An experimental study of the partitioning of trace elements between rutile and silicate melt as a function of oxygen fugacity (original) (raw)
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Geochimica et Cosmochimica Acta, 2000
Fractionation of some or all of the high field strength elements (HFSE) Nb, Ta, Zr, Hf, and Ti relative to other trace elements occurs in igneous rocks from convergent margins and in the average continental crust, and is generally attributed to a process occurring during subduction. The experimental partitioning of an extensive array of trace elements between rutile/melt pairs is presented which enables the effect of rutile during melting in subduction zones to be directly assessed. D Nb and D Ta are in the range 100 -500, D Zr and D Hf are about 5, whereas all other trace elements analyzed have D rutile/melt less than 0.1. Published D patterns for Nb and Ta between rutile and water-rich fluids are similar to those for melt, whereas the values for Zr and Hf are significantly higher. D Nb and D Ta values for clinopyroxene and garnet are much lower than for rutile, and cannot cause the fractionation of HFSE from other elements seen in island arcs. The presence of rutile in the subducted slab residue during dehydration may be essential in the production of the geochemical signatures of arc magmas, whereas that of the continental crust, including higher Zr/Sm, may be produced by melting of eclogite.
Earth and Planetary Science Letters, 2008
Keywords: rutile high field strength elements subduction plumes eclogite recycled crust This experimental study examines the solubility of rutile in silicate melts and presents a model for rutile saturation as a function of temperature, pressure and melt composition. Rutile saturation experiments were carried out in the system SiO 2 -TiO 2 -Al 2 O 3 -MgO-CaO-Na 2 O-K 2 O at 1 bar to 35 kbar and 1150 to 1450°C on model rhyodacite (∼69 wt.% SiO 2 ) and haplobasalt (∼54 wt.% SiO 2 ) base melt compositions. At rutile saturation, the concentration of TiO 2 in the model rhyodacite base melt increases at 1 bar from 3.27 ± 0.03 wt.% at 1150°C to 13.88 ± 0.04 wt.% at 1450°C. At 1350°C it decreases from 8.89 ± 0.08 wt.% at 1 bar to 2.06± 0.13 wt.% at 35 kbar. Rutile solubility is significantly higher in the haplobasalt base melt at a given pressure and temperature. The concentration of TiO 2 in rutile-saturated haplobasalt increases at 1 bar from 20.9 ± 0.3 wt.% at 1300°C to 39.0 ± 0.3 wt.% at 1450°C. At 1350°C it decreases from 25.8 ± 0.3 wt.% at 1 bar to 15.67 ± 0.16 wt.% at 15 kbar. Results from these experiments were combined with data from the literature to formulate a model for rutile saturation in silicate melts. Application of this model to partial melting of MORB-type eclogite indicates that beneath volcanic arcs low-degree, hydrous partial melts of rutile-bearing subducted oceanic crust contain only ∼600 ppm TiO 2 . Therefore, rutile will remain as a residual phase in the eclogite and the amount of TiO 2 that will be transferred to the mantle wedge will be small because so little TiO 2 is dissolved in the melt. Partial melting of recycled oceanic crust in an upwelling mantle plume can exhaust rutile from the residual solid at moderate degrees of partial melting (∼22%). The retention of rutile in subducted oceanic lithosphere during dehydration and/or partial melting, combined with exhaustion of rutile during partial melting of eclogite in mantle plumes suggests that HFSE enrichments in recycled crust that were established during subduction may be detectible in OIB.
