近日，美国佛罗里达州立大学Mainak Mookherjee团队从玄武岩和安山岩岩浆的高压导电性研究次弧岩浆的化学演化。相关论文于2025年3月19日发表在《地质学》杂志上。

大陆地壳富含铝硅酸盐，是由弧形岩浆结晶形成的。然而，在亚弧深处产生的岩浆通常硅含量低。弧下岩浆从贫硅到富铝硅酸盐的化学演化令人困惑。俯冲带中的大地电磁（MT）观测和基于实验室的电导率（σ）补充约束对于理解这种化学演化至关重要。岩浆的σ对压力（P）、温度（T）和化学（X）很敏感。迄今为止，基于实验室的硅酸盐熔体σ测量有助于解释P≤2 GPa时的MT观测结果。然而，俯冲带的融化可能发生在更深的地方，P≤6-7GPa。熔体在这种压力下的σ约束较差。

为了解决这个问题，研究组在P≤6 GPa下进行了实验，以检查俯冲带中常见的玄武岩到安山质熔体的σ。他们限制了二氧化硅、氧化铝、碱、碱和水（H₂O）含量对熔体σ的影响。σ的活化体积随着二氧化硅含量的增加而增加。因此，玄武岩熔体的σ总体上大于安山质熔体的σ。玄武质岩浆的σ对磷的敏感性也低于安山质岩浆。水降低了所有熔体成分的活化能并提高了σ。该研究结果有助于限制岩浆的电性质如何随着俯冲带成分的演变而变化。

附：英文原文

Title: Insights into the chemical evolution of sub-arc magmas from the high-pressure electrical conductivity of basaltic and andesitic magmas

Author: Aaron Wolfgang Ashley, Geeth Manthilake, Federica Schiavi, Mainak Mookherjee

Issue&Volume: 2025-03-19

Abstract: The continental crust is rich in aluminosilicates and formed by the crystallization of arc magmas. However, the magma produced at sub-arc depths is often silica-poor. The chemical evolution of sub-arc magma from silica-poor to aluminosilicate-rich is perplexing. Magnetotelluric (MT) observations in subduction zones and complementary laboratory-based constraints of electrical conductivity (σ) are crucial to understanding this chemical evolution. The σ of a magma is sensitive to pressure (P), temperature (T), and chemistry (X). To date, laboratory-based measurements on the σ of silicate melts have helped to interpret MT observations at P ≤ 2 GPa. Yet, the melting in subduction zones could occur deeper, at P ≤ 67 GPa. The σ of melt at such pressures is poorly constrained. To address this, we performed experiments at P ≤ 6 GPa to examine the σ of basaltic to andesitic melts, which are common in subduction zones. We constrained the effects of silica, alumina, alkali, alkaline, and water (H2O) contents on the σ of melt. The activation volume of σ increases with silica contents. Hence, the σ of basaltic melt is overall greater than that of an andesitic counterpart. The σ of basaltic magma is also less sensitive to P than andesitic magma. Water lowers the activation energy and enhances σ for all melt compositions. Our results help constrain how the electrical properties of a magma change with an evolving composition in a subduction zone.

DOI: 10.1130/G52545.1

Source: https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G52545.1/653191/Insights-into-the-chemical-evolution-of-sub-arc