Mantle composition, age and geotherm beneath the Darby kimberlite field, west central Rae Craton

作者：Harris, Garrett A.^*; Pearson, D. Graham; Liu, Jingao; Hardman, Matthew F.; Snyder, David B.; Kelsch, Dave

来源：Mineralogy and Petrology, 2018, 112(S1): 57-70.

DOI：10.1007/s00710-018-0609-4

• Northern Canada
• Peridotite
• Pyroxenite
• Xenolith
• Kimberlite eruption
• Lithosphere

The Rae Craton, northern Canada, contains several diamondiferous kimberlite fields that have been a focus of episodic diamond exploration. Relatively little is known about the deep mantle lithosphere underpinning the architecturally complex crust. We present bulk and mineral element and isotopic compositional data for peridotite and pyroxenite/eclogite xenoliths from the Darby kimberlites representing fragments of the west central Rae lithosphere, as well as the first kimberlite eruption age of 542.2 +/- 2.6Ma (2 sigma; phlogopite Rb-Sr isochron). Darby peridotites have low bulk Al2O3 contents with highly-depleted olivine (median Mg#=92.5) characteristic of cratonic lithosphere globally, but more depleted than peridotites from other Rae Craton localities. One peridotite xenolith contains a harzburgitic G10D garnet. Re-Os T-RD model ages appear to be the oldest measured to date from peridotites of the Rae lithosphere, having a mode in the early Neoarchean and ranging to the Paleoproterozoic (2.3Ga). Concentrate clinopyroxene defines a well constrained mantle geotherm indicating the existence of a 200km thick lithosphere at the time of kimberlite eruption, greater than the lithospheric thickness beneath Somerset Island and in good agreement with modern seismic constraints. Nickel-in-garnet thermometry in grains that record temperatures below the mantle adiabat, indicates mantle sampling dominantly in the graphite stability field whereas Al-in-olivine thermometry shows a distinct mantle sampling mode in the diamond stability field. Abundant pyroxenite and eclogite xenoliths are recovered across the Darby property and low-Cr garnet (Cr2O3<1wt%) is the most abundant garnet type recovered in kimberlite concentrate. Based on thermometry, these rocks range in likely depths of equilibration, from the lower crust into the shallow lithospheric mantle. They give variable Os model ages, with the oldest ages in the Mesoarchean. The abundance of this mafic material reflects derivation from a large mafic body possibly evident in the layered structure of the Rae Craton mantle root defined by new seismic studies.

• 出版日期2018-12
• 单位中国地质大学（北京）