Behavior of rare earth elements in a paleoweathering profile on granodiorite in the Front Range, Colorado, USA

Abstract

A Paleoweathering profile on the Boulder granodiorite in northern Colorado provides an opportunity to trace the behavior of REEs from parent rock, through a weathering profile, into unconformably overlying Permian sediments. With progressive upward weathering of the granodiorite, Na 2O, CaO, SiO 2, Ta Hf , Co Th , Cr Sc , Cr Th , Zr Hf , La Sc , Zr Y , and La Th decrease; Al 2O 3 and Fe 2O 3T increase; and TiO 2, MgO, K 2O, P 2O 5, Rb, Zr, Sc, Cr, Co Hf, Nb, Ta, Y, Th, U, REE, Ti Nb , and Zr Nb increase to maximum values and then either level off or decrease. LREE enrichment is less in the weathering profile than in the parent granodiorite and although the parent does not have an Eu anomaly (or only a slight positive anomaly), all samples from the weathering profile and overlying sediments have significant negative Eu anomalies. This observation is especially important in that it shows conclusively that a negative Eu anomaly can be produced during chemical weathering of granitoids. We suggest these Eu anomalies are due to relative enrichment of the other REEs and partial loss of Eu during the breakdown of plagioclase. The Boulder weathering profile also has a very minor negative Ce anomaly that is within error of a Ce anomaly in the parent. In the unweathered parent, >50% of the REE are contained in sphene, and in the case of La, also in allanite. From 10–20% of the REE are contained in apatite and biotite (± hornblende), and from 7–10% of the HREEs are in zircon. With exception of Eu, for which feldspars contribute about 8%, negligible amounts of REEs occur in the feldspars. In weathered samples, >75% of the REEs are contained in clay minerals. The crossover between sphene and clay control of REEs occurs over a distance of 1 m near the contact with fresh rock. Except for their small negative Eu anomalies, the clay minerals have REE patterns very similar to those of the parent rock. Isocon plots suggest apparent enrichments of many elements in the Boulder weathering profile result from losses of Na, Ca, and Si during plagioclase weathering. In addition, variable amounts of Sr, Eu, Ta, Nb, P, and Ba were lost during weathering. Although Th U , Zr Y , Th Sc , Zr Hf , Lu Hf , and Ti Zr may have been transferred relatively unchanged from granodiorite parent to the bulk weathering profile, most other element ratios and REE distributions were significantly changed during weathering. This observation implies that caution needs to be exercised when using REE patterns and element ratios to trace sediment provenance. The fact that most element ratios and REE distributions also differ between Fountain sediments and the bulk weathering profile may be related to one or a combination of four factors, listed in order of probable decreasing importance: contribution of other sources to the Fountain sediments, sorting of minerals during sediment deposition, remobilization of elements during diagenesis, and leaching of elements by water flow through the upper meter of the weathering profile.