GEMOC ARC National Key Centre

Morgan, P. and O'Reilly, S.Y.
GEMOC Macquarie

The chemical heterogeneity of continental crust, in particular the horizontal and vertical variability in the heat-producing isotopes of thorium, uranium, and potassium, presents a challenge in the understanding of heat flow in terms of the heat loss from the underlying mantle lithosphere. Approximately thirty years ago, Francis Birch and his colleagues at Harvard, and Art Lachenbruch's group in the USGS in Menlo Park made a major contribution to the understanding of this problem with the discovery and interpretive suggestions of a linear relation between surface heat flow and surface heat production in plutonic terrains. They suggested that continental heat flow could be divided into two components, a crustal radiogenic component, and a component from below the upper crustal zone of enrichment of the radiogenic elements, which they called the reduced heat flow component. The exact relation between reduced heat flow and heat flow through the uppermost mantle is not clear, however, as heat production in the lower crust is unconstrained.

We have used mantle xenoliths, erupted in essentially stable provinces, to make an independent estimate of heat flow through the uppermost mantle. When suitable mineralogies are present, xenolith geochemistry may be used to determine both pressure and temperature of last equilibration conditions of these rocks. Extensive xenolith studies, and internal consistency among the data, suggest that these equilibration conditions commonly represent pre-eruption mantle conditions. Analyses of suites of xenolith samples allow the calculation of an array of pressure-temperature conditions that can be converted into a geotherm. Xenoliths from a number of different sites have yielded geotherms typically of 6+1mKm-1. High temperature thermal conductivity measurements on typical mantle xenolith samples yield thermal conductivity values typical of those determined for single-crystal olivine samples. These data indicate uppermost mantle heat flow values of 23+4mWm-2. We do not suggest that this is a universally applicable stable mantle heat flow value, but it does indicate that at least in some regions, a significant portion of the reduced heat flow component is derived from below the Moho.