Compositional and kinetic controls on liquid immiscibility in ferrobasalt–rhyolite volcanic and plutonic series

Abstract

We present major element compositions of basalts and their differentiation products for some major tholeiitic series. The dry, low-pressure liquid lines of descent are shown to approach or intersect the experimentally-defined compositional space of silicate liquid immiscibility. Ferrobasalt-rhyolite unmixing along tholeiitic trends in both volcanic and plutonic environments is supported by worldwide occurrence of immiscible globules in the mesostasis of erupted basalts, unmixed melt inclusions in cumulus phases of major layered intrusions such as Skaergaard and Sept Iles, and oxide-rich ferrogabbros closely associated with plagiogranites in the lower oceanic crust. Liquid immiscibility is promoted by low-pressure, anhydrous fractional crystallization that drives the low Al 2 O 3 , high FeO liquids into the two-liquid field. Kinetic controls can be important in the development of two-liquid separation. The undercooling that occurs at the slow cooling rates of plutonic environments promotes early development of liquid immiscibility at higher temperature. In contrast rapid cooling in erupted lavas leads to large undercoolings and liquid immiscibility develops at significantly lower temperatures. Unmixing leads to the development of a compositional gap characterized by the absence of intermediate compositions, a feature of many tholeiitic provinces. The compositions of experimental unmixed silica-rich melts coincide with those of natural rhyolites and plagiogranites with high FeO tot and low Al 2 O 3 , suggesting the potential role of large-scale separation of immiscible Si-rich liquid in the petrogenesis of late-stage residual melts. No trace of the paired ferrobasaltic melt is found in volcanic environments because of its uneruptable characteristics. Instead, Fe-Ti ± P-rich gabbros are the cumulate products of immiscible Fe-rich melts in plutonic settings. The immiscibility process may be difficult to identify because both melts crystallize the same phases with the same compositions. The two liquids might form incompletely segregated emulsions so that both liquids continue to exchange as they crystallize and remain in equilibrium. Even if segregated, both melts evolve on the binodal surface and exsolve continuously with decreasing temperature. The two liquids do not differentiate independently and keep crystallizing the same phases with differentiation. Further evolution by fractional crystallization potentially drives the bulk liquid out of the twoliquid field so that very late-stage liquids could evolve into the single melt phase stability field.

Fe–Ti–V–P ore deposits associated with Proterozoic massif-type anorthosites and related rocks

“…Immiscibility between two silicate melts (ferrobasalt-rhyolite) has been recognized experimentally and in natural systems as a potential differentiation mechanism in evolved basaltic magmas (Roedder, 1978;Philpotts, 1982;Charlier and Grove, 2012;Charlier et al, 2013). Philpotts (1967) proposed that immiscibility is responsible for the origin of certain Fe-Ti oxide and apatite rocks.…”

Trace element compositions of apatite from the middle zone of the Panzhihua layered intrusion, SW China: Insights into the differentiation of a P- and Si-rich melt

“…8b), indicating that P could be highly under-saturated in the Fe-rich melt at high temperature. It is suggested that liquid immiscibility can take place at high temperature in plutonic environments on slow cooling, or at lower temperatures in lavas on fast cooling (Charlier et al, 2013), and liquid immiscibility is easier to be induced in turbulent rather than static environments (Veksler et al, 2007). The parental high-Ti basaltic magmas of the Panzhihua intrusion have whole-rock compositions plotting in the immiscibility field on the pseudo-ternary diagram of the silica-fayalite-leucite system ( Fig.…”

Magma chamber–scale liquid immiscibility in the Siberian Traps represented by melt pools in native iron

“…This process results in major petro-and geochemical differentiation, because the compositional divergence between unmixed phases can be extreme (e.g., Charlier et al, 2013;Kamenetsky and Kamenetsky, 2010;Roedder, 1992;Thompson et al, 2007;Veksler et al, 2007). Even though the physical amounts of one of the immiscible phases may be small, its separation and transport can be important, especially with respect to formation of some rock types and orthomagmatic ore deposits (e.g., Charlier et al, 2013;De, 1974;Philpotts, 1967;Roedder, 1992;Thompson et al, 2007;Veksler, 2004;Veksler et al, 2007). For example, volumetrically signifi cant immiscibility between high-Si and Fe-Ti-Ca-P-rich conjugate melts has been suggested to explain compositional bimodality of some intrusive layered rocks (Charlier et al, 2011(Charlier et al, , 2013 and the origin of certain magnetite-apatite deposits (Philpotts, 1967).…”