Insights Into Deep Mantle Thermochemical Contributions to African Magmatism From Converted Seismic Phases
Insights Into Deep Mantle Thermochemical Contributions to African Magmatism From Converted Seismic Phases
复制标题
DOI：
10.1029/2020gc009478
复制
发表时间：
2021-03-01
影响因子：
3.5
通讯作者：
Cottaar, S.
中科院分区：
文献类型：
--
作者：
Boyce, A.;Cottaar, S.
不同空间范围的地幔上涌对整个非洲新生代岩浆活动的贡献存在争议，因为用于询问它们的地球化学和地震学工具主要对成分或温度敏感。热化学条件控制着地幔物质发生相变的深度，从而导致地震不连续。绘制横跨地幔过渡带（MTZ）及以下的地震不连续面，可以深入了解上升流的可变热化学性质。我们介绍了从1990-2019年非洲地震仪网络记录的P-to-s接收函数（rf；使用Pds、PPds和PKPds阶段）汇编中获得的非洲地下地震不连续的观测结果。我们利用最近的高分辨率非洲大陆p波速度模型，将我们的rf迁移到共同转换点堆栈的深度。沿东非裂谷的新生代岩浆活动在很大程度上由薄的MTZ支撑，这意味着裂谷岩浆活动来自MTZ深度或以下的来源。埃塞俄比亚裂谷下部为d410凹陷和d660凸起，表明在MTZ深度（类似于100-150 K）存在中度正热异常。东非裂谷南部表现出较大的d410坳陷和区域性d660坳陷，表明MTZ深度存在较强的热化学异常。在这里，类似于1025公里深度的地震转换与非洲超级羽流内的慢波速度相匹配，证实了成分异常的证据。我们认为，埃塞俄比亚正下方的纯热地幔柱的贡献增加了地幔融化和裂谷的条件。明显的上升流也可能影响喀麦隆和马达加斯加新生代岩浆作用下的MTZ。
The contribution of mantle upwellings of varying spatial extent to Cenozoic magmatism across Africa is debated because geochemical and seismological tools used to interrogate them are primarily sensitive to either composition or temperature. Thermochemical conditions control the depth at which mantle materials undergo phase changes, which cause seismic discontinuities. Mapping seismic discontinuities across the mantle transition zone (MTZ) and below provides insight into the variable thermochemical nature of upwellings. We present observations of seismic discontinuities beneath Africa obtained from a compilation of P-to-s receiver functions (RFs; using Pds, PPds, and PKPds phases), recorded at seismograph networks across Africa between 1990-2019. We exploit a recent high-resolution African continental P-wavespeed model to migrate our RFs to depth in a common conversion point stack. Cenozoic magmatism along the East African Rift is largely underlain by a thin MTZ implying a contribution to rift magmatism from sources at or below MTZ depths. The Ethiopian rift is underlain by a depressed d410 and uplifted d660 indicating a moderate positive thermal anomaly at MTZ depths (similar to 100-150 K). The southern East African Rift displays a greater d410 depression and a regional d660 depression, suggesting a stronger thermochemical anomaly at MTZ depths. Here, seismic conversions at similar to 1,025 km depth are collocated with slow wavespeeds within the African Superplume, corroborating evidence for a compositional anomaly. We suggest that the contribution of a purely thermal plume directly below Ethiopia augments conditions for mantle melting and rifting. Distinct upwellings may also affect the MTZ below Cenozoic magmatism in Cameroon and Madagascar.