Origin and ascent history of unusually crystal-rich alkaline basaltic magmas from the western Pannonian Basin

Abstract

The last eruptions of the monogenetic Bakony-Balaton Highland Volcanic Field (western Pannonian Basin, Hungary) produced unusually crystal- and xenolith-rich alkaline basalts which are unique among the alkaline basalts of the Carpathian–Pannonian Region. Similar alkaline basalts are only rarely known in other volcanic fields of the world. These special basaltic magmas fed the eruptions of two closely located volcanic centres: the Bondoró-hegy and the Füzes-tó scoria cone. Their uncommon enrichment in diverse crystals produced unique rock textures and modified original magma compositions (13.1–14.2 wt.% MgO, 459–657 ppm Cr, and 455–564 ppm Ni contents). Detailed mineral-scale textural and chemical analyses revealed that the Bondoró-hegy and Füzes-tó alkaline basaltic magmas have a complex ascent history, and that most of their minerals (∼30 vol.% of the rocks) represent foreign crystals derived from different levels of the underlying lithosphere. The most abundant xenocrysts, olivine, orthopyroxene, clinopyroxene, and spinel, were incorporated from different regions and rock types of the subcontinental lithospheric mantle. Megacrysts of clinopyroxene and spinel could have originated from pegmatitic veins/sills which probably represent magmas crystallized near the crust–mantle boundary. Green clinopyroxene xenocrysts could have been derived from lower crustal mafic granulites. Minerals that crystallized in situ from the alkaline basaltic melts (olivine with Cr-spinel inclusions, clinopyroxene, plagioclase, and Fe–Ti oxides) are only represented by microphenocrysts and overgrowths on the foreign crystals. The vast amount of peridotitic (most common) and mafic granulitic materials indicates a highly effective interaction between the ascending magmas and wall rocks at lithospheric mantle and lower crustal levels. However, fragments from the middle and upper crust are absent from the studied basalts, suggesting a change in the style (and possibly rate) of magma ascent in the crust. These xenocryst- and xenolith-rich basalts yield divers tools for estimating magma ascent rate that is important for hazard forecasting in monogenetic volcanic fields. According to the estimated ascent rates, the Bondoró-hegy and Füzes-tó alkaline basaltic magmas could have reached the surface within hours to few days, similarly to the estimates for other eruptive centres in the Pannonian Basin which were fed by “normal” (crystal and xenoliths poor) alkaline basalts.

Appendix: magma ascent rate estimates

• 1. Modeling—fluid filled crack-propagation velocity:

  Ascent rate for alkaline basaltic magmas in general: we followed the method that Mattsson (2012) performed for melilititic magmas. Equation (8) in Sparks et al. (2006) was used, which is based on fluid filled crack propagation under turbulent conditions and assumes that magma ascending in dykes is mainly buoyancy driven (Lister and Kerr 1991). The equation contains the half-width of the feeder dyke (w) and the density contrast between the magma and the wall rock. According to Sparks et al. (2006), a dyke width (2w) of 1 m is comparable to those observed in kimberlite and basalt dykes. We studied the effects of variations in ∆ρ (ascending magma propagates through different wall rocks characterized by different densities, so ∆ρ changes during ascent) and in 2w on ascent rates. Therefore, we made three presumed scenarios using different ∆ρ (100, 200 and 300 kg/m³) and 2w values (0.5, 1 and 1.5 m) (Appendix Fig. 12a). The magma viscosity was assumed as 5.5 Pa s.

• 2. Modeling—xenolith settling rate:

  According to Sparks et al. (1977), the presence of xenoliths in alkaline basalts is due not to their higher ascent rates compared with other basaltic types, but instead due to their higher yield strengths and, thus, ability to suspend solids. Spera (1984) did not agree with this theory but argued that the occurrence of ultramafic nodules in alkaline basaltic volcanics is indicative of relatively high average flow rates. He calculated xenolith settling velocities (taking into account the rheological behaviour of the magma) that give minimum estimates of magma ascent rates, and published 0.1–5 m/s ascent velocities for spinel peridotite-bearing alkaline magmas. Klügel (1998) concluded that the presence of mantle xenoliths does not necessarily imply single-stage ascent (i.e. rapid and direct ascent of their host magma), therefore xenoliths do not always indicate the absence of crustal reservoirs where magmas are stored temporarily. However, he agreed with Spera (1984) that magma ascent rates must exceed the settling rates of the largest and densest xenoliths during each ascent stage.

