XENOCRYST DISSOLUTION IN CONTINENTAL BASALTS: EXPERIMENTALLY DETERMINED RATES AND TEXTURAL COMPARISON WITH FIELD EXAMPLES FROM COLORADO AND ANTARCTICA

Xenolith dissolution is an important contamination process affecting mafic magmas, supported by isotopic and geochemical evidence in a variety of settings. We present preliminary results from a series of experiments aimed at quantifying the rates of xenolith dissolution in two different continental basalts, and compare these with actual dissolution textures observed in the field examples.

Two basaltic compositions were used, a tholeiitic dolerite from the Jurassic Ferrar Large Igneous Province, Antarctica, and a Tertiary alkali basalt from the Raton Basin, Colorado. Cylinders were cored from remelted basalt that was quenched to glass, polished, and placed below polished single-crystal quartz disks. As described by others, dissolution rates are controlled by diffusion, compositional convection and kinetic limitations. Rates are slowest when controlled by kinetics and fastest when controlled by convection. Quartz was placed on top of basalt to obtain rates of diffusion-controlled dissolution. Experiments were conducted at atmospheric pressure with oxygen fugacity buffered at QFM. Temperatures of 1225, 1275 and 1300°C were used, with experiment durations from 15 minutes to 10 hours.

Dissolution rates of quartz into the alkali basalt increased from ~40 µm per hour at 1225°C to ~60 µm/h at 1275°C and ~80 µm/h at 1300°C. Some convection was observed at 1300°C, which may contribute to the observed rate. Diffusion rims of ~150µm width were established within 15 minutes, with silica contents in the liquid close to 70% at the contact, diminishing to background values of ~48% at the far end of the reaction rim. Within this zone most elements are depleted relative to the basalt but sodium and potassium increase. This pattern has been observed by others, and reflects preferential partitioning of alkalis into silicic liquids.

Dissolution into the tholeiitic basalt was not detectable after 4 hours at 1300°C, despite the observation of reaction coronas between rounded and embayed quartz xenocrysts and basalt in thin sections from outcrop. Similar reaction textures are preserved in the alkali basalt. We are presently conducting experiments designed to investigate development of reaction textures, using disks of sandstone (both) and granite (tholeiite only) in place of single quartz crystals.