Trace Element Partitioning in Immiscible Silicate-Carbonate Liquid Systems: an Initial Experimental Study Using a Centrifuge Autoclave

Abstract

The origin of carbonatites remains a contentious topic. However, INTRODUCTION an important role for liquid immiscibility between silicate and Silicate–carbonate liquid immiscibility has often been carbonate liquids has often been proposed. To understand and proposed as a possible mechanism for the origin of constrain the role this process may play, it is important to have carbonatites associated with ultramafic alkaline rocks in trace element partitioning data available. Few experimental studies composite volcanic and subvolcanic complexes. Field on trace element partitioning between silicate and carbonate liquids observations (Ferguson & Currie, 1971; Kjarsgaard & have been undertaken, reflecting both analytical and experimental Peterson, 1991; Church & Jones, 1995) and data on difficulties. To achieve better phase separation new two-liquid melt inclusions in minerals (Rankin & Le Bas, 1974; experiments have been performed utilizing the rotating centrifuge Romanchev & Sokolov, 1979) support the role of liquid autoclave. Trace elements in the run products were analysed in situ immiscibility in carbonatite petrogenesis. Since the piusing laser ablation microprobe–inductively coupled plasma mass oneering work of Koster van Groos & Wyllie (1963) spectrometry. Partition coefficients (D) have been determined for the separation of carbonatitic immiscible liquids from selected rare earth elements (La, Nd, Sm, Tb, Er, Tm), high field carbonated silicate melts has been demonstrated exstrength elements (Zr, Hf, Nb, Ta), and for Sr, Ba and Y. Most perimentally in many synthetic silicate–carbonate systems of the rare earth elements partition preferentially into the silicate and for natural rock compositions [e.g. see review by liquid. La, Sr and Ba, however, strongly partition into the carbonate Kjarsgaard & Hamilton (1989)]. However, there is no liquid. The high field strength elements, although all preferentially consensus about the role of liquid immiscibility in the partitioning into the silicate liquid, are characterized by a wide development of carbonatite magmas (see Gittins, 1989). range of D values. Zr and Hf have similar D values, which are In this connection, quantitative experimental data on one to two orders of magnitude lower than those of Nb, Ta and trace element partitioning between the immiscible liquids Ti. Ti and Nb behave similarly, whereas Ta demonstrates behaviour are important for constraining the origin of carbonatites. intermediate to that of Zr and Hf. Nb/Ta ratios are strongly Experimental studies in silicate–carbonate systems are fractionated by two-liquid partitioning. hampered by the rapid crystallization of carbonate-rich