ALBERT

Notes: Previous work in nonequilibrium molecular dynamics (NEMD) has been restricted to systems subject only to pair interactions. We use methods of homogenous NEMD to investigate the nature of liquid sulfur under extreme shear using the potential model developed by Stillinger and Weber which involves explicitly three-body interaction. Simulations with up to 2048 particles have been carried out at a temperature of 1583 K and a density of 1.805 g cm−3 for shear rates between 0.005 and 1.75 in reduced units. We find that the fluid separates in sheets alternating from high to low density in planes perpendicular to the velocity gradient. No evidence is seen for the transition to the "string'' phase as exhibited by two-body systems. The molecules show a tendency to align in the direction of shear. Data are presented describing the magnitude of this effect.

Notes: Abstract Petrographic, fluid inclusion, geochemical and isotopic evidence from xenoliths in alkali basalts suggests that low-viscosity fluids rich in O-H-C, dissolved silicates and especially the incompatible elements may ascend, decompress and precipitate crystalline phases and/or induce partial fusion in the upper mantle. Such mantle metasomatic fluids (MMF) may be important in generating isotopic heterogeneity and in transporting and focusing mantle heat. In order to model the movement of MMF, the ordinary differential equations governing the variation ofP, T, ascent velocity and fluid density of a compressible, viscous, single-phase (H2O or CO2) non-reacting fluid ascending through a vertical crack of constant width have been solved. A large number of numerical simulations were carried out in which the significant factors affecting flow behavior (thermodynamic and transport fluid properties, roughness and width of cracks, geothermal gradient, initial conditions, etc.) were systematically varied. The calculations show that: (1) MMF tends to move at uniform rates following a short period of rapid initial acceleration, (2) MMF ascends nearly isothermally, (3) MMF acts as an efficient heat transfer agent; numerical experiments show that transport of heat into regions undergoing metasomatism can lead to partial fusion. The heat transported by movement of MMF averaged over the age of the Earth is sufficient to generate about 0.1 km3 of basaltic magma per year, which is approximately equal to the production rate of alkaline magma. If an intense period of mantle degassing occured early in the history of the Earth, the transport of heat and mass (K, U, Rb, LREE) by migrating fluids might have been important.

Notes: Abstract The Mogan and Fataga formations on the island of Gran Canaria, Canary Islands, represent a sequence of approximately 30 intercalated pyroclastic and lava flows (total volume about 500 km3 dense-rock equivalent) including subalkaline rhyolitic, peralkaline rhyolitic and trachytic pyroclastic flows, nepheline trachyte lavas and a small volume of alkali basaltic lavas and tephra deposits. The eruption of the intermediate to silicic rocks of the Mogan and Fataga formations follows the roughly 4 Ma duration of basaltic shield volcanism. The most common assemblage in the evolved (Mogan and Fataga) rocks is anorthoclase+ edenitic amphibole+ilmenite+magnetite±augite±hypersthene +apatite+pyrrhotite. A few flows also contain plagioclase, biotite, or sphene. Coexisting Fe-Ti oxides yield equilibrium temperatures between 835 and 930° C and log $$f_{O_2 } $$ between −11.2 and −12.6. The lowermost pyroclastic flow of the Mogan formation is zoned from a rhyolitic base (848° C) to a basaltic top (931° C). Unit P1 has an oxygen isotope feldspar-magnetite temperature (850° C) very close to its Fe-Ti oxide temperature. One of the youngest Mogan flows is zoned from a comendite (836° C) at the base to a comenditic trachyte (899° C) at the top. The Fataga formation pyroclastic flows show only slight compositional zonation, and one flow has the same Fe-Ti oxide compositions at top and base. Calculations using the reaction 1/3 magnetite+SiO2 (melt)=ferrosilite+1/6 O2 indicate total pressures of 1–4 (±3) kb for six of the Mogan flows and one of the Fataga flows. For four of the pyroclastic flows, equilibria involving tremolite-SiO2-diopside-enstatite-H2O and phlogopite-SiO2-sanidine-enstatite-H2O imply water contents of 0.9 to 2.6 (±0.5) wt% and $$f_{H_2 O} $$ between 80 and 610 bars, which indicates that magma within the Tejeda reservoir was H2O-undersaturated throughout the entire history of Mogan to Fataga volcanism. The fluorine contents of amphibole, biotite, and apatite, and chlorine contents of apatite reveal thatf HF/ $$f_{H_2 O} $$ andf HCl/ $$f_{H_2 O} $$ high compared to most igneous rocks and are consistent with the peralkaline nature of most of the volcanics. Thef HCl estimate for one flow is 10−2 to 10−1 bars andf HF for six of the flows ranges from about 10−1 to 6 bars. Pyrrhotite compositions yield estimates for log $$f_{S_2 } $$ between −1 and −3, log $$f_{SO_2 } $$ between −2 and 1.5, and log $$f_{H_2 S} $$ between 0.5 and 3, which fall in the range of most intermediate to silicic systems. The lack of a systematic trend with time for magma composition, Fe-Ti oxide temperatures, water contents, phenocryst abundances, and ferromagnesian phase composition indicate that the Tejeda magmatic system was open and kept at nearly the same conditions by the periodic addition of more primitive melts. The intensive thermodynamic parameters estimated from coexisting phenocryst equilibria are used to constrain the eruption dynamics based on solution of the conservation equations for a vapor plus pyroclast mixture. The estimates of magma reservoir temperature, pressure, and water concentration, when combined with a one-dimensional fluid dynamical model of a pyroclastic eruption, imply that the velocities of the ash flows at the vent exit were on the order of 100 to 200 m s−1, and the mass flow rates were about 107 kg s−1 for an assumed vent radius of 10 m.

