The role of amphibole in crust-mantle magma differentiation: its trace-element signatures and the halogen effect

Abstract

Amphibole is an important hydrous mineral involved in lithospheric processes and plays a crucial role in magma differentiation. Amphibole-melt trace element partition coefficients have been studied for over 60 years. This thesis: (1) introduces the results of a new method for plotting rare earth element (REE) patterns, which allows distinguishing between amphibole and garnet (2) examines the effects of minor elements on trace element partitioning between amphibole and silicate melt and (3) explores its applications to amphiboles from REE-bearing carbonatites. The first part focuses on distinguishing between the effects of garnet vs amphibole fractionation. The garnet signature observed in the REE abundances of adakites has been considered a key genetic indicator of these controversial rocks, whose proposed origins include direct melting of subducted oceanic crust ("slab melts"). The present study shows that the garnet signature may be quantified using the shape coefficients of chondrite-normalized REE patterns. This method is applied to a global dataset of volcanic samples described as "adakites". The results reveal that many, but not all, suites of rocks labelled as adakites have undergone fractional crystallization of garnet, starting from parental melts attributable to partial melts of garnet-bearing sources. The extreme garnet signatures seen in many examples require hybrid sources, consisting of subducted sediment as well as igneous oceanic crust. However, the extensive deep-crustal differentiation implied by garnet fractionation obscures the major and trace-element characteristics of these sources, casting doubt on their identification as primitive slab melts. In the second part, the effects of F on REE partition coefficients between synthetic amphibole, clinopyroxene and melt with variable F contents has been determined at 10 kbar in the system Na2O-(K2O)-Al2O3-CaO-(TiO2)-MgO-SiO2-(OH, F, Cl) for F in the melt from 0 to ~7 wt.%. Experiments at 950 celsius degree crystallized amphibole (OH, F), and in some cases contained clinopyroxene, orthopyroxene, olivine and accessory titanite or rutile coexisting with melt, while only fluoro-amphibole, clinopyroxene and melt were found at 1015 and 1050 celsius degree. There is a strong negative correlation between F and REEs in amphibole. The effect of F can be ascribed to REE-F bonding stabilizing REE in the melt rather than to the F/OH in the pargasite since a similar effect is observed in the clinopyroxene/melt partition coefficients. This implies that F would influence mineral/melt REE partition coefficients generally. The third part explores correlations between F and REE in natural amphiboles from carbonatites including global data collection and new data from Mt Weld, one of the world's largest carbonatite-associated REE deposits. Two distinct groups of amphiboles were identified in fresh carbonatite. One group, referred to as "Fe-amphibole", is found within ferro-carbonatite, while the other group, the "Ca-amphiboles", correspond to calcite carbonatite. These amphibole compositions align with the characteristics of the host carbonatite. Fe-amphiboles with higher REE contents contain lower F compared with Ca-amphibole. It is important to note that the lower fluorine contents of Fe-amphiboles do not necessarily imply a low fluorine system. This is evident from the high fluorine contents observed in coexisting phlogopite. These results indicate that amphibole and phlogopite together can potentially provide indications on formation conditions of host carbonatites. Furthermore, certain geochemical signatures of amphibole and phlogopite have potential to assist in targeting carbonatite associated REE-deposits. Show more