Ilya V. Veksler

Immiscible iron- and silica-rich melts in basalt petrogenesis documented in the Skaergaard intrusion

Silicate liquid immiscibility in basalt petrogenesis is a contentious issue. Immiscible iron- and silica-rich liquids were reported in melt inclusions of lunar basalt and in groundmass glasses of terrestrial volcanics. In fully crystallized plutonic rocks, however, silicate liquid immiscibility has yet to be proven. Here we report the first finding of natural, immiscible iron- and silica-rich melts in a plutonic environment documented in the Skaergaard intrusion, East Greenland. Primary melt inclusions (now finely crystallized) in apatite are either dark or light colored. The predominant dark colored type contains 30.9 ± 4.2 wt% FeO t and 40.7 ± 3.6 wt% SiO 2 , whereas the light colored type contains 8.6 ± 5.9 wt% FeO t and 65.6 ± 7.3 wt% SiO 2 . Similar light colored melt inclusions in olivine and fine-grained dark and light colored interstitial pockets also give evidence of crystallization from emulsion of silica and iron-rich liquids. On the outcrop scale, silica-rich (melanogranophyre) pods and layers in iron-rich ferrodiorite of the Upper Zone of the Skaergaard intrusion witness segregation of the two liquids. These findings demand that silicate immiscibility is considered in basalt petrogenesis. Some granitic rocks may represent unmixed silica-rich melt, whereas the dense, iron-rich melt is likely to sink in the crust and could mix with hot mantle-derived magma to form unusual rocks, like ferropicrites, otherwise interpreted as products of heterogeneous mantle sources.

Experimental evidence of three coexisting immiscible fluids in synthetic granitic pegmatite

Abstract We present an experimental study of synthetic peraluminous granite doped with H2O, B, P, and F, which confirms that aluminosilicate melt, hydrous fluid, and hydrosaline melt (high-temperature brine) can stably coexist at 450-900 ∞C and 0.1-0.2 GPa in bulk compositions similar to those of natural granitic pegmatites. Hydrosaline melt is not quenchable, unstable at room conditions, and requires special techniques for synthesis and preservation. Raman spectroscopy and electron microprobe analyses of hydrosaline melt synthesized in our experiments show that it is composed of H3BO3, Na3AlF6, AlPO4, H2O, and aluminosilicate components. Aluminosilicate melt saturated in both hydrosaline liquid and hydrous fluid at 850 °C and 0.2 GPa contains 3.6 wt% F, 4.2 wt% P2O5, and 4 wt% B2O3. Natural hydrosaline melts have previously been found as inclusions trapped in rock-forming minerals. They are not restricted to granites and can be effective agents for enhanced crystal growth, metasomatism, and ore formation. In addition, hydrosaline melts may account for many characteristic features of rare-element and miarolitic pegmatites, such as giant size and perfect shapes of crystals in pegmatite cores, diverse mineralogy, and strong enrichment in rare elements.

On the significance of ultra-magnesian olivines in basaltic rocks

Karoo LIP in Antarctica (Heinonen and Luttinen, 2010), and the Parana-Etendeka LIP in northwest Namibia (Thompson and Gibson, 2000), the subject of this paper. Shared features of ultra-magnesian olivines in these provinces are that they are rare, occurring in just a few localities, and they are too Mg rich to be considered equilibrium phenocrysts in the basaltic host rocks. It is signifi cant that these olivines coexist in the same hand specimens with much more abundant, less magnesian crystals (Fo 85‐90 ), the compositions of which are close to equilibrium with the Mg# of the host rocks, and can therefore be considered as phenocrysts. It follows that the ultra-magnesian olivines are not phenocrysts in the present host, but represent xenocrysts. The high CaO contents in these olivines clearly distinguish them from entrained crystals of mantle olivine, and most studies have concluded that the ultra-magnesian crystals originated from an unerupted komatiitic melt and were incorporated into a second, more evolved magma. We emphasize that this conclusion is based only on olivine and bulk-rock compositions. However, olivine compositions constrain only the Mg# [MgO/(MgO + FeO) in molar percent] of the equilibrium liquid and not its MgO concentration, on which the inference of a komatiitic affi nity and high temperature of the parental melts critically depend. To calculate the melt MgO requires information on its FeO content. This study presents an example where original melt compositions are recovered directly by analysis of inclusions trapped in the ultra-magnesian olivines. This removes the dependence of the results on the bulk rock composition and