American Mineralogist, 2006
The distribution of V in magmatic rocks is controlled primarily by spinel stability. Extensive previous experimental work at oxidized conditions on doped (V-rich) compositions has led to the recognition of the importance of temperature, oxygen fugacity, and spinel composition, but also left ambiguity with respect to the relative importance of these variables in controlling D V spinel/melt . One major uncertainty has been the valence of V in the spinel and glass. Spinel-melt pairs were equilibrated at low and variable oxygen fugacities, with a range of V and Ti contents. XANES spectra were measured on the spinel and glass products, and pre-edge peaks measured and calibrated against valence with the use of glass and oxide standards. The valence of V is always greater in the glass than in the spinels. In spinel, V is dominantly 3+ at oxygen fugacities near the FMQ (fayalite magnetite quartz) buffer, but we Þ nd evidence for mixed 3+, 4+, and 5+ at oxidized conditions (FMQ to air), and 2+ and 3+ at very reduced conditions [FMQ to IW-1 (1 log f O 2 unit below the iron wüstite buffer)]. Increased V contents in spinels are correlated with increased D V spinel-melt , at constant temperature and oxygen fugacity. However, increased Ti content causes only a slight decrease in D V spinel-melt and a shift to more reduced V (smaller pre-edge peak), which may be related to Fe-V exchange equilibria. Using the new partition coefÞ cients, together with published results and valence information, expressions have been derived to predict D V spinel/melt for basaltic systems. Application of these expressions to natural suites illustrate their utility and also the great range of D V spinel/melt values relevant to natural systems. Calculation of V depletions in planetary mantles from basalt suites must take silicate, oxide, and metal fractionation into account, as is demonstrated using terrestrial, lunar, martian, and eucritic samples.
Earth and Planetary Science Letters, 2004
Partition coefficients between rutile and silicate melts were determined experimentally for Nb and Ta with melt compositions varying from rhyolite to basalt. Experimental conditions were 1.7-2.5 GPa, 950-1300 8C, at oxygen fugacities between QFMÀ2 and QFM+3.5. Both rt/melt D Nb and rt/melt D Ta increase by almost one order of magnitude with SiO 2 content and polymerization, but decrease with TiO 2 content in the melt. The ratio rt/melt D Nb /D Ta is 0.45-0.55 for basaltic melt compositions, around 0.6 for andesitic melts and 0.8-1.0 for more silicic melts, remaining V1 for all examined silicate melts. The fact that rt/melt D Nb /D Ta is smaller than unity can be explained by a slightly smaller ionic radius of Ta 5+ than Nb 5+ and thus a preferred incorporation of Ta into rutile. The variation of rt/melt D Nb , rt/melt D Ta , and rt/melt D Nb /D Ta strongly depends on melt composition without any significant correlation with rutile composition. The strong positive correlation of rt/melt D Nb and rt/melt D Ta with rt/melt D Ti and SiO 2 contents is explained with the decreasing solubility of high charge cations in an increasingly polymerized melt where the concentration of non-bridging oxygens decreases. The positive correlation of rt/melt D Nb /D Ta with rt/melt D Ti and SiO 2 contents is more difficult to understand and might be related to the higher polarizibility of Nb 5+ compared to Ta 5+ .
Chemical Geology, 2012
We have performed partitioning experiments to assess the role of chromium-rich spinel in controlling the behavior of the platinum-group elements (PGEs) during igneous differentiation. Spinels were equilibrated with natural and synthetic iron-bearing basalt at 0.1 MPa and 2 GPa at 1400-1900°C over an fO 2 range of IW+ 1.6 to IW+ 7. Results from relatively reduced, graphite-encapsulated experiments done at 2 GPa indicate that Ru is compatible in Cr-spinel (mineral/melt partition coefficient, D, of~4), followed by Rh and Ir, which are moderately incompatible (D range of 0.04 to~1), with Pt and Pd the most incompatible (D b 0.2). Partition coefficients for Ir, Ru and Rh measured at more oxidizing conditions in this and previous studies are 10 to 1000 times higher than results from experiments using graphite capsules. We account for the variation in spinel-melt partitioning with a model which considers both the affinity of the PGE cation for a particular spinel lattice site, and the change in site occupancy accompanying the increase in ferric iron component with fO 2 . Assuming that Ir and Rh are present as divalent species, with a strong affinity for VI-fold coordination, D Ir and D Rh are predicted to rise rapidly with the ferric iron component, explaining the large D-values for magnetite-rich spinels. Model results indicate that D Ir ≤ 20 and D Rh are ≤100 for ferric-iron poor, Cr-rich compositions, as would crystallize in komatiites, some layered intrusions, and ophiolites. The overall compatibility of Ru for chromite is consistent with the predominance of Ru 3+ at experiment conditions and the similarity in the size of Ru 3+ to Cr 3+ and Fe 3+ . The increase in D Ru with the ferric iron content of the spinel likely involves a strong effect of mineral composition superimposed on a change in melt speciation (Ru 2+ to Ru 3+ ) with increased fO 2 . The effect of mineral composition is a consequence of the difference in octahedral site preference energy (OSPE) between Ru 3+ , Fe 3+ and Cr 3+ , with stronger partitioning of Ru into Fe 3+ -rich compositions due to the enhanced reduction in energy gained by the Ru 3+ substitution. Ru partition coefficients for ferric-iron poor spinel are expected to be~30, which is somewhat lower than values estimated from natural samples obtained from in situ chromite analyses. Results indicate that the ferric iron content of chromite exerts a strong control on the partitioning of some PGEs which should be taken into account in both future experimental work and in models of igneous differentiation.
