Materiały konferencyjne (WNGiG)

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The effect of water activity on the eutectic point in natural granitic system
(Mineralogical Society of Poland, 2012-09) Ciazela, Jakub; Bolte, Torsten; Holtz, Francois; Muszynski, Andrzej; Machowiak, Katarzyna
The water activity except for pressure is known to be the most important parameter affecting the eutectic point in granitic systems. Water saturated magmas have lower solidus temperatures than water undersaturated magmas (Johannes, Holtz 1996). Water activity influences also Ab-Or relation in granitic systems (Holtz et al. 1992). Mineral phases present in water saturated conditions differ from those present in water poor conditions (Dall’Agnol et al. 1999). This effect of water activity on the granitic system has so far been analyzed on synthetic materials. Other parameters which would affect the eutectic point might exist in a natural granitic system. Therefore, two natural samples of Wangrah granites from Australia were taken to analyze their crystallization path. They were chosen so they had very similar composition in terms of Qz-Ab-Or, but different in terms of Ca and Fe contents. The homogenous glass powder was used as a starting material. The water was added to reach water saturated conditions. The water and silver oxalate were added to reach water undersaturated conditions. Crystallization experiments with both samples were performed in cold seal pressure vessel (CSPV) in the temperature range of 680-720ºC for water saturated conditions and 765-815ºC for water undersaturated conditions. The pressure of 2 kbar and oxygen buffer NNO were established for 15 days. The phases were identified and glass composition was determined by an electron microprobe. The water content of glass was measured with the use of near infrared spectroscopy. The results were plotted on the Qz-Ab-Or diagram with application of Blundy and Cashman corrections (Blundy, Cashman 2001) for the An content. Two very similar compositions in terms of Qz-Ab-Or show a different position of their cotectic lines for water saturated conditions. One possible explanation of this fact is that the projection of Blundy and Cashman does not account accurately for the effect of Ca. The alternative explanation is that there must be other parameters controlling the position of the cotectic line, one of them may be Fe. In any case, one cannot simply project natural granite or rhyolites on the Qz-Ab-Or diagram to extract quantitative information on pressure or water activity of granites or rhyolites. A new set of experiments should be performed to clarify this problem.
Experimental petrology: a new insight into the deep Earth composition
(Mineralogical Society of Poland, 2012-10) Ciazela, Jakub
Meteorites remain the main data source for inferring deep Earth composition (McDonough 2003; Palme, O’Neill 2003) whereas seismic waves give fundamental information on the structure of our planet (Bina 2003; McDonough 2003). However, the constant development of high pressure methods in recent decades has allowed us to simulate core and mantle conditions on the surface of the Earth. As a consequence, we can examine the mineralogical composition, crystal structure and physical structure of experimental samples of a meteorite composition and compare these features to those which are expected from geophysical and computational data (Li, Fei 2003). Two main approaches are used to reach high pressure conditions. One is maximizing the force and second is minimizing the area. The first approach is based on the multianvil press technology, and pressures up to 26 GPa can at present be achieved in a moment (Frost et al. 2004). The second approach is connected with the use of diamond anvil cells. It allows us to reach pressures greater than 365 GPa (Bassett 2009) which correspond to the pressure in the centre of the Earth. In the presented poster, an explanation of the two methods mentioned above will be followed by a review of experimental results obtained through application of these techniques in the last decade.
Mantle-crust differentiation of chalcophile elements in the oceanic lithosphere
(2014) Ciazela, Jakub; Dick, Henry; Koepke, Juergen; Kuhn, Thomas; Muszynski, Andrzej; Kubiak, Marta
The chalcophile elements, as associated with sulfides, are believed mainly from the study of ophiolites to be generally enriched in the upper mantle, but depleted by magmatic processes in the lower and upper ocean crust. However, studies of some orogenic lherzolites suggest a copper depletion of peridotites in relation to the primitive mantle, suggesting that a portion of the sulfides is melted during decompression and incorporated into the ascending magmas. The rarity of abyssal peridotites and the high degree of their alteration have not allowed these results to be verified in situ in the oceans. Here, we present the first complete study of chalcophile elements based on a suite of rocks from an oceanic core complex (OCC), the Kane Megamullion at 22°30’N at the MidAtlantic Ridge. OCCs provide large exposures of mantle and lower crustal rocks on the seafloor on detachment fault footwalls at slow and ultraslow spreading ridges. The Kane Megamullion is one of the best sampled OCCs in the world, with 1342 rocks from 28 dredge sites and 14 dives. We have made XRF, TDMS and INAA analyses of 129 representative peridotites, gabbroic rocks, diabases and basalts. Our results suggest a depletion of some peridotites in relation to the primitive mantle (28 ppm Cu). Dunites, troctolites and olivine gabbros are relatively enriched in chalcophile elements. The amount of sulfides decreases gradually with progressive differentiation, reaching a minimum in gabbronorites and diabases. The highest bulk abundance of chalcophile elements in our sample suite was observed in dunites (up to ~ 300 ppm Cu in several samples) and a contact zone between residual peridotite and a mafic vein (294 ppm Cu). Plagioclasebearing harzburgites, generally formed by latestage melt impregnation in the mantle, are typically more enriched in Cu than unimpregnated residual peridotites. For these reasons, our initial results indicate sulfide melting during mantle melting, and their local precipitation in the mantle lithosphere due to late-stage melt impregnation.
Cu refertilization of abyssal peridotites by melt percolation
(Copernicus Publications, 2015) Ciazela, Jakub; Dick, Henry; Koepke, Juergen; Botcharnikov, Roman; Muszynski, Andrzej; Kuhn, Thomas
Primitive 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).
Why primary copper enrichment could be expected at the Moho Transition Zone
(2015-05-16) Ciazela, Jakub; Botcharnikov, Roman; Dick, Henry; Kuhn, Thomas; Muszynski, Andrzej
Highly 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.

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