Physical and chemical interactions between coexisting acid and basic magmas at Elizabeth Castle, Jersey, Channel Islands
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Complex subvolcanic magma plumbing system of an alkali basaltic maar-diatreme volcano (Elie Ness, Fife, Scotland)Upton, B.G.J.; Ugra, R.; Yücel, C.; Taylor, R.N.; Elliott, Holly; University of Southampton (Elsevier, 2016-08-17)Alkali basaltic diatremes such as Elie Ness (Fife, Scotland) expose a range of volcanic lithofacies that points to a complex, multi-stage emplacement history. Here, basanites contain phenocrysts including pyrope garnet and sub-calcic augites from depths of ~60km. Volcanic rocks from all units, pyroclastic and hypabyssal, are characterised by rare earth element (REE) patterns that show continuous enrichment from heavy REE (HREE) to light REE (LREE), and high Zr/Y that are consistent with retention of garnet in the mantle source during melting of peridotite in a garnet lherzolite facies. Erupted garnets are euhedral and unresorbed, signifying rapid ascent through the lithosphere. The magmas also transported abundant pyroxenitic clasts, cognate with the basanite host, from shallower depths (~35–40km). These clasts exhibit wide variation in texture, mode and mineralogy, consistent with growth from a range of compositionally diverse melts. Further, clinopyroxene phenocrysts from both the hypabyssal and pyroclastic units exhibit a very wide compositional range, indicative of polybaric fractionation and magma mixing. This is attributed to stalling of earlier magmas in the lower crust — principally from ~22 to 28km — as indicated by pyroxene thermobarometry. Many clinopyroxenes display chemical zoning profiles, occasionally with mantles and rims of higher magnesium number (Mg#) suggesting the magmas were mobilised by juvenile basanite magma. The tuffs also contain alkali feldspar megacrysts together with Fe-clinopyroxene, zircon and related salic xenoliths, of the ‘anorthoclasite suite’ — inferred to have crystallised at upper mantle to lower crustal depths from salic magma in advance of the mafic host magmas. Despite evidence for entrainment of heterogeneous crystal mushes, the rapidly ascending melts experienced negligible crustal contamination. The complex association of phenocrysts, megacrysts and autoliths at Elie Ness indicates thorough mixing in a dynamic system immediately prior to explosive diatreme-forming eruptions.
Origin and evolution of silicic magmas at ocean islands: Perspectives from a zoned fall deposit on Ascension Island, South Atlantic.Chamberlain, Katy J.; Barclay, Jenni; Preece, Katie; Brown, Richard J.; Davidson, Jon P.; University of Durham; University of East Anglia (2016-11-15)Ascension Island, in the south Atlantic is a composite ocean island volcano with a wide variety of eruptive styles and magmatic compositions evident in its ~ 1 million year subaerial history. In this paper, new observations of a unique zoned fall deposit on the island are presented; the deposit gradationally changes from trachytic pumice at the base, through to trachy-basaltic andesite scoria at the top of the deposit. The key features of the eruptive deposits are described and are coupled with whole rock XRF data, major and trace element analyses of phenocrysts, groundmass glass and melt inclusions from samples of the compositionally-zoned fall deposit to analyse the processes leading up to and driving the explosive eruption. Closed system crystal fractionation is the dominant control on compositional zonation, with the fractionating assemblage dominated by plagioclase feldspar and olivine. This fractionation from the trachy-basaltic andesite magma occurred at pressures of ~ 250 MPa. There is no evidence for multiple stages of evolution involving changing magmatic conditions or the addition of new magmatic pulses preserved within the crystal cargo. Volatile concentrations range from 0.5 to 4.0 wt.% H2O and progressively increase in the more-evolved units, suggesting crystal fractionation concentrated volatiles into the melt phase, eventually causing internal overpressure of the system and eruption of the single compositionally-zoned magma body. Melt inclusion data combined with Fe–Ti oxide modelling suggests that the oxygen fugacity of Ascension Island magmas is not affected by degree of evolution, which concentrates H2O into the liquid phase, and thus the two systems are decoupled on Ascension, similar to that observed in Iceland. This detailed study of the zoned fall deposit on Ascension Island highlights the relatively closed-system evolution of felsic magmas at Ascension Island, in contrast to many other ocean islands, such as Tenerife and Iceland.
Timescales of mixing and mobilisation in the Bishop Tuff magma body: perspectives from diffusion chronometry.Chamberlain, Katy J.; Morgan, Daniel J.; Wilson, Colin J. N.; Victoria University of Wellington; University of Leeds (Springer, 2014-07-01)We present two-feldspar thermometry and diffusion chronometry from sanidine, orthopyroxene and quartz from multiple samples of the Bishop Tuff, California, to constrain the temperature stratification within the pre-eruptive magma body and the timescales of magma mixing prior to its evacuation. Two-feldspar thermometry yields estimates that agree well with previous Fe–Ti oxide thermometry and gives a ~80 °C temperature difference between the earlier- and later-erupted regions of the magma chamber. Using the thermometry results, we model diffusion of Ti in quartz, and Ba and Sr in sanidine as well as Fe–Mg interdiffusion in orthopyroxene to yield timescales for the formation of overgrowth rims on these crystal phases. Diffusion profiles of Ti in quartz and Fe–Mg in orthopyroxene both yield timescales of <150 years for the formation of overgrowth rims. In contrast, both Ba and Sr diffusion in sanidine yield nominal timescales 1–2 orders of magnitude longer than these two methods. The main cause for this discrepancy is inferred to be an incorrect assumption for the initial profile shape for Ba and Sr diffusion modelling (i.e. growth zoning exists). Utilising the divergent diffusion behaviour of Ba and Sr, we place constraints on the initial width of the interface and can refine our initial conditions considerably, bringing Ba and Sr data into alignment, and yielding timescales closer to 500 years, the majority of which are then within uncertainty of timescales modelled from Ti diffusion in quartz. Care must be thus taken when using Ba in sanidine geospeedometry in evolved magmatic systems where no other phases or elements are available for comparative diffusion profiling. Our diffusion modelling reveals piecemeal rejuvenation of the lower parts of the Bishop Tuff magma chamber at least 500 years prior to eruption. Timescales from our mineral profiling imply either that diffusion coefficients currently used are uncertain by 1–2 orders of magnitude, or that the minerals concerned did not experience a common history, despite being extracted from the same single pumice clasts. Introduction of the magma initiating crystallisation of the contrasting rims on sanidine, quartz, orthopyroxene and zircon was prolonged, and may be a marker of other processes that initiated the Bishop Tuff eruption rather than the trigger itself.