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.
Geochemical and geodynamical constraints on subduction zone magmatism
Earth and Planetary Science Letters, 1991
The geochemical and geodynamical parameters that may influence the composition of island-arc basalts (IAB's) are evaluated. Systematic correlations amongst high-field strength (HFS) elemental ratios (Zr/Nb, Sm/Nb and TiO2/Zr) relative to Nb abundances, indicate that HFS element systematics are not controlled by the presence of residual Nb-bearing phases in the slab. This provides confirmation of models whereby high-field strength (HFS) and HREE elements remain immobile during slab-fluxing processes and are thus derived from the mantle wedge without additional enrichments from the slab. In contrast enrichment of large-ion-lithophile elements (LIL) such as Rb, Cs, Ba, Sr, Pb, U and LREE (i.e., La, Ce) in lAB's is consistent with slab involvement, with their relative enrichment, being due to a combination of both their high rock-melt incompatibility and slab-" fluid" mobility. As a consequence, the low abundances of HFS elements such as Nb, Ti, Zr, and Hf in lAB's reflect a depleted (relative to MORB source) mantle wedge overlying the subduction slab. Depletion of the arc mantle wedge in HFS elements is attributed to previous melting events in the mantle wedge, and to geodynamic conditions associated with the formation and evolution of coupled island arcs and back-arc basins. These processes ensure a budgetary deficit in the HFS elements relative to those elements derived from the subducted slab (predominantly LILE and LREE). Thus, although in MORB's and OIB's, Nb has a similar incompatibility to U, in subduction zones the main factor controlling its abundance is its highly immobile character, particularly relative to elements like U which are mobile during prograde dehydration reactions in the slab. Based on these observations, a quantitative model has been developed for lAB petrogenesis with the transfer of trace elements from the slab to the mantle wedge being modelled with empirical slab-"fluid" partition coefficients whilst the mantle-wedge to arc-crust transfer is constrained by melt-solid partitioning. The empirically derived slab-" fluid" partition coefficients indicate that the enrichment factors characteristic of slab fluxing processes have a distinctive pattern particularly for the elements Nb, U, Th, and Sr.
