Zircon saturation in terrestrial basaltic melts and its geological implications

摘要

Zircon is a widely used accessary mineral, and zircon saturation in silicate melts has many applications in geology. Based on the original model proposed by Watson and Harrison (Earth Planet. Sci. Lett. 64:295–304, 1983), the quantification of the zircon saturation in melts with felsic and intermediate compositions has recently become a major research topic, resulting in some disagreements and different models. Theoretically, the addition of new data, especially regarding the zircon solubility in a basaltic (peralkaline) melt that can provide an upper limit below which zircon is likely to crystallize in igneous rocks, is thus critical to the development of a new, refined model that can be applied to a wide range of compositions and provide a resolution to the ongoing debate. Here, the zircon saturation in a terrestrial basaltic melt was systematically investigated for the first time using a piston-cylinder apparatus across the temperature range of 1050–1350?°C at pressures of 0.5, 1.0 and 1.5?GPa. We combined our new data on mafic melts with data from previous studies on mafic to felsic melts to investigate the factors affecting zircon saturation in basaltic melts. Our results confirm an extremely high zircon saturation in mafic (peralkaline) melts in addition to its strong dependence on the melt composition and temperature and its weak dependence on the pressure and water content. We used all available data that can be used to calculate compositional parameter G [=(3·Al2O3?+?SiO2)/(Na2O?+?K2O?+?CaO?+?MgO?+?FeO), molar ratio] to evaluate fit to a previous model designed to work with more alkaline compositions and proposed a new refined model, given by (with 1σ errors): lnCZr(melt)?=?(3.313?±?0.349)–(1.35?±?0.10)·lnG?+?(0.0065?±?0.0003)·T, whereCZr(melt) is the Zr concentration in the melt at zircon saturation andTis temperature in °C. Additionally, we reintroduced the previously proposed dominance of the temperature and melt composition on the degree of polymerization and thus the ability of zircon to enter the melt, thereby leading to large differences in the zircon saturation between intermediate-felsic magmas and mafic magmas. Although mantle-derived basaltic magmas have a low Zr concentration, they are capable of dissolving the zircon in surrounding rocks during their ascent. Zircons are generally rare in extrusive or hypabyssal basaltic rocks but can form during the late crystallization stage of plutonic basaltic magmas; thus, zircon crystallization has little impact on the distribution of trace elements in mafic magmas.