SPOT DATING OF DETRITAL RUTILE BY LA-Q-ICP-MS: A POWERFUL PROVENANCE TOOL

Spot dating of detrital zircon is now routinely used for provenance work and recently pitfalls of this approach have been published. Zircon, because of its robustness, tends to preserve the age of the igneous event from which it crystallized, and not ensuing metamorphisms that it might have experienced. Rutile, in contrast, is chiefly a high-grade metamorphic mineral that tends not to survive multiple tectonic/metamorphic events. This makes rutile a better target mineral for dating if the question is “From what tectonic highlands was rock eroded to form this sand?”

Rutile commonly contains 2 to 200 ppm U making 206Pb/238U and 207Pb/206Pb dating possible. An advantage of rutile dating is its distain of Th. Essentially if 208Pb is detected, it is common, and with a model composition, common Pb can be accurately corrected for even if measurement of 204Pb is not possible. In addition concentrations of Ti and 11 trace elements are measured during dating analysis. Some trace elements (Cr, V, and Zr) show strong correlation with age.

The power of rutile dating as a provenance tool is shown by a lower Mississippi River sand sample. U/Pb dating of zircon yields a dominant Grenville age peak (33%). Appalachian (650-280 Ma) and Cordilleran ages (<250 Ma) make up 7 and 19%, respectively, from the total range of concordant ages (3350 to 23 Ma). The ratio of numbers of Grenville to Appalachian ages is 4.7. (U+Th)/He dating of some of the same zircon grains quoted above indicates that a significant portion of zircon was reheated by, and probably released in Appalachian orogenies. For instance, 5/12 of zircons with Grenville U/Pb ages yielded Appalachian (U+Th)/He ages, indicating that many of these zircons were derived from the Appalachian Mountains. U/Pb dating of rutile from the same sample leads to the conclusion of a major Appalachian source directly. For rutile U/Pb dating, Appalachian ages are dominant (51%) trailed by Grenville-aged ones (26%), and ones older than Grenville (18%). The entire age spread for concordant rutile grains is 2230 to 140 Ma. The ratio of Grenville to Appalachian rutile ages is 0.5, the converse of the zircon ages.