THE ROLE OF MONAZITE WEATHERING ON PHOSPHORUS BUDGETS OF THE LOCH VALE WATERSHED, ROCKY MOUNTAIN NATIONAL PARK, COLORADO, USA

The Loch Vale watershed located in Rocky Mountain National Park is a 660 ha glaciated headwater experimental watershed currently experiencing algal blooms. Atmospheric nitrogen deposition attributable to anthropogenic activities along the Colorado Front Rage have been determined to be responsible for these algal blooms. However, phosphorus, a lithogenic macronutrient also involved in algal blooms, has not been as thoroughly studied as nitrogen in the watershed.

Previous research in the Loch Vale watershed has identified accessory bedrock apatite [Ca₅(PO₄)₃(Cl,F,OH)] as a major source of phosphorus to stream waters. However, the rare Earth element (REE) phosphate mineral monazite [(REE)PO₄] has also been recognized in the Loch Vale bedrock and in similar abundances as apatite. Scanning electron micrographs of weathered monazite show grains displaying crystallographically controlled dissolution etch pits. The REEs in the monazite also have similar stoichiometric proportions as those dissolved in stream waters implicating monazite as the REE source.

Monazite dissolution rates were determined using two independent methods. First, a plug-flow column reactor of Loch Vale bedrock was used to quantify the percentages of phosphorus released by apatite and monazite during dissolution. A chemical weathering rate was then calculated using the PO₄^3- release flux from the previously determined watershed-scale apatite weathering rate. Second, the monazite weathering rate was determined using REE watershed flux values and mass-balance methods. The reactor-based monazite weathering rates of 0.014 – 0.050 mol ha^-1 yr^-1 compare very favorably with those of 0.019 – 0.030 mol ha^-1 yr^-1 determined from the watershed REE flux values.

The watershed-based monazite weathering rates correspond to a PO₄^3- contribution to surface waters of 0.018 – 0.028 mol ha^-1 yr^-1. However, these PO₄^3- release rates are three orders of magnitude lower than those from apatite of 13 – 47 mol ha^-1 yr^-1. Despite being present in bedrock in similar trace abundances as apatite, the contribution of phosphorus to stream waters from monazite is negligible relative to apatite. With increasing interest in phosphorus budgets of surface waters more work is needed to understand natural primary mineralogical sources including monazite.