High pressure fluids in the system MgO–SiO2–H2O under upper mantle conditions

Fluids and melts have been trapped and analysed in high pressure experiments in the model mantle system MgO-SiO₂-H₂O at 6 to 10.5 GPa and 900 to 1,200 °C. The fluid/melt traps consisted of a diamond layer that was added to the experimental charge and was separate from the silicate phases. The recovered diamond traps were analysed by laser ablation - ICP - MS. Starting materials were synthetic mixtures of brucite, talc and silica with variable Mg/Si containing 11-31 wt% H₂O. Experiments on a serpentine starting composition [Mg₃Si₂O₅(OH)₄] result in MgO/SiO₂ weight ratios in the subsolidus fluids close to 1 at 6 GPa and close to 2 at 9 GPa. Melt compositions at 6 and 9 GPa have MgO/SiO₂ ratios close to that of forsterite. At a single pressure the amount of dissolved silicate in the fluid increases steadily with increasing temperature up to 1,150 °C, where a sudden increase of both SiO₂ and MgO is observed. This discrete step marks the solidus, which is more clearly developed at 6 than at 9 GPa. Thus, hydrous melts within the model mantle subsystem Mg₂SiO₄-Mg₂Si₂O₆-H₂O are chemically distinct from aqueous fluids up to at least 9 GPa, corresponding to 300 km depth. Extrapolation of the current data set implies that total convergence between fluid and melt along the solidus probably occurs at 12-13 GPa (~400 km), i.e. close to the Earth's mantle transition zone. Beneath cratons, interactions of hydrous fluids with upper mantle lithologies cause relative silica depletion (olivine enrichment) at depths greater than 200 km and silica (orthopyroxene) enrichment at shallower depths.     

A rocking multianvil: elimination of chemical segregation in fluid-saturated high-pressure experiments

Fluid saturated high-pressure experiments often result in strongly zoned experimental charges, this hinders experimentation in chemically homogeneous systems which in turn has serious consequences on equilibration, reaction progress, and (apparent) phase stabilities. In order to overcome these problems, a 600-ton press accommodating either a multianvil or end-loaded piston cylinder module has been mounted in such a way that it can be turned by 180°, thus inverting its position in the gravity field. During turning, hydraulic pressure, heating power, and cooling water remain connected allowing fully controlled pressures and temperatures during experiments.A series of experiments at 13 GPa, 950°C, on a serpentine bulk composition in the MgO-SiO₂-H₂O system demonstrates that continuous turning at a rate of 2 turns/min results in a nearly homogeneous charge composed of phase E + enstatite. The same experiment at static conditions resulted in four mineral zones: quench phase E, enstatite, enstatite + phase E, and phase E + phase A. Phase A disappears in experiments at a turning rate ≥1 turn/min. A static 15-min experiment shows that zonation already forms within this short time span. Placing two short capsules within a single static experiment reveals that the fluid migrates to the hot spot in each capsule and is not gravitationally driven toward the top. The zonation pattern follows isotherms within the capsule, and the degree of zonation increases with temperature gradient (measured as 10 °C within a capsule) and run time.Our preferred interpretation is that Soret diffusion causes a density-stratified fluid within the capsule that does not convect in a static experiment and results in temperature dependant chemical zonation. The aggravation of zonation and appearance of additional phases with run time can be explained with a dissolution-reprecipitation process where the cold spot of the capsule is relatively MgO enriched and the hot spot relatively SiO₂ and H₂O enriched (at 13 GPa and 950°C). Rocking and tilting of a stratified fluid induces Rayleigh-Taylor instabilities, causing chemical rehomogenization. If turning is faster than the time required to build significant chemical potential gradients in the fluid, chemical zonation in the distribution of the solids is suppressed.     

High pressure single-crystal synthesis, structure and compressibility of the garnet Mn2+ 3Mn3+ 2[SiO4]3

