Glimpses of Oceanic Lithosphere of the Challenger Deep Forearc Segment in the Southernmost Marianas: The 143°E transect, 5800–4200 m
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Authors
Stern, Robert
Ohara, Yas
Ren, Minghua
Leybourne, Matthew
Bowers, Brent
Date
2020-06-16
Type
journal article
Language
en
Keyword
Forearc , Igneous Rock , Mariana Trench
Alternative Title
Abstract
We studied nine samples of igneous rocks from the inner wall of the Mariana Trench above the Challenger Deep from 4150 to 6100 m depth recovered by manned submersible and ROV. Samples from two regions that bracket the Moho were studied: (i) 7 samples from a N‐S transect a few km to the west of the Shinkai Seep Field; and (ii) 2 samples from the Shinkai Seep Field. Transect samples include olivine‐2 pyroxene hornblendites, amphibole basalts, basaltic andesite, and hornblende andesite. We analyzed three transect samples for 40Ar/39Ar ages; two yielded good plateau ages of 46.5 ±0.5 Ma (hornblendite) and 46.60 ±0.15 Ma (hornblende andesite). These results combined with previously published results, indicate that this crust formed during an intense 46–47 Ma magmatic episode that occurred 5–6 my after subduction initiation. Hornblendites and hornblende basalts formed from primitive magmas, as shown by high MgO (11–21 wt%), Ni (222–885 ppm) and Cr (412–1145 ppm) contents. Electron microprobe analyses indicate that hornblende is Na‐rich (up to 3.0 wt% Na2O) and that many samples have an atypically large range in plagioclase composition (i.e. individual samples have An < 10 to An 90 plagioclase). Two subgroups can be identified: a mostly deeper depleted suite and a mostly shallower enriched suite. These results indicate that (i) the crust–mantle boundary in this region is transitional, occurring over a ~ 1.5 km interval, with interlayered peridotite and hornblendites between 5800 and 4300 m; and (b) extension to form the Challenger Deep forearc segment occurred by combined stretching of old crust and injection of young basaltic magmas. In contrast to the mostly fresh nature of transect samples, the two samples from the Shinaki Seep Field are intensely altered peridotite and basalt.
Description
This is the peer reviewed version of the following article: Stern, RJ, Ohara, Y, Ren, M, Leybourne, M, Bowers, B. Glimpses of oceanic lithosphere of the Challenger Deep forearc segment in the southernmost Marianas: The 143°E transect, 5800–4200 m. Island Arc. 2020; 29:e12359. https://doi.org/10.1111/iar.12359, which has been published in final form at https://doi.org/10.1111/iar.12359. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.
Citation
Stern, RJ, Ohara, Y, Ren, M, Leybourne, M, Bowers, B. Glimpses of oceanic lithosphere of the Challenger Deep forearc segment in the southernmost Marianas: The 143°E transect, 5800–4200 m. Island Arc. 2020; 29:e12359. https://doi.org/10.1111/iar.12359
Publisher
Wiley