Origin of spongy textures in clinopyroxene and spinel from mantle xenoliths, Hessian Depression, Germany

Summary Spongy textures are observed in anhydrous Group 1 mantle xenoliths (harzburgite, lherzolite and wehrlite) hosted in Tertiary alkali basaltic lavas from the Hessian Depression, Germany. These textures are developed only on clinopyroxene and spinel, and occur as rims or cross-cutting veinlets and patches showing optical continuity with the host grain. They are often associated with pools of amorphous glassy material. There is no preferential development of spongy domains against the xenolith-lava contact suggesting that the host magma did not play any significant role in their formation. Spongy clinopyroxene and spinel occur in all rock types, but, are more pervasive in wehrlite. Chemically, spongy domains of clinopyroxene and spinel are more refractory than unaffected areas, which is consistent with their formation through a partial melting event. The associated glassy material shows chemical characteristics which suggest that the melt pools are genetically related to the development of the spongy textures. The partial melting event was probably triggered by the infiltration of a low-density fluid. The fluid may have evolved from a silicate melt responsible for the metasomatic Fe-enrichment recorded in wehrlite. In this context, the more pervasive development of spongy clinopyroxene in wehrlite may be explained by a higher concentration of the evolved fluid phase at proximity to its silicate melt source. Received March 15, 2000; revised version accepted September 6, 2001     

Book review of ENCYCLOPEDIA OF NONLINEAR SCIENCE,edited by Alwyn Scott,Routledge, Taylor and Francis Group, 2005, VII +1053 pp., 135, $225 hardback (ISBN 1-57958-385-7).

We present three-dimensional (3D) solutions of the magnetohydrostatic equations in the co-rotating frame of reference outside a magnetized rigidly rotating cylinder. We make no symmetry assumption for the magnetic field, but to be able to make analytical progress we neglect outflows and specify a particular form for the current density. The magnetohydrostatic equations can then be reduced to a single linear partial differential equation for a pseudo-potential U, from which the magnetic field can be calculated by differentiation. The equation for U can be solved by standard methods. The solutions can also be used to determine the plasma pressure, density and temperature as functions of all three spatial coordinates. Despite the obvious limitations of this approach, it can, for example, be used as a simple tool to create 3D models for the closed field line regions of rotating magnetospheres without rotational symmetry.     

Origin of glass in upper mantle xenoliths from the quaternary volcanics of Gees,West Eifel,Germany

Glasses have been analysed from six mantle-derived xenoliths (5 orthopyroxene and/or olivine-rich, 1 clinopyroxene-rich) from the Quaternary volcanics S.E. of Gees, West Eifel, Germany. The glasses in these xenoliths occur as pools surrounding and embaying spinels, as inclusions in spinels, as veins and stringers within phlogopiterich veins, and as jackets partially surrounding some of the xenoliths. Glasses analysed are felsic and characterised by low to intermediate SiO₂ (40–62 wt.%), variable CaO (1–11 wt.%) and MgO (1–4 wt.%), high Al₂O₃ (14–21 wt.%), and up to 11 wt.% Na₂O + K₂O. The jacket glasses have the lowest SiO₂, highest CaO and MgO. Variations in all of the glass compositions are similar and imply a unifying factor or process in their formation. Glass as pools and stringers within veins of phlogopite forms part of the same trends shown by all the glasses when plotted on bivariate (oxide vs SiO₂) diagrams but can be distinguished from glass surrounding and enclosed by spinels. Glasses occurring as jackets are similar in composition to those in pools and veinlets associated with phlogopite but are of quite different composition to the glasses found within the xenoliths that they partially enclose. The occurrence and chemistry of the glasses do not support such glasses as representing original or differentiated magma trapped during formation of the xenolithic assemblages. The chemistry of the glasses also makes it unlikely that they were produced by dissociation of phlogopite during ascent of the xenoliths. The most likely origin for the glasses is that they represent volatile-rich melts which migrated through upper mantle material. These melts are likely to be responsible for the heterogeneous nature of the mantle underlying this part of the West Eifel region.     

Mechanisms of orthopyroxene dissolution in silica-undersaturated melts at 1 atmosphere and implications for the origin of silica-rich glass in mantle xenoliths

