GEMOC ARC National Key Centre

S. Graham¹, W. L. Griffin¹, S. R. Shee² and D. D. Lambert³.

1. sgraham@laurel.ocs.mq.edu.au, GEMOC Key Centre Macquarie University Sydney 2109 Australia,
2. DeBeers Australia Exploration Limited 60 Wilson Street South Yarra 3141 Australia,
3. VIEPS Monash University 3168 Australia.
 

Introduction:
Low-Cr megacryst suite minerals are present in most southern African kimberlites, but are absent from orangeites.  In general it is accepted that the suite crystallised under high T-P conditions (~1250^oC, ~50 kbar) from an alkaline picrite magma. Lithophile element isotope systematics (Sr-Nd-Hf) [1,2,3] of megacrysts and kimberlites are similar, and these data have been used to imply that the megacryst magma was the "proto-kimberlite" magma. However, isotopic similarity of kimberlites and megacrysts is not always the case.  Sm-Nd isotopes of the Premier kimberlite (1180 Ma) and its megacrysts [2,3,4] are not in isotopic equilibrium, there are 4 Lu-Hf populations of zircon megacrysts at Orapa and 2 different populations in the Monastery and Leicester kimberlites [2]. Despite this isotopic disequilibrium Re-Os isotopic data from two megacryst ilmenites, from different kimberlites on different cratons [5,6], have similar initial γOs values, suggesting that megacryst magmas may ultimately have similar formation processes.

Re-Os Results and Discussion:
Melnoite: Average Re and Os concentrations of the Sutherland melnoite (66 Ma) are 0.31 and 0.37 ppb respectively.  For a mafic rock this sample has a low ¹⁸⁷Re/¹⁸⁸Os ratio (4.34) and its initial γOs value of +40 is significantly higher than mantle plumes.
Kimberlites: The Wesselton kimberlite (84 Ma) has high Os (~1.8) and moderate Re (~0.24) concentrations, with a low ¹⁸⁷Re/¹⁸⁸Os ratio 0.65 and an initial γOs value of +4.6. The Premier kimberlite has variable Re and Os contents (Re = 0.13 - 0.28; Os = 0.62 - 0.98) and ¹⁸⁷Re/¹⁸⁸Os ratios vary from 0.66 - 1.8.  Initial γOs values range from -11 to +13.
Orangeites: Four orangeites (118 Ma) yield highly variable Re-Os systematics.  Re and Os concentrations range from 0.1 - 1.1 ppb and 0.63 - 4.9 ppb respectively. Initial γOs values are similarly variable, ranging from -6.5 to +61, and Re/Os and initial γOs values are negatively correlated (r² = 0.6, n = 7).  Coupled high γOs values and low Re/Os ratios characterise ¹⁸⁷Os enrichment during subduction processes [e.g. 7].
Mg-Ilmenites: Individual Mg-ilmenite megacrysts have low, but variable, Re (0.01 - 0.31 ppb) and Os (0.01 - 0.40 ppb) concentrations. ¹⁸⁷Re/¹⁸⁸Os ratios range from 1.80 to 25.0 (Ave. = 6.2).  Initial (90 Ma) γOs values range from +35 - +41, but two ilmenites from the transitional Frank Smith kimberlite have γOs(i) values of +72, suggesting the presence of 2 ilmenite populations. These γOs values are significantly radiogenic for samples that crystallised in the mantle.
The ilmenites yield a Re-Os isochron age of 110 ± 7 Ma, and initial γOs value of 36.3 ± 0.6 (MSWD = 1.5). A two-point isochron from the more radiogenic ilmenites suggest an age of 130 Ma and an initial γOs = 70.4. Within individual intrusions the ilmenites yield ages ranging from 100 ± 26 Ma (γOs = 36.7 ± 1.0; MSWD = 0.3) to 132 ± 76 Ma  (γOs = 36.6 ± 2.6; MSWD = 0.4). For silicates the Re-Os closure temperature is lower than mantle temperatures [8], but for homogeneous ilmenites the temperature may be higher.

Conclusions: The orangeite negative Re/Os and γOs(i) correlation is consistent with mixing between 2 mantle reservoirs: a subduction related ¹⁸⁷Os enriched and Re-enriched lithospheric peridotite (Re/Os = 0.1 - 0.5, γOs = -13 - +12). Based on orangeite Nd TDM model ages ¹⁸⁷Os enrichment may have occurred during Namaqua-Natal orogenesis.  The kimberlites studied also contain a Re enriched lithospheric mantle component, but not the subduction-related endmember, suggesting that removal of orangeites from the lithospheric mantle largely exhausted this source.
Both populations of ilmenites suggest ilmenite crystallisation close to, but preceding kimberlite emplacement by >20 Ma. The similar ilmenite γOs(i) values between localities (spanning 350 km) suggests the ilmenite source was largely homogenous over this range.  Moreover, the γOs values are higher than mantle reservoirs and do not suggest interaction with melt-depleted lithospheric mantle.
The megacryst petrogenetic model we therefore propose is that a sub-lithospheric mantle magma was contaminated by enriched mantle, which may have formed during Karoo or Bushveld magmatism, at, or near, the base of the Kaapvaal lithosphere.  While there is no evidence for a megacryst-orangeite genetic link our date suggest they may be related to the same thermal source.  For example there is a correspondence of the megacryst ilmenite isochron age (~110 Ma) and orangeite emplacement (~115 Ma).  This coincidence suggests that the parental megacryst melt that impinged on the base of the Kaapvaal lithosphere also caused melting of the enriched orangeite lithospheric mantle source, higher in the lithospheric mantle column. These results therefore argue that megacrysts are xenocrysts and probably not representative of a "proto-kimberlite" magma.
 

References:

[1] Jones R. A. (1987) Mant. Xeno. (Nixon P. H. ed), 711-738.
[2] Griffin et al. (2000) GCA, 64, 133-147.
[3] Nowell, G. M. (1999) 7th Int. Kimb. Conf. Proc.Vol2 616-636.
[4] Basu A. R. & Tatsumoto M. (1980) Cont.M.&P 75, 43-54.
[5] Graham S. et al. (1999) Geology, 27, 431-434.
[6] Carlson R. W.  & Bell D. R. (1997) 7th Ann. Gold. Conf. Abs. 43-44.
[7] McInnes et al. (1999) Science 286, 512-516.
[8] Burton K. W. et al. (2000) EPSL 183, 93-106.