Trevor H. Green

Experimental studies of trace-element partitioning applicable to igneous petrogenesis — Sedona 16 years later

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 rare-earth 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. The effects of pressure, temperature and melt composition on trace-element partition coefficients have been evaluated, as well as crystal-chemical controls on the substitution of trace elements in minerals. This has led to formulation of relationships between mineral compositions and trace-element partition coefficients for olivine, low-Ca pyroxene and calcic pyroxene. These studies have been motivated by the need of geochemists for partition coefficient data to apply to models of igneous petrogenesis and mantle evolution. Overall, the new data show systematic and consistent behaviour, as determined in different laboratories. Also, partitioning relationships for key elements between selected mantle minerals and H2O-rich fluids have been established experimentally, although further work exploring the effect of variable fluid composition, for a wider range of mantle minerals is needed. Future work will allow refinement and more precise “fingerprinting” of geochemical processes, including the role of fluids and trace element-enriched accessory minerals in metasomatic events. Also, experiments are needed to extend the partition coefficient determinations to much greater pressure, to resolve controversies over the geochemical evolution of the mantle at depths greater than ≈ 130 km.

SIMS determination of trace element partition coefficients between garnet, clinopyroxene and hydrous basaltic liquids at 2–7.5 GPa and 1080–1200°C

Abstract Trace element partition coefficients (Ds) 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 and a lightly doped (200 ppm Nb, Ta and Hf) quartz tholeiite. Experiments were conducted at 2–7.5 GPa, achieving near-liquidus crystallization at relatively low temperatures of 1080–1200°C under strongly hydrous conditions (5–27 wt.% added water). Garnet and pyroxene DREE show a parabolic pattern when plotted against ionic radius, and conform closely to the lattice strain model of Blundy and Wood (Blundy, J.D., Wood, B.J., 1994. Prediction of crystal–melt partition coefficients from elastic moduli. Nature 372, 452–454). Comparison, at constant pressure, between hydrous and anhydrous values of the strain-free partition coefficient (D0) for the large cation sites of garnet and clinopyroxene reveals the relative importance of temperature and melt water content on partitioning. In the case of garnet, the effect of lower temperature, which serves to increase D0, and higher water content, which serves to decrease D0, counteract each other to the extent that water has little effect on garnet–melt D0 values. In contrast, the effect of water on clinopyroxene–melt D0 overwhelms the effect of temperature, such that D0 is significantly lower under hydrous conditions. For both minerals, however, the lower temperature of the hydrous experiments tends to tighten the partitioning parabolas, increasing fractionation of light from heavy REE compared to anhydrous experiments. Three sets of near-liquidus clinopyroxene–garnet two-mineral D values increase the range of published experimental determinations, but show significant differences from natural two-mineral Ds determined for subsolidus mineral pairs. Similar behaviour is observed for the first experimental data for orthopyroxene–clinopyroxene two-mineral Ds when compared with natural data. These differences are in large part of a consequence of the subsolidus equilibration temperatures and compositions of natural mineral pairs. Great care should therefore be taken when using natural mineral–mineral partition coefficients to interpret magmatic processes. The new data for strongly hydrous compositions suggest that fractionation of Zr–Hf–Sm by garnet decreases with increasing depth. Thus, melts leaving a garnet-dominated residuum at depths of about 200 km or greater may preserve source Zr/Hf and Hf/Sm. This contrasts with melting at shallower depths where both garnet and clinopyroxene will cause Zr–Hf–Sm fractionation. Also, at shallower depths, clinopyroxene-dominated fractionation may produce a positive Sr spike in melts from spinel lherzolite, but for garnet lherzolite melting, no Sr spike will result. Conversely, clinopyroxene 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.

