The sources and time-integrated evolution of diamond-forming fluids – Trace elements and isotopic evidence

Abstract

Sub-micrometer inclusions in fibrous diamond growth zones carry high-density fluids (HDF) from which the host diamonds have precipitated. The chemistry of these fluids is our best opportunity of characterizing the diamond-forming environment. The major and trace element patterns of diamond-forming fluids vary widely. Such elemental signatures can be easily modified by a variety of mantle processes whereas radiogenic isotopes give a clear fingerprint of the time-integrated evolution of the fluid source region. Thus, the combination of elemental and isotope data is a powerful tool in constraining the origin of fluids from which diamonds precipitate. Here we present combined trace element composition (34 diamonds) and Sr isotopic data (23 diamonds) for fluid-rich diamonds from six worldwide locations. The Nd and Pb isotopic composition of two of the diamonds were also obtained. Several of the samples were analyzed in at least 2 locations to investigate variations in the fluid during diamond growth. The data was acquired using an off-line laser sampling technique followed by solution ICPMS and TIMS analysis.The Sr isotopic compositions of diamond fluids from the different suites range between convecting mantle values for Udachnaya (87Sr/86Sr363=0.70300±16 to 0.70361±4), to highly enriched values, up to 87Sr/86Sr=0.72330±3, for a diamond from Congo. No isochronous relationships were observed in any of the suites. The lowest Nd isotopic composition recorded so far in a diamond is from Congo (εNd71=−40.4), which also contains the most radiogenic Sr isotopic composition. In contrast, a less enriched but still rather unradiogenic Nd isotope composition (εNd540=−11) was obtained for a diamond from Snap Lake, which has moderately radiogenic Sr isotopic enrichment (87Sr/86Sr540=0.70821±1). The Pb isotopic system measured in one diamond indicates a complex evolution for the fluid source, with extreme 207Pb/204Pb ratio (15.810±3) and moderate, kimberlite-like 206Pb/204Pb and 208Pb/204Pb ratios. A multi-stage evolution of the diamond-forming fluids source can be constrained from our new isotopic data, indicating an Achaean enrichment event resulting in elevated U/Pb, Rb/Sr ratios and enrichment in LREEs. This source underwent a more recent fractionation, in the last 500Myr that may have been related to the diamond-forming event.There is a strong correspondence between fluids with relatively unradiogenic Sr isotopes and relatively low (La, Nd, Sm)/(Nb, Zr) and (Ba, Th)/(Nb) ratios. Sr isotopic enrichment is accompanied by an increase in these ratios. The least trace element enriched and most isotopically depleted fluids are from the high-Mg carbonatitic suite. Thus, HDFs could be derived from asthenospheric mantle as low degree melts that interact to varying degrees with an ancient, metasomatized, rutile- and phlogopite bearing, sub continental lithosphere mantle. The internal heterogeneity in the Sr isotopic ratios within a single diamond suite and even within single diamonds may indicate fluid-mixing processes. Such mixing may occur during migration through preferred mantle veins and may be affected by the small-scale geochemical variability within them.