14:30 〜 14:45
[SEM14-04] Garnet pyroxenites can explain high electrical conductivity in the deep lithospheric mantle
キーワード：Electrical conductivity, Garnet pyroxenites, Laboratory experiments, Craton, Lithospheric mantle, Magnetotellurics
In the past few decades, magnetotellurics (MT) has revealed numerous electrically conductive anomalies in the uppermost asthenosphere (e.g. Evans et al., 2005; Baba et al., 2006) and in the deep lithospheric mantle, including the root of the Tanzanian Craton (Selway, 2015) and the oceanic Cocos plate offshore Nicaragua (Naif et al., 2013). Conductive anomalies in the asthenosphere can well be explained by partial melting and local melt pooling at the lithosphere-asthenosphere boundary (LAB), e.g. in the vicinity of the East Pacific Rise (Evans et al., 2005; Baba et al., 2006). However, deep intra-lithospheric anomalies are demonstrably inconsistent with the presence of any connected melt layer, neither within the Cocos plate (Ferrand, 2020) nor within the Tanzanian Craton (Ferrand & Chin, 2021).
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 P-T conditions relevant to the cratonic lithosphere (1.5 and 3 GPa; from 400 to > 1500°C) on xenoliths from Engorora, Northern Tanzania (Ferrand & Chin, 2021). Once garnet becomes stable in fertile mantle rocks (> 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 Sm-1) and high (> 10-1 Sm-1) conductivity values are best 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.
MT surveys have also identified anisotropic conductive anomalies in the mantle of the Cocos and Nazca oceanic plates, respectively, offshore Nicaragua and in the neighborhood of the East Pacific Rise (EPR). Both the origin and nature of these anomalies are controversial as well as their role in plate tectonics. The high electrical conductivity has been hypothesized to originate from partial melting and melt pooling at the LAB. The anisotropic nature of the anomaly likely highlights conductive channels in the spreading direction, often interpreted as the persistence of a stable liquid silicate throughout the whole oceanic cycle, on which the lithospheric plates would slide by shearing. Here I show that the observed electrical anomaly offshore Nicaragua does not correlate with the LAB but instead with the top of the garnet stability field and that networks of garnet-rich rocks suffice to explain the reported conductivity values (Ferrand, 2020). I further propose that this anomaly corresponds to the fossilized trace of the early-stage LAB that formed near the EPR about 23 million years ago. Melt-bearing channels and/or pyroxenite underplating at the bottom of the young Cocos plate would have transformed into garnet-rich pyroxenites with decreasing temperature, forming solid-state high-conductivity channels between 40 and 65 km depth (1.25-1.9 GPa, 1000-1100°C), consistently with experimental petrology.
- Ferrand, T. P. & Chin, E. J. (2021). Garnet pyroxenites explain high electrical conductivity in the East African deep lithosphere. Preliminary preprint arXiv:2112.11559.
- Ferrand, T. P. (2020). Conductive channels in the deep oceanic lithosphere could consist of garnet pyroxenites at the fossilized lithosphere-asthenosphere boundary. Minerals 10(12), 1107.
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 P-T conditions relevant to the cratonic lithosphere (1.5 and 3 GPa; from 400 to > 1500°C) on xenoliths from Engorora, Northern Tanzania (Ferrand & Chin, 2021). Once garnet becomes stable in fertile mantle rocks (> 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 Sm-1) and high (> 10-1 Sm-1) conductivity values are best 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.
MT surveys have also identified anisotropic conductive anomalies in the mantle of the Cocos and Nazca oceanic plates, respectively, offshore Nicaragua and in the neighborhood of the East Pacific Rise (EPR). Both the origin and nature of these anomalies are controversial as well as their role in plate tectonics. The high electrical conductivity has been hypothesized to originate from partial melting and melt pooling at the LAB. The anisotropic nature of the anomaly likely highlights conductive channels in the spreading direction, often interpreted as the persistence of a stable liquid silicate throughout the whole oceanic cycle, on which the lithospheric plates would slide by shearing. Here I show that the observed electrical anomaly offshore Nicaragua does not correlate with the LAB but instead with the top of the garnet stability field and that networks of garnet-rich rocks suffice to explain the reported conductivity values (Ferrand, 2020). I further propose that this anomaly corresponds to the fossilized trace of the early-stage LAB that formed near the EPR about 23 million years ago. Melt-bearing channels and/or pyroxenite underplating at the bottom of the young Cocos plate would have transformed into garnet-rich pyroxenites with decreasing temperature, forming solid-state high-conductivity channels between 40 and 65 km depth (1.25-1.9 GPa, 1000-1100°C), consistently with experimental petrology.
- Ferrand, T. P. & Chin, E. J. (2021). Garnet pyroxenites explain high electrical conductivity in the East African deep lithosphere. Preliminary preprint arXiv:2112.11559.
- Ferrand, T. P. (2020). Conductive channels in the deep oceanic lithosphere could consist of garnet pyroxenites at the fossilized lithosphere-asthenosphere boundary. Minerals 10(12), 1107.