INSIGHTS INTO THE ORIGIN OF MANTLE GRAPHITE FROM A NEW OCCURENCE IN GARNET PYROXENITES FROM THE EXTERNAL LIGURIDE PERIDOTITES (NORTHERN APENNINE, ITALY)

Abstract

The occurrence of elemental carbon in the mantle, either as diamond or, more rarely as graphite, is almost uniquely restricted to peridotite and eclogite xenoliths derived from cratonic lithospheric mantle. In non-cratonic areas, the occurrence of graphite-bearing garnet pyroxenites has been only reported for mantle bodies exhumed in orogenic settings, namely the Ronda (Davies et al., 1993) and Beni Bousera (Pearson et al., 1993) massifs, where the graphite may occur as diamond pseudomorphs and was interpreted as originated from crustally-derived, biogenic carbon recycled into mantle through subduction on the basis of 13C-depleted isotopic composition. Mantle peridotites of the External Liguride units represent slices of subcontinental lithospheric mantle exhumed during the rifting phases leading to the Jurassic Ligurian Tethys formation. These mantle rocks locally enclose up to meter-sized layers of graphite-bearing garnet pyroxenites. The high-pressure protoliths were characterised by coarsegrained isotropic texture and anhydrous bimineralic assemblage, consisting of pyrope-rich garnet (Prp53-45Alm29- 38Grs11-20) + sodic Al-augite (Na2O ~2.5 wt%, Al2O3 ~ 12.5 wt%), with accessory amounts of Fe-Ni sulphides and rutile. Geothermobarometric estimates for the high-pressure equilibration have yielded T = 1150-1200 °C and P = 2.6-3.0 GPa. Graphite (up to 3 vol%) occurs as flakes and stacks of flakes with grain size up to 2-3 mm. No pseudomorphs with octahedral or cubic simmetry suggesting the former ocurrence of diamond have been observed, in agreement with pressure estimates. The enclosing peridotites do not contain any vein or disseminated graphite. Sulphides (mainly pyrrhotine and pentlandite derived from high-T monosulphide solid solutions) occur as tiny rounded blebs into garnet and clinopyroxene or as interstitial crystals; their abundance may locally reach up to ~ 5 vol % . Graphite crystals were analysed by microRaman spectrometry on fresh fracture surfaces. Well defined first (O-) and second order (S-) peaks, and the absence of first order D-peaks indicate a highly ordered structure similar to that of graphite crystallised at high temperature in peridotite and eclogite xenoliths (Pearson et al., 1994). C isotope composition (d13C= - 4.5) reflects a typical mantle signature. Whole-rock and mineral trace element evidence indicate that the graphite-bearing garnet pyroxenites have suffered some partial melting (Tribuzio et al., this meeting). The observed sulphides may have originated from trapped (immiscible) sulphide melt. We propose that graphite precipitation was possibly related to carbon saturation reached in these sulphide liquids or to carbon nucleation on a sulphide medium, in agreement with experimental evidence and common diamond-sulphide association in higher pressure peridotite and eclogite xenoliths (Bulanova, 2002; Bulanova et al., 1998; Haggerty, 1986; Jacob et al., 2004; Litvin et al., 2002).