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The role of carbonated eclogite in kimberlite and carbonatite petrogenesis

Ekaterina S. Kiseeva1, Gregory M. Yaxley1 and Vadim S. Kamenetsky2

1 Research School of Earth Sciences, Australian National University , Canberra , ACT 0200, Australia
2 CODES and School of Earth Sciences, University of Tasmania, Hobart, TAS, Australia

 

The exotic and rare rock types - kimberlites and carbonatites are undoubtedly of mantle origin and are sometimes considered to be genetically related, but the compositions of their parental melts and melting conditions are still widely debated. A component in the source of kimberlitic and carbonatitic melts may be carbonate-bearing eclogite, reflecting heterogeneity in the mantle derived from recycling during subduction of oceanic crust.


Backscattered electron image of GA+10%cc 1350°C 3.5 GPa (A, B) and 1200°C, 3.5 GPa (C, D) run material. A. Melt (left) and solid phase (right). B. Magnified view of part of the solid material in A, showing clinopyroxene (Cpx) and garnet (Grt) crystals. C. Solid phases represented by Grt and Cpx crystals. D. Magnified area of melt distributed within the capsule. Melt is in contact with residue.

The aim of this study is to use high pressure experimental petrology to investigate the behaviour of carbonate-bearing eclogite in upper-mantle conditions and to test its possible involvement in the mantle sources of kimberlite and related magmas. The first part of our investigation aims to locate the solidus positions and partial melt compositions as functions of bulk compositional parameters such as SiO2 and CO2 contents, Ca/Mg, Na2O/CO2, etc.

Altered oceanic crust typically contains a few % calcite, formed during hydrothermal alteration. The starting experimental composition (GA1) is an average "altered" MORB composition. Ten% CaCO3 was added to GA1 in the experiments to facilitate detection of carbonate in the runs. The second mix, Volga, is GA1 minus 6.5% SiO2, to which 10% CaCO3 was also added (Volga+10‰cc). Experiments were run at 3.5 to 5.5 GPa and 1000-1400°C in piston-cylinder presses at RSES. The run results were analyzed with a HITACHI 4300 SE/N FESEM and JEOL 6400 at the ANU Electron Microscopy Unit, using EDS detectors for quantitative analysis of mineral phases.

Experimental runs after quenching contained the three main phases: garnet, clinopyroxene and melt (Fig. 1, A, B), and sometimes various accessory phases such as K-feldspar, rutile, coesite and carbonate.

Several types of melt were observed in our experiments. A large fraction (>30%) of silicate melt is present in higher temperature runs (T≥1250 °C). In these cases silicate melt segregated to a pool at one end of the capsule (Fig. 1, A). The totals are about 88-92%, suggesting 8-12% CO2 dissolved in the melt. At T = 1150 to 1250°C and 3.5 GPa in GA1+10%cc and 1100 to 1200°C in Volga+10%cc experiments tiny particles of incompatible-element rich material (ie K-rich and P-rich) are distributed throughout much of the graphite capsules (Fig. 1, C, D) which was often vesiculated or fragmented, indicating decarbonation of a silicate-carbonate melt during quenching. Capsule piercing of some of these runs into a gas chromatograph detected significant CO2-fluid. With decreasing temperatures at high pressures (4.5 and 5 GPa) carbonate-silicate melts formed small pools of melt within the graphite spheres. Interestingly, at near-solidus conditions in all the investigated pressure intervals two immiscible carbonate and silicate melts are formed. Potassium usually fractionates into the silicate melt, while phosphorous prefers the carbonate.

In sub-solidus conditions in GA1 + 10‰cc 1050°C and 3.5 GPa experiment apatite (Ap), potassium feldspar (K-Fspar), rutile (Ru) and carbonate were observed. At higher pressures potassium feldspar as well as apatite is no more stable and K and P presumably fractionate into the fluid. GA1 + 10%cc solidus at 3.5 GPa is at about 1075°C, Volga + 10%cc solidus is at least 50°C lower at all the studied pressures.


A. Results of experiments with GA1+10%cc. Approximate solidus is shown by the red dashed line.
B. Phase proportions for GA1+10%cc at 3.5 GPa pressure. The sole liquidus phase is Cpx.

Carbonate-rich melts formed from a low degree of eclogite melting will infiltrate neighbouring peridotite, resulting in metasomatism and refertilization. Our next aim is to investigate how these melts would interact with peridotite and compare the outcomes with melt compositions that may be parental to kimberlites.