Cliff S. J. Shaw

The temporal evolution of three magmatic systems in the West Eifel volcanic field, Germany

Abstract The Quaternary West Eifel volcanic field is the surface expression of a mantle plume that is thought to contain ∼1% melt. The volcanoes range in age from 0.7 Ma to ∼10 000 yr BP and the average recurrence rate of volcanism is around 3.5×10−4 eruptions per year, giving a mean time between eruptions of 2875 years, assuming that the field is monogenetic. The presence of mafic and ultramafic xenoliths is taken as evidence for rapid, single-stage emplacement of magmas at velocities around 0.5 km/h. However, the presence of clinopyroxene that preserves evidence of polybaric crystallisation belies this simple model. This study uses variations in the Fo content of olivine in mantle-derived xenoliths and in olivine xenocrysts enclosed in cumulate xenoliths to calculate the time scale of evolution of three volcanic centres in the Eifel field. Core–rim variations in olivine are interpreted to result from Fe–Mg interdiffusion between it and a surrounding melt. Using experimental determinations of the interdiffusion coefficient and approximations of magma temperature, contact times have been calculated using planar and radial diffusion models. The three volcanic centres studied were Meerfelder Maar, Gemundener Maar and Baarley. The cumulate xenoliths at Meerfelder Maar formed at a depth of ∼13 km and record a magma emplacement event at between 1 and 3 years prior to eruption. Two suites of mantle-derived olivine clinopyroxenites record a further two events at 1–8 days and ∼3 h prior to eruption. The final event is interpreted to record the eruption of this volcano and the 3-h contact time records the time required for transport of peridotite xenoliths from the mantle to surface, giving an average emplacement velocity of around 15 km/h. The cumulate xenoliths at Gemundener Maar crystallised at depths of between 4 and 22 km. There are four distinct events recorded by the zoned olivine grains in the clinopyroxene-rich cumulates from this locality. The maximum residence time is similar to that at Meerfelder Maar. However, in this case the final magma resided in the chamber for between 23 days and 2.7 months prior to eruption, allowing enough time for any entrained peridotite xenoliths to be filtered out. The cumulate xenoliths from the Baarley locality record a much longer history than those at the other localities. In this case, initially peridotitic olivine has been completely re-equilibrated over >175 years prior to eruption. In common with the other two localities, there is evidence of multiple intrusive events, at depths up to 7 km, prior to final eruption. The duration of the final eruption event and the interpreted depth to source of the xenoliths indicate an emplacement velocity of around 3 km/h. The data derived from modelling of diffusion profiles in olivine from cumulate xenoliths indicate that the magmatic systems feeding these three volcanoes were dynamic and very short-lived. In addition, these data can be used to place constraints on the rate of cumulate formation in subvolcanic magma chambers. On the basis of the data presented here, it would seem that an appreciable thickness of cumulates can form over relatively short time spans (days to a few years).

Determination of the concentration of water dissolved in glasses and minerals using nuclear microprobe

In Earth Sciences, the global water cycle is of fundamental importance. For this reason, the H2O content of volcanic glass and mantle minerals must be analysed: usually by micro-infrared spectroscopy (FTIR) or secondary ion mass spectrometry (SIMS). However, both of these methods require calibration using standards of known water content. To avoid matrix effects, the standards and unknowns must be otherwise identical in composition. In this study we have determined the water content of geological samples, in the range 10 ppm–5wt.%H2O, using an absolute analytical technique: a combination of elastic recoil detection analysis (ERDA) and Rutherford backscattering spectrometry (RBS). We compared the results obtained by this method to data obtained by FTIR on the same samples. We discuss the limitations of the method and use the results to calibrate IR extinction coefficients for FTIR spectroscopy.

In situ mapping of high-pressure fluids using hydrothermal diamond anvil cells

We present new results combining high pressures and temperatures attainable in a diamond anvil cell with in situ synchrotron radiation induced micro-X-ray fluorescence measurements. Hydrothermal diamond anvil cells experiments have been performed by measuring the partitioning of Pb between aqueous fluids (pure water or NaCl-enriched water) and hydrous silicate melts of haplogranite composition using synchrotron X-ray fluorescence. The in situ measurements were performed in the range 0.3–1.2 GPa and 730–850 °C both in the aqueous fluid and in the silicate melts being in equilibrium. Pb is strongly partitioned into high-pressure–temperature hydrous melts when Cl is present in either the hydrous melt or the aqueous fluid. Moreover, our comparisons of in situ results with post-mortem results show that significant changes take place during rapid quenching especially when samples are small (few hundred of microns in diameter). Water exsolution is induced by the quench in the silicate melt showing the high mobility of Pb which immediately partitions into the water vapor phase during the quench. The current in situ approach offers thus a pertinent complementary method to the classical experimental petrology investigations.