  We used Eq. (1) of Spera (1984). The largest peridotite xenoliths found at Bondoró-hegy and Füzes-tó are 20 cm in diameter. The calculation requires the knowledge of magma viscosity (η) and the density difference (∆ρ) between xenolith and melt. The viscosity of alkaline basaltic magmas ranges between ∼10 and 550 poise (1–55 Pa s) as a function of the temperature (e.g. Best 2003). We studied the effects of variations in ∆ρ (the studied basalts contain various xenoliths which can have different densities) and in η (viscosity increases with increasing crystal content in the ascending, crystallizing magma) on ascent rates. Thus, we used three hypothetical scenarios involving different ∆ρ (400, 500, and 600 kg/m³) and η values (5.5, 10, and 35 Pa s) (Fig. 12b). 50 N/m² yield strength (corresponding to ∼15 vol% crystallinity) was used after Spera (1984). According to Fig. 12b, the changes of these parameters cause some differences in the magma ascent rates but they are in the same order of magnitude.

• 3. Residence time of olivine xenocrysts in basaltic melt:

  Time estimation from chemical profiles of crystals in magmatic rocks is based on the theory that if the melt incorporates a foreign crystal (antecryst or xenocryst), diffusion occurs to diminish the chemical disequilibrium between the host melt and the foreign crystal, and there is new growth from the host melt or a reaction texture. A diffusion concurrent with growth causes a moving-boundary problem, therefore ascertaining the single effect of each process is difficult (Costa et al. 2008). Olivine phenocryst cores crystallized in equilibrium conditions do not show zoning in Fo. However, with decreasing Mg content and temperature, they will have a smoothly curved profile (Maaloe and Hansen 1982; Larsen and Pedersen 2000). It is a similar case for a xenocrystic olivine incorporated by magma. If the melt and the grain are in equilibrium at the time of the incorporation, a simple overgrowth occurs. At disequilibrium conditions, when the liquid has too low Mg content, Fe-Mg exchange occurs between magma and xenocryst, as described by Zhang (2005). After they have equilibrated, a rim will grow on the crystal. Larsen and Pedersen (2000) mentioned that Fe–Mg exchange could modify the Fo content of olivines after a simple crystallization, i.e. the curved Fo profiles at crystal rims will straighten.

  Growth and Fe–Mg exchange could be accompanied by diffusion of minor elements, like Ca and Ni. Costa et al. (2008) described that beside numerical modeling methods there is a simple way to recognize concurrent diffusion and crystal growth of olivine. The relation between Fo and Ni content of olivine is non-linear at fractionation (crystal growth), and in contrast, the linear trend is caused by diffusion only. Based on the work of Jurewicz and Wattson (1988), Lasaga (1998) created a one-dimensional model for Ca diffusion into olivine: T _1/2 = (X _1/2)²/2D, where T _1/2 is the time necessary to reach half of the equilibration concentration of Ca in olivine and X _1/2 is the distance from crystal rim to this point. The diffusion coefficient (D) for Ca in olivine is 3.18*10⁻¹² cm²/s at 1,200 °C and fO₂ = 10⁻⁸ Pa. The Ca diffusion barely depends on the orientation of the olivine (Lasaga 1998).

  Our studied olivine xenocryst profiles show a Fo-rich inner plateau characterized by Ca contents lower than 500 ppm and a thin rim with sharply increasing Ca (Fig. 6a, b). According to Gurenko et al. (1996), they could be identified as mantle olivines. Relationships between Fo and NiO toward rims of olivine xenocrysts are linear (Fig. 7) indicating that diffusion was the main driving force after the incorporation. Our interpretation is that after the crystals were incorporated by the ascending magma, equilibration by diffusion, and precipitation of an overgrowth rim began.

  Figure 13 shows that the Ca profile of the olivine xenocryst in Fig. 6a has a slightly tilted plateau (i.e. characterized by slightly increasing Ca contents from 343 to 515 ppm) from the inner part of the crystal towards the rim which changes abruptly at a distance of 22 μm from the crystal rim. After this inflection point the Ca content increases to 2,130 ppm. The strong enrichment of Ca in the rims corresponds to the above mentioned processes. However, the tilt in the inner plateau could be the result of a heating event—probably related to the build up of the magmatic system, when the olivine xenocryst was an integral part of the mantle and heat has reached it conductively from the accumulating basaltic magma—before the incorporation. The X _1/2 for this period is 108 μm and the evaluated heating time is 1.16 years.