Igneous Layering in Basaltic Magma Chambers

Layering is a common feature in mafic and ultramafic layered intrusions and generally consists of a succession of layers characterized by contrasted mineral modes and/or mineral textures, including grain size and orientation and, locally, changing mineral compositions. The morphology of the layers is commonly planar, but more complicated shapes are observed in some layered intrusions. Layering displays various characteristics in terms of layer thickness, homogeneity, lateral continuity, stratigraphic cyclicity, and the sharpness of their contacts with surrounding layers. It also often has similarities with sedimentary structures such as cross-bedding, trough structures or layer termination. It is now accepted that basaltic magma chambers mostly crystallize in situ in slightly undercooled boundary layers formed at the margins of the chamber. As a consequence, most known existing layering cannot be ascribed to a simple crystal settling process. Based on detailed field relationships, geochemical analyses as well as theoretical and experimental studies, other potential mechanisms have been proposed in the literature to explain the formation of layered igneous rocks. In this study, we review important mechanisms for the formation of layering, which we classify into dynamic and non-dynamic layer-forming processes.

Crystallization of AlPO4-SiO2 solid solutions from granitic melt and implications for P-rich melt inclusions in pegmatitic quartz

Abstract Aluminum orthoposphate (AlPO4) has polymorphs isostructural with tridymite, cristobalite, and quartz. Berlinite is the low-temperature form that corresponds to α-quartz. We report berlinite-quartz solid solutions to crystallize from a synthetic P-rich peraluminous granitic melt, similar in composition to the most volatile-rich silicate melt inclusions found in pegmatites. The crystallization took place in experiments performed in cold-seal pressure vessels at 450-700 °C and 0.1-0.2 GPa H2O pressure. At these conditions, the berlinite-quartz mutual solubility is limited to 5-7 mol% SiO2 on the phosphate side of the solvus and to the maximum of 1 mol% AlPO4 on the silica-rich side. The mutual solubility appears to decrease with falling temperature. At low T the crystals of berlinitequartz solid solutions are strongly zoned and show complex intergrowths between the P-rich and silica-rich phases. They were studied by electron microprobe, transmission electron microscopy, and Raman spectroscopy. In the light of our new experimental results, the extreme P enrichment reported earlier for some natural quartz-hosted melt inclusions may be explained as a post-entrapment contamination by the berlinite-bearing host.

Element partitioning between immiscible borosilicate liquids: A high-temperature centrifuge study

Abstract The main objectives of this study were to investigate conditions for stable and metastable liquid immiscibility in dry borosilicate synthetic systems and to evaluate effects of temperature and bulk melt composition on two-liquid element partitioning and boron speciation. To distinguish between the stable immiscibility and quench heterogeneity, we used high-temperature centrifuge phase separation. For the case of stable liquid immiscibility, silica-rich (LS) and borate-rich (LB) conjugate liquids formed two distinct layers separated by a sharp meniscus. The liquids were quenched into glasses, which were analysed by electron microprobe. Some of the glasses were also studied by Raman spectroscopy. We used several synthetic mixtures along the danburite-anorthite (CaB 2 Si 2 O 8 -CaAl 2 Si 2 O 8 ) and danburite-reedmergnerite (CaB 2 Si 2 O 8 -NaBSi 3 O 8 ) joins. In addition, we studied four complex, six-component, Mg-bearing compositions with variable Na 2 O and Al 2 O 3 contents. The experiments show that the width of the LS-LB miscibility gap decreases more rapidly with the B-Al substitution (in the danburite-anorthite join) than with the Ca-Na substitution, implying that interactions between network-forming elements have a greater effect on borate-silicate unmixing than the nature of network-modifying cations. Ca and Mg partition strongly to the depolymerised borate-rich liquid with LB-LS partition coefficients of ∼40 and higher. On the other hand, two-liquid partition coefficients of Na and Al in most cases are close to 1 and show complex variations with temperature and bulk melt composition. Raman spectra of LB glasses quenched at different temperatures suggest that the proportion of trigonal boron in bulk boron content decreases with decreasing temperature. The change in boron speciation appears to affect Al and Na two-liquid partitioning in such a way that at low temperatures, the latter element becomes more compatible with LS.