Single crystals of the garnet Mn²⁺ ₃Mn³⁺ ₂[SiO₄]₃ and coesite were synthesised from MnO₂-SiO₂ oxide mixtures at 1000°C and 9 GPa in a multianvil press. The crystal structure of the garnet [space group Ia3¯d, a=11.801(2) Å] was refined at room temperature and 100 K from single-crystal X-ray data to R1=2.36% and R1=2.71%, respectively. In contrast to tetragonal Ca₃Mn³⁺ ₂[GeO₄]₃ (space group I4₁/a), the high-pressure garnet is cubic and does not display an ordered Jahn-Teller distortion of octahedral Mn³⁺. A disordered Jahn-Teller distortion either dynamic or static is evidenced by unusual high anisotropic displacement parameters. The room temperature structure is characterised by following bond lengths: Si-O=1.636(4) Å (tetrahedron), Mn³⁺-O=1.995 (4) Å (octahedron), Mn²⁺-O=2.280(5) and 2.409(4) Å (dodecahedron). The cubic structure was preserved upon cooling to 100 K [a=11.788(2) Å] and upon compressing up to 11.8 GPa in a diamond-anvil cell. Pressure variation of the unit cell parameter expressed by a third-order Birch-Murnaghan equation of state led to a bulk modulus K ₀=151.6(8) GPa and its pressure derivatives K′=6.38(19). The peak positions of the Raman spectrum recorded for Mn²⁺ ₃Mn³⁺ ₂[SiO₄]₃ were assigned based on a calderite Mn²⁺ ₃Fe³⁺ ₂[SiO₄]₃ model extrapolated from andradite and grossular literature data.     

Entrained Macrocryst Minerals as a Key to the Source Region of Olivine Nephelinites: Humberg, Kaiserstuhl, Germany

Olivine nephelinites commonly contain macrocrysts of olivineand clinopyroxene. Some of these macrocrysts might representfragments of the source region of the host magma transportedto the Earth's surface. If this hypothesis is correct thesefragments can be used to characterize the composition of thesource region and to put constraints on the magma generationprocess. In this study, we investigate the origin of macrocrystsand mineral aggregates from an olivine nephelinite from theKaiserstuhl, Germany. We focus on clinopyroxenes (Cpx), whichcan be divided into three groups. Cpx I is relict Cpx from aggregateswith deformed olivine that is depleted in Ca and characterizedby strong light rare earth element (LREE) fractionation, lowTi/Eu and negative high field strength element (HFSE) anomalies.Its geochemical signature is consistent with formation by carbonatitemetasomatism and with equilibration in the presence of orthopyroxene.Cpx II is Ca-rich Cpx, forming both aggregates with deformedolivine and individual macrocrysts. The LREE, as for Cpx I,are strongly fractionated. Convex REE patterns may be present.The depletion in HFSE is less pronounced. Cpx III is oscillatoryzoned Cpx phenocrysts showing enrichment in Ca, convex REE patternsand no HFSE anomalies. The transition in the trace element abundancesbetween the Cpx of the three groups is gradual. However, CpxI and II did not crystallize from the host magma, as demonstratedby the presence of kink-bands and undulose extinction in theassociated olivine and by the composition of alkali aluminosilicateglass inclusions in Cpx II. Based on the Cpx relationships,we interpret the studied suite of macrocrysts and mineral aggregatesas a mixture of disintegrated fragments of the source regionof the host olivine nephelinite. The process of melt generationwas multi-stage. A primary carbonatite melt ascending from deeperlevels in the mantle, probably from the dolomite–garnetperidotite stability field, reacted with mantle peridotite alongthe solidus ledge in the system lherzolite–CO₂ (< 20–22kbar) and started to crystallize carbonate minerals. Becauseof its low solidus temperature, the resulting carbonate-wehrliteassemblage melted incongruently with the formation of additionalclinopyroxene. The carbonatite melt evolved during crystallizationof carbonate minerals and concomitant incongruent melting ofthe carbonate-wehrlite, accompanied by the segregation of incipientalkali aluminosilicate melts. As a consequence of fast reactionrates in the presence of a carbonatite melt, this process probablytook place under disequilibrium conditions. Further meltingof the assemblage wehrlite + alkali aluminosilicate melt ledto the generation of the olivine nephelinite magma. It entrainedfragments of the wehrlite and brought them to the surface. KEY WORDS: carbonatite; metasomatism; source region; clinopyroxene macrocrysts; wehrlite; olivine nephelinite; Kaiserstuhl     

Petrology and Mineral Chemistry of Lower Crustal Intrusions: the Chilas Complex, Kohistan (NW Pakistan)

