Diamonds - Time capsules of volatiles and the key to dynamic Earth evolution

Abstract

Noble gas analyses of basalts indicate that the present-day structure of the Earth comprises a slightly degassed lower mantle and highly degassed upper mantle. The extent and timing of mantle in and out-gassing and sources of volatiles are, however, not well-constrained and require quantification. The objective of this thesis is to address these questions through a study of the noble gas composition of diamonds as diamonds, being chemically inert, can preserve information on the fluid and mantle composition at the time of diamond formation. Specific objectives are to explore the potential of U-Th/He systematics for dating fibrous diamonds, the influence of volatile subduction on the heterogeneity in the sub-continental lithospheric mantle (SCLM), and how noble gas compositions have evolved over time in the SCLM.

High-density fluid (HDF) inclusions with different major element compositions found in South African fibrous diamonds have different noble gas compositions that show these fluids originated from subducted sediments and oceanic crust and had either limited interaction with the SCLM (silicic-low Mg carbonatitic fluids) or significant interaction with the SCLM (saline fluids). A positive correlation between 3He/4He and d13C values in monocrystalline lithospheric diamonds from Argyle (Australia), together with low 40Ar/36Ar and He/Ne isotopic compositions, demonstrates a subduction influence caused by high U-Th/3He ratios and thus low 3He/4He ratios in subducted organic material. A noble gas depth profile based on N systematics of these diamonds shows fluid-rock interaction over scales of at least 15 kilometres above the accreted subducted material. Sub-lithospheric diamonds from Brazil, in contrast, show a negative correlation between 3He/4He and d13C values, but 3He/4He ratios are decoupled from trace elements and Pb-Sr isotope systematics that have characteristics of subducted material. This indicates that the subducted sediments have been deprived of U-Th-He and a high 3He/4He source, located in the deep mantle, is dominating the helium budget.

Fibrous diamonds are generally assumed to have formed shortly before kimberlite eruption but there is currently no way to date these diamonds. The U-Th-Sm/He systematics of fibrous Congo and Jwaneng diamonds showed that in most cases U-Th/He ratios are sufficiently high to produce significant radiogenic 4He to provide age constraints and some fibrous diamonds are up to several 100 Myr older than the kimberlite eruption age. Previously dated monocrystalline diamonds (with multiple ages ranging from 0.07 to ~3.4 Ga) were analysed for trace elements and He-Ar isotopic compositions and showed similar trace element patterns for all diamonds but a correlation between the age and 4He-40Ar*-3He/4He values. After correcting for radiogenic ingrowth since diamond formation a large variation remains in 3He/4He values at ~1.0 Ga in eclogitic diamonds that can be explained by mixing between mantle and subducted components. Given the preservation of heterogeneities, it is difficult to develop a simple noble gas evolution curve for the SCLM. From examination of fibrous and monocrystalline diamonds from different formation depths, formation ages, and geographic locations this study showed the large influence of subduction at the base of the lithosphere and a more SCLM-like noble gas composition at shallower depths. Show more