A global compilation of kimberlite carbonate data (未13C = 鈭?#xA0;11.9 to + 0.2鈥? median 未13C = 鈭?#xA0;5.0鈥? relative to VPDB; 未18O = 1.2-26.6鈥? median 未18O = 13.2鈥? relative to VSMOW) reveals that the majority of results (86%) plot within a range of 未13C ~ 鈭?#xA0;2 to 鈭?#xA0;8鈥? which is considered representative of mantle carbon, but only 15% of analyses are in the field of oxygen isotopic values for mantle carbonates (未18O ~ 6-9鈥?. Variations in kimberlite carbon isotopic compositions occur on regional scales, implying widespread mantle heterogeneity, possibly related to input of carbon from recycled crustal material and/or partial overprinting by secondary processes at the local scale. Carbonates in southern African Group I (or archetype) and Group II kimberlites (or orangeites) show different 未13C distributions (median values of 鈭?#xA0;5.3鈥?and 鈭?#xA0;6.5鈥? respectively). This is consistent with distinct mantle sources, as demonstrated previously by radiogenic isotope studies. Kimberlite breccia carbonates commonly have higher 未18O values than carbonates in massive and hypabyssal kimberlites, which suggests more extensive interaction of kimberlite rocks with hydrous fluids in the brecciated parts of kimberlite pipes. Modelling of the stable isotope compositions of carbonates from the Kimberley, Lac de Gras and Udachnaya-East kimberlites reveals that several processes are capable of modifying these compositions, including interaction with H2O-rich deuteric (i.e. late-stage magmatic) fluids, meteoric waters and/or hydrothermal fluids, and incorporation of sedimentary material. However, these processes can produce similar variations of the carbonate C-O isotopic compositions, which means that carbonate isotopes alone cannot provide tight constraints on the alteration of kimberlite rocks. Only few carbonates in hypabyssal kimberlites show isotopic compositions consistent with abundant CO2 degassing (i.e. increasing 未18O with decreasing 未13C values), thus implying that kimberlite magmas that are not emplaced explosively retain most of their CO2 concentrations prior to carbonate crystallisation.
In kimberlitic rocks early-formed serpentine exhibits higher 未18O values (~ + 4-+ 6鈥? than later serpentine rims and segregations (未18O values as low as ~ 鈭?#xA0;2鈥?. These variations are consistent with serpentine crystallisation from hydrous fluids derived from mixing between deuteric fluids and meteoric/hydrothermal fluids, with progressive enrichment in the latter component. Serpentine is considered to have formed under hydrothermal conditions when externally derived hydrous fluids infiltrated the cooling kimberlite volcanic system.
Only limited sulphur isotopic data are available for kimberlitic bulk rocks and sulphide and sulphate phases. Of these, relatively few sulphur isotopic ratios approach the 未34S values considered representative of the mantle (0 卤 2鈥? relative to VCDT). Elevated 未34S values (~ 14鈥? characteristic of sulphates in the Udachnaya-East kimberlite are consistent with equilibration with sulphides (未34S ~ 1-2鈥? at temperatures of ~ 500-550 掳C, after kimberlite melt outgassing under oxidising conditions. Conversely, the large 未34S range shown by some southern African and Yakutian kimberlites (鈭?#xA0;3-+ 12鈥?and + 15-+ 53鈥? respectively) may be largely due to alteration and crustal contamination.
In conclusion, the stable isotopic compositions of carbonates, serpentine and S-rich minerals in kimberlites, can be used in conjunction with detailed petrographic and geochemical analyses, to constrain processes affecting kimberlite magmas prior to, during, and subsequent to crystallisation. The available stable isotopic data indicate that externally derived (i.e. non-magmatic) hydrothermal fluids have affected the compositions of most kimberlites, including the hypabyssal varieties often used to reconstruct the compositions of primary kimberlite melts. This discrepancy remains a major obstacle in the quest for the primary composition of kimberlite melts.