Significance of Nb/Ta as an indicator of geochemical processes in the crust-mantle system

Abstract A mantle value of ∼ 17.5 for Nb/Ta appears well established; less well established are crustal values of ∼ 11–12, although it appears that Nb/Ta for crustal-derived melts is less than mantle Nb/Ta, demonstrating fractionation of these two elements during crustal evolution, and suggesting that Nb/Ta variation may be indicative of a particular chemical process within the crust-mantle system. Experimental studies on silicate and carbonatitic liquids at high pressure indicate that, although silicate minerals such as garnet, amphibole and clinopyroxene do fractionate Nb and Ta, the partition coefficients (Ds) for both elements are very low. Thus involvement of these minerals may explain relatively small changes in Nb/Ta, but appears inadequate to explain the crust-mantle variation. However, high-quality data for Nb, Ta may be used to provide information on mantle melting or metasomatic processes (e.g., amphibole in the source region decreases Nb/Ta in derived melts, while carbonatitic metasomatism will increase Nb/Ta in affected mantle). Titanate minerals have high Ds for Nb and Ta, and do fractionate these elements (e.g., D Nb D Ta rutile/liquid of 0.5–0.8), and their involvement in crystal fractionation would increase Nb/Ta in derivative liquids. In contrast, D Nb D Ta for rutile/fluid is ∼ 1.25, so that rocks affected by fluid equilibrated with residual rutile will show a decrease in Nb/Ta Some Archaean gneisses appear to have high Nb/Ta, and may be a complementary component to that part of the crust which has a relatively low Nb/Ta, such as crustal-derived magmas (e.g., A- ad I-type granites and silicic volcanics). Within the crustal system pegmatites are known to have extremely high and variable Nb, Ta contents, often with low Nb/Ta. A fluid is generally considered to be involved in the generation of these rocks. Thus it is possible that fluid/melt partitioning may be the key to fractionating Nb and Ta, with preference for Ta in the fluid, and enrichment of Ta relative to Nb into the mid-upper crustal system, as the crust evolved, through upward movement of fluid.

Apatite/liquid partition coefficients for the rare earth elements and strontium

Abstract Sixteen sets of apatite/liquid partition coefficients (Dap/liq) for the rare earth elements (REE; La, Sm, Dy, Lu) and six values for Sr were experimentally determined in natural systems ranging from basanite to granite. The apatite + melt (glass) assemblages were obtained from starting glasses artificially enriched in REE, Sr and fluorapatite components; these were run under dry and hydrous conditions of 7.5–20 kbar and 950–1120°C in a solid-media, piston-cylinder apparatus. An SEM-equipped electron microprobe was used for subsequent measurement of REE and Sr concentrations in coexisting apatites and quenched glasses. The resulting partition coefficient patterns resemble previously determined apatite phenocryst/groundmass concentration ratios in the following respects: (1) the rare earth patterns are uniformly concave downward (i.e., the middle REE are more compatible in apatite than the light and heavy REE); (2) DREEap/liq is much higher for silicic melts than for basic ones; and (3) strontium (and therefore Eu2+) is less concentrated by apatite than are the trivalent REE. The effects of both temperature and melt composition on DREEap/liq are systematic and pronounced. At 950°C, for example, a change in melt SiO2 content from 50 to 68 wt.% causes the average REE partition coefficient to increase from ∼7 to ∼30. A 130°C increase in temperature, on the other hand, results in a two-fold decrease in DREEap/liq. Partitioning of Sr is insenstitive to changes in melt composition and temperature, and neither the Sr nor the REE partition coefficients appear to be affected by variations in pressure or H2O content of the melt. The experimentally determined partition coefficients can be used not only in trace element modelling, but also to distinguish apatite phenocrysts from xenocrysts in rocks. Reported apatite megacryst/host basalt REE concentration ratios [12], for example, are considerably higher than the equilibrium partition coefficients, which suggest that in this particular case the apatite is actually xenocrystic. A reversal experiment incorporated in our study yielded diffusion profiles of REE in apatite, from which we extracted a REEαCa interdiffusion coefficient of 2–4×10−14 cm2/s at 1120°C. Extrapolated downward to crustal temperatures, this low value suggests that complete REE equilibrium between felsic partial melts and residual apatite is rarely established.

