Browsing by Author "McDonough, W. F."

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    Gold distribution in Archean continental crust: Evaluating the effects of intracrustal differentiation in the Tanzanian Craton
    (2013-12) Long, K.; Rudnick, R. L.; McDonough, W. F.; Manya, Shukrani
    We have evaluated the vertical distribution of gold in variably metamorphosed igneous rocks in the Tanzanian Craton: 2.6 Ga upper-crustal greenschist-facies greenstone belt basalts and andesites from the Lake Victoria Gold Field of northern Tanzania, and compositionally similar 2.6 Ga lower-crustal mafic granulite-facies xenoliths that were carried in rift-related basalts that erupted nearby. We implemented the preconcentration method of Pitcairn et al. (2006), which utilizes chromatographic separation of gold from acid-digested rocks using diisobutyl ketone (DIBK), followed by standard addition ICP-MS to determine the distribution of gold in the crust. Repeat analyses of the certified reference material TDB-1, a whole-rock powder diabase dike from Tremblay Lake, Saskatchewan, Canada (certified gold concentration = 6.3 × 1.0 ng/g), yielded an average gold concentration of 6.5 × 1.1 ng/g. Results were reproducible to within 17% for rock powder aliquots between 200-600 mg (n=38), where 400 mg sample aliquots were reproducible to within 6% (n=9), and 600 mg aliquots were reproducible to within 4.5% (n=4). Better reproducibility for the greater sample aliquots likely reflects the 'nugget' effect. Rock samples in the 0.1-0.8 ng/g gold concentration range reproduced to within 27% for 400-600 mg sample aliquots. Although the lavas come from an area containing gold deposits, all were more than 5 km from any gold mine. The Tanzanian greenstone belt basalts have the highest gold concentrations (9 ng/g to 62 μg/g, ave. = 40 (+68/-25) ng/g, 1σ (n=10)), followed by the greenstone belt andesites (0.4 to 120 ng/g, ave. = 1.1 (+0.9/-0.5) ng/g, 1σ (n=14)). The lowest concentrations were observed in the granulite-facies lower-crustal xenoliths (0.1 to 3.3 ng/g, ave. = 0.3 (+0.3/-0.1) ng/g, 1σ (n=21)). Gold is incompatible in silicates and can partition into hydrothermal and/or magmatic fluid or vapour during high-grade metamorphic dehydration reactions or partial melting, particularly if sulfides break down during these processes. Rise of these buoyant mobile phases may explain the observed depletion of gold in the lower crust. Oxidative breakdown of sulfides was observed in some of the lower-crustal xenoliths, whereas some xenoliths did not contain any visible sulfides. Gold concentration in the samples did not, however, correlate with the presence of sulfides, which may indicate that the existing sulfides crystallized after the gold depletion had occurred. References: Pitcairn, I.K., Warwick, P.E., Milton, J.A., and Teagle, D.A.H. Anal. Chem. 2006, 78, p.1290-1295
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    A Xenolith Perspective on the Composition and Age of the Northern Tanzanian Lithosphere
    (2009-12) Manya, Shukrani; Rudnick, R. L.; Aulbach, S.; Bellucci, J. J.; Blondes, M. S.; Chesley, J.; Lee, C.; Mansur, A. T.; McDonough, W. F.
    Study of deep crustal and upper mantle xenoliths from rift volcanoes throughout northern Tanzania provides insights into the architecture of the Tanzanian lithosphere, as well as the interaction of this lithosphere with rift magmas. Like the upper crust, the lower crust and mantle lithosphere of the Tanzanian Craton (TC) and Mozambique Belt (MB) are Archean, but the lower crust of the MB has been thermally reactivated during the pan-African Orogeny, whereas that of the craton has not. In addition, both mantle sections have experienced interaction and heating associated with rift magmas. Cratonic lithospheric mantle is compositionally stratified, with highly refractory but strongly LREE-enriched peridotite comprising the bulk of the section (40-130 km depth), underlain by more fertile and deformed peridotites (130-150 km depth), which are also LREE-enriched. Lithospheric mantle of the MB is highly variable in thickness, ranging from a maximum of ˜150 km at Lashaine to <50 km within the Rift axis near the Kenyan border. Like the cratonic lithosphere, this mantle is also refractory and yields Archean Os model ages throughout. Mantle lithospheres of both the TC and MB have interacted with rift magmas, including carbonatites (at Olmani) and alkali basalts (s.l.), which, in some cases, precipitated veins containing phlogopite or amphibole. Late Pleistocene zircons in one of these veins testify to the youth of this interaction. Rift basalt precipitates that formed in the mantle (pyroxenites and glimmerites) and have, thus, never interacted with continental crust, have radiogenic Os isotopic compositions (γOs = +9), providing strong evidence for a plume source of the rift magmas. Sr and Nd isotopes in cpx from peridotites are highly variable: in some they are completely overprinted by rift magmas, whereas others contain Archean components. Granulite-facies xenoliths throughout northern Tanzania are generally mafic (including anorthositic compositions), with a few intermediate compositions; no granulite-facies metapelites have been found. Marbles, schists, quartzites and amphibolites from the MB likely derive from middle-crustal depths. All zircon U-Pb ages are Archean (≥ 2.6 Ga) and many of the samples fall along a 2.6 Ga Sm-Nd reference line. U-Pb thermochronology largely records slow cooling in the MB following the Pan-African Orogeny and is consistent with a present-day conductive geotherm of 47 mW/m2 in a crust with very low heat production (see Blondes et al., this meeting). Despite the fact that ɛNd varies from -4 to -32 in the lower crustal xenoliths, 87Sr/86Sr is much less variable and the data fall along a near-vertical trend in a Sr vs. Nd isotope plot, reflecting ancient Rb depletion relative to Sr. Similarly, the unradiogenic Pb in granulite feldspars from both TC and MB is consistent with ancient U depletion. Collectively, such distinctive radiogenic isotope characteristics can serve as a diagnostic signature of crustal assimilation in rift magmas from northern Tanzania.