Geochemistry, …, 2000
A new set of partitioning data for rare earth elements (REE: La, Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb), Y, Th, U, and Pb has been obtained for 25 calcic amphiboles (pargasites and kaersutites) crystallized from alkali-basaltic and basanitic bulk rock compositions at f O 2 AˊFMQ−2,pressureP=1.4GPa,andtemperatureTbetween9508and10758C.Thevariationsofamphibole/liquidpartitioncoefficientsandoftheirratiosrelevanttopetrogeneticstudiesarediscussedwithreferencetothemajorelementcompositionoftheamphibolesandofthecoexistingmelt,andtothecrystalchemicalmechanismsfortraceelementincorporation.OurresultssupporttheconclusionsthatREEandactinidesareincorporatedintotheM4cavityincalcicamphibolesanddistributedbetweenthetwoavailablesiteswithinthatcavityandthatPbisincorporatedintotheAsite.Inoursamplepopulation,REEpatternsaresystematicallyenrichedinheavyREE(HREE),asexpectedfromthepresenceofsignificantcummingtonitecomponent.NosignificantfractionationisobservedbetweenThandU.ThemajorfactorcontrollingtheamountoftraceelementincorporationistheSiO2contentofthemelt.ThemajorimplicationofthisstudyisthatHREEcanbecomecompatibleinamphiboleinsystemswithSiO2contentgreaterthanÁFMQ-2, pressure P = 1.4 GPa, and temperature T between 9508 and 10758C. The variations of amphibole/liquid partition coefficients and of their ratios relevant to petrogenetic studies are discussed with reference to the major element composition of the amphiboles and of the coexisting melt, and to the crystal chemical mechanisms for trace element incorporation. Our results support the conclusions that REE and actinides are incorporated into the M4 cavity in calcic amphiboles and distributed between the two available sites within that cavity and that Pb is incorporated into the A site. In our sample population, REE patterns are systematically enriched in heavy REE (HREE), as expected from the presence of significant cummingtonite component. No significant fractionation is observed between Th and U. The major factor controlling the amount of trace element incorporation is the SiO 2 content of the melt. The major implication of this study is that HREE can become compatible in amphibole in systems with SiO 2 content greater than AˊFMQ−2,pressureP=1.4GPa,andtemperatureTbetween9508and10758C.Thevariationsofamphibole/liquidpartitioncoefficientsandoftheirratiosrelevanttopetrogeneticstudiesarediscussedwithreferencetothemajorelementcompositionoftheamphibolesandofthecoexistingmelt,andtothecrystalchemicalmechanismsfortraceelementincorporation.OurresultssupporttheconclusionsthatREEandactinidesareincorporatedintotheM4cavityincalcicamphibolesanddistributedbetweenthetwoavailablesiteswithinthatcavityandthatPbisincorporatedintotheAsite.Inoursamplepopulation,REEpatternsaresystematicallyenrichedinheavyREE(HREE),asexpectedfromthepresenceofsignificantcummingtonitecomponent.NosignificantfractionationisobservedbetweenThandU.ThemajorfactorcontrollingtheamountoftraceelementincorporationistheSiO2contentofthemelt.ThemajorimplicationofthisstudyisthatHREEcanbecomecompatibleinamphiboleinsystemswithSiO2contentgreaterthan50 wt %, whereas LREE always remain incompatible. We use the new D REE amph/l values to calculate the effects of amphibole crystallization during melt migration in the upper mantle by reactive porous flow as well as fractional crystallization of amphibole during melt migration in veined systems. We show that both processes will lead to residual liquids and solids with extremely variable La N /Yb N ratios.
Journal of Petrology, 2011
and Ta between lawsonite and fluid, and zoisite and fluid at 3·0^3·5 GPa and 650^8508C. The aim is to provide data bearing on the trace element contents of fluids released during dehydration of subducting oceanic crust. Experimental trace element partition coefficients for lawsonite indicate a preference for the light rare earth elements (LREE) over the heavy REE (HREE) and for Be.These characteristics are consistent with the chemical composition of lawsonite in natural rocks. Experimental trace element partition coefficients for zoisite indicate a preference for HREE relative to LREE.This observation, consistent with earlier experimental data, is the reverse of the observed trace element compositions of natural zoisites, indicating the influence of other factors on the trace element contents of this phase. Lattice strain theory explains well the experimentally derived partitioning of divalent cations in the Ca-site between lawsonite and fluid. However, the weak relative fractionation of REE between lawsonite and fluid cannot be explained by lattice strain theory, as previously observed for zoisite^fluid REE partitioning. We combine our experimental data with thermodynamic models of mineral stability to model the compositions of fluids released during subduction of altered normal mid-ocean ridge basalt. The low La/Sm ratio associated with very high Ba/Th in arc magmas can be explained only if allanite is stable in the subducting oceanic crust. This suggests that the crustal fluid component involved in arc magma petrogenesis results from processes occurring in the warm, top part of the subducting slab. Decreasing lawsonite modal proportion with depth is associated with a large release of fluid characterized by low B/Be ratios that could explain the decreasing B/Be ratios in arc magmas with increasing distance from the trench. This implies that an important Be input in arc magma originates from the fluid generated during oceanic crust dehydration.