LITHOSPHERE MANTLE DENSITY BENEATH THE SIBERIAN CRATON

We use free-board modeling to examine density heterogeneity of lithospheric mnatle beneath the Archean–Proterozoic Siberian Craton. We calculate a vertically-averaged mantle density in the layer from the Moho down to base of the chemical boundary layer (CBL) for two models. Model 1 assumes that the base of the CBL is vraiable and coincides with the LAB, whereas in Model 2 the base of the CBL is fixed at a 180 km depth. The results are presented at in situ and at room temperature (SPT) conditions, and have an uncertainty of less than 0.02 g/cm3. They indicate a strong heterogeneity in the density structure of the Siberian lithospheric mantle with a strong correlation between mantle density variations and the tectonic setting.

Our analysis suggests that Model 2 fits better petrological data. From mantle density anomalies, we distinguish three types of cratonic mantle.

1) ‘Pristine’ cratonic regions not sampled by kimberlites have the strongest depletion with density deficit of 1.8–3.0% (and SPT density of 3.29–3.33 g/cm3 as compared to 3.39 g/cm3 of primitive mantle).

2) Cratonic mantle affected by magmatism (including the kimberlite provinces) has a typical density deficit of 1.0–1.5%, indicative of a metasomatic melt-enrichment.

3) Intracratonic sedimentary basins have a dense mantle (3.38–3.40 g/cm3 at SPT) which suggests, at least partial, eclogitization. Based on moderate density anomalies beneath the Tunguska Basin, we speculate that the source of the Siberian LIP lies outside of the Craton.

We test the isopycnic condition of the Siberian Craton and conclude that there are significant lateral variations in the isopycnic state, correlated with mantle depletion and best achieved for the Anabar Shield region and other intracratonic domains with a strongly depleted mantle.

We also calculate synthetic Mg# for the bulk lithospheric mantle from density values, by using data from petrological studies of peridotite xenoliths from the Siberian kimberlites. Our results indicate that melt migration may produce local patches of metasomatic material in the overall depleted cratonic mantle.

Reference:

Cherepanova Y. and Artemieva I., 2015. Density heterogeneity of the cratonic lithosphere: A case study of the Siberian Craton. Gondwana Research 28, 1344–1360.