29-28 What Processes Formed Challenger Deep Forearc Segment Pyroclastic Breccias? UTD-QU Undergraduate Student Research.

Session: Undergraduate Research, Part II (Posters)

Poster Booth No.: 79

Presenting Author:

Julia Anderson

Authors:

Anderson, Julia Kay¹, Rehman, Shumyla², Rosselot, Renee³, Caracalas, Ty ⁴, Stern, Robert J.⁵, Leybourne, Matthew⁶, Pujana, Ignacio⁷

(1) Sustainable Earth Systems Sciences, University of Texas at Dallas, The Colony, TX, USA, (2) Queen's University, Kingston, Ontario, Canada, (3) University of Texas at Dallas, The Colony, TX, USA, (4) University of Texas at Dallas, USA, (5) University of Texas at Dallas, , (6) Queen's University, Kingston, ON, , (7) The University of Texas at Dallas, Department of Geosciences, ROC 21, Richardson,, TX, ,

Abstract:

The Challenger Deep in the southernmost Mariana Trench is the deepest point on Earth’s solid surface and the reasons why it is so deep is not known. We studied the northern (over-riding plate) trench slope in late 2024 during the TN438 research cruise using the ROV Jason/Medea. We call this the Challenger Deep Forearc Segment (CDFS), and we recovered 164 samples of mostly igneous rocks. Of these, 40 were young pyroclastic rocks formed by explosive eruptions at 5-6.5km water depth. This is surprising because volcanism in a trench is unheard of, and because these eruptions happened at much greater depths than previously reported for submarine pyroclastics. Explosive eruptions at this depth are likely to differ strongly from those near the surface because of the much higher hydrostatic pressure (50-65MPa, equivalent to 500-650 atm). Such great pressures suppress the expansion of exsolved magmatic volatiles (esp. H₂O, CO₂) because these volatiles will be supercritical fluids (with small molar volumes), not steam (with large molar volumes). Six samples from Jason/Medea dives 1674-1676 are being studied using methods including thin section petrography, electron microprobe analysis, and whole-rock major and trace element compositions by undergraduates at the University of Texas at Dallas and Queen's University, Canada. Thin sections display distinctive clasts and matrixes indicating multiple brecciation events, as well as abundant altered and fresh glass. Between the six samples, glass makes up roughly 70% with subequal proportions of fresh and altered glass. Minerals within the samples include approximately 90% plagioclase, 5% olivine, and 5% pyroxene, suggesting that these are basaltic glasses. There is currently no scientific explanation for how these “double breccias” form, and in studying these samples we aim to better understand this process. Through chemical analysis we will better understand the nature of this unusual volcanic activity in the inner trench slope of the western CDFS.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

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