CARBON CONCENTRATIONS AND ISOTOPIC-RATIOS IN FLUID-INCLUSION-BEARING UPPER-MANTLE XENOLITHS ALONG THE NORTHWESTERN MARGIN OF NORTH-AMERICA

At least three and perhaps four carbon components have been identified by step-heating extraction of upper-mantle xenoliths from Nunivak Island and Prindle Volcano (Alaska), Fort Selkirk and Alligator Lake (Yukon), Jacques Lake and Castle Rock (British Columbia). One component, released mostly between 200° and 600°C, comprises 30-95% of the total carbon extracted and has δ13C-values between -28 and -23‰. This carbon corresponds mostly to complex carbon compounds condensed on surface minerals and cracks and will be referred to as isotopically light carbon (ILC). CO2 released between 600° and 800°C is derived from interstitial carbonates. It has variable δ13C and concentrations that generally do not exeed 24 ppm C. The carbon extracted from 1000° to 1450°C is a mixture of CO2 from fluid inclusions and the ILC fraction. The amount of ILC released in this temperature interval never exceeds 10 ppm. Because part of this ILC may have been introduced from the host lava, the δ13C variations of this carbon extracted above 800°C is not only characteristic for the original carbon present in fluid inclusion-poor xenoliths. However, xenoliths enriched in CO2 fluid inclusions show C isotopic variations at the highest temperature steps that fall in a relatively narrow range between -10 and -4‰. Depending upon the fluid-inclusion abundance, the concentration of high-temperature C derived from fluid inclusions varies from 0.1 to 66 ppm. Anhydrous spinel lherzolites and harzburgites generally contain <2 ppm C as fluid inclusions. Pyroxenite intruding anhydrous spinel lherzolites have δ13C-values lighter than their host rocks. Anhydrous pyroxenites and tectonized harzburgite xenoliths with numerous fluid inclusions contain between 18 and 66 ppm C as fluid inclusions and the heaviest δ13C-values (-5 to -4‰). Fluid-inclusion-rich amphibole-bearing clinopyroxenite, spinel lherzolite and harzburgite xenoliths contain 8-24 ppm C as fluid inclusions with δ13C ranging from -10 to -6‰. The range of CO2 isotopic compositions observed in fluid inclusions corresponds to those measured in other mantle carbon material such as diamonds, kimberlites, carbonatites and volcanic gases. The observed carbon isotopic variations reflect isotopic fractionation between fluids and carbon dissolved in melts present in the upper mantle. © 1990.