Experiments dissolving orthopyroxene (En₉₃) in a variety of Si-undersaturated alkaline melts at 1 atmosphere and variable f _O2 demonstrate that orthopyroxene dissolves to form olivine, Si-rich melt and clinopyroxene. These phases form a texturally and chemically distinct boundary layer around the partly dissolved orthopyroxene crystals. The occurrence of clinopyroxene in the boundary layer is due to inward diffusion of Ca from the solvent melt to the boundary layer causing clinopyroxene saturation. Compositional profiles through the solvent and the boundary layer for a number of experiments demonstrate rapid diffusion of cations across the boundary layer – solvent interface. SiO₂ diffuses outward from the boundary layer whereas CaO and Al₂O₃ diffuse toward the Si-enriched boundary layer melt. The rate of Al diffusion is slower under reducing conditions compared to the rates in experiments performed in air. Concentrations of FeO and MgO in the boundary layer and solvent are approximately equal indicating rapid diffusion and attainment of equilibrium despite ongoing crystallisation of clinopyroxene within the boundary layer. The behaviour of Na₂O and K₂O is strongly affected by f _O2. Under reducing conditions Na₂O and K₂O concentrations are approximately equal in the boundary layer and solvent indicating normal diffusion down the concentration gradient and attainment of equilibrium. Under oxidising conditions, K₂O and to a lesser extent Na₂O, have compositional profiles indicative of uphill diffusion likely due to their preference for more polymerised Si- and Al-rich melts. Under reduced conditions Al-enrichment in the boundary layer melt is not as extreme and uphill diffusion did not occur. The composition of the solvent melt after the experiments indicates that it was contaminated by the boundary layer by convective mixing due to the onset of hydrodynamic instabilities brought on by density and viscosity contrasts between the two melts. Despite using a wide variety of solvent melt compositions we find that the boundary layer melts converge toward a common composition at high SiO₂ contents. The composition of glass generated by orthopyroxene dissolution at 1 atmosphere is similar in many respects to Si-rich glass found in many orthopyroxene-rich mantle xenoliths that have been attributed to high pressure in situ processes including mantle metasomatism. The results of this study suggest that at least some Si-rich melts are likely to have formed by dissolution of xenolith orthopyroxene at low pressure possibly by their Si-undersaturated host magmas. Received: 30 August 1996 / Accepted: 15 April 1998     

Electron paramagnetic resonance,optical absorption,and magnetic circular dichroism studies of the CO 3 − molecular-ion in irradiated natural beryl

Room temperature X-irradiation of some natural beryls produced several new absorption lines in the electron paramagnetic resonance (EPR) spectrum, a known series of optical absorption lines in the 500–700 nm range, and a shift of the absorption edge to lower energies. Several of the new EPR lines and part of the irradiation-induced shift of the absorption edge disappeared after a few days at room temperature, and were not examined in detail. However, three of the paramagnetic centres responsible for the new EPR lines were stable at room temperature and two of these have previously been identified as atomic hydrogen and the methyl radical, CH₃. These species were stable to ～150 and ～450°C respectively. The third stable species, hitherto unreported, showed a single-line EPR spectrum of axial symmetry, with g∥=2.0051 and g⊥=2.0152. This spectrum was found to be intensity-correlated with the series of optical bands in the 500–700 nm range, after thermal bleaching at 175°C. The EPR and optical spectra are therefore assigned to the same species. It is argued that this species is the CO ₃ ^? molecular ion, located in the widest part of the structural channel and aligned with the plane of the molecule perpendicular to the c axis. The EPR spectrum is consistent with a ² A′₂ ground state of a CO ₃ ^? molecule with trigonal symmetry, and this requires that the optical transition has a ² A′₂ → ² E′ character. Most of the features in the optical spectrum can be assigned to coupling of a totally symmetric mode of frequency ～1020 cm^?1 onto a zero-phonon line at 14,490 cm^?1 and a second weaker line at 16,020 cm^?1. However, both of these two fundamental lines are structured, and the two components show strong temperature-dependent derivative-shaped magnetic circular dichroism (MCD). Furthermore, the overall sign of the MCD for the line at 16,020 cm^?1 is opposite to that at 14,490 cm^?1. The separation (～120 cm^?1) of the two components of the 14,490 cm^?1 line is much larger than that expected from spin-orbit interaction, and the origin of this splitting is not yet understood.     

An experimental study of partitioning of fluorine between K-richterite,apatite, phlogopite,and melt at 20 kbar

Phase relations have been determined at 20 kbar and primarily under suprasolidus conditions in the Fe−Ti-free F-bearing K-richterite—phlogopite and K-richterite—apatite systems in order to assess the partitioning of F among phlogopite, K-richterite, apatite, and melt under upper-mantle conditions. Both systems are pseudoternary because they contain forsterite, enstatite and a diopside-rich clinopyroxene from the breakdown of the mica and K-richterite. The F-bearing K-richterite systems have lower minimum melting temperatures than the F-bearing phlogopite —apatite system at the same pressure. However in the systems studied, F in phlogopite appears the most effective component in altering minimum liquid compositions whereas comparison between the present study and previous systems suggests that the presence of P₂O₅ during melting may result in more K-enriched melts. Variations in the compositions of the F-bearing phases are primarily controlled by the bulk compositions of the end-member minerals and by temperature, although buffering by non-F bearing minerals (e.g. clinopyroxene) may be effective. Distribution coefficients (as wt% ratios) between F-bearing minerals and coexisting liquids have been determined as functions of bulk composition and temperature for products of experiments. Distribution coefficients between K-richterite—liquid, apatite—liquid, and phlogopite—liquid are ≥1 to slightly <1 for most bulk compositions, indicating thatF is generally a compatible element. This conclusion is in agreement with the sequence ofF distribution for similar phases in ultrapotassic rocks. These results preclude F-bearing mineral reservoirs in the mantle, at depths corresponding to 20 kbar, being capable of producing F-enrichment in ultrapotassic magmas, or being effective in redox melting processes. Editorial responsibility: K. Hodges