John Adam

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.

The effects of pressure and temperature on the partitioning of Ti, Sr and REE between amphibole, clinopyroxene and basanitic melts

An electron microprobe was used to measure partition coefficients (D-values) for Ti, Sr, La, Sm, Ho and Lu in amphiboles, clinopyroxenes and Sr- and REE-enriched basanitic melts. D-values were found to be negatively correlated with pressure at pressures from 0.5 to 3.0 GPa. This effect is greatest for Ti and smallest for Lu. Changes in D-values accompanying changes in temperature are less apparent than those caused by changes in pressure. Only DLu for clinopyroxene shows a small but consistent increase as temperature increases from 1050° to 1200°C. D-values for amphibole are not noticeably affected by changes in fO2. In contrast, D-values for clinopyroxene become larger as fO2 increases. The correlations between D-values and pressure may be related to changes in site volumes and electrostatic potentials accompanying shifts in the relative concentrations of AlIV, AlVI and Na+.

Proton microprobe determined partitioning of Rb, Sr, Ba, Y, Zr, Nb and Ta between experimentally produced amphiboles and silicate melts with variable F content☆

Abstract A proton microprobe was used to measure partition coefficients for Rb, Sr, Ba, Y, Zr, Nb and Ta between experimentally produced amphiboles and hydrous basaltic melts. A limited amount of data was also obtained for the distribution of trace elements in clinopyroxene and mica. Partition coefficients for trace elements in amphibole and basanite melts are (at 1σ): Rb 0.34 ± 0.14; Sr 0.33 ± 0.07; Ba 0.46 ± 0.16; Y 0.6 ± 0.2; Ti 0.95 ± 0.19; Zr 0.25 ± 0.06; Nb 0.08 ± 0.01; and Ta 0.09 ± 0.03. Only small, generally non-systematic differences in these values are observed with variation in pressure (10–20 kbar) and temperature (1000–1050°C), but large differences accompany changes in melt composition and F content. For a (F-free) basaltic andesite melt, at 20 kbar and 950°C, amphibole/melt distribution coefficients are: Rb 0.07 ± 0.01; Sr 0.35 ± 0.03; Y 1.3 ± 0.1; Ti 1.75 ± 0.12; Zr 0.35 ± 0.06; Nb 0.21 ± 0.01; and Ta 0.19 ± 0.02. The data support proposals that residual amphibole in mantle source regions for some nephelinites explains their relatively high HFSE/LILE ratios. In contrast, the data do not favour amphibole as the cause of characteristically low HFSE/LILE observed in mantle-derived island arc basalts. The increases in partition coefficients (excepting for Rb) for the more SiO2-rich melt are consistent with similar trends observed in phenocryst-matrix pairs from volcanic rocks. The compositional dependence of HFSE partition coefficients increases with increasing field strength. This trend can be related to steric effects within polymerised aluminosilicate units of the melt phase. Amphiboles grown from F-enriched melts are relatively depleted in TiO2, Al2O3, CaO and incompatible trace elements. These effects are only large, however, at high F concentrations (> 2 wt%). The effects of F on incompatible elements will be least in melts containing high concentrations of Al2O3, FeO, MgO and CaO. For these reasons, it is unlikely that concentrations of HFSE and other incompatible elements in natural magmas (with the possible exception of some rare F-and SiO2-rich magmas) are significantly affected by F.

Experimentally-determined trace element characteristics of aqueous fluid from partially dehydrated mafic oceanic crust at 3.0 GPa, 650–700°C

