Effect of chemical weathering on the RbSr date of feldspar in neogene till, Mount Fleming, South Victoria Land, Antarctica

Abstract

Cenozoic till deposits occur in presently ice-free areas at high elevations in the Transantarctic Mountains. These tills could have been deposited by local ice caps that no longer exist, or by ancestral outlet glaciers of the Antarctic ice sheet. Because the possible sources of the sediment differ significantly in age, RbSr dates of detrital feldspar in till can be used to evaluate these hypotheses. However, chemical weathering products adversely affect RbSr dates and must be removed to obtain dates that reflect the time of formation of the feldspar and, hence, the provenance of the sediment. A feldspar-quartz concentrate of till from Mr. Fleming was cleaned ultrasonically in 36 incremental steps for a total of 85 hr. in order to remove weathering products from the feldspar. Aliquots of the feldspar grains and the weathering products removed from them were collected after each cleaning step for SEM, XRD and RbSr analysis. Illite is the primary weathering product of K-feldspar; kaolinite forms from plagioclase, and gibbsite seems to occupy the outermost layer on both feldspars. The average total thickness of the altered layer is 4.5μm. Progressive cleaning causes the position of the feldspar on a RbSr isochron diagram to move in a way that affects the date derived from it. Illite has the greatest effect and causes RbSr dates of weathered feldspar to be lowered. The presence of an X-ray amorphous precursor of illite apparently has no effect on the RbSr date. Cleaned feldspar from Mt. Fleming till yields a date of 506 Ma, which is similar to the age of the granitic basement rock of the Transantarctic Mountains. There is no evidence that the till contains feldspar from other sources. We conclude that deeply weathered feldspar should be ultrasonically cleaned for at least 30 hr. to remove the alteration products that affect the RbSr date.