An experimental study of Nb and Ta partitioning between Ti-rich minerals and silicate liquids at high pressure and temperature

Experimental determination of partition coefficients (D) for Nb and Ta between Ti-rich minerals (sphene, rutile, ilmenite and Ti-magnetite) and coexisting silicate liquids of basaltic andesite, andesite and trachyte composition, indicate that these elements partition strongly into sphene and rutile, and moderately into ilmenite and Ti-magnetite. The D values for sphene increase with decreasing temperature and with increasing SiO2 content of the coexisting liquid, but appear unchanged by pressure between 7.5 and 16 kb, and oxygen fugacity between magnetite-wustite and haematite-magnetite buffers. Also, DNbDTa. varies from 0.3–0.4 for sphene, increasing to 0.6–0.7 for rutile, 0.7–0.8 for ilmenite and 0.8 for Ti-magnetite. Thus fractionation involving these minerals may cause an increase in the NbTa ratio of any derivative liquid; this will be most noticeable where sphene is involved.

FeMg partitioning between coexisting garnet and phengite at high pressure, and comments on a garnet-phengite geothermometer

Abstract The FeMg exchange reaction between coexisting garnet and phengite has been studied by reacting a natural phengite ( mg = 67 ) in the presence of quartz and water at pressures of 20–35 kb and temperatures of 800–1000°C. Compositions of coexisting garnet and mica indicate a linear relation between both the InKD((Fe/Mg) garnet/(Fe/Mg) phengite) and temperature, and InKD and pressure in the above P.T range. This FeMg exchange reaction between garnet and phengite is shown to be dependent on the Ca-content of the garnet, and on the mg number of the bulk composition. These two composition effects have been studied by usin phengitic mica mixes with mg numbers of 23 and 46, and by using a synthetic basaltic composition. The overall results allow broad empirical calibration of separate geothermometers for pelitic and basaltic systems, respectively. However, because of non-ideality in the exchange reaction, this geothermometer should not be used in any practical application outside the composition ranges studied. Also, careful consideration of the presence of Fe3+ in phengite must be made. If the Fe3+ content of the natural phengite is unknown, then the temperatures obtained will be maximum temperatures only.

Crystallization of calc-alkaline andesite under controlled high-pressure hydrous conditions

A series of experimental runs has been conducted on a glass prepared from a natural island are calc-alkaline andesite from Fiji. The crystallization sequence was determined for the pressure interval 9–36 kb under anhydrous conditions and with 2, 5, and 10% by weight of water carefully added.Addition of water markedly lowers the liquidus, depresses the appearance of quartz and plagioclase in the crystallization sequence, and greatly enlarges the field of garnet-clinopyroxene crystallization above 25 kb. Amphibole crystallizes in hydrous runs up to 25 kb.Electron microprobe analyses of critical phases allows calculation of controls on crystal fractionation trends. For hydrous conditions at 5–15 kb amphibole-clinopyroxene dominate fractionation and a moderate decrease in Mg/Fe and a slight increase in K/Na occurs. At 15–25 kb garnet also affects the fractionation and a moderate decrease in Mg/Fe and an increase in K/Na results. Above 25 kb garnet-clinopyroxene control the fractionation and there is a slight decrease in Mg/Fe but a significant increase in K/Na and a pronounced silica enrichment.In terms of major element chemistry, the derivation of the Fijian dacites in the second period of eruption may be satisfactorily explained by the fractionation of hydrous andesite at pressures >25 kb. Alternatively the dacites may result from lower degrees of melting of the down-going hydrous lithosphere. Similarly other members of this eruptive period may be derived according to a model of eclogite-controlled fractional melting or crystallization. Models involving amphibole fractionation at lower pressures are less satisfactory for explaining compositions in the Fijian second period of eruption, but in other environments models including amphibole-controlled fractionation may form part of a continuum of melting processes in subduction zones.

