GEMOC ARC National Key Centre

S. Aulbach¹, W.L. Griffin1,2, S.Y. O'Reilly1, B.J. Doyle³ and K. Kivi³

1. GEMOC ARC National Key Centre, Macquarie University, NSW 2109, Australia,
2. CSIRO Exploration and Mining, P.O.Box 136, N. Ryde, NSW 1670, Australia,
3. Kennecott Canada Inc., 200 Granville St., Vancouver, B.C., Canada V6C1S4.
 

 Introduction: The Archean Slave craton comprises the 2.7-2.8 Ga Contwoyto and Hackett River Arc terrane and the 4.0-2.6 Ga Central Slave Basement Complex (CSBC), which collided at 2.7 Ga [1,2]. 100 km east of the crustally defined collision boundary, a north-south trending "Nd isotopic line" based on granitoids indicates input of ancient material in the west and of juvenile material in the east [3]. Lac de Gras is situated in the Contwoyto terrane, at the southern extrapolation of the Nd isotopic line. Investigation of kimberlite-entrained xenoliths [4] and concentrates [5] from Lac de Gras reveals a cool, 220 km thick, stratified lithosphere, with a sharp transition from an ultra-depleted shallow to a less depleted deep layer. The presence of abundant diamonds of the super-deep paragenesis [6] suggests that this deep layer represents subcreted plume-type material. An increase of fertility with depth is observed also for other mantle sections [cf. 5] and might be explained by polybaric melting [e.g. 7], imbrication of subducted slabs [e.g. 8], refertilisation of deeper mantle [e.g. 9] or conductive cooling of asthenospheric mantle [e.g. 10]. A trace element and multi-isotope study is under way to test these different hypotheses. We present here preliminary Re-Os data obtained by in-situ multi-collector ICPMS analysis of sulfides included in mineral grains from kimberlite concentrate.

Major and Trace Elements: Of 72 sulfide inclusions recovered, 80% are assemblages of Fe-rich monosulfide solid solution (MSS) and/or pentlandite ± chalcopyrite. 20% belong to a group of unusual Co-Ni-rich MSS. Preliminary PGE data show that Os (0.52-25.11 ppm) is negatively correlated with Pd/Ir (0.33-26.98), suggesting that the sulfides are related by igneous processes. A comparison of host mineral with xenolith major element composition allows assignment of a majority of the sulfides to the deep layer.

Re-Os Systematics and Geochronology: Re-Os isotope data (¹⁸⁷Os/¹⁸⁸Os = 0.1003 to 0.4732; γOs=-21.03 to +272.49, ¹⁸⁷Re/¹⁸⁸Os = 0.013 to 1.40) suggest there are three populations of sulfides. Regression of a group (including unpubl. data of Alard et al.) with correlated Re and Os isotope ratios yields an isochron age of 3.45 ± 0.25 Ga (N=6, MSWD = 0.81) and an initial ¹⁸⁷Os/¹⁸⁸Os of 0.1072 ± 0.0016 (γOs = +3.3 ± 1.5). A second group of sulfides with low ¹⁸⁷Os/¹⁸⁸Os and elevated ¹⁸⁷Re/¹⁸⁸Os  lies on a trend of recent Re addition. Their TR_D model ages of 3.0-3.9 Ga support ancient formation ages. The third group has unsupported, superchondritic ¹⁸⁷Os/¹⁸⁸Os, giving young or future T_CHUR model ages, attesting to recent Re-depletion or radiogenic Os addition.

Discussion:  The positive γOs of +3.3 at 3.45 Ga is higher than that of the outer core at that time (+1.66, values of [11]) and can therefore not be explained by mixing of such material into the source. Rather, input of higher Re/Os material, which can evolve to highly radiogenic compositions within a short time span, is required. Mantle-derived melts and their derivatives are the only known reservoirs that meet this criterion. For example, addition of <3% of 100 Ma old (at 3.45 Ga) basaltic crust to chondritic mantle will raise the 187Os/188Os of the source to the required value. If we accept this explanation, it supports initiation of plate tectonics in the Archaean [8]. The fact that the isochronous sulfides all have present-day subchondritic 187Os/188Os indicates that they have evolved at lower Re/Os than the enriched source. We interpret them as residues from polybaric decompression melting in an ascending diapir at 3.45 Ga, which led to variable Re-Os ratios in the sulfides and separation of a crustal volume. Subcretion of this moderately depleted diapir resulted in thickening of the cratonic root and emplacement of the lower mantle diamonds.

The isochron age exceeds that of the overlying crust, which forms part of the 2.7 Ga Contwoyto terrane, by ~ 0.7 Ga, but coincides within error with a period of extensive crust formation in the adjoining CSBC [12]. The "Nd isotope line" of [3] is interpreted as the leading edge of the suture between these two terranes  [2] and can be extrapolated to the Lac de Gras area. Our data suggest that this suture is a trans-lithospheric structure and that ancient CSBC-type mantle underlies at least a portion of the younger Contwoyto terrane. This model further implies that a diamondiferous deep mantle layer may exist beneath the CSBC.
 

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