• 4. Dissolution time of orthopyroxene xenocrysts:

  Interaction between the alkaline basalt and orthopyroxene is a dynamic disequilibrium process, which possibly depends on the composition of the orthopyroxene, the rate of supply of undersaturated melt and other processes (Daines and Kohlstedt 1994). The reaction zone can also be interpreted as a melt-mixing zone, where the Si-rich secondary melt, derived from the incongruent melting of orthopyroxene, is mixed with the surrounding, less silicic alkaline basaltic melt (Arai and Abe 1995). In the system Fo + SiO₂, enstatite breaks down at 1559 °C and at 1 atmosphere to form forsterite and silica, and the amount of olivine that should be formed on incongruent melting is approximately 5-6 % (Bowen and Anderson 1914). However, the observed amount of olivine is much more in the reaction rims. The additional olivine could crystallize from the melt of the hybrid boundary layer, formed by the mixing of the basanite melt and the liquid derived from orthopyroxene breakdown (Shaw et al. 1998). Clinopyroxene is not a normal product of the incongruent breakdown of orthopyroxene, and therefore the components required for its formation must have diffused into the boundary layer from the basanite melt (Shaw et al. 1998).

  Based on the experimental results of Shaw (1999), the dissolution of orthopyroxene, i.e. the formation of the reaction rim, is a function of time and pressure. Dissolution rates were determined for anhydrous solvent melt at 0.4, 1 and 2 GPa. The experimental results indicate that the dissolution rates at 1 and 2 GPa are much faster than at 0.4 GPa under anhydrous conditions, which is in accordance with previous observations of increasing dissolution rate with increasing pressure (Brearley and Scarfe 1986). The relationship between the reaction rim thickness and the time can be expressed by a power law function of the form y = ax ^b where y = rim thickness, x = time, a and b are coefficients distinct for each pressure. The a and b coefficients can be obtained by fitting power law functions on the experimental data of Shaw (1999). After this, reaction rates can be determined from the thicknesses of the orthopyroxene reaction rims. The reaction rim forms where the crystal is in direct contact with the melt. Thus, the calculated reaction time gives the time that the orthopyroxene spent in the melt. This reaction rim has to form long before the eruptions and during magma ascent when the melt is capable of reaction. The reaction rims around single xenocrysts are characterized by roughly unvarying thicknesses (intracrystalline thickness), whereas the reaction rim thicknesses of the distinct xenocrysts (intercrystalline thickness) are diverse. This implies that the magma could have incorporated discrete orthopyroxene xenocrysts at different times/levels.

  In the studied basalts, we determined the change in radius (i.e. the reaction rim thicknesses) of numerous orthopyroxene xenocrysts (0.05–0.48 mm): multiple measurements of the thickness of the same reaction rim were performed on a given crystal and the results were averaged. The reaction rims are frequently overgrown by a phenocrystic clinopyroxene mantle, therefore the orthopyroxene-melt interaction could be stopped much earlier, before the quenching. It is important to note that the dissolution rate of orthopyroxene largely depends on the prevailing conditions, mainly on the pressure. Thus, during magma ascent, the reaction rate can change and the resulting reaction rim thicknesses may preserve a derivative time of interaction.

Fig. 12

Calculations of magma ascent rates for the Bondoró-hegy and Füzes-tó alkaline basaltic magmas. a Method 1: magma ascent rate computations based on Eq. (8) in Sparks et al. (2006); the grey shaded area indicates the average width of the dyke observed in the capping scoria cone at Bondoró-hegy; b method 2: xenolith settling rates (i.e. minimum magma ascent rates) based on Eq. (1) in Spera (1984). In both calculations, 2,800 kg/m³ liquid density was applied after Spera (1984)

Fig. 13

Plot of Ca (in parts per million) versus distance from olivine rim (a part of the Ca profile of the olivine xenocryst in Fig. 6a) with the calculated heating event (1.16 years) and residence time (3.5 days) of the olivine xenocryst in the basaltic magma. See Appendix text (method 3) for details