Mineral major and trace element data are presented for the mainrock units of the Chilas Complex, a series of lower crustalintrusions emplaced during initial rifting within the MesozoicKohistan (paleo)-island arc (NW Pakistan). Detailed field observationsand petrological analysis, together with geochemical data, indicatethat the two principal units, ultramafic rocks and gabbronoritesequences, originate from a common parental magma, but evolvedalong different mineral fractionation trends. Phase petrologyand mineral trace element data indicate that the fractionationsequence of the ultramafic rocks is dominated by the crystallizationof olivine and clinopyroxene prior to plagioclase, whereas plagioclaseprecedes clinopyroxene in the gabbronorites. Clinopyroxene inthe ultramafic rocks (with Mg-number [Mg/(Fe_tot + Mg] up to0·95) displays increasing Al₂O₃ with decreasing Mg-number.The light rare earth element depleted trace element pattern(Ce_N/Gd_N 0·5–0·3) of primitive clinopyroxenesdisplays no Eu anomaly. In contrast, clinopyroxenes from thegabbronorites contain plagioclase inclusions, and the traceelement pattern shows pronounced negative anomalies for Sr,Pb and Eu. Trace element modeling indicates that in situ crystallizationmay account for major and trace element variations in the gabbronoritesequence, whereas the olivine-dominated ultramafic rocks showcovariations between olivine Mg-number and Ni and Mn contents,pointing to the importance of crystal fractionation during theirformation. A modeled parental liquid for the Chilas Complexis explained in terms of mantle- and slab-derived components,where the latter component accounts for 99% of the highly incompatibleelements and between 30 and 80% of the middle rare earth elements.The geochemical characteristics of this component are similarto those of a low percentage melt or supercritical liquid derivedfrom subducted mafic crust. However, elevated Pb/Ce ratios arebest explained by additional involvement of hydrous fluids.In accordance with the crystallization sequence, the subsolidusmetamorphic reactions indicate pressures of 0·5–0·7GPa. Our data support a model of combined flux and decompressionmelting in the back-arc. KEY WORDS: Kohistan; Island arc; gabbro; trace element modelling; lower crustal intrusion     

Propagation of P and S-waves in magmas with different crystal contents: Insights into the crystallinity of magmatic reservoirs

Increasing amount of crystals tends to reduce the mobility of magmas and modifies its elastic characteristics (e.g. [Caricchi, L. et al., 2007. Non-Newtonian rheology of crystal-bearing magmas and implications for magma ascent dynamics. Earth and Planetary Science Letters, 264: 402–419.; Bagdassarov, N., Dingwell, D.B. and Webb, S.L., 1994. Viscoelasticity of crystal- and bubble-bearing rhyolite melts. Physics of the Earth and Planetary Interior, 83: 83–99.]). To quantify the effect of crystals on the elastic properties of magmas the propagation speed of shear and compressional waves have been measured at pressure and temperatures relevant for natural magmatic reservoirs. The measurements have been performed in aggregates at variable particle fractions (? = 0–0.7). The measurements were carried out at 200 MPa confining pressure and temperatures between 300 K and 1273 K (i.e. across the glass transition temperature (Tg) from glass to melt). The specimens were mixtures of a haplogranitic melt containing 5.25 wt.% H₂O and variable amounts of sub-spherical alumina particles. Additional experiments were carried out on a sample containing both, crystals and air bubbles. The temperature derivatives of the shear (dVs/dT) and compressional wave (dVp/dT) velocities for pure glass and samples with a crystal fraction of 0.5 are different below and above the glass transition temperature. For a crystal fraction 0.7, only dVp/dT changed above the Tg. In the presence of gas bubbles, Vp and Vs decrease constantly with increasing temperature. The bubble-bearing material yields a lower bulk modulus relative to its shear modulus. The propagation velocities of compressional and shear waves increase non-linearly with increasing crystal fraction with a prominent raise in the range 0.5 < ? < 0.7. The speed variations are only marginally related to the density increase due to the presence of crystals, but are dominantly related to the achievement of a continuous crystal framework. The experimental data set presented here can be utilized to estimate the relative proportions of crystals and melt present in a magmatic reservoir, which, in turn, is one of the fundamental parameters determining the mobility of magma and, consequently, exerting a prime control on the likelihood of an eruption from a sub-surficial magma reservoir.     

Application of High Spatial Resolution Laser Ablation ICP-MS to Crystal-Melt Trace Element Partition Coefficient Determination

In this contribution we evaluate the capabilities of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) using a 12 μm spot size. Precision, accuracy and detection limits were assessed on the USGS BCR-2G reference material. We demonstrate that the 12 μm LA-ICP-MS analyses of experimentally-grown amphibole and garnet are in excellent agreement with secondary ion mass spectrometry (SIMS) trace element determinations on the same crystals. The 12 μm spot size configuration was subsequently used to determine trace element crystal-melt partition coefficients (D_c/m) for a wide range of trace elements in amphibole in equilibrium with a basanitic melt. The following strategy to determine accurately and evaluate D_c/m is proposed. One or more major elements determined previously by electron probe microanalysis (EPMA) was used to ensure consistency between EPMA and the composition of the aerosol produced by the laser ablation. Measured D_c/m values were successively evaluated using the lattice strain model. The use of this strategy significantly improved the precision and accuracy of D_c/m determination when a LA-ICP-MS configuration with a high spatial resolution was employed.     