Ti-rich accessory phase saturation in hydrous mafic-felsic compositions at high P, T

Abstract Experiments on hydrous mafic, intermediate and felsic compositions at 7.5–30 kbar and 900–1100°C allow broad delineation of TiO 2 contents for a range of silicate liquids coexisting with a Ti-rich accessory phase. Different starting compositions, including unadulterated fused natural rocks, and rocks enriched with various Ti-rich phases, give mutually consistent results. The TiO 2 content of liquids saturated in a Ti-rich accessory phase is shown to decrease with decreasing temperature, increasing pressure, increasing SiO 2 , alkali and rare-earth element content (at % level) of the liquid. Water content has little observable effect, while increasing oxygen fugacity lowers the TiO 2 content of liquid coexisting with a Ti-rich phase. A separate Ti-rich accessory phase may be expected to crystallize from liquids at 1000°C in the deep crust (pressures corresponding to 7.5–12 kbar) if the TiO 2 content exceeds ∼ 3.0 wt.% for mafic, ∼ 1.8 wt.% for intermediate, and ∼ 1.2 wt.% for felsic compositions. At the same depth but at 950°C, these TiO 2 values change to ∼ 1.7, 1.2 and 0.9 wt.%, respectively. At upper-mantle depths (20–30 kbar) the corresponding TiO 2 saturation levels at 1000°C are ∼ 2.0, 1 and 0.7 wt.%, respectively. In natural igneous rock series the contrasting TiO 2 SiO 2 systematics in alkaline and tholeiitic series compared with calc-alkaline series are well established, as are the characteristic, though non-unique, low TiO 2 values for rock series in convergent plate regions. The present results allow experimental confirmation of observed and/or predicted crystallization of Ti-rich phases in these rock series. They also provide reasonable constraints on conditions under which TiO 2 contents of convergent plate magmas may be buffered by a residual Ti-rich phase in the source region, and indicate that TiO 2 contents of mafic parent magmas in these areas are unlikely to be controlled in such a manner. In contrast, it is almost inevitable that felsic magmas, generated by partial melting of the lower crust, are saturated with respect to a Ti-rich phase in their source regions.

Determination of partition coefficients for trace elements in high pressure-temperature experimental run products by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS)

Abstract This paper reports the first trace element partition coefficients measured on experimentally produced products (clinopyroxene, garnet, rutile, and glass) by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS). A 266 nm (UV) laser microprobe was used to improve ablation characteristics and to achieve a fourfold reduction in ablation pit diameter compared to the previously used 1064 nm beam. Results are compared with PIXE analyses on the same experimental products, and literature values, where available, for similar systems, and include the first simultaneously measured partition coefficients for Zr, Nb, and Ta between rutile and glass. Advantages of the LAM technique include rapid results and simultaneous determination of a wide range of major and trace elements, thus ensuring sampling integrity through time-resolved analysis of the sampled material.

Proton microprobe-determined partitioning of Nb, Ta, Zr, Sr and Y between garnet, clinopyroxene and basaltic magma at high pressure and temperature

Abstract Application of a proton microprobe to experimentally produced mineral-liquid pairs has enabled determination of Nb and Ta partition coefficients ( D ) between garnet, clinopyroxene, and a Nb- and Ta-enriched hydrous basaltic liquid at 2.5 GPa, 1100°C. For garnet D Nb =0.02, D Ta =0.06, and for clinopyroxene D Nb =0.005 and D Ta =0.013. Both minerals significantly fractionate Nb and Ta from each other and may have an important role in modifying the Nb/Ta ratio of evolving magmas from that of their parent magma or their source region. For eclogite melting up to 40% change could occur in the Nb/Ta ratio. Additional partition coefficients obtained using the proton microprobe include values of: for garnet, D Zr (0.4–0.7), D Sr ( D Y (>2.5–9); for clinopyroxene, D Zr (0.10), D Sr (0.06) and D Y (0.9); and for orthopyroxene D Zr (0.18), D Sr (0.04) and D Y (0.18). These were determined for natural element abundance levels, and are consistent with data obtained for artificially enriched compositions, verifying Henrys law behaviour and the valid application of these data to geochemical modelling of natural systems.