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The Behavior of Orthopyroxene in Carbonatitic Melts Open Access


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Type of item
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University of Alberta
Author or creator
Stone, Rebecca S
Supervisor and department
Luth, Robert (Earth and Atmospheric Science)
Examining committee member and department
Luth, Robert (Earth and Atmospheric Science)
Pearson, Graham (Earth and Atmospheric Science)
Chacko, Tom (Earth and Atmospheric Science)
Department of Earth and Atmospheric Sciences

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Master of Science
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The composition of primary kimberlitic melts has been the subject of much debate because of the heterogeneous nature of kimberlites, their ubiquitous modification by mantle and crustal contamination, their high susceptibility to alteration during and after emplacement, and the fact that the melts do not quench to a glass. Recently Russell et al. (2012) proposed that primary kimberlitic melts are originally carbonatitic. In their model, the melt assimilates orthopyroxene during ascent because of its low silica activity, which causes the melt to evolve to more kimberlitic compositions and triggers the massive exsolution of CO2, which in turn explains the rapid ascent of the kimberlite magma. Russell et al. (2012) supported their model with experiments performed at atmospheric pressures that used Na2CO3 as the model carbonatitic melt. To better simulate the conditions of a rising kimberlite melt in the mantle I investigated the assimilation of orthopyroxene in a variety of carbonatitic melts in the system CaO-MgO-Al2O3-SiO2-CO2 ± H2O at pressures between 2.5 and 6.0 GPa. These melts had been determined to be in equilibrium with lherzolite assemblages (Opx + Cpx + Ol ± Grt) at pressures of 6, 6.9, 10, and 16.5 GPa in previous studies (Dalton and Presnall, 1998; Girnis et al., 2011; Keshav and Gudfinnsson, 2014; Ghosh et al., 2014). At pressures between 4 GPa and 6 GPa, orthopyroxene remained in equilibrium with the carbonatitic melt with no signs of assimilation into the melt, nor were there any signs of CO2 exsolution. At 2.5 GPa the experiments show that carbonatitic melt and orthopyroxene reacted to form olivine and diopside, exsolving CO2. The formation of clinopyroxene in this orthopyroxene assimilation reaction, but its absence in kimberlite at the surface, can be explained by a second decarbonation reaction at lower pressure where clinopyroxene reacts with a carbonatitic melt to form more forsterite and additional CO2 exsolution further driving kimberlite ascent. This reaction could also cause the evolution of the melt towards more calcitic compositions or the crystallization of calcite, explaining why calcite rather than dolomite is the common groundmass mineral in kimberlite. Xenocrystic clinopyroxene could also contribute to this reaction, making it an important secondary mineral for kimberlite melt evolution. In addition to the assimilation experiments, a number of orthopyroxene and xenolith assimilation calculations using the same carbonatitic melts in our experiments were done in order to determine whether carbonatites can indeed evolve to kimberlites via mantle contamination. Even at 50 wt.% mantle contamination none of the melts reach kimberlite compositions by orthopyroxene assimilation. However, by adding clinopyroxene and garnet to the assimilation calculation, the Girnis et al. (2011) composition which lies more towards a transitional melt composition (intermediate between that of a carbonatite and a kimberlite) can evolve to kimberlitic melts. It is also possible through extensive orthopyroxene assimilation, fuelled not only by bulk mantle contamination but by wallrock reactions as well, that a range of carbonatitic melts can evolve to kimberlite compositions. In supplementary calculations (Appendix 5), the possible correlation of kimberlite composition to the orthopyroxene content of the lithospheric mantle was explored. The results show that weak correlation exists between the compositions of kimberlites and the petrology of xenoliths at the craton scale, but no correlation is present when compared on the scale of individual kimberlite fields. Future work must look at the contribution of other mantle minerals in addition to orthopyroxene in more compositionally complex systems to fully understand how kimberlites form.
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