HYDROTHERMAL FLUID CIRCULATION THROUGH THE SEDIMENT OF CRATER-LAKE, OREGON - PORE-WATER AND HEAT-FLOW CONSTRAINTS

We present evidence for porewater flow through the sediment of Crater Lake, Oregon based on systematic variations in pore water chemical com positions and thermal gradients. Pore water was extracted from sedimen t by centrifugation and diffusive exchange using a gravity corer deplo yed from a surface vessel and a box corer and peepers deployed from th e submersible Deep Rover in a known geologic context. Depth profiles o f sediment temperature were measured using two probes deployed from th e submersible. One probe was connected to the submersible whereas the other was self-contained and deployed for 7 days. On the basis of meas ured and calculated depth profiles of pore water Na, Ca, Mg, K, Li, an d temperature, we show that pore water upwells in zones of focused upf low at speeds of meters to hundreds of meters per year. These zones of focused flow are patchy and usually cover several square meters to hu ndreds of square meters and are marked by iron-manganese-rich crusts, bacterial mats, and saline pools. In contrast, most of the lake floor consists of sediment derived from the caldera walls and has a low heat flow with pore water velocities of millimeters per year. The chemical composition of the pore water that upwells through the sampled sectio n of the sediment column differs from core to core. This difference re sults from mixing a hydrothermal fluid in igneous basement below the l ake with lake water before the final ascent through the sediment colum n. Elemental ratios of this thermally and chemically altered fluid in basement match those calculated based on mass balance considerations. Calculation of mass balance and geothermometry constrain the temperatu re in basement and ultimately the power output, which is about 30 MW. This power output is in agreement with two other estimates that were c alculated using temperature data from the water column and measurement s of sediment heat flow.