Aqueous vapour, released from subducted dehydrated oceanic crust, has the potential to remove relatively large amounts of trace elements and to modify key trace element ratios in the overlying mantle wedge, an important source region for subduction zone magmatism. To experimentally assess this process an oceanic tholeiite (MORB) has been enriched (10–100 ppm) in 25 trace elements and crystallized at 3.0 GPa, 650 and 700°C for 2 weeks, with 30–50wt.% added water. At 650°C the fluid coexists with omphacite, lawsonite, chloritoid, coesite and accessory phengite, rutile, Ti-magnetite and talc. Changes at 700°C are that garnet is conspicuous, chloritoid absent and lawsonite, staurolite and talc are rare. LAM ICP-MS analyses establish patterns of trace element partitioning between minerals (data for omphacite, lawsonite, phengite, chloritoid and garnet) and fluid, but not absolute values. Similarly ratios of trace elements in the fluid can be well constrained. Relative to the bulk composition the fluid shows strong enrichment of LILE/HFSE and LILE/REE at both 650 and 700°C, but also shows significant decrease in Rb/Sr, Cs/Sr and Ba/Sr from 650 to 700°C, apparently controlled by residual lawsonite, and increase in La/Lu, clearly controlled by garnet. In addition, omphacite strongly fractionates both Zr/Nb and La/Lu, contributing to a decrease in Zr/Nb and an increase in La/Lu in the fluid. Residual rutile (analyzed with an electron microprobe) strongly takes up Nb and Ta, the latter more readily, causing an almost a 4-fold increase in the Nb/Ta ratio in the fluid relative to the bulk. Omphacite, garnet and chloritoid all have Nb/Ta less than and lawsonite greater than the starting composition. Omphacite and garnet strongly reject Sr, Ba, Rb and Cs; lawsonite favours all REE (HREE slightly more so), Sr, U and Th, whereas phengite favours Rb, Ba and Cu. Chloritoid most readily accommodates V, Cr, Ni and Zn. Involvement of a fluid derived by dehydration processes may increase LILE/HFSE and LILE/REE and decrease Zr/Nb in subduction-linked magmas, relative to MORB. Lowest Zr/Nb ratios point to the greatest relative involvement of clinopyroxene in the residue. Absence of major change to La/Lu restricts the role of garnet. Higher Rb/Sr, Cs/Sr and Ba/Sr may point to a low geothermal gradient in the subduction zone such that lawsonite remains stable during the dehydration process that produced the fluid. Residual rutile may explain Nb and Ta depletion and cause an increase in Nb/Ta of the coexisting fluid.

Trace element partitioning between silicate minerals and carbonatite at 25 kbar and application to mantle metasomatism

SummaryExperiments at 25 kbar and 1000°C, on a model trace element-enriched carbonatite-eridotite mix, produced augite + pargasite ± garnet ± dolomite coexisting with a carbonatite melt. Proton microprobe analysis of the phases showed that key trace elements (Rb, Ba, Sr, Nb, Ta, Zr, Y and REE) all partitioned strongly into the melt (with the exception of Y, Ho and Lu in garnet), verifying that carbonatite is potentially a highly effective metasomatizing agent. The data also indicate that carbonatitic metasomatism will impart higher Ba/Rb, Ba/Nb, Nb/Ta, Sr/Ta, La/Ta, and lower Zr/Y, with little change to Sr/Nb, in affected mantle.ZusammenfassungExperimente mit einer Modell-mischung von Karbonatit-Peridotit, angereichert mit Spurenelementen, produzierten bei 25 kbar und 1000°C Augit + Pargasit ± Granat ±Dolomit coexistierend mit einer Karbonatitschmelze. Protonmikrosonden-Analyse der Phasen zeigte, dass alle Schlüsselspurenelemente (Rb, Ba, Sr, Nb, Ta, Zr, Y and REE) stark in der Schmelze angereichert werden (mit der Ausnahme von Y, Ho und Lu in Granat), was beweist, dass Karbonatit potentiell ein sehr effektives Agens für Metasomatose ist. Die Daten zeigen weiterhin, dass karbonatitische Metasomatose in betroffenen Mantel höhere Ba/Rb, Ba/Nb, Nb/Ta, Sr/Ta, La/Ta und niedrigere Zr/Y produziert, mit geringen Äderungen für Sr/Nb.