Clinopyroxene geobarometry of magmatic rocks Part 1: An expanded structural geobarometer for anhydrous and hydrous, basic and ultrabasic systems

Crystal-structure modeling of experimental Ca-rich clinopyroxenes [Ca + Na > 0.5 apfu; Mg/(Mg + Fe²⁺) > 0.7] coexisting with basic and ultrabasic melts was utilized for calibration of geobarometers based on unit-cell volume (V_cell) vs M1-site volume (V_M1). The clinopyroxene database includes over one hundred experiments from literature and sixteen previously unpublished experiments on basanite and picrobasalt starting materials. The coexisting melts span a wide range of petrologically relevant anhydrous and hydrous compositions (from quartz-normative basalt to nephelinite, excluding high-Al basalts and melts coexisting with garnet or melilite) at pressure conditions pertinent to the earth's crust and uppermost mantle (P= 0–24 kbar) in a variety of fO ₂ conditions (from CCO-buffered to air-buffered) and mineral assemblages (Cpx ± Opx ± Pig ± Ol ± Plag ± Lc ± Ne ± Spl ± Amp ± Ilm). As previously found for near-liquidus products of basaltic melts, the experimental clinopyroxenes follow two distinct trends: (i) at a given P, V_cell is linearly and negatively correlated with V_M1. This corresponds with the extent of Tschermak-type substitutions, which depends strongly on aSiO₂ and a _CaO; (ii) for a fixed melt composition, V_cell and V_M1 decrease linearly as P increases, due to a combination of M₁, M₂ and T site exchanges. Despite the chemical complexity of these relationships, P could be modeled as a linear function of V_cell and V_M1. A simplified solution for anhydrous magmas reproduced the experimental pressures with an uncertainty of 1.75 kbar (=1 ; max. dev. = 5.5 kbar; N = 135). An expanded T-dependent solution capable of recovering the measured pressures of both anhydrous and hydrous experiments with an uncertainty of 1.70 kbar (=1 ; max. dev. = 5.4 kbar; N = 157) was obtained by correcting unit-cell and M1-site volumes for thermal expansivity and compressibility. The corrected formulation is more resistant to the effects of temperature variations and is therefore recommended. Nevertheless, it requires an independent, accurate estimate of crystallization T. Underestimating T by 20 °C propagates into a 1-kbar increase of calculated P. The applicability of the T-dependent formulation was tested on hydrous ultramafic to gabbroic rocks of the southern Adamello batholith for which P-T evolution could independently be constrained by field observation, petrography and experimentally determined phase relations. The pressure estimates obtained by clinopyroxene structural geobarometry closely matched those predicted by phase equilibria of a picrobasaltic melt parental to the investigated magmatic rocks. To facilitate application of the present geobarometers, both anhydrous and corrected solutions were implemented as MS-DOS^® and UNIX^® software programs (CpxBar) designed to permit retrieval of the pressure of crystallization directly from a chemical analysis or from uncorrected unit-cell and M1-site volume X-ray data.     

A high-pressure experimental study on the evolution of the silicic magmatism of the Main Ethiopian Rift

In this contribution we report the results of an experimental study that investigated equilibrium and fractional crystallization of hydrous, transitional alkaline basalt at low oxygen fugacity, under lower to middle crustal conditions to constrain the generation of subaluminous and peralkaline differentiation products that typically occur in rift systems. The experiments reveal that liquids produced by equilibrium crystallization in the range 0.7–1 GPa cannot cross the subaluminous/peralkaline compositional divide. In contrast, fractional crystallization experiments under isobaric and polybaric conditions approach closer the naturally observed trend from subaluminous to evolved peralkaline products suggesting that polybaric differentiation starting at elevated pressures can indeed lead to the transition from subaluminous to peralkaline derivative liquids. The presence of water in the parental magmas of silicic derivative products is of prime importance for the fractionation equilibria as well as for the mobility of such magmas toward shallow crustal levels.

We suggest that peralkaline magmas in rift environments are indicative for differentiation under relatively low oxygen fugacity conditions in an extensional environment characterized by a high degree of crustal fracturing that allows rapid upward migration of mafic parental magmas and formation of shallow magma reservoirs. Crystallization–differentiation of parental, hydrous transitional alkaline basalt in such reservoirs is controlled by low pressure phase equilibria that typically evolve through early saturation of anorthite-rich plagioclase and suppressed amphibole crystallization resulting in ‘low-alumina’, peralkaline derivative liquids.