Notes: [Auszug] Various possibilities for buoyancy-induced flows along the walls of a magma chamber are summarized in Fig. 1. In the simple case, where only thermal buoyancy is important (Fig.?a), negative buoyancy along the cold chamber wall dictates a boundary layer region of downwelling flow there. The velocity ...

Notes: Abstract Thermochemical calculations and laboratory phase equilibration experiments on lavas of the 131 day 1983 Mt. Etna flank eruption of 0.1 km3 were undertaken to investigate possible systematic variations in inferred melt-phenocryst equilibration conditions as a function of time. The 1983 Mt. Etna lavas are multiply saturated; plagioclase, clinopyroxene and olivine, the dominant phenocrysts, occur in the ratio 1:1/2:1/4. Melts (glasses) plot close to the plagioclase saturated olivine-clinopyroxene low pressure cotectic on a Walker-O'Hara diopside-forsterite-silica diagram suggesting equilibration of melt and phenocrysts in a high level magma reservoir. Total pressures, temperatures and dissolved H2O concentrations were calculated using the isoactivity method of Carmichael and coworkers based on about 300 elelctron microprobe analyses of coexisting olivine, clinopyroxene and plagioclase phenocrysts, microphenocrysts and groundmass microlites for samples collected 6, 46 and 125 days after the start of the eruption. Total pressures (P t), temperatures and H2O contents based on representative olivine-clinopyroxene pairs are 140 MPa, 1105°C, 2.4 wt% H2O; 255 MPA, 1112°C, 1.0 wt% H2O and 85 MPa, 1096°C, 1.8 wt% H2O respectively for the early (283), middle (I83) and late (L83) samples. Corresponding equilibration depths are in the range 3 to 10 kilometers. Plagioclase feldspar phenocrysts, while showing more evidence of disequilibrium, provide compatible estimates of P t and T when analysis is restricted to the low anorthite mode of the plagioclase frequency-composition histograms: 133 MPa and 1115°C; 260 MPa and 1117°C and 103 MPa and 1104°C, repectively for 283, I83 and L83. The pre-eruptive (i.e., in situ) temperature-pressure gradient calculated from olivine-clinopyroxene equilibria is 10.6 K/kbar. This compares well with independent estimates of the temperature-pressure derivative of the (pseudo) invariant point composition (10 to 12 K/kbar) in both model (e.g., diopside-forsterite-anorthite, Presnall et al. 1978) and natural (e.g., Walker et al. 1979; Grove et al. 1982) systems. Apparently, magma within the Etna reservoir was in a quasiequilibrium state buffered by its multiply-saturated character immediately preceding eruption. The temporal variation of computed P t, T and H2O concentrations for melt-phenocryst equilibrium agrees well with predictions based on simulations of the withdrawal of magma from a body zoned with respect to dissolved H2O provided the temporal record of magma discharge is taken into account. Discharge varied by a factor of about 100 during the sample collection interval. The intermediate P t but high H2O content inferred for sample 283 reflects the withdrawal of H2O enriched magma during an early phase of high average discharge of about (3∼50 m3/s) before evaculation isochrons became quasistationary. The high P t and relatively dry I83 magma reflects the deepening of the evacuation isochrons after 50 days of intermediate discharge with the development of quasi-stationary isochrons in time and space. Sample L83 from day 125 near the end of the eruption reflects the “shoaling” of evacuation isochrons (hence low P t and relatively high H2O content) associated with the observed low (0.5 m3/s) discharge. Our results show that thermochemical modeling efforts provide important opportunities for testing the predictions of magma with-drawal simulations.

