Geochemical data from inclusions within diamonds from the Northwest Territories, Canada, indicate that saline fluids are parental to silicic and carbonatitic deep mantle melts, via fluid–rock interaction; a subducting plate under western North America is suggested to be the source of the fluids.

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 (Sr-87/Sr-86(363) = 0.70300 +/- 16 to 0.70361 +/- 4), to highly enriched values, up to 87 Sr/86 Sr = 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 (epsilon(Nd71) = 40.4), which also contains the most radiogenic Sr isotopic composition. In contrast, a less enriched but still rather unradiogenic Nd isotope composition (epsilon(Nd540) = -11) was obtained for a diamond from Snap Lake, which has moderately radiogenic Sr isotopic enrichment (Sr-87/Sr-86(540) = 0.70821 +/- 1). The Pb isotopic system measured in one diamond indicates a complex evolution for the fluid source, with extreme Pb-207/Pb-204 ratio (15.810 +/- 3) and moderate, kimberlite-like Pb-206/Pb-204 and Pb-208/Pb-204 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 500 Myr 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. (C) 2013 Elsevier Ltd. All rights reserved.

X-ray diffraction, Raman and infrared spectroscopic evidence for the inclusion of water-rich ringwoodite in diamond from Juína, Brazil, indicates that, at least locally, the Earth’s transition zone is hydrous to about 1 weight per cent.

Monocrystalline gem-quality diamonds from Akwatia, Ghana and De Beers Pool, South Africa were analyzed for their trace element content using an ultra-sensitive ‘off-line’ LA-ICP-MS method. Concentrations of 27 elements analyzed range from ppt to ppm level but mostly are below the LOQ. CeN/EuN (1–6; N=chondrite normalized) and CeN/TiN (0.6 to 12) indicate mildly elevated LREEN/MREEN and variable LREEN/TiN. One diamond from Ghana is distinct (G103) by having a much steeper LREEN/MREEN slope than the other samples and overall matches the trace element pattern of low-Mg carbonatitic melt inclusions in fibrous diamonds from Kankan, Guinea (Weiss et al., 2009). Syngenetic garnet inclusions indicate that the diamond growth medium must have been highly enriched in LREE, with CeN/EuN and CeN/TiN from 9 to 370 and 10 to 3400, respectively. This extreme discrepancy in trace element ratios between these gem diamonds and inclusion-based melt compositions is inconsistent with the commonly made interpretation that trace impurities in gem diamond represent trapped inclusions of the diamond forming fluid/melt.Mixtures of submicroscopic inclusions of common peridotitic and metasomatic phases in equilibrium with a basaltic melt mimic the relative Ce, Eu and Ti abundances of the studied diamonds. But models are dominated by orthopyroxene and hence, do not correspond to modal relationships expected for peridotitic sources. The models also require at least 0.5% of the sample volume be occupied by inclusions, which would affect diamond transparency. Incorporation of exotic minerals that are normally rare or as yet undiscovered in peridotite (e.g. crichtonite group minerals and REE phosphates) could significantly lower the required inclusion abundance to an extent that preserves the transparent nature of gem diamonds. The principal finding of our study is that, with one exception, the trace element patterns of these gem diamonds do not yield a faithful representation of the diamond growth medium and hence caution is required when extrapolating gem quality diamond trace element measurements into “fluid” compositions.

Sub-micrometer inclusions in diamonds 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 trace element patterns of diamond fluids vary within a limited range and are similar to those of carbonatitic/kimberlitic melts that originate from beneath the lithospheric mantle. A convecting mantle origin for the fluid is also implied by C isotopic compositions and by a preliminary Sr isotopic study (Akagi, T., Masuda, A., 1988. Isotopic and elemental evidence for a relationship between kimberlite and Zaire cubic diamonds. Nature 336, 665–667.). Nevertheless, the major element chemistry of HDFs is very different from that of kimberlites and carbonatites, varying widely and being characterized by extreme K enrichment (up to ∼39wt.% on a water and carbonate free basis) and high volatile contents. The broad spectrum of major element compositions in diamond-forming fluids has been related to fluid–rock interaction and to immiscibility processes.

The geochemical signature of diamond-forming fluids can be used to unravel diamond-forming processes and is of potential use in the detection of so-called 'conflict' diamonds. While fluid-rich fibrous diamonds can be analyzed by a variety of techniques, very few data have been published for fluid-poor, gem-quality diamonds because of their very low impurity levels. Here we present a new ICPMS-based (ICPMS: inductively coupled plasma mass spectrometry) method for the analysis of trace element concentrations within fluid-poor, gem-quality diamonds. The method employs a closed-system laser ablation cell. Diamonds are ablated and the products trapped for later pre-concentration into solutions that are analyzed by sector-field ICPMS. We show that our limits of quantification for a wide range of elements are at the sub-pg to low pg level. The method is applied to a suite of 10 diamonds from the Cullinan Mine (previously known as Premier), South Africa, along with other diamonds from Siberia (Mir and Udachnaya) and Venezuela. The concentrations of a wide range of elements for all the samples (expressed by weight in the solid) are very low, with rare earth elements along with Y, Nb, Cs ranging from 0.01 to 2 ppb. Large ion lithophile elements (LILE) such as Rb and Ba vary from 1 to 30 ppb. Ti ranges from ppb levels up to 2 ppm.From the combined, currently small data set we observe two kinds of diamond-forming fluids within gem diamonds. One group has enrichments in LILE over Nb, whereas a second group has normalized LILE abundances more similar to those of Nb. These two groups bear some similarity to different groups of fluid-rich diamonds, providing some supporting evidence of a link between the parental fluids for both fluid-inclusion-rich and gem diamonds.