(U-Th)/He dating of kimberlites-A case study from north-eastern Kansas

Abstract

Dating kimberlite intrusive rocks by radiogenic isotope geochronology often is a difficult task, complicated by both the lack of dateable minerals within kimberlite as well as significant sample alteration that can degrade samples and alter parent-daughter ratios. This study presents a new geochronologic tool for timing the emplacement of kimberlites using the (U-Th)/He system to date the cooling of common kimberlite phenocrystic and xenocrystic minerals. To demonstrate the use of this technique, new apatite, titanite, zircon, magnetite and garnet (U-Th)/He ages constrain the timing of emplacement for the Stockdale, Tuttle, Baldwin Creek, Bala, and Leonardville kimberlite pipes, located in Riley County, Kansas. Zircon from the Tuttle pipe and titanite from the Stockdale pipe yield (U-Th)/He ages of 108.6 ± 9.6 Ma and 106.4 ± 3.1 Ma, respectively. These data are consistent with new Tuttle kimberlite Rb-Sr analyses of phlogopite megacrysts that give a five point isochron age of 106.6 ± 1.0 Ma. Similarly, an apatite (U-Th)/He age of 85.3 ± 2.3 Ma from the Baldwin Creek kimberlite is in agreement with a Rb-Sr phlogopite age of 88.4 ± 2.7 Ma. These dates demonstrate that (U-Th)/He thermochronometry provides reliable timing constraints on the cooling of common kimberlite xenocrystic phases, thereby timing kimberlite emplacement. In addition to the use of more commonly used apatite and zircon (U-Th)/He thermochronometers, we produced reliable emplacement ages of 103.0 ± 7.5 Ma for the Bala kimberlite using (U-Th)/He dating of phenocrystic magnetite and an age of 98.8 ± 8.9 Ma for the Tuttle kimberlite using (U-Th)/He dating of megacrystic garnet. In contrast, kimberlitic apatite (U-Th)/He ages from the Stockdale, Bala, Tuttle, and Leonardville kimberlites yield ages ranging from 67.3 ± 4.4 Ma to 64.3 ± 5.6 Ma, suggesting a local, possibly hydrothermal reheating event resulting in resetting of the apatite (U-Th)/He clock in latest Cretaceous to earliest Tertiary time. Additional (U-Th)/He analyses of apatite from nearby sandstone and basement rocks suggest regional cooling below ~ 70 °C at ~ 165 Ma. These (U-Th)/He and Rb-Sr age data imply that the kimberlites were emplaced over a period of time from ~ 85-110 Ma with several pipes subjected to local reheating at ~ 65 Ma. © 2008 Elsevier B.V. All rights reserved.