Notes: Abstract This paper is a companion to Clark (1988; hereafter Part I) which described the evolution of the Tejeda Magmatic System (TMS), a Miocene caldera complex, Gran Canaria, Spain, based on geochronologic, paleomagnetic and field data. In this study, petrochemical data are used to corroborate the history out-lined in Part I. Geochemical discriminant analysis shows that whereas the Extra-Caldera (EC) Mogan/Fataga volcanics are separated by a composition gap, no composition gap exists within the Intra-Caldera (IC) sequence. IC ignimbrites change rapidly but progressively from pantellerites and comendites to comenditic trachytes and finally to trachytes in a 0.47 Ma time interval. Significantly, the lower pantelleritic part of the IC series is similar to the EC pantellerites (units B, C and D) as expected based on results from Part I. The appearance of a compositional gap in the EC sequence is the result of flows having been trapped within the caldera during the 0.47 Ma Mogan-Fataga transition interval. The transitional IC sequence may be geochemically modelled by mixing of Mogan comendites and Fataga trachytes. The mixing was most probably induced by the high discharge of magma from the compositionally-zoned Tejeda magma body. The rate of change in erupted composition is best explained by imagining a continuous influx of Fataga or parental Fataga magma into a chamber whose previous silicic component (Mogan composition) was no longer being replenished and that the two magmas did not convectively mix prior to eruption. Repose times between successive eruptions in the lower to middle Mogan (from P1/T1 to A) were of order 30 000 a; the upper Mogan pantellerites and comendites/comenditic trachytes (B to F?) erupted once every 125 000 years or so. The longer repose time for the upper units is consistent with their more differentiated character.

Notes: Abstract Kinetic and fluid dynamic constraints on deep-seated magma migration rates suggest ascent velocities in the range 10 to 30 m/s, 10−1 to 10 m/s and 10−2 to 5 m/s for kimberlitic, garnet peridotite-bearing and spinel peridotite-bearing alkalic magmas. These rates virtually demand translithospheric magma transport by a fracture as opposed to diapiric mechanism. The hypothesis that volatile exsolution accelerates magma through the deep lithosphere is tested by solution of the appropriate set of conservation, mass balance and volatile component solubility equations governing the steady ascent (decompression) of compressible, two-phase magma (melt+H2O+CO2) in which irreversible phenomena (friction, heat transfer) are accounted for. The results of the numerical experiments were designed to test the importance of melt bulk composition (kimberlite, nephelinite, alkali basalt), initial conditions (mass flux (M), heat transfer coefficient (B), lumped friction factor (C f )), conduit width (D), initial magma volatile content and geothermal gradients. The fractional increase in ascent rate (Δu/u i ) is rarely greater than approximately 2 during translithospheric migration. The propellant hypothesis is rejected as a first-order mechanism driving magma acceleration during ascent. The most influential parameters governing ascent dynamics are M, C f , D, B and the geotherm. Because of the relatively incompressible nature of the magmatic volatile phase at P〉100 MPa, the initial magma volatile content plays a secondary (although demonstrable) role. The main role of volatiles is in controlling the initial magma flux (M) and the magma pressure during ascent. In adiabatic (B=0) simulations, magma ascends nearly isothermally. Generally, however, the assumption of adiabaticity is a poor one especially for flow through narrow (0.5 to 2 m) conduits in old (cold) lithosphere at rates ∼10−1 m/s. The proposed fluid dynamic model is consistent with and complementary to the magma-driven crack propagation models. The generation of mantle metasomatic fluid is a corollary of the non-adiabatic ascent of volatile-bearing magma through the lithosphere. Magma heat death is an important process for the creation of mantle heterogeneity.

