CARBON ISOTOPES IN XENOLITHS FROM THE HUALALAI VOLCANO, HAWAII, AND THE GENERATION OF ISOTOPIC VARIABILITY

The isotopic composition of carbon has been determined in a suite of xenoliths from lava of the 1800-1801 Kaupulehu eruption of Hualalai Volcano, Hawaii. Several lithologies are represented in the suite, including websterite, dunite, wehrlite, pyroxenite, and gabbro. In addition, there are composite xenoliths in which contacts between lithologies are preserved. Most of the xenoliths represent deformed cumulates. The contact relations in the composite samples indicate that the lithologies originated from the same source region, which, based on pressures determined from fluid inclusions, is estimated to be at a depth of ≈20 km, or near the crust-mantle boundary. Samples were heated in steps from 200 to 1475°C to obtain separation of the different carbonaceous phases, and the isotopic composition of carbon released at each step was determined. Grossular glass was found to be a suitable flux to fuse refractory samples. Upon heating, carbon exhibits the typical bimodal evolution behavior observed in other studies of xenoliths and basalts. Carbon extracted from all samples at temperatures below 900°C is characterized by a δ13C of about -25%. vs PDB and is thought to be composed dominantly of graphitic and organic material, which is known to be present on virtually all cracks. The δ13C of the carbon fraction extracted at 1200°C and above from wehrlite and dunite is in the range-1.5%. to 5.2%., whereas that extracted from websterite ranges from -22%. to -26‰ Similarly, in one composite sample, the compositions of dunite and websterite were found to be -2.4%. and -7.0%., respectively. The large difference can be associated with specific petrographic features unique to each lithology. In wehrlite and dunite, carbon exists mostly as CO2-rich inclusions in arrays representing partially annealed microcracks. The websterite xenoliths contain megascopic zones of large, irregularly-shaped inclusions. The zones traverse entire thinsections and are interpreted to represent fractures annealed at depth. Most of the carbon is believed to exist in the inclusion-rich zones and to consist of carbonaceous material precipitated from fluid. The observations and isotopic results demonstrate that isotopic variability can be generated by multistage fractionation processes such as degassing of CO2 from magma and precipitation of CO2-rich fluids to form graphitic compounds. Such processes operated over regions the scales of which were determined by style and intensity of deformation and by lithology. © 1990.