Interfacial tension between immiscible liquids in the system K2O-FeO-Fe2O3-Al2O3-SiO2 and implications for the kinetics of silicate melt unmixing

Abstract Interfacial tension between immiscible liquids is an important thermodynamic parameter of silicate melt unmixing and a property that determines the kinetics of phase separation. In this study, we present experimental measurements of interfacial tension between immiscible Fe-rich and silica-rich melts in the system K2O-FeO-Fe2O3-Al2O3-SiO2. We have also measured densities and surface tensions of the individual immiscible liquid phases. The measurements were carried out in air at 1500-1550 °C by the maximum detachment force method employing vertical cylinder geometry and using a gravimetric balance system. We have chosen the most oxidized and contrasting liquid compositions containing 73 and 17 wt% SiO2 and 14 and 80 wt% FeOt, respectively, that have been shown to coexist in air at and above 1465 °C. Interfacial tension between the synthetic immiscible liquids decreases with increasing temperature from 16.4 ± 3.1 mN/m at 1500 °C to 7.8 ± 1.1 mN/m at 1550 °C. Interfacial tension between natural, less compositionally contrasting ferrobasaltic and rhyolitic melts should be even lower by a factor of 2 or 3. Very low interfacial tension implies easy nucleation of immiscible liquid droplets and very slow coarsening of resulting silicate emulsions.

Silicate Liquid Immiscibility in Layered Intrusions

More and more evidence for the development of silicate liquid immiscibility during cooling of magmas in layered intrusions have been presented. Here, we review some theoretical principles with a focus on the separation of two silicate melts, i.e. silica-rich vs. iron-rich. We discuss the role of melt structure and present phase equilibria relevant to stable and metastable immiscibility. The understanding of immiscibility in magmas has strongly benefited from recent progress in experimental approaches. Kinetics studies evidence the importance of nucleation barriers in producing unmixing , coarsening and potential separation of equilibrium melts. Improvement of analytical tools has also enabled detailed study of major and trace element partitioning. The study of immiscible emulsion in volcanic rocks also brings important information on the evolution of plutonic systems and on the potential formation of compositional gap along liquid lines of descent. We then present the most recent evidence for immiscibility in some major layered intrusions, i.e. the Skaergaard, Sept Iles, intrusions of the Emeishan province, and the Bushveld complex. Paired melts are identified as contrasted melt inclusions trapped in apatite and their segregation can be responsible for the formation of Fe–Ti–P-rich rocks. We finally discuss more broadly the potential effect of immiscibility in interstitial melt and the implications on the evolution of the crystal mush.

A fundamental dispute: A discussion of "On some fundamentals of igneous petrology" by Bruce D. Marsh, Contributions to Mineralogy and Petrology (2013) 166: 665-690

Marsh (Contrib Miner Petrol 166:665–690, 2013) again claims that crystal-free basalt magmas are unable to differentiate in crustal magma chambers and regards layered intrusions as primarily due to the repeated emplacement of crystal suspensions. He ignores an earlier critique of his unconventional inferences (Latypov, J Petrol 50:1047–1069, 2009) as well as a wealth of petrographic, geochemical and experimental evidence supporting the dominant role of fractional crystallization in the solidification of layered intrusions. Most tellingly, the cryptic variations preserved in the Skaergaard and many other basaltic layered intrusions would require an exceedingly implausible sequence of phenocrystic magmas but are wholly consistent with in situ fractional crystallization. A major flaw in Marsh’s hypothesis is that it dismisses progressive fractional crystallization within any magma chamber and hence prohibits the formation of crystal slurries with phenocrysts and melts that change systematically in composition in any feeder system. This inherent attribute of the hypothesis excludes the formation of layered intrusions anywhere.

Experimental confirmation of high-temperature silicate liquid immiscibility in multicomponent ferrobasaltic systems

Abstract Here we report the results of an experimental study aimed at testing the existence of stable, superliquidus immiscibility between silica- and Fe-rich multicomponent melts at temperatures above 1100 °C. Four pairs of the potentially immiscible compositions were tested in a 1-atm gas-mixing furnace (Ar/H2-CO2 gas mixture) at 1150 and 1200 °C and at the oxygen fugacity corresponding to that of the QFM buffer. Pre-synthesized pairs of the silica-rich and Fe-rich starting compositions were loaded in Pt wire loops, fused separately at 1300 °C, then brought in contact and kept at constant experimental temperature for more than 24 h. Three pairs of compositions out of four used in this study did not mix. Some temperature-dependent chemical re-equilibration was observed in the Fe-rich liquid phase but, in the cases of immiscibility, the two liquids remained compositionally distinct and showed sharp compositional gradients at contacts. One pair of liquids crystallized some tridymite, whereas the other compositions were clearly above the liquidus. Overall, the results of the experiments are in good agreement with the earlier centrifuge study and confirm the existence of stable, super-liquidus immiscibility in some Fe-rich basaltic-andesitic compositions at temperatures up to 1200 °C.