Materiały konferencyjne (WNGiG)
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Item Cu refertilization of abyssal peridotites by melt percolation(Copernicus Publications, 2015) Ciazela, Jakub; Dick, Henry; Koepke, Juergen; Botcharnikov, Roman; Muszynski, Andrzej; Kuhn, ThomasPrimitive mantle is depleted in many elements by partial melting processes, but it can be subsequently refertilized by impregnation with percolating melts. It is known that Cu can be enriched in primitive melts, depleting mantle residue, due to the former process (Patten et al. 2013). However, the behavior of Cu in the processes of mantle-melt interaction is poorly understood. The only comprehensive study is based on compositions of orogenic peridotites, representing the subcontinental mantle (Lorand et al. 1993; 2013), where a moderate enrichment of the mantle in Cu (up to 50 ppm) has been observed. Here, we present the first results obtained for a suite of rocks from an oceanic core complex (OCC), the Kane Megamullion at 22 30’N at the Mid-Atlantic Ridge (Dick et al. 2008). OCC’s provide large exposures of mantle and lower crustal rocks on the seafloor on detachment fault footwalls at slow and ultraslow spreading ridges. The mantle rocks are composed of spinel and plagioclase harzburgites. The spinel harzburgites represent depleted mantle, whereas the plagioclase harzburgites were formed by subsequent late-stage melt impregnation in the depleted mantle (Dick et al. 2010). We have determined Cu concentrations in 22 residual spinel harzburgites and 4 plagioclase harzburgites using total digestion ICP-MS. The average Cu concentration in spinel harzburgites is 35 11 ppm Cu (2 ). The average Cu concentration obtained for plagioclase harzburgites is 131 33 ppm Cu (2 ). Additionally, we have analyzed one 1.5 cm thick contact zone between an oxide gabbro vein and residual peridotite. The contact zone, which has been heavily impregnated by the melt, contains 284 ppm Cu. In contrast, the neighboring oxide gabbro vein and the hosting peridotite contain 147 and 68 ppm Cu, respectively. Furthermore, we have determined the concentration of Cu in a dunite (118 ppm), formed in a reaction between the mantle and melt ascending through the lithosphere (Dick et al. 2010). Magmatic processes in the rocks coming from OCCs can be obscured by deformation and alteration. Plastically deformed rocks are common in the damaged zone related to the detachment fault. Metaperidotites from these zones, which show protomylonitic to ultramylonitic textures, are systematically depleted in Cu (15 5 ppm, 2 ) in comparison to non-deformed spinel harzburgites. We have not included the values obtained from non-deformed harzburgites in the calculation of the averages presented above. Thus, the effect of deformation processes does not influence our results. The relatively narrow 0.95 confidence intervals of the means obtained for non-deformed spinel and plagioclase harzburgite species and a large difference between the two means indicate a relatively low influence of alteration. Therefore, we believe the significant enrichment in Cu exhibited by the refertilized mantle rocks is caused exclusively by mantle impregnation with late-stage melts. Enhanced Cu concentrations indicate that the scale of this enrichment can be significantly underestimated in previous studies (Lorand et al. 2013).Item Why primary copper enrichment could be expected at the Moho Transition Zone(2015-05-16) Ciazela, Jakub; Botcharnikov, Roman; Dick, Henry; Kuhn, Thomas; Muszynski, AndrzejHighly increased chalcophile element concentrations in harzburgites which underwent interaction with MORB melts in comparison to normal abyssal harzburgites have been observed from the Kane Megamullian oceanic core complex (OCC; Mid-Atlantic Ridge, 23°30’ N; Ciazela et al. 2014). Ciazela et al. 2015 quantified the Cu enrichment based on the bulk rock analyses of plagioclase peridotites and a contact zone between mafic vein and hosting mantle, obtaining a four times higher concentration in the former and a nine times higher concentration in the latter with respect to unaffected spinel harzburgites (36 ppm Cu). Here, we provide a hypothesis for this enrichment, based on the S determination of bulk samples, and the in situ microscopy and electron microprobe (EMPA) analyses of two contact zones between mafic veins and host peridotites. We determined the S concentrations in six mantle samples from the Kane Megamullion oceanic core complex, that interacted with mafic melt. The Cu and S concentrations correlate well (r=0.95) for this set of the samples. This implies that sulfides are the main phases concentrating Cu and probably other chalcophile elements. Moreover, we investigated by in-situ methods two thin sections containing peridotites that exhibit distinct contact zones (8 and 15 mm wide) adjacent to gabbroic veins. By using reflected light microscopy, we have estimated the density of large (>40 µm) sulfides in the contact zones that is ~3.3 grains/cm2, whereas it is only ~0.3 grains/cm2 in the background peridotite. No large sulfide occurs in the gabbro vein. A similar analysis for the medium (10-40 µm) sulfides shows they are more common but similarly distributed, with densities of ~6.0 grains/cm2, ~1.5 grains/cm2 and ~0.7 grains/cm2, respectively. Subsequently, we performed the EMPA mapping of selected areas (1 cm2) crossing the contact zones in both thin sections. Based on S distribution in the given areas, we have discovered that the density of small (<10 µm) sulfides overcomes the density of large and medium sulfides, and the main crystallization front of the sulfides is ~ 3 mm wide, and is located on the margins of the contact zones adjacent to the mafic veins. A similar pattern of sulfide distribution was observed in an experiment performed under high pressure (2 kbar) and high temperature (1150 °C) in an internally heated pressure vessel, using a sulfur-saturated basaltic melt which was filled in an capsule of olivine (olivine from San Carlos; Fo. 90). Most of sulfides (~90%) in the experimental product crystallized at the contact of the basaltic glass adjacent to olivine capsule material. The narrow sulfide crystallization fronts observed in the two thin section represent an example of a small-scale melt/rock interaction at the margins of local melt channels transporting MORB-type melt during its ascent through the lithospheric mantle. However, we suppose that this process may also operate on a broader scale, considering that the Cu concentration in 14 dunites from the Kane Megamullion area is 118 ± 17 ppm Cu (1σ; Ciazela et al. 2014), which is four times higher than that found in the associated spinel harzburgites. Dunites are usually formed due to reaction of mantle and MORB melts during their ascent through the lithosphere. That this processes may be of broader significance is indicated from observations in the Samail ophiolite in the Sultanate Oman. Here, some dunites, preferentially found in the up to several hundred meter thick Moho transitions zone, are associated with ancient copper deposits. At least 12 sites of ancient Cu excavations have been found throughout this zone (Boudier, personal communication). Although Cu has partially been redeposited in secondary processes, the data from our study and the characteristic distribution of the ancient mining sites imply that the first stage of Cu enrichment could be a primary magmatic process. The upcoming SlowMo project gives us a unique opportunity to verify this hypothesis. Ciazela, J., Dick, H., Koepke, J., Kuhn, T., Muszynski, A., & Kubiak., M., 2014. Mantle-crust differentiation of chalcophile elements in the oceanic lithosphere. Abstract V31B-4756 presented at AGU Fall Meeting, San Francisco, Calif., 15-19 Dec. Ciazela, J., Dick, H., Koepke, J., Botcharnikov, R., Kuhn, T. & Muszynski, A., 2015. Cu refertilization of harzburgites by melt percolation. Geophysical Research Abstracts 17, 1044.