The nature and co-behaviour of volatile and non-volatile elements in the sub-continental lithospheric mantle

The lithospheric mantle represents a critical interface in global volatile cycles because it separates Earth’s convecting mantle from the rigid crust. Nevertheless, the source, abundance and flux of volatiles stored in this reservoir are poorly constrained. In this work, the concentrations of volatiles stored both within the crystal lattice (H, Li, B, F, P and Cl) and fluid-inclusions (He and C) in mantle phases, together with their isotope compositions (3He/4H and 13C/12C), have been determined in 55 well-characterised peridotites and pyroxenites predominantly from off-craton mantle. The peridotites include spinel- and garnet-bearing lherzolites to harzburgites and are representative of the suites found at Kilbourne Hole, Colorado Plateau (SW USA), Tariat (Mongolia), West Eifel (Germany), Antarctic Peninsula, S. Patagonia, N.Tanzania and Ichinomegata (Japan). Their major and trace elements record a wide range in melt extraction (from 5 to 40%) but most xenoliths have undergone subsequent enrichment, via fluid or melt-related metasomatism. Fluid-inclusions are isotopically highly variable, with 3He/4He = 5.3 to 8.4 Ra and δ13C = -25.0 to 5.6 ‰ (V-PDB). 3He/4He-major-trace-element systematics shows that the subcontinental lithospheric mantle dominantly reflects formation from a temporally evolving upper-mantle that has been overprinted by radiogenic processes. Furthermore, the influence of metasomatism is enhanced by 3He/4He-13C/13C systematics which show the dominant role of small-fraction volatile-rich upper-mantle melts with variable inputs from melts/fluids derived from recycled oceanic lithosphere containing organic material and carbonates. Coupled links between volatile and non-volatile elements suggest that the CO2 hosted in fluid inclusions in the off-craton lithospheric mantle is approximately 41 ppm equating to an influx of 2.5 x 1019 g Myr-1. The estimated concentration for CO2 in off-craton mantle is lower than H2O (85 to 100 ppmw), similar to P (40 ppm) add higher than for the halogen (F = 15 to 27 ppm; Cl = 3.4 to 5.4 ppm), B (0.09 to 0.22 ppm) and Li (1.77 ppm). In comparison with the depleted mantle, the off-craton mantle is more concentrated in Li, B and F has similar CO2, P and Cl and lower amounts of H2O. Finally, this study expands the importance of pyroxenites in the off-craton lithospheric mantle as important hosts for volatiles to including 3He, CO2, Li and P. Overall, this study provides the first, fully internally-consistent appraisal of the petrography, mineral and calculated whole-rock major, trace and volatile element chemistry, and helium and carbon compositions of fluid-inclusions to advance the understanding of volatiles in the sub-continental lithospheric mantle on a global scale.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2021

Author dc:creator

• Crosby, James

Advisors dc:contributor.advisor

• Gibson, Sally Anne
• Stuart, Finlay Stuart
• Riley, Teal

• Geochemistry
• Noble Gas Geochemistry
• Stable Isotope Geochemistry
• Mantle Xenoliths
• Lithospheric Mantle

Licence

• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.90255

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/342836