Tracking lower crustal events with Re-Os isotopes of granulite sulfides

S. Aulbach, Cristen Krauss, Robert A. Creaser, T. Stachel, Larry M. Heaman, Thomas Chacko

Research output: Contribution to journalMeeting abstract

Abstract

The composition of the lower crust, sampled by granulite xenoliths, provides critical constraints on the tectonothermal evolution of continents. Granulite whole rock Re-Os isotope systematics are corrupted by crustal assimilation and do therefore not yield meaningful ages [1]. While in situ U-Pb geochronology on zircon or monazite in granulites has proven to be a useful tool [2] to date events such as initial crust formation and later mafic underplating or accretion [3], not all granulites carry these minerals. We are analysing Re-Os isotopes and major-elements of single sulfides (pyrites) in kimberlite-entrained granulite xenoliths from the central Slave craton in order to assess whether useful age and petrogenetic constraints can be obtained from these minerals. The granulites have mafic to peraluminous compositions, with variable δ18O, SiO2 and Al2O3/(CaO + Na2O + K2O) [4].

Preliminary results show Ni contents of most sulphide grains <1 at%, whereas one grain has 6.7 at% Ni, compared to <<1 at% in kimberlitic pyrite. Re and Os contents are distinctly lower (median 135 and 9 ppb, respectively) and 187Re/188Os higher (140) than in sulfides in eclogite xenoliths from the same locality (620 and 70 ppb, respectively, 187Re/188Os = 50), interpreted to have MORB protoliths.

In the Re-Os isochron diagram sulfides show some scatter likely due to assimilation of older crust, and several generations of sulfides may occur in a single xenolith. The Nirich grain lies on a trend of pre-entrainment (ca 90 Ma) sulphide introduction into a high-temperature mantle eclogite, possibly
documenting translithospheric effects of events precursory to kimberlite magmatism. Several sulfide grains lie along 3.3 or 2.65 Ga model isochrons corresponding to the age of lower crustal formation/metamorphism [5]. Others plot about a 2.2 Ga model isochron, the age of the Malley-McKay dike swarms previously linked to zircon growth in lower crustal xenoliths [3]. Lower Re and Os contents but higher 187Re/188Os may indicate that the protoliths to the granulites crystallised sulfides prior to emplacement in the central Slave lower crust.
Original languageEnglish
Pages (from-to)A63-A63
Number of pages1
JournalGeochimica et Cosmochimica Acta
Volume73
Issue number13, Supplement
DOIs
Publication statusPublished - Jun 2009
Externally publishedYes
EventGoldschmidt Conference (19th : 2009) - Davos, Switzerland
Duration: 21 Jun 200926 Jun 2009

Cite this

Aulbach, S., Krauss, C., Creaser, R. A., Stachel, T., Heaman, L. M., & Chacko, T. (2009). Tracking lower crustal events with Re-Os isotopes of granulite sulfides. Geochimica et Cosmochimica Acta, 73(13, Supplement), A63-A63. https://doi.org/10.1016/j.gca.2009.05.001