Kc. Condie et al., BEHAVIOR OF RARE-EARTH ELEMENTS IN A PALEOWEATHERING PROFILE ON GRANODIORITE IN THE FRONT RANGE, COLORADO, USA, Geochimica et cosmochimica acta, 59(2), 1995, pp. 279-294
A Paleoweathering profile on the Boulder granodiorite in northern Colo
rado provides an opportunity to trace the behavior of REEs from parent
rock, through a weathering profile, into unconformably overlying Perm
ian sediments. With progressive upward weathering of the granodiorite,
Na2O, CaO, SiO2, Ta/Hf, Co/Th, Cr/Sc, Cr/Th, Zr/Hf, La/Sc, Zr/Y, and
La/Th decrease; Al2O3 and Fe2O3T increase; and TiO2, MgO, K2O, P2O5, R
b, 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 enrichm
ent 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 overlyi
ng 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. W
e suggest these Eu anomalies are due to relative enrichment of the oth
er REEs and partial loss of Eu during the breakdown of plagioclase. Th
e Boulder weathering profile also has a very minor negative Ce anomaly
that is within error of a Ce anomaly in the parent. In the unweathere
d 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 zirco
n. With exception of Eu, for which feldspars contribute about 8%, negl
igible 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 similiar to those of the pa
rent 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, E
u, 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 u
nchanged 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 a
lso differ between Fountain sediments and the bulk weathering profile
may be related to one or a combination of four factors, listed in orde
r 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 elemen
ts by water flow through the upper meter of the weathering profile.