Equilibrium and Disequilibrium Trace Element Partitioning in Hydrous Eclogites (Trescolmen, Central Alps)

Despite the widespread presence of hydrous phases in subduction-related systems, experimental D^Min/Fluid trace element valuesfor many hydrous phases are lacking. To fill this gap, we presenta set of D^{Min/Clinopyroxene} values (where Min indicates amphibole,zoisite, phengite, paragonite or apatite) derived from equilibriumparageneses of eclogites from Trescolmen (Central Alps, Switzerland).These data can be combined with experimental data for D^{Clinopyroxene/Fluid},to estimate D^Min/Fluid values for the hydrous phases, thus circumventingexperimental problems with the direct determination of suchvalues. We analysed Li, Be, B, Sr, Y, Zr, Nb, Ba, Ce, Nd, Sm,Pb, Th and U in coexisting phases by laser ablation microprobeinductively coupled plasma mass spectrometry. Many of the valuesare extremely low; for example, Nb, Ba, Ce, Th and U are inthe lower ppb range in clinopyroxene. Attainment of equilibriumwas evaluated by textural, and major and trace element characteristics.Non-equilibrated assemblages are common in most eclogite localities,including Trescolmen, and using such samples would lead to thederivation of erroneous values for equilibrium partitioning.However, four of the 10 studied eclogites from Trescolmen havinghomogeneous clinopyroxene compositions and preferred orientationof high-pressure phases yielded consistent D^{Min/Clinopyroxene}values in all four samples (where Min indicates amphibole, phengite,paragonite, apatite), and hence were studied in detail. Thelow abundances in some phases result from strong preferentialincorporation of trace elements into other minor phases. Fromthe investigated hydrous phases (amphibole, zoisite, clinozoisite,phengite, paragonite, apatite and talc), zoisite was found tobe the most important carrier of Sr, light rare earth elements,Pb, Th and U, whereas phengite hosts Ba and is, along with clinopyroxeneand paragonite, an important phase for B. However, because oftheir low modal abundance in eclogite-facies rocks, phengiteand paragonite do not control the B whole-rock budget. We inferthat estimated D^{Min/Clinopyroxene} values from equilibrium assemblagescan be used as a good approximation for partition coefficientsunder the given PT conditions (     

Evolution of Crystallizing Interstitial Liquid in an Arc-Related Cumulate Determined by LA ICP-MS Mapping of a Large Amphibole Oikocryst

High-resolution chemical mapping of a large, single, oikocrysticamphibole grain from the Eriksberg gabbro, by laser ablationinductively coupled plasma mass spectrometry (LA ICP-MS), documentsthe progressive evolution of interstitial liquid in a hydrousbasaltic system. The amphibole has a nearly constant mg-numberand only minor variations in most major elements, in part becauseof the resorption of olivine buffering the liquid composition.However, non-buffered trace elements including Sc, V, Nb, Zr,Th, U, and the rare earth elements (REE) show at least an orderof magnitude variation. For example, V varies from <1 to>1000 ppm, Zr from 4·3 to >1000 ppm, Nb from 0·14to 12·3 ppm, and the REE patterns range from bowed upwith a negative Eu anomaly to bowed down with a positive Euanomaly—all within a single crystal. The distributionof amphibole compositions indicates that the interstitial liquidwas not uniformly distributed as crystallization proceeded.Rather, the compositional variations reflect progressively morechannelized flow of interstitial liquid during compaction. Whenthe interstitial liquid evolved so that the crystallizing amphibolehad     

Petrogenesis and Implications of Calc-Alkaline Cryptic Hybrid Magmas from Washburn Volcano, Absaroka Volcanic Province, USA

The petrogenesis of calc-alkaline magmatism in the Eocene AbsarokaVolcanic Province (AVP) is investigated at Washburn volcano,a major eruptive center in the low-K western belt of the AVP.New ⁴⁰Ar/³⁹Ar age determinations indicate that magmatism atthe volcano commenced as early as 55 Ma and continued untilat least 52 Ma. Although mineral and whole-rock compositionaldata reflect near equilibrium crystallization of modal phenocrysts,petrogenetic modeling demonstrates that intermediate compositionmagmas are hybrids formed by mixing variably fractionated andcontaminated mantle-derived melts and heterogeneous siliciccrustal melts. Nd and Sr isotopic compositions along with traceelement data indicate that silicic melts in the Washburn systemare derived from deep-crustal rocks broadly similar in compositionto granulite-facies xenoliths in the Wyoming Province. Our preferredexplanation for these features is that mantle-derived basalticmagma intruded repeatedly in the deep continental crust leadingto fractional crystallization, silicic melt production, andhomogenization of magmas, followed by ascent to shallow reservoirsand crystallization of new plagioclase-rich mineral assemblagesin equilibrium with the intermediate hybrid liquids. The implicationsof this process are that (1) some calc-alkaline magmas may onlybe recognized as hybrids on purely chemical grounds, particularlyin systems where mixing precedes and is widely separated fromcrystallization in space and time, and (2) given the role ascribedto crustal processes at Washburn volcano, the variation betweenrocks that follow calc-alkaline trends in the western AVP andthose that follow shoshonitic trends in the east cannot simplyreflect higher pressures of fractionation to the east in Moho-levelmagma chambers in the absence of crustal interaction. KEY WORDS: petrogenesis; magma mixing; calc-alkaline; Absaroka Volcanic Province; ⁴⁰Ar/³⁹Ar dates     

