The effects of pressure and temperature on the partitioning of Ti, Sr and REE between amphibole, clinopyroxene and basanitic melts (original) (raw)
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Contributions to Mineralogy and Petrology, 2013
We present the variation in trace element partition coefficients measured at the interface between rapidly cooled clinopyroxene crystals and co-existing melts. Results indicate that, as the cooling rate is increased, clinopyroxene crystals are progressively depleted in Si, Ca and Mg counterbalanced by enrichments in Al (mainly tetrahedral Al iv ), Na and Ti. Partition coefficients (Ds) for rare earth elements (REE), high field strength elements (HFSE) and transition elements (TE) increase with increasing cooling rate, in response to clinopyroxene compositional variations. The entry of REE into the M2 site is facilitated by a coupled substitution where either Na substitutes for Ca on the M2 site or Al iv substitutes for Si in the tetrahedral site. The latter substitution reflects an increased ease of locally balancing the excess charge at M2 as the number of surrounding Al iv atoms increases. Due to the lower concentration of Ca in rapidly cooled clinopyroxenes, divalent large ion lithophile elements (LILE) on M2 decrease at the expense of monovalent cations. Conversely, higher concentrations of HFSE and TE on the M1 site are facilitated as the average charge on this site increases with the replacement of divalent-charged cations by Al vi . Although crystallization kinetics modify clinopyroxene composition, deviations from equilibrium partitioning are insufficient to change the tendency of a trace element to be compatible or incompatible. Consequently, there are regular relationships between ionic radius, valence of the trace element and D. At both equilibrium and cooling rate conditions, Ds for isovalent cations define parabola-like curves when plotted against ionic radius, consistent with the lattice strain model, demonstrating that the partitioning of trace elements is driven by charge balance mechanisms; cation substitution reactions can be treated in terms of the energetics of the various chargeimbalanced configurations.
Contributions to Mineralogy and Petrology, 2006
Thirty five minor and trace elements (Li, Be, B, Sc, Cu, Zn, Ga, Ge, As, Rb, Nb, Mo, Ag, Cd, In, Sn, Sb, Cs, Ba, La, Ce, Nd, Sm, Tb, Ho, Tm, Lu, Hf, Ta, W, Tl, Pb, Bi, Th and U) in experimentally produced near-liquidus phases, from a primitive nelpheline basanite from Bow Hill in Tasmania (Australia), were analysed by LAM ICP-MS. A number of halogens (F, Cl and I) were also analysed by electron microprobe. The analyses were used to determine mineral/melt partition coefficients for mica, amphibole, garnet, clinopyroxene, orthopyroxene and olivine for conditions close to multiple saturation of the basanite liquidus with garnet lherzolite (approximately 2.6 GPa and 1,200°C with 7.5 wt% of added H2O). A broader range of conditions was also investigated from 1.0 GPa and 1,025°C to 3.5 GPa and 1,190°C with 5-10 wt% of added H2O. The scope and comprehensiveness of the data allow them to be used for two purposes, these include the following: an investigation of some of the controlling influences on partition coefficients; and the compilation of a set partition coefficients that are directly relevant to the formation of the Bow Hill basanite magma by partial melting of mantle peridotite. Considering clinopyroxene, the mineral phase for which the most data were obtained, systematic correlations were found between pressure and temperature, mineral composition, cation radius and valence, and Δ G coulb (the coulombic potential energy produced by substituting a cation of mismatched valence into a crystallographic site). Δ G coulb is distinctly different for different crystallographic sites, including the M2 and M1 sites in clinopyroxene. These differences can be modelled as a function of variations in optimum valence (expressed as 1 sigma standard deviations) within individual M1 and M2 site populations.
Ar and K partitioning between clinopyroxene and silicate melt to 8 GPa
Geochimica et Cosmochimica Acta, 2002
The relative incompatibility of Ar and K are fundamental parameters in understanding the degassing history of the mantle. Clinopyroxene is the main host for K in most of the upper mantle, playing an important role in controlling the K/Ar ratio of residual mantle and the subsequent time-integrated evolution of 40 Ar/ 36 Ar ratios. Clinopyroxene also contributes to the bulk Ar partition coefficient that controls the Ar degassing rate during mantle melting. The partitioning of Ar and K between clinopyroxene and quenched silicate melt has been experimentally determined from 1 to 8 GPa for the bulk compositions Ab 80 Di 20 (80 mol% albite-20 mol% diopside) and Ab 20 Di 80 with an ultraviolet laser ablation microprobe (UVLAMP) technique for Ar analysis and the ion microprobe for K. Data for Kr (UVLAMP) and Rb (ion probe) have also been determined to evaluate the role of crystal lattice sites in controlling partitioning. By excluding crystal analyses that show evidence of glass contamination, we find relatively constant Ar partition coefficients (D Ar ) of 2.6 ϫ 10 Ϫ4 to 3.9 ϫ 10 Ϫ4 for the Ab 80 Di 20 system at pressures from 2 to 8 GPa. In the Ab 20 Di 80 system, D Ar shows similar low values of 7.0 ϫ 10 Ϫ5 and 3.0 ϫ 10 Ϫ4 at 1 to 3 GPa. All these values are several orders of magnitude lower than previous measurements on separated crystal-glass pairs. D K is 10 to 50 times greater than D Rb for all experiments, and both elements follow parallel trends with increasing pressure, although these trends are significantly different in each system studied. The D K values for clinopyroxene are at least an order of magnitude greater than D Ar under all conditions investigated here, but D Ar appears to show more consistent behavior between the two systems than K or Rb. The partitioning behavior of K and Rb can be explained in terms of combined pressure, temperature, and crystal chemistry effects that result in changes for the size of the clinopyroxene M2 site. In the Ab 20 Di 80 system, where clinopyroxene is diopside rich at all pressures, D K and D Rb increase with pressure (and temperature) in an analogous fashion to the well-documented behavior of Na. For the Ab 80 Di 20 system, the jadeite content of the clinopyroxene increases from 22 to 75 mol% with pressure resulting in a contraction of the M2 site. This has the effect of discriminating against the large K ϩ and Rb ϩ ions, thereby countering the effect of increasing pressure. As a consequence D K and D Rb do not increase with pressure in this system.
Chemical Geology, 2004
Trace element compositions of clinopyroxene and amphibole from three rift-related alkaline to peralkaline igneous complexes (syenites and granites) in South Greenland reflect evolving magma composition as well as crystal-chemical control on partitioning. Clinopyroxenes and amphiboles evolve from Ca-Mg-dominated members via intermediate to Na-Fe 3+dominated members. Most trace elements are highly enriched compared to