Layered rhyolite bands in a thick North Mountain Basalt flow: the products of silicate liquid immiscibility?

Greenough, John D.; Dostál, J.

doi:10.1180/minmag.1992.056.384.02

Cited by 17 publications

(12 citation statements)

References 32 publications

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“…While silicic and mafic magmas are clearly miscible at temperatures above their liquidi [e.g., Kouchi and Sunagawa, 1985], the miscibility of such liquids at subliquidus temperatures is not well constrained. However, we note that plausible cases of immiscibility between mafic and silicic magmas have been observed both in the field [Philpotts, 1971[Philpotts, , 1972Bender et al, 1982;Greenough and Dostal, 1992] and in the laboratory under sub-liquidus conditions [Martin and Kushiro, 1991]. The observation of immiscibility is consistent with the enthalpy of mixing of basalt and dacitic melts being positive under hydrous conditions [Russell, 1990].…”

Section: Basic Methodssupporting

confidence: 76%

Microgranitic enclave shapes and magmatic strain histories: Constraints from drop deformation theory

Williams¹,

Tobisch²

1994

J. Geophys. Res.

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Microgranitic (mafic) enclaves are common components of plutonic bodies and may represent the result of mingling of mafic and felsic magmas. These enclaves are often ellipsoidal or elongate in shape. In order to constrain the conditions under which such enclaves were generated and deformed by their host pluton, we apply drop deformation theory to model their distortion. Our model takes into account the change in viscosity of both the host and enclave with temperature, composition and the amount of suspended solids in enclave/host systems, and it assumes that simple shear flow predominates in the plutonic system. We focus on the deformational history of enclaves over the time period in which they remain liquid and are either more or less viscous than their granitic host. We find that the most stable enclaves with respect to being broken up by their entraining flow are generally the most mafic enclaves: those with silica contents near those of the host granite are highly unstable. This is in reasonable accord with field observations of the abundance of enclaves of different chemistry. Most deformation of microgranitic enclaves is likely to occur at relatively high temperatures, probably between 950°C and 1050°C, and therefore, the enclaves record magmatic strain of the host over only a limited temperature‐time range in the host's cooling history. Furthermore, the observation of apparent deformation of these enclaves in a liquid regime coupled with our calculated dependences of enclave deformation on flow velocity implies that magmatic flow velocities are likely to be below 10 m/yr in enclave‐bearing plutonic systems.

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Section: Basic Methodssupporting

confidence: 76%

Microgranitic enclave shapes and magmatic strain histories: Constraints from drop deformation theory

Williams¹,

Tobisch²

1994

J. Geophys. Res.

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“…Similar textures have been recognized by other authors (e.g. Kontak et al, 2002) and immiscible thin fine-grained rhyolite bands have even been shown to have segregated from differentiated layers of lavas (Greenough and Dostal, 1992).…”

Section: Volcanic Provincessupporting

confidence: 86%

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

Charlier¹,

Namur²,

Grove³

2013

Geochimica et Cosmochimica Acta

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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.

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“…Many thick flood-basalt flows [1][2][3][4][5][6][7] and lava lakes [8][9][10][11] contain horizontal sheets of coarse-grained rock whose composition corresponds to liquid formed by approximately one-third fractional crystallization of the host basalt. Several mechanisms have been proposed for separating this liquid from the basalt, one of which involves expulsion of liquid from a crystal mush that undergoes compaction in the lower part of the flow 7 .…”

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confidence: 99%

Role of plagioclase crystal chains in the differentiation of partly crystallized basaltic magma

Philpotts

Shi

Brustman

1998

Nature

170

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Melting experiments on samples of basaltic rock from a thick lava flow reveal that when this lava originally began to crystallize, feldspar crystals linked together to form a continuous three-dimensional network of chains when the lava was no more than 25% crystallized. Formation of this network has profound implications for the behaviour and differentiation of basaltic magma. Much of the compositional variation of igneous rocks results from the separation of liquid from crystals, a process that is dramatically affected according to whether crystals occur separately or are linked together in networks.Many thick flood-basalt flows 1-7 and lava lakes 8-11 contain horizontal sheets of coarse-grained rock whose composition corresponds to liquid formed by approximately one-third fractional crystallization of the host basalt. Several mechanisms have been proposed for separating this liquid from the basalt, one of which involves expulsion of liquid from a crystal mush that undergoes compaction in the lower part of the flow 7 . Compaction can occur only after crystals interconnect to form a network but before the network grows too strong to be deformed. Previous partial melting experiments 12 on the thick Jurassic Holyoke flood basalt of Connecticut and Massachusetts indicate that such a network forms when this basalt is only 25% crystallized.Here we present the results of experiments that indicate that the crystal mush in the Holyoke basalt owed its strength to the presence of a remarkable three-dimensional (3D) network of plagioclase crystal chains. We have determined the shape of this network from 3D models constructed from serial sections through partially melted samples of the basalt. The deformation of the network that resulted from compaction during solidification of the lava can be measured from the chain distribution in the serial sections. These measurements provide a direct means of determining the amount of compaction that has occurred during the solidification of an igneous body. Partial melting experimentsSamples of the Holyoke basalt have been partially melted so that the textural relations between the refractory minerals involved in compaction could be studied. By partly melting and then rapidly quenching the samples, the low-melting, late-crystallizing minerals are converted to glass in which the refractory minerals are embedded. One-centimetre cubes of the rock were heated in graphite crucibles under a reducing atmosphere. Graphite is not wetted by silicate melts and therefore does not wick away the liquid from the partially melted rock. The furnace atmosphere was not controlled precisely, but the cores of the rock cubes remained close to their intrinsic oxygen fugacity (quartz-fayalite-magnetite buffer) for the duration of the experiments (Ͻ24 hours).Previous melting experiments 12 reveal that cubes of basalt from the central (slowly cooled) part of this flow retain their cubic shape when at least 70% melted, which indicates that a continuous network of crystals still exists at this high degree of melt...

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Layered rhyolite bands in a thick North Mountain Basalt flow: the products of silicate liquid immiscibility?

Cited by 17 publications

References 32 publications

Microgranitic enclave shapes and magmatic strain histories: Constraints from drop deformation theory

Microgranitic enclave shapes and magmatic strain histories: Constraints from drop deformation theory

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

Role of plagioclase crystal chains in the differentiation of partly crystallized basaltic magma

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