Abundance and distribution of volatile elements in the subcontinental lithospheric mantle
Creators: name: Ananuer, Halimulati
Publisher: name: Macquarie University
Year: 2024
Subjects: classificationCode: 370302
subject: Inorganic geochemistry

description: The Earth's mantle may contain an order of magnitude higher volatile (C, H ±S) abundances than the exospheres (crust, hydrosphere, atmosphere and biosphere). The subcontinental lithospheric mantle (SCLM), as the upper part of the mantle, is a critical reservoir balancing the outer and inner Earth volatiles. However, how the cold and long-lived SCLM may modulate volatile flux remains debated and depend mostly on its storage capacity and the nature of the volatile repository phase(s). 
Concepts about water storage in the mantle have evolved dramatically during the last decades. It is now widely accepted that the main host phases for H(±C) in the mantle are the main forming silicates (olivine, orthopyroxene, clinopyroxene) which were first believed to be dry but contain few tens to hundreds μg•g<sup>-1</sup> of water, and are consequently now called nominally anhydrous minerals (NAMs). However, the SCLM also includes minerals, referred to as metasomatic minerals, in which volatiles are a major constituent (i.e., <em>n</em>•wt.%) such as phlogopite and/or amphibole for H, carbonate or diamond for C, apatite for both C and H. Further, it has been shown that the budget of incompatible elements is dominated by the elusive but pervasive grain boundary and interstitial components (GBIC, i.e., the "stuff between the grains"). Volatiles are incompatible elements. However, for the last 20 years, the mantle's volatile (H mostly) content has been almost exclusively considered through the NAM's water budget, relegating metasomatic phases and the GBIC to the back burner. 
This work explores another perspective by considering a more inclusive mantle assemblage and investigates the distribution and behaviour of C and H among and between NAMs, metasomatic minerals and GBIC. A new analytical protocol has been developed to assess the C, H, ±S distribution and content of the whole mantle assemblage. This approach combined in-situ analyses such as FTIR (Fourier Transform Infra-Red Spectroscopy) and simultaneous thermal analysis of emitted gas (STA) for C, H, and S obtained on whole-rock and hand-picked mineral fractions. Water and carbon contents in eight xenolith suits, equilibrated in the spinel-facies hosted in alkali basalts, from worldwide occurrences, have been investigated. 
Volatile contents are highly variable in the SCLM but overall much higher than previously estimated through the study of NAMs only. These results suggest that the old, stable, cold but "botoxed" SCLM could act as a "volatile siphon" between the deep Earth and the exospheres. 
Further, this work unfolds counter-intuitive relationships between NAMs, volatile-rich metasomatic phases, and GBIC and unexpected findings: i) hydrogen concentration in NAMs is independent of the occurrence and abundance of hydrous phase; (ii) NAMs host less than 10% of the volatile content of the total mantle assemblage; (iii) a large amount of water and carbon is stored in GBIC; (iv) metasomatic phases compete with the GBIC for volatile elements; (v) amphiboles (or phlogopites) may host significant amounts of CO2; (vi) the relationship between the abundance of volatiles in the mantle and metasomatic processes is not straightforward.