Temperatures and Heat Production in the Slave Craton Lower Crust - Evidence from Xenoliths in the Diavik A-154 Kimberlite
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2019-01-01 |
| Authors | Benjamin Gruber |
Abstract
Section titled āAbstractāLower crustal heat production is poorly constrained due to the relative inaccessibility of lower crustal samples and their inherent complexity. To obtain the requisite information, the current project conducts spatially resolved geochemical analyses on minerals in 15 lower crustal xenoliths erupted via the Diavik A-154 kimberlite of the Northwest Territories, Canada. The aims are to: 1) conduct geothermometric measurements on lower crustal minerals, 2) construct a heatproducing element budget of the lower crust of the Slave craton, and 3) test the validity of these measurements in a parameter space relevant to geodynamic modeling and diamond exploration. The Diavik lower crustal xenolith suite comprises two main lithologies, mafic granulite (garnet-plagioclase-clinopyroxene Ā± orthopyroxene) and metasedimentary granulite (garnetplagioclase- orthopyroxene Ā± quartz Ā± K-feldspar Ā± kyanite), which are present in proportions of approximately 80:20, respectively. Application of mineral-pair, iron-magnesium exchange geothermometers (garnet-biotite, garnet-amphibole, and garnet-clinopyroxene) to these xenoliths indicates that the lower crust was at a maximum temperature of roughly 500 Ā°C at the time of kimberlite eruption (~ 55 Ma). The actual temperature of the lower crust is likely lower than 500 Ā°C as the geothermometers probably record the closure temperature of diffusional Fe2+-Mg exchange between touching mineral pairs rather than the ambient temperature of the rocks prior to their entrainment in the kimberlite magma. Heat-producing element (HPE) concentration measurements show that the lower crustal heat production of the Slave craton is likely 0.14 Ā± 0.02 Ī¼W/m3, which is lower than most values in the literature but broadly comparable to some geophysical estimates. This estimate is the result of (20:80) bimodal mixing of idealized lower crustal endmembers: a metasedimentary lower crust (0.37 Ā± 0.06 Ī¼W/m3) and a mafic lower crust (0.08 Ā± 0.01 Ī¼W/m3). These endmembers were iii calculated via a reconstructed bulk rock calculation utilizing trace element concentrations of constituent lower crustal minerals and idealized lithologies from the lower crustal xenoliths. Using these heat production estimates and other crustal parameters such as continental heat flux, mantle heat flux, crustal thickness, and crustal thermal conductivity, I modeled a Moho temperature for the Slave craton of 425 Ā°C, which is consistent with maximum lower crustal temperature estimate given by geothermometry. Adjusting the lower crustal heat production in the geotherm modeling program FITPLOT changes the temperature of the Moho in a similar fashion to the calculated models; however, the diamond propensity of the mantle lithosphere (partially a function of Moho temperature and heat production) does not appear to be strongly affected by a changing Moho temperature and is more strongly controlled by the conditions of the mantle P-T array.