Crustal Evolution in the SW Part of the Baltic Shield: the Hf Isotope Evidence

The results of a laser ablation microprobe–inductivelycoupled plasma mass spectrometry Lu–Hf isotope study ofzircons in 0·93–1·67 Ga rocks from southNorway indicate that early Proterozoic protoliths of the BalticShield have present-day ¹⁷⁶Hf/¹⁷⁷Hf     

Analogue study of particle segregation in pyroclastic density currents, with implications for the emplacement mechanisms of large ignimbrites

Abstract Analogue flume experiments were conducted to investigate the transport and sedimentation behaviour of turbulent pyroclastic density currents. The experimental currents were scaled approximately to the natural environment in three ways: (1) they were fully turbulent; (2) they had a very wide range of particle sizes and associated Rouse numbers (the ratio of particle settling velocity to effective turbulent eddy velocity in the current); and (3) they contained particles of two different densities. Two sets of surge‐type experiments were conducted in a 5 m long, water‐filled lock‐exchange flume at five different volumetric particle concentrations from 0·6% to 23%. In one set (one‐component experiments), the currents contained just dense particles; in the other set (two‐component experiments), they contained both light and dense particles in equal volume proportions. In both sets of experiments, the population of each component had a log‐normal size distribution. In the two‐component experiments, the size range of the light particle population was selected in order to be in hydrodynamic equivalence with that of the dense particles. Dense particles were normally graded, both vertically and downstream, in the deposits from both sets of experiments. The mass loading (normalized to the initial mass of the suspension) and grain size of the dense component in the deposits decreased with distance from the reservoir and were insensitive to initial total particle concentration in the currents. On the other hand, in the two‐component experiments, the light particles were extremely sensitive to concentration. They were deposited in hydrodynamic equivalence with the dense particles from dilute currents, but were segregated efficiently at concentrations higher than a few per cent. With increasing particle concentration, the large, light particles were carried progressively further down the flume because of buoyancy effects. Deposits from the high‐concentration currents exhibited reverse vertical grading of the large, light particles. Efficient segregation of the light component was observed even if the bulk density of the current was less than that of the light particles. In both sets of experiments, marked inflexions in the rate of downstream decline in mass loading and maximum grain size of the dense component can be attributed to the presence of two different particle settling regimes in the flow: (1) particles with Rouse numbers >2·5, which did not respond to the turbulence and settled rapidly; and (2) particles with Rouse numbers <2·5, which followed the turbulent eddies and settled slowly. The results are applied to the transport and sedimentation dynamics of pyroclastic density currents that generate large, widespread ignimbrites. Field data fail to reveal significant departures from aerodynamic equivalence between pumice and lithic clasts in three such ignimbrites: the particulate loads of some large ignimbrites are transported principally in turbulent suspensions of low concentration. In some ignimbrites, the well‐developed inflexions in curves of maximum lithic (ML) size vs. distance can be attributed to the existence of distinct high and low Rouse number particle settling regimes that mark the transition from an overcharged state to one in which the residual particulate load is transported more effectively by turbulence.     

An experimental study of grain scale melt segregation mechanisms in two common crustal rock types

