODP (Ocean Drilling Program) Leg 153, undertaken at the western flank of the Mid-Atlantic Ridge, south of the Kane transform, at 23 degrees 31'N-45 degrees 00'E. Recovered gabbros exhibit considerable primary heterogeneity and a post-magmatic evolution from granulite to prehnite-actinolite facies. Shorebased studies on measured and calculated thermal parameters performed on gabbros from Hole 923A are compared with physical properties data obtained during Leg 153 ; shipboard measurements of the thermal conductivity were carried out by means of the modified half space needle-probe device. The thermal conductivity was calculated with the empirical relation of Woodside and Messmer (1961), based on the weighted geometric mean of the conductivity of minerals, and the theoretical model of Hashin and Shtrikman (1962) devised for the determination of magnetic permeability, extended for the calculation of the thermal conductivity. The two calibrations are in accordance and yield an average k = 2.0 W/(m K). The reliability of calculated thermal conductivity was verified with shorebased measures on 10 samples from Hole 923A and values compared with Hashin and Shtrikman (1962) calibration. Modelling of thermal conductivity downcore i) represents a contribution to constrain the physical properties in the lower oceanic crust; ii) elaboration evidences that calculated conductivities are in good accordance with measured values; iii) the calibrations and the results result alteration - independent; the structural, compositional and thermal conductivity logs are widely consistent.