The partitioning of barium and lead between silicate melts and aqueous fluids at high pressures and temperatures

Abstract The origin of subduction-related magmas is still a matter of debate in the Earth Sciences. These magmas are characterised by their distinctive trace element compositions compared to magmas from other tectonic settings, e.g. mid-ocean ridges or rifts. The distinct trace element composition of these magmas is generally attributed to alteration of the source region by a contaminating agent: either a silicate melt or a hydrous fluid, possibly chlorine-enriched. In this study, we have used μPIXE (proton induced X-ray emission) to analyse synthetic samples obtained from a micro-experimental petrology study that aims to determine the partitioning behaviour of two key elements, Ba and Pb, between silicate melt and both pure water and saline fluids. Our experiments were performed at high-pressure (>0.34–1.53 GPa) and high-temperature (697–1082 °C) in a hydrothermal diamond anvil cell, that was used as a transparent rapid quench autoclave. We observed that at high pressure and temperature, in the presence of pure water, Ba and Pb are not strongly fractionated into one phase or the other. The partition coefficient of Pb is ranging from 0.46 to 1.28. Results from one experiment performed at 0.83 GPa and 847 °C, in the presence of a saline fluid indicate that the presence of Cl induces strong fractionation of Pb and moderate fractionation of Ba both into the silicate melt. In addition, our data indicate that Cl is strongly partitioned into the fluid phase.

The effects of potassium addition on the rate of quartz dissolution in the CMAS and CAS systems

Quartz dissolution in melts in the KCAS and KCMAS systems results in the formation of a silica- and potassium-enriched boundary layer next to the dissolving crystals. The presence of potassium in CAS melts has no discernible effect on dissolution rate compared with that in K-free melts with otherwise similar composition despite a small decrease in the diffusivity of silica in the potassium-bearing melts. The decrease in silica diffusivity is offset by an increase in the solubility of silica in the K-bearing melts. Addition of potassium to CMAS melts results in a large decrease in the dissolution rate of quartz. Even though the solubility of silica is enhanced, the addition of potassium leads to large changes in the structure of the melt in the boundary layer (as measured by NBO/T), which results in a large decrease in the diffusivity of silica and thus slower dissolution. There is significant diffusive coupling of Al2O3, CaO and MgO during dissolution, which leads to local uphill diffusion of these components. K2O is decoupled from the other components, as shown by its much thicker diffusion zone. Potassium moves through the boundary layer as a result of two homogeneous reactions: uphill diffusion in which potassium diffuses into the silica-enriched melt adjacent to the dissolving quartz crystal and downhill diffusion in the region furthest from the crystal–melt interface where SiO2 and K2O diffuse away from the interface together.

Thermodynamic Modelling of Mantle–Melt Interaction Evidenced by Veined Wehrlite Xenoliths from the Rockeskyllerkopf Volcanic Complex, West Eifel Volcanic Field, Germany

The oldest volcanic center of the Rockeskyllerkopf Volcanic Complex (RVC) in the West Eifel volcanic field hosts three distinct compositional groups of mantle xenolith: two groups of lherzolite and harzburgite and a group of wehrlite xenoliths that are cross-cut by phlogopite–clinopyroxene veins and variably impregnated by these same minerals. The lherzolite and harzburgite xenoliths represent mantle that was affected by metasomatism prior to the Quaternary magmatic activity below the RVC. We interpret the wehrlites to be the result of infiltration of orthopyroxene-undersaturated alkaline melt into the orthopyroxene-bearing lithospheric mantle. Reaction between mantle and melt consumed orthopyroxene and precipitated olivine and clinopyroxene as well as phlogopite. Models of the equilibration of peridotite and infiltrated melt created with alphaMELTS produce wehrlite with olivine, phlogopite and spinel compositions that are similar to those observed in the natural samples for melt:rock ratios between 0 11 and 1 11. Model clinopyroxene compositions do not match the observed range of compositions in wehrlite. The intra-sample range in clinopyroxene composition in wehrlite xenoliths suggests that clinopyroxene did not reach equilibrium, whereas olivine compositions in the same samples show much less variation, suggesting that they attained equilibrium. We calculate the time required for olivine homogenization to be around 3300 years, whereas the time needed for clinopyroxene to be homogenized is 100 times longer. We suggest that the range of clinopyroxene compositions within and between the wehrlite xenoliths is the result of varying degrees of equilibration of the initial lherzolite or harzburgite clinopyroxene with clinopyroxene formed by reaction with the melt. Melt compositions from the alphaMELTS models suggest that the differences in lava composition observed in the RVC can be produced by mixing between melts from a garnet peridotite source with secondary melts produced by peridotite–melt reaction in the spinel-facies mantle. We suggest that mixing occurred during melt transport through the lithospheric mantle as secondary melts produced during wehrlite formation were drawn into fractures that were exploited by several generations of magma rising from the garnet peridotite source.