Creation of pathways for melt to migrate from its source is the necessary first step for transport of magma to the upper crust. To test the role of different dehydration‐melting reactions in the development of permeability during partial melting and deformation in the crust, we experimentally deformed two common crustal rock types. A muscovite‐biotite metapelite and a biotite gneiss were deformed at conditions below, at and above their fluid‐absent solidus. For the metapelite, temperatures ranged between 650 and 800 °C at P_c=700 MPa to investigate the muscovite‐dehydration melting reaction. For the biotite gneiss, temperatures ranged between 850 and 950 °C at P_c=1000 MPa to explore biotite dehydration‐melting under lower crustal conditions. Deformation for both sets of experiments was performed at the same strain rate (ε.) 1.37×10^?5 s^?1. In the presence of deformation, the positive ΔV and associated high dilational strain of the muscovite dehydration‐melting reaction produces an increase in melt pore pressure with partial melting of the metapelite. In contrast, the biotite dehydration‐melting reaction is not associated with a large dilational strain and during deformation and partial melting of the biotite gneiss melt pore pressure builds more gradually. Due to the different rates in pore pressure increase, melt‐enhanced deformation microstructures reflect the different dehydration melting reactions themselves. Permeability development in the two rocks differs because grain boundaries control melt distribution to a greater extent in the gneiss. Muscovite‐dehydration melting may develop melt pathways at low melt fractions due to a larger volume of melt, in comparison with biotite‐dehydration melting, generated at the solidus. This may be a viable physical mechanism in which rapid melt segregation from a metapelitic source rock can occur. Alternatively, the results from the gneiss experiments suggest continual draining of biotite‐derived magma from the lower crust with melt migration paths controlled by structural anisotropies in the protolith.     

Rates of Deglaciation during the Last Glaciation and Holocene in the Cordillera Vilcanota-Quelccaya Ice Cap Region, Southeastern Perú

Moraine chronology is combined with digital topography to model deglacial rates of paleoglacier volumes in both the Huancané Valley on the west side of the Quelccaya Ice Cap and the Upismayo Valley on the northwest side of the Cordillera Vilcanota. The fastest rates of deglaciation (39×10⁻⁵ to 114×10⁻⁵ km³ yr⁻¹ and 112×10⁻⁵ to 247×10⁻⁵ km³ yr⁻¹ for each valley, respectively) were calculated for the most recent paleoglaciers, corresponding to the last few centuries. These results are consistent with observations in the Venezuelan Andes showing high rates of deglaciation since the Little Ice Age. These rates also fall within the range of 20th century rates of deglaciation measured on the Quelccaya Ice Cap (29×10⁻⁵ to 220×10⁻⁵ km³ yr⁻¹, Brecher and Thompson, 1993; Thompson, 2000). These results imply that rates of deglaciation may fluctuate significantly over time and that high rates of deglaciation may not be exclusive to the late 20th century. Equilibrium line altitude (ELA) depressions for the ice volumes of the last glaciation modeled here were computed as 230 m for the Quelccaya Ice Cap and 170 m for the Cordillera Vilcanota. Maximum ELA depressions are lower than previously published: <500 m for the Cordillera Vilcanota and <400 m for the Quelccaya Ice Cap. These lower values could imply a topographic control over paleoglacier extent.     

Incorporation of Cr<Superscript>3+</Superscript> in dickite: a spectroscopic study

 We have investigated a well-ordered sample of natural Cr-bearing dickite from Nowa Ruda (Lower Silesia, Poland) using electron paramagnetic resonance (EPR) at X- and Q-band frequencies (9.42 and 33.97 GHz, respectively) and optical diffuse reflectance spectroscopy. The observation of the spin-forbidden transitions at 15500 and 14690 cm⁻¹ allows us to unambiguously identify the major contribution of octahedrally coordinated Cr³⁺ ions in the optical spectrum. The X- and Q-band EPR spectra show two superposed Cr³⁺ signals. The corresponding fine-structure parameters were determined at room temperature and 145 K. These results suggest the substitution of Cr³⁺ for Al³⁺ in equal proportions in the two unequivalent octahedral sites of the dickite structure. In kaolin group minerals, the distortion around Cr³⁺ ions (λ≈ 0.2–0.4) in Al sites is significantly less rhombic than that observed around Fe³⁺ ions (λ≈ 0.6–0.8). Received: 29 June 2001 / Accepted: 22 October 2001     

A three-dimensional parametric study of the use of soil nails for stabilising tunnel faces

Applying an effective nailing system at a tunnel heading, not only improves the stability of the tunnel heading and limits deformation at the tunnel face, but it also reduces volume loss during excavation and hence reduces ground surface settlement. The effectiveness of a soil nail system is affected by many factors such as the diameter and stiffness of the nails. In this paper, a systematic parametric study was conducted to study the axial rigidity of a nail, E_nA_n, for improving the stability of tunnel headings and reducing ground movements in stiff clay. The parametric study involved a series of three-dimensional elasto-plastic coupled-consolidation finite element analyses. The stability of the tunnel face is improved with increasing E_nA_n. For a given nail density applied at the tunnel face, an optimum axial rigidity of the nail (E_nA_n)_opt can be identified. The efficiency of the nailing system diminishes when (E_nA_n)_opt is reached. The use of a soil nailing system reduces the magnitude of stress relief at the tunnel heading during excavation. Thus, this reduction of stress relief minimises the amount of soil yielding and excess pore water pressure generated in the soil around the tunnel heading.