Rare-earth element and mineralogic changes in Holocene soil and stream sediment: A case study in the Wet Mountains, Colorado, U.S.A.

Abstract

Size-fractions of soil and stream sediment developed on plutonic, metamorphic, or mixed plutonic-metamorphic sources in the Wet Mountains, Colorado, have been analyzed for REE (rare-earth elements) and mineralogy. The clay-sized fractions (<0.5- μm and 0.5–2-μm, or <2 μm) have REE contents that are similar to more enriched than the source, Eu/Sm ratios that are similar to the source and La/Lu ratios that are higher than their corresponding source. The <0.5- μm fractions have lower REE contents than the corresponding 0.5–2-μm fraction in every stream sediment. For example, unweathered San Isabel batholith near the sampled stream sediment contains ΣREE = 511–712 ppm, and Eu Sm ratio = 0.17–0.19 and ( La Lu ) cn = 9.8–10.3 . The corresponding <0.5- and 0.5–2-μm fractions of sediment formed from San Isabel batholith are enriched in the REE ( ΣREE = 1135 and 2225 ppm, respec.), are slightly lower in Eu/Sm ratios (0.16 and 0.14, respec.) and are higher in La/Lu ratios (12.2 and 16.3, respec.) than the unweathered rocks. The silts have LREE (light REE) contents and Eu/Sm ratios very similar to the source (due to similar mineralogy), but silts have HREE (heavy REE) contents higher than the source (due to high zircon). For example, silt-sized fractions in stream sediment draining the San Isabel batholith contain ΣREE = 808 ppm, Eu Sm ratio = 0.18 and ( La Lu ) cn = 7.4 . In contrast, the sands contain lower REE contents, similar to higher Eu/Sm ratios and lower La/Lu ratios than the corresponding source due to depletion in heavy minerals (concentrated in the REE and containing lower Eu/Sm ratios) relative to quartz and feldspar (depleted in the REE and containing higher Eu/Sm ratios). For example, the sand fraction from stream sediment from the San Isabel batholith has ΣREE = 206 ppm, Eu Sm ratio = 0.17 and ( La Lu ) cn = 5.6 . Small variations of accessory minerals concentrated in the REE like allanite or sphene may cause drastic variation in REE content in the sand fraction. Clays and silts derived only from the plutonic rocks have higher REE and similar to lower Eu/Sm ratios than corresponding fractions derived only from migmatites (e.g., ΣREE of silt fractions from migmatites = 224 ppm and from plutons = 375–808 ppm; Eu/Sm ratios from migmatites = 0.17 and from plutons = 0.11–0.18). The REE content and the Eu/Sm ratio of clays and silts derived from migmatites are much more similar to the average of MCPS (mid-continent platform sediment) than corresponding fractions derived from the plutons (MCPS has ΣREE = 192 ppm; Eu Sm = 0.19 ; La Lu = 8.0 ). The Eu Sm ratio of sands derived from plutons and migmatite are lower to mostly higher than the MCPS. The REE content of sands from plutons are not necessarily higher than the MCPS due to variation in heavy minerals like allanite or sphene concentrated in the REE. Sediments derived from mixed sources have REE contents in fine-fractions that may not necessarily reflect the REE content of the nearest source. For example, 0.5–2-μm and silt fractions developed on the San Isabel batholith with a migmatite upstream have the high REE content of the batholith (ΣREE in 0.5–2- μm fraction = 1100 ppm; silt = 573 ppm) while the corresponding <0.5- μm fraction has the very low REE content of the migmatites ( ΣREE = 110 ppm). These results suggest fine-grained material near the source may sample a much larger area than coarser material near the source. Thus, analysis of fine-grained sediment associated with coarse-grained material near the source may give a much different impression of the source than would determination of the source using classic petrographic examination of the sand or gravels.