Award Date

5-15-2026

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geoscience

First Committee Member

Yan Hu

Second Committee Member

Andrew Martin

Third Committee Member

Arya Udry

Fourth Committee Member

Luqing Wang

Number of Pages

57

Abstract

When oceanic plates bend and sink into the mantle at subduction zones, they carry sediments from the surface down into the mantle. The recycled sediments contain elements, that are depleted in the mantle, thereby influencing the composition of the mantle wedge and the formation of arc magmas, which are the building blocks of juvenile continental crust. However, it remains poorly constrained how much these elements, particularly fluid-mobile elements such as rubidium (Rb), are lost into fluids during metamorphic dehydration, and how much is transported to sub-arc depths where arc magmas are generated. Rubidium in subducting sediments is mainly hosted in detrital phases (e.g., clay and mica), but is variable diluted by quartz and carbonate minerals. As a result, variations in Rb concentration may reflect both dehydration loss and dilution by Rb-poor phases, making it difficult to isolate the effects of metamorphic dehydration. This study uses the 87Rb/85Rb ratio (expressed as δ87Rb) as a more robust tracer of fluid-mobile element behavior during prograde metamorphic dehydration, where Rb loss during subduction is expected to produce systematic fractionation of 87Rb/85Rb with depth, whereas Rb conservation would not. We present high-precision δ87Rb data for eleven metamorphosed sediment samples from the Schistes Lustrés complex (Western Alps) and compare them with four similar, but unmetamorphosed sedimentary rocks from the Lavagna nappe (Northern Apennines, Italy). The Schistes Lustrés samples were subducted to depths of up to ~90 km along a relatively cold geothermal gradient (8 °C/km), allowing us to track how fluid-mobile elements behave during high-pressure to ultrahigh-pressure metamorphism. We find that Rb isotopes show minimal change during subduction and are not correlated with the degree of metamorphism or indicator of elemental loss (e.g., Rb/K2O, Rb/Cs, and H2O contents). δ87Rb values range from −0.17 to 0.04‰ in the Schistes Lustrés samples and −0.12 to −0.01‰ in the Lavagna samples. This limited variability suggests that most Rb is retained in subducted sediments, likely because it is stored in minerals that are stable at high pressures, such as phengite. As a result, subducted sediments can carry significant amounts of Rb to sub-arc depths with little loss during early stages of subduction and metamorphic dehydration. These findings have important implications for how we interpret volcanic rocks at subduction zones. The relatively low δ87Rb values previously observed in arc lavas from Mariana and Lesser Antilles are more likely caused by partial melting of subducted sediments at sub-arc depths, rather than loss of Rb during metamorphic dehydration. In addition, the preferential incorporation of isotopically light Rb into sediment melts contrasts with the isotopically heavy Rb released during dehydration of altered oceanic crust. Consequently, Rb isotopes provide a useful tool for distinguishing between crustal inputs from sediment melts and fluids released from the subducted oceanic crust, allowing us to better understand how different recycled crustal components contribute to arc magmas.

Keywords

Arc magmatism; HP-UHP metamorphism; Metasediments; Rubidium isotope ratio; Subduction zone

Disciplines

Geochemistry | Geology | Physical Sciences and Mathematics

File Format

PDF

File Size

1647 KB

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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