Notes: Abstract The simplified model of basalt genesis described in Part I of this series, equilibrium partial melting followed by Rayleigh-type fractional crystallization, is applied to a stratigraphically controlled sequence of basalt flows from Kohala volcano. Major-element compositions were determined for 52 samples and show a time-stratigraphic progression from tholeiites through transitional basalts to alkali basalts. Twenty-six of these samples were analyzed by isotope dilution for K, Rb, Cs, Sr, Ba and the REE, 13 for87Sr/86Sr, and 19 for Co, Cr, Ni and V by atomic absorption. After a simple, first-order correction for the effects of fractional crystallization (involving mostly olivine and aluminous clinopyroxene), the major element concentrations cluster tightly, and the incompatible trace elements show monotonic increases in concentration as a function of stratigraphic height. The process identification plot shows that all the (fractionation corrected) melt compositions can be explained by equilibrium partial melting of compositionally identical batches of source material. The REE and Sr are fractionated because of the presence of residual clinopyroxene. Garnet may also be present but in much smaller amounts. In this respect our results differ significantly from those of Leeman et al. (1980). The calculated chondrite-normalized REE patterns of the source are nearly flat to slightly convex upward. Therefore there is no need to invoke special mechanisms, such as metasomatic REE preenrichment of the source, in order to explain the petrogenesis of the suite of lavas. Specifically, Ce concentrations ranging from 20 to 250 times chondritic are all explained by the same calculated source pattern having a chondrite-normalized ratio of Ce/Sm=0.9±0.2. However, the normalized ratio Ce/Ba≅2 shows that the source is not simply primitive mantle.

Notes: Abstract A number of experimental CO2 solubility data for silicate and aluminosilicate melts at a variety of P- T conditions are consistent with solution of CO2 in the melt by polymer condensation reactions such as SiO 4(m 4− +CO2(v)+Si n O 3n+1(m) (2n+1) ⇌Si n+1O 3n+4(m) (2n+4)− +CO 3(m )2− . For various metalsilicate systems the relative solubility of CO2 should depend markedly on the relative Gibbs free change of reaction. Experimental solubility data for the systems Li2O-SiO2, Na2O-SiO2, K2O-SiO2, CaO-SiO2, MgO-SiO2 and other aluminosilicate melts are in complete accord with predictions based on Gibbs Free energies of model polycondesation reactions. A rigorous thermodynamic treatment of published P- T-wt.% CO2 solubility data for a number of mineral and natural melts suggests that for the reaction CO2(m) ⇌ CO2(v) (1) CO2-melt mixing may be considered ideal (i.e., { $$a_{{\text{CO}}_{\text{2}} }^m = X_{{\text{CO}}_{\text{2}} }^m $$ ); (2) $$\bar V_{{\text{CO}}_{\text{2}} }^m $$ , the partial molal volume of CO2 in the melt, is approximately equal to 30 cm3 mole−1 and independent of P and T; (3) Δ C p 0 is approximately equal to zero in the T range 1,400° to 1,650 °C and (4) enthalpies and entropies of the dissolution reaction depend on the ratio of network modifiers to network builders in the melt. Analytic expressions which relate the CO2 content of a melt to P, T, and $$f_{{\text{CO}}_{\text{2}} } $$ for andesite, tholeiite and olivine melilite melts of the form $$\ln X_{{\text{CO}}_{\text{2}} }^m = \ln f_{{\text{CO}}_{\text{2}} } - \frac{A}{T} - B - \frac{C}{T}(P - 1)$$ have been determined. Regression parameters are (A, B, C): andesite (3.419, 11.164, 0.408), tholeiite (14.040, 5.440,0.393), melilite (9.226, 7.860, 0.352). The solubility equations are believed to be accurate in the range 3〈P〈30 kbar and 1,100°〈T〈1,650 °C. A series of CO2 isopleth diagrams for a wide range of T and P are drawn for andesitic, tholeiitic and alkalic melts.