Garnet pyroxenites explain high electrical conductivity in the East African deep lithosphere

Abstract

In Tanzania, the deep lithospheric mantle (> 70 km depth) is characterized by significantly higher electrical conductivity within the cratonic root than in the Mozambique belt. Such contrasts are typically attributed to changes in volatiles and/or melt content, with changes in mineralogy deemed insufficient to impact conductivity. To test this assumption, electrical conductivity measurements were conducted at pressure-temperature conditions relevant to the Tanzanian lithosphere (1.5 and 3 GPa; from 400 to >1500 °C) on dunite (depleted) and pyroxenite (fertile) xenoliths from Engorora, Northern Tanzania. Once garnet becomes stable in the fertile mantle rock (> 60 km, 1.7 GPa), it nucleates at grain boundaries, forming the backbone of a conductive network. At 3 GPa, such garnet-rich networks increase conductivity by a factor of 100 regardless of temperature. Numerical models demonstrate that the observed low (< 10−2 S⋅m−1) and high (> 10−1 S⋅m−1) conductivity values can be explained by low and high degrees of garnet connectivity, respectively. Such high electrical conductivities in cratonic roots can be explained by the presence of connected garnet clusters or garnet pyroxenites, suggesting mantle fertilization. This new source for electromagnetic signal generation at appropriate pressures and temperatures must be factored in where interpreting magnetotelluric signals at relevant depths in the lithosphere. Plain language summaryThe deep root of the Tanzanian craton is characterized by high electrical conductivity. In this study, we reproduce this anomaly in the laboratory. We use powders of Tanzanian xenoliths, which are natural rocks similar to what is expected in the deep Tanzanian lithosphere. Two extremes of mantle composition are tested: dunite and pyroxenite, respectively typical of a depleted mantle (already produced melts) and fertile mantle (high chemical diversity). Experiments at high pressures and temperatures representative of the deep Tanzanian lithosphere reveal that the high electrical conductivity is generated by the establishment of a connected network of garnet pyroxenites. Minerals such as garnet and pyroxenes are known to conduct electricity much better than olivine – the major component of most mantle rocks – especially when they contain limited amounts of hydrogen. This “water” is incorporated in crystals and at their boundaries and promotes high-conductivity pathways. A significant increase in conductivity occurs once the pressure is high enough for garnet to become stable. These results suggest that the composition of the deep cratonic root in Tanzania is closer to a pyroxenite than a peridotite. This difference in composition compared to the average mantle is expected as an impact of the East African superplume.