Hadean greenstones from the Nuvvuagittuq fold belt and the origin of the Earth's early continental crust

To investigate formation of the Earth9s earliest continental crust, partial-melting experiments were conducted (at 900–1100 °C and 0.5–3.0 GPa) on two greenstones from the 4.3 Ga Nuvvuagittuq complex of Quebec, Canada. For comparison, experiments were also conducted on a compositionally similar but modern arc volcanic (a Tongan boninite). At 1.5–3.0 GPa and 950–1100 °C, the experimentally produced melts are compositionally similar to the tonalite-trondhjemite-granodiorite (TTG) granitoids that compose most of Earth9s early continental crust, including a 3.66 Ga tonalite that encloses the Nuvvuagittuq Complex. Because the degree of melting needed to produce the TTG-like melts is comparatively high (>30%), the relative concentrations of most incompatible elements in the melts are similar to those in their greenstone parent rocks. These greenstones have compositional affinities with modern subduction zone magmas and do not resemble mid-oceanic ridge basalts. That arc-like mafic rocks could have been selectively involved in TTG formation (in spite of their volumetrically subordinate status in most greenstone terrains) must reflect tectonic circumstances that were specific to their generation. These must have enabled accumulations sufficiently deep to melt at the 1.5–3.0 GPa needed to generate TTG magmas from eclogitic sources. They are also likely to have been related to some form of crustal recycling whereby mafic crust and water were returned to the mantle and arc-like mafic magmas generated as a consequence. To what degree these circumstances replicated modern plate tectonics is difficult to say, but it seems likely that, as in the modern Earth, the Hadean crust was organized into different tectonic environments and that one of these gave rise to the first continental crust.

The influence of pressure, mineral composition and water on trace element partitioning between clinopyroxene, amphibole and basanitic melts

Trace and minor elements in clinopyroxenes, amphiboles and basanitic melts produced in experiments at 0.5 to 2.0 GPa and 1000 to 1050°C were analysed by LAM-ICP-MS. The analyses reproduce the results of previous electron microprobe analyses by Adam & Green (1994) but also provide data for many new elements. The expanded data set was used to evaluate the trace element partitioning models of Blundy & Wood (1994) and Wood & Blundy (1997). Although the lattice-strain model of Blundy & Wood (1994) could be successfully fitted to our data, the thermodynamic model of Wood & Blundy (1997) does not predict the negative correlations that we found between DREE and pressure. This is not explained by variations in the amounts of H2O in our experiments. Instead, we attribute the variations in DREE to volume-reducing substitutions in octahedral and tetrahedral sites that increase the activities of +3 and +4 ions, and decrease those of +1 ions, in sites of 8-fold coordination as pressure increases. We were able to quantitatively model some of these effects using a combination of simple electrostatics, Pauling bond strengths and bond-valence calculations. Notwithstanding this, it is apparent that pressure, temperature and the properties of the melt phase also influence partition coefficients independently of mineral composition and crystal-chemical effects.

An electron microprobe, LAM-ICP-MS and single-crystal X-ray structure refinement study of the effects of pressure, melt-H2O concentration and fO2 on experimentally produced basaltic amphiboles

Amphiboles were crystallized in sub-liquidus experiments at 0.5–2.0 GPa and 1000–1050 °C from hydrous nepheline basanite and olivine basalt starting compositions. The amphiboles and coexisting (quenched) melts were analysed for major, minor and trace elements by a combination of electron microprobe, laser ablation microprobe and inductively-coupled plasma mass-spectrometry (LAM ICP-MS). Individual amphiboles were also characterized by single-crystal X-ray structure refinement, and empirical estimates of dehydrogenation were obtained based on M1–M2 distances. The amphiboles display compositional variation that can be interpreted as crystal-chemical responses to: (1) increasing pressure, and (2) changes in oxygen fugacity ( f O 2 ) and the activity of H 2 O. As pressure increases, Al moves from the T1 tetrahedron (where it is replaced by Si) to the octahedral M2 site. This coupled substitution, which implies an increase in coordination number for Al, results in a decrease in the c and b unit-cell edges. The overall decrease in unit-cell volumes is kept small, however, by an increase in the B (Fe, Mg) content with increasing pressure, which in turn decreases the volume occupied by the B-cations but increases the sin β value. In this way, the entrance of minor K at the A site and Cl at the O3 site (K D s for both increase with pressure) is allowed, resulting in a slight lengthening of the a edge. The degree of dehydrogenation at O3 correlates inversely with the H 2 O concentration in coexisting melts. Generally, dehydrogenation is locally balanced by M1 Ti, with the Ti excess with respect to ½ O 2− ordered at the M2 site. In one sample, crystallized under more oxidizing conditions, O 2− is > 2Ti, and local charge balance requires the presence of Fe 3+ ordered at the M1 (and M3) sites. D amph/melt values measured for the high field strength elements Ti, Zr, Hf, Nb and Ta ( D HFSE ) correlate positively with O 2− and with [4] Al, suggesting that Ti, Zr, Hf, Nb and Ta (HFSE) are incorporated in both the M1 and the M2 sites. Partition coefficients for rare earth elements ( D REE ) correlate positively with [4] Al and negatively with [6] Al. Increased f O 2 results in increased Fe 3+ , [4] Al and D REE , but does not produce a noticeable increase in O 2 − or in D HFSE .

An experimental study of two garnet pyroxenite xenoliths from the Bullenmerri and Gnotuk Maars of western Victoria, Australia

Experiments were conducted at 6–30 kb and 875–1200°C on two garnet pyroxenite xenoliths from the Bullenmerri and Gnotuk Maars of western Victoria, Australia. The (garnet + clinopyroxene + plagioclase + spinel) assemblage of DR9734 was stable between 10 and 12.5 kb, and 950 and 1,050°C. The compositions of its natural mineral phases were most closely approximated in experiments at 12.5 kb and 1,000–1,050°C. The (garnet + spinel + clinopyroxene + orthopyroxene + amphibole) assemblage of DR10165 was stable at pressures > 8 kb and temperatures > 950°C. However, differences between natural and experimental mineral compositions indicate that the mineral assemblage of this xenolith persisted metastably after cooling below 950°C with chemical exchange continuing down to approximately 850–900°C. When the experimental data for DR9734 and DR10165 are applied to mineralogical data for other mafic and ultramafic xenoliths from the Bullenmerri and Gnotuk Maars, they indicate that previous pressure and temperature estimates for individual xenoliths are 2–3 kb and ∼ 50°C too high. These corrections increase average temperatures for the geotherm beneath western Victoria by about 50°C over a depth range of 30–45 km and confirm its perturbed (high-temperature) character.

Pressure effect on Ti- or P-rich accessory mineral saturation in evolved granitic melts with differing K2O/Na2O ratios

The solubility of Ti- and P-rich accessory minerals has been examined as a function of pressure and K2O/Na2O ratio in two series of highly evolved silicate systems. These systems correspond to (a) alkaline, varying from alkaline to peralkaline with increasing K2O/Na2O ratio; and (b) strongly metaluminous (essentially trondhjemitic at the lowest K2O/Na2O ratio) and remaining metaluminous with increasing K2O/Na2O ratio (to 3). The experiments were conducted at a fixed temperature of 1000 °C, with water contents varying from 5 wt.% at low pressure (0.5 GPa), increasing through 5–10 wt.% at 1.5–2.5 GPa to 10 wt.% at 3.5 GPa. Pressure was extended outside the normal crustal range, so that the results may also be applied to derivation of hydrous silicic melts from subducted oceanic crust. For the alkaline composition series, the TiO2 content of the melt at Ti-rich mineral saturation decreases with increasing pressure but is unchanged with increasing K content (at fixed pressure). The P2O5 content of the alkaline melts at apatite saturation increases with increased pressure at 3.5 GPa only, but decreases with increasing K content (and peralkalinity). For the metaluminous composition series (termed as “trondhjemite-based series” (T series)), the TiO2 content of the melt at Ti-rich mineral saturation decreases with increasing pressure and with increasing K content (at fixed pressure). The P2O5 content of the T series melts at apatite saturation is unchanged with increasing pressure, but decreases with increasing K content. The contrasting results for P and Ti saturation levels, as a function of pressure in both compositions, point to contrasting behaviour of Ti and P in the structure of evolved silicate melts. Ti content at Ti-rich mineral saturation is lower in the alkaline compared with the T series at 0.5 GPa, but is similar at higher pressures, whereas P content at apatite saturation is lower in the T series at all pressures studied. The results have application to A-type granite suites that are alkaline to peralkaline, and to I-type metaluminous suites that frequently exhibit differing K2O/Na2O ratios from one suite to another.