Partitioning of elements between high-temperature, low-density aqueous fluid and silicate melt as derived from volcanic gas geochemistry (original) (raw)
Related papers
Chemical Geology, 1994
In the 16 years since the Sedona Conference on the behaviour of trace elements in silicate systems, numerous studies providing new data have filled many of the gaps in knowledge of trace-element partitioning evident at that conference. The advent of new microbeam techniques for in situ trace-element analysis has provided great impetus for this work. For example, values for large ion lithophile element (LILE) and high field strength element (HFSE) partitioning between olivine, pyroxene, garnet, amphibole and titanate minerals and silicate liquids have been determined. When plotted on mantle normalizing geochemical diagrams, partition coefficients for the main mantle silicate minerals show steeply inclined patterns (over several orders of magnitude) from LILE to heavy rareearth elements (HREE). Amphibole, however, has a relatively flat pattern (though still favouring HREE over LILE by about an order of magnitude). Also, there is a notable flattening of the patterns for HREE in pyroxenes and garnet relative to olivine.
Geochimica et Cosmochimica Acta, 1992
The effects of composition and temperature on the partitioning behavior of SC, Y, and the rare earth elements (REEs) between high-Ca clinopyroxene and natural silicate melts were evaluated from doped experiments on natural mafic to intermediate composition lavas at 1 atmosphere pressure. Partition coefficients for these elements were found to be dependent on temperature and composition. The most important compositional parameters controlling clinopyroxene-melt partitioning for Y, SC, and REEs are Al content of the liquid and pyroxene Ca content. Towards the goal of deriving expressions describing partitioning behavior, approximations were made of equilibrium constants for reactions involving a REE-Al component in the pyroxene. Regression of these equilibrium constants over the experimental temperature range (1180-1050°C) produced expressions which, when applied to the experimental glasses, reproduced the clinopyroxene trace element contents with precisions between 9 and 32% (1 u). The most important conclusion of this work is that pyroxene-melt partition coefficients for trivalent cations have different compositional dependencies than divalent cations because of the participation of Al in paired substitution. Values for high-Ca pyroxene SC, Y, and REE partition coefficients have a range of over a factor of two between alkali basalts, andesites, and tholeiitic basalts (e.g., 0.2-0.7 for Sm at 1100°C). This represents a large proportion of the total range for D values from all m&c and intermediate magmas. Our contribution is to describe the parameters that control partitioning behavior. This will allow us to more accurately determine REE partitioning for specific systems. Matma Kea ankammitePotter et al. (1987) Kohala hawaiite-Porter et al. (1987) Galapagos fen&&salt-Melson (petwnal cotnmunication) Kilauea tboleiite-Neal et al. (1988) Kilawa tboleiite-Moore et al. (1980) Uwekabuna Bluff pictite-Casadevall and Dzwisin (1987) Maotta Loa tboleiiteelectron m&opt&e analysis of fused glass beads Mauna I-xx tboleiite-Rhodes (1988) Mattna Loa tboleiite-Rhodes (1988) Kilauea tboleiite-Wright and Fiske (1971) Mount Hop, AZ alkali olivine basalt.-elecaon tnicroptube analysis Of fused glaSt
Journal of Geophysical Research, 1981
A wide range of rock types (abyssal tholeiite, Fe-Ti-rich basalt, andesitc, and rhyodacite) were dredged from near 95øW and 85øW on the Galapagos spreading center. Computer modeling of major element compositions has shown that these rocks could be derived from common parental magmas by successive degrees of fractional 6rystallization. However, the P2Os/K20 ratio averages 0.83 at 95øW and 1.66 at 85øW and implies distinct mantle source compositions for the two areas. These source regions also have different rare earth element (REE) abundance patterns, with [La/Sm]oeF = 0.67 at 95øW and 0.46 at 85øW. The sequence of fractionated lavas differs for the two areas and indicates earlier fractionation of apatite and titanomagnetite in the lavas from 95øW. The mantle source regions for these two areas are interpreted to be depleted in incompatible (and volatile?) elements, although the source region beneath 95øW is less severely depleted in La and K. Incompatible trace element abundances in 26 samples are used to infer that the range of Fe-Ti-rich basalt from 85øW represents 19 to 35% residual liquid following crystal fractionation of a mineral assemblage of plagioclase, clinopyroxene, and lesser olivine. The most highly differentiated samples have also had less than 1% titanomagnetite removed. Most samples from 85øW can be related to a common parental magma that contained approximately 9 wt % FeO*, 1 wt % TiO2, and had an Mg number (Mg# --100 Mg/(Mg + Fe2+)) of about 65. Although the samples from 95øW cannot all be derived from a common parental magma, the inferred parental magmas may have been derived by varying degrees of partial melting of a common source. The fractionation sequence consists of two parts: an initial iron enrichment trend followed by a silica enrichment trend. We interpret the trace element data to indicate that the most iron rich lavas represent about 32% residual liquid derived by crystal fractionation of plagioclase, clinopyroxene, and lesser olivine from a parental magma with an Mg number of about 66. The silica enrichment trend results from crystallization of titanomagnetite and some apatite. Fractionation of pigeonitc, which is a minor phase in the major element models, cannot be distinguished from clinopyroxene fractionation by using trace elements.
Geochimica et Cosmochimica Acta
The diffusive exchange of 30 trace elements (Cs, Nb) during the interaction of natural mafic and silicic alkaline melts was experimentally studied at conditions relevant to shallow magmatic systems. In detail, a set of 12 diffusion couple experiments have been performed between natural shoshonitic and rhyolitic melts from the Vulcano Island (Aeolian archipelago, Italy) at a temperature of 1200 °C, pressures from 50 to 500 MPa, and water contents ranging from nominally dry to ca. 2 wt. %. Concentration-distance profiles, measured by Laser Ablation ICP-MS, highlight different behaviours, and trace elements were divided into two groups: (1) elements with normal diffusion profiles (13 elements, mainly low field strength and transition elements), and elements showing uphill diffusion (17 elements including Y, Zr, Nb, Pb and rare earth elements, except Eu). For the elements showing normal diffusion profiles, chemical diffusion coefficients were estimated using a concentrationdependent evaluation method, and values are given at four intermediate compositions (SiO2 equal to 58, 62, 66 and 70 wt. %, respectively). A general coupling of diffusion coefficients to silica diffusivity is observed, and variations in systematics are observed between mafic and silicic compositions. Results show that water plays a decisive role on diffusive rates in the studied conditions, producing an enhancement between 0.4 and 0.7 log units per 1 wt.% of added H2O. Particularly notable is the behaviour of the trivalent-only REEs (La to Nd and Gd to Lu), with strong uphill diffusion minima, diminishing from light to heavy REEs. Modelling of REE profiles by a modified effective binary diffusion model indicates that activity gradients induced by the SiO2 concentration contrast are responsible for their development, inducing a transient partitioning of REEs towards the shoshonitic melt. These results indicate that
Partition coefficients between a synthetic andesite melt and 1.5 and 3.0 molal (m) NaCi-H20 fluids have been measured at 1.5-2.0 GPa and 1250°C. Use of the double-capsule method allowed collection of silicate glass and of solute quenched from the fluid. Measured solubilities of silicate melt in both 1.5m and 3.Om NaCl fluid are 51 wt% at 1.5 GPa and 53 wt% at 2.0 GPa, much higher than solubilities measured for pure HZ0 at the same conditions. Wet chemical analysis of the run products yielded fluid/ melt partition coefficients (D Ruid'me't) ranging from 0.43 to 1.31 and Ti, suggesting a near-congruent dissolution of the melt in fluid. D values for many elements increase in direct proportion to melt solubility, as expected for congruent solution. Concentrations of alkalis in the fluid positively correlate with total Cl concentration, suggesting that Cl complexes with alkalis in the fluid. Results imply that NaCl-bearing aqueous fluids can dissolve large amounts of silicate material, but do not strongly fractionate elements in equilibrium with silicate melts. Thus, the geochemical signature left by metasomatic H20-NaCl fluids will not be distinctly different from that of silicate melts.
Variability of trace elements in basaltic suites
Earth and Planetary Science Letters, 1993
This paper discusses the distribution of trace element concentrations in cogenetic suites of lavas, based on the study of trace element histograms. For each histogram, the variability is calculated (V = s/m, where m is the mean concentration and s its standard deviation). The study of two basaltic series, each specific of a single igneous process (fractional crystallization and partial melting), shows that trace element variabilities are directly related to bulk partition coefficients. Considering different trace element behaviour models, quantitative relationships are established between the variability and the descriptive parameters for the processes (bulk partition coefficients, mean degree of fractionation and related standard deviation). An important feature is that the variability is independent of the initial concentrations in the source. This method is applied for comparison to several basalt suites on which case studies of magmatic processes have been conducted. It is also applied to study the behaviour of trace elements in different geodynamical contexts (hotspot, mid-ocean ridge and arc volcanism). Finally, a set of numerical values of bulk partition coefficients is proposed that accounts for N-MORB genesis.
Earth and Planetary Science Letters, 2006
We report volatile (H 2 O, CO 2 , F, S, Cl) and trace element data for submarine alkalic basalt glasses from the three youngest Samoan volcanoes, Ta'u, Malumalu and Vailulu'u. Most samples are visibly sulfide saturated, so have likely lost some S during fractionation. Cl/K ratios (0.04 -0.15) extend to higher values than pristine MORBs, but are suspected to be partly due to source differences since Cl/K roughly varies as a function of 87 Sr/ 86 Sr. There are no resolvable differences in the relative enrichment of F among sources, and compatibility of F during mantle melting is established to be nearly identical to Nd. Shallow degassing has affected CO 2 in all samples, and H 2 O only in the most shallowly erupted samples from Vailulu'u. Absolute water contents are high for Samoa (0.63 -1.50 wt%), but relative enrichment of water compared to equally incompatible trace elements (Ce, La) is low and falls entirely below normal MORB values. H 2 O/Ce (58 -157) and H 2 O/La (120 -350) correlate inversely with 87 Sr/ 86 Sr compositions (0.7045 -0.7089). This leads us to believe that, because of very fast diffusion of hydrogen in mantle minerals, recycled lithospheric material with high initial water and trace element content will lose water to the drier ambient mantle during storage within the inner Earth.
Lithos, 2000
. Trace element partition coefficients D's for up to 13 REE, Nb, Ta, Zr, Hf, Sr and Y have been determined by SIMS analysis of seven garnets, four clinopyroxenes, one orthopyroxene and one phlogopite crystallized from an undoped basanite Ž . and a lightly doped 200 ppm Nb, Ta and Hf quartz tholeiite. Experiments were conducted at 2-7.5 GPa, achieving Ž near-liquidus crystallization at relatively low temperatures of 1080-12008C under strongly hydrous conditions 5-27 wt.% . added water . Garnet and pyroxene D show a parabolic pattern when plotted against ionic radius, and conform closely to REE ) Corresponding T.H. Green . 0024-4937r00r$ -see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 0 2 4 -4 9 3 7 0 0 0 0 0 2 3 -2 ( ) T.H. Green et al.r Lithos 53 2000 165-187 166 megacrysts with negative Sr spikes may crystallize from magmas without anomalous Sr contents when plotted on mantle compatibility diagrams. Because the characteristics of strongly hydrous silicate melt and solute-rich aqueous fluid converge at high pressure, the hydrous data presented here are particularly pertinent to modelling processes in subduction zones, where aqueous fluids may have an important metasomatic role. q
Geochimica Et Cosmochimica Acta, 1998
To constrain the trace element composition of aqueous fluids in the deep crust and upper mantle, mineral-aqueous fluid partition coefficients (D min/fluid) for U, Th, Pb, Nb, Ba, and Sr have been measured for clinopyroxene, garnet, amphibole, and olivine in experiments at 2.0 GPa and 900°C. Ciinopyroxeneand garnet-fluid partition coefficients are similar for Nb (0.01-0.7) and Ba ( ~ 10-4-10-5), whereas values of D cpx/ttuid for Sr (0.5-4), Th (0.6-9), and Pb (0.04-0.09) are -10× (Th, Pb) to ~ 1000× (Sr) higher than O garnet/fluid. At the same fO2 (FMQ + 1), garnet-fluid partition coefficients for U are -10× higher than those for clinopyroxene. Amphibole-fluid partition coefficients are uniformly high ( -1 ) for all elements studied, and, with the exception of Ba, interelement fractionations are similar to clinopyroxene. The olivine-fluid partition coefficient for Nb is similar to values measured for the other silicates, whereas D °~Vm~/~u~J for U, Th, Pb, Sr, and Ba are significantly lower.