Primary Igneous Graphite in Ultramafic Xenoliths: II Isotopic Composition of the Carbonaceous Phases Present in Xenoliths and Host Lava at Tissemt (Egg?r?, Alegerian Sahara)

¹³C/¹²C ratios have been analysed in graphite adcumulate crystalsassociated with the olivine and orthopyroxene of a suite ofultramafic xenoliths in the alkali basalts of Tissemt (Egg?r?,Algerian Sahara). The ¹³C of these graphites varies from –24?6to – 14?4 per milk with internal variations of up to 1?6per mille within a given sample. Carbonates, linked to the volcanicphase and precipitated at medium temperatures have ¹³C valuesfrom –0?8 to + 3?5 and ¹⁸O values from 21?6 to 31?5 Thehost lava has a high carbon content of 675 p.p.m. with a global¹³C of –4?2. It can be separated into three parts: a majorpeak is obtained between 670 and 985 ?C and also liberated byslow acid attack with a ¹³C of up to +7?3 and representing CO₂trapped in the natrolite; a high temperature peak (above 1160?C with 47 p.p.m at –25?8) will represent carbon trappedin silicates and there is an additional carbon not well determinedextracted from 300 ?C to high temperature which represent 215p.p.m. at –23?0 with a possible fraction as low as –68?0.The graphite is interpreted as the result of the precipitationof carbon from an interstitial liquid previously outgassed thensupersaturated by the ongoing crystallization process. The volcanicevent can be considered to a first approximation as remobilizingthe carbon from the xenoliths by oxidation to CO₂, subsequentpartial reduction upon cooling and precipitation of carbonateand trapping of residual CO₂ within zeolite. Only volatilereducedspecies can be significantly outgassed so that the total ¹³Cof –4?2 is probably close to (or a little higher than)the original ¹³C.