Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA.
Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA.
Nature. 2022 Apr;604(7906):491-494. doi: 10.1038/s41586-022-04483-w. Epub 2022 Apr 20.
Plate tectonics requires a low-viscosity layer beneath the lithosphere-asthenosphere boundary (LAB), yet the origin of this ductile transition remains debated. Explanations include the weakening effects of increasing temperature, mineral hydration or partial melt. Electrical resistivity is sensitive to all three effects, including melt volatile content, but previous LAB constraints from magnetotelluric soundings did not simultaneously consider the thermodynamic stability of the inferred amount of melt and the effect of uncertainty in the estimated resistivity. Here we couple an experimentally constrained parameterization of mantle melting in the presence of volatiles with Bayesian resistivity inversion and apply this to magnetotelluric data sensitive to a LAB channel beneath the Cocos Plate. Paradoxically, we find that the conductive channel requires either anomalously large melt fractions with moderate volatile contents or moderate melt fractions with anomalously large volatile contents, depending on the assumed mantle temperature. Large melt fractions are unlikely to be mechanically stable and conflict with melt-migration models. As large volatile contents require a highly enriched mantle source inconsistent with mid-ocean-ridge estimates, our results indicate that a mantle plume emplaced volatile-rich melts in the LAB channel. This requires the presence of a previously undetected nearby plume or the influence of the distant Galápagos hotspot. Plumes that feed thin, hydrous melt channels may be an unrecognized source of LAB anomalies globally.
板块构造需要在岩石圈-软流圈边界(LAB)之下存在一个低粘度层,但这个韧性转换的起源仍存在争议。解释包括温度升高、矿物水合或部分熔融的减弱效应。电阻率对所有三种效应都很敏感,包括熔体挥发物含量,但以前的大地电磁测深 LAB 限制并没有同时考虑推断出的熔体数量的热力学稳定性以及估计电阻率的不确定性的影响。在这里,我们将挥发性存在下地幔熔融的实验约束参数化与贝叶斯电阻率反演相结合,并将其应用于对 Cocos 板块下 LAB 通道敏感的大地电磁数据。矛盾的是,我们发现导电通道要么需要异常大的熔体分数和中等的挥发物含量,要么需要中等的熔体分数和异常大的挥发物含量,这取决于假设的地幔温度。大的熔体分数不太可能在机械上稳定,并且与熔体迁移模型相冲突。由于大的挥发物含量需要一个高度富集的地幔源,与大洋中脊的估计不一致,因此我们的结果表明,地幔柱在 LAB 通道中放置了富含挥发物的熔体。这需要存在一个以前未检测到的附近地幔柱或遥远的加拉帕戈斯热点的影响。为薄的、含水熔体通道提供养分的地幔柱可能是全球 LAB 异常的一个未被认识到的来源。