CHEMICAL TOMOGRAPHY: IMAGING COMPOSITIONAL VARIATION WITHIN THE LITHOSPHERIC MANTLE

W.L. Griffin1,2, C.G. Ryan2 and S.Y. O'Reilly1

1 GEMOC Macquarie, 2CSIRO Exploration and Mining

The subcontinental lithospheric mantle (SCLM) carries a geochemical, thermal and chronological record of large-scale tectonic events that have shaped the Earth's crust, but little of it is accessible to geological mapping. However, with microbeam analytical techniques, garnet and chromite xenocrysts in mantle-derived volcanic rocks can be used to construct realistic geological sections of the SCLM. Trace-element thermometers place each grain, and the information contained in its geochemistry, in a stratigraphic context, and where enough samples are available, we can determine the paleogeotherm, the detailed distribution of rock types with depth, the spatial distribution of fluid-related (metasomatic and anatectic) processes and the depth to the lithosphere-asthenosphere (LAB) boundary within the tectosphere for each section. A series of such "mantle drill holes" can be used to map SCLM geology in 2 or 3 dimensions, and the 4th dimension (time) can be added where multiple episodes of volcanism occur in one region.

Paleozoic kimberlites intruding two Archean nuclei within the Sino-Korean craton, separated by the TanLu fault zone, have sampled mantle sections with markedly different bulk composition, rock-type stratigraphy and fluid-related metasomatic signatures. This finding suggests that the TanLu fault separates two originally distinct terranes within the craton. Tertiary parakimberlites and basalts in the same region sample thin, fertile Phanerozoic-type SCLM, implying the removal or dispersal of the Archean SCLM root.

Contouring of data collected from sampling points along a traverse reveals regional trends in SCLM composition and stratigraphy, which can be interpreted geologically. Detailed images of the SCLM along a 1000-km traverse in the Siberian Platform, constructed using garnet data (>4000 analyses from Å50 kimberlites) show mantle domains of distinct rock type distribution and composition. These domains correspond to closely crustal terranes mapped from surface geology and geophysics, suggesting that the terrane boundaries are translithospheric, that each terrane (microcontinent) had its own SCLM, and that these survived amalgamation into the Siberian Craton. Along the traverse, the Archean-Proterozoic boundary is clearly defined by changes in SCLM composition and lithospheric thickness. Changes in thickness and composition with time in the northern part of the traverse are related to Devonian rifting.

Detailed mapping in northern Canada reveals an unusual 2-layered lithosphere beneath the central part of the Slave Craton, with a shallow (<150 km) ultradepleted layer and a deeper more normal layer; this structure has been mapped over an area of >9000 km2. Toward the outer parts of the craton the deeper layer, interpreted as a plume head, rises to shallower depth, displacing the ultradepleted material. Proterozoic SCLM is found around the craton margins and along the trace of a major rift zone (Kilohigok Basin), and is inferred to have replaced the Archean SCLM by rifting and asthenospheric upwelling. The Chemical Tomography technique can be combined with geophysical analysis to extend the mapping to areas away from the mantle sampling points provided by kimberlites and other volcanic rocks.

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