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Segregated Melts Below the 660 in the Central Pacific: Implications on Water Transport in Mantle Upwellings

Description: 

Global water cycle has been of great interest to the community in the past several decades, and the majority of seismic observations, along with experimental studies, appears to support the transport of water down to the deep Earth through mantle downwelling near subducting slabs. In contrast, there is limited seismic observation in facilitating the argument for or against the transport of water from the deep Earth back to the upper mantle. As a consequence, the key mechanism(s) or/and mineral(s) responsible for potential water transport during mantle upwelling remain enigmatic.

In the context of receiver function technique, here we present a novel observation of P660sP and PS660P phases, which involve P-to-S conversion at the 660 and arrive as postcursors of surface reflected PP waves. Specifically, we present robust detection of PP postcursors, P660sP and PS660P, in teleseismic receiver function stacks that sample the 660 beneath the central Pacific near Hawaii. Interestingly, their amplitude systematics against the epicentral distance are inconsistent with the predictions from isotropic media. Since the receiver function stacks in different distance ranges effectively sample the same proximity and such unique amplitude systematics is unlikely caused by heterogeneous sampling and lateral variation of the structure.

We search for an effective VTI (vertically transversely isotropic, or radially anisotropic) layer either below or above the 660-km discontinuity through a Markov Chain Monte Carlo method and validate the model against other observations of P660s, SS precursors, and ScS multiples in neighbouring regions. We find that the properties of such anisotropic layer can only be reconciled with a tilted segregated melt layer beneath the 660, presumably due to strong shear deformation induced by the mantle upwellings against the 660. We propose that such deep melts are likely hydrous melts induced by the breakdown of hydrous aluminous poststishovite, carried by hot mantle plume, into the less-soluble stishovite in the uppermost lower mantle.

Session: Anisotropy Across Scales - II

Type: Oral

Date: 5/3/2024

Presentation Time: 11:30 AM (local time)

Presenting Author: Teh-Ru Alex

Student Presenter: No

Invited Presentation: 

Authors