Abstract
Using a thermodynamic-geophysical approach, we map the 2-D interior structure of the Siberian cratonic mantle along the ultra-long cross-cutting seismic profiles Kimberlite and Meteorite carried out in Russia with peaceful nuclear explosions. There is a systematic decrease in temperature from west to east for the Kimberlite profile and a weak decrease from NW to SE for the Meteorite profile. Cratonic mantle shows a significant heterogeneity in the distribution of seismic velocities, temperature and density, topography of seismic boundaries and degree of layering at depths up to ~200 km reflecting somewhat different thermal state along both seismic profiles. Temperatures in the central part of the craton are somewhat lower than those at the periphery and 300-400°C lower than the average temperature in the surrounding mantle. A change of composition from depleted to fertile material reveals a negligible effect on seismic velocities that is practically unresolved by seismic methods, but remains the most important factor for the density increase of cratonic root. The distribution of density in the mantle cannot be attributed to any single composition, either depleted or enriched in basaltic components. This finding suggests a significant fertilization with depth and is compatible with the chemical stratification in the root of the craton. The thickness of the thermal boundary layer (TBL, conductive lid + transition layer) can be estimated as 300 ± 30 km thick along the Kimberlite and Meteorite profiles. At the base of the TBL, the temperature is close to 1450 ± 100°C isotherm; the calculated density corresponds to the PREM model density
Highlights
It is believed that cratons are stable domains, which experienced practically no changes in the process of subsequent tectonic evolution
We find that P-velocities for the fertile primitive mantle composition are slightly greater (≤ 0.3%) than those for the depleted rocks (Figure 4a)
Average temperature profile for the surrounding mantle (TP(AK135)), which is approximated by the primitive mantle (PM) composition is inferred from the AK135 model [29]
Summary
It is believed that cratons are stable domains, which experienced practically no changes in the process of subsequent tectonic evolution. Knowledge of the thermal regime, thickness and composition of the mantle beneath ancient cratons is important to both fundamental geochemical-geophysical research and diamond exploration. Investigations of the mantle beneath the Siberian craton (SC) have been performed in a number of thermal, seismic and tomographic studies [13,14,15,16,17,18]. Thermobarometric results for Siberian mantle xenoliths of garnet, garnet–spinel, spinel peridotites, and pyroxenites [2,3,7,8,19] provide unique information about the compositional heterogeneity and evolution of the cratonic mantle, but do not give direct information about its seismic structure. Thermal and seismic studies provide only indirect information about the composition and temperature of cratonic mantle. Combinations of surface heat flow measurements, geophysical data, xenolith thermobarometry and additional thermodynamic constraints reduce some of the ambiguity in interpretations of mantle structure and provide the tighter constraints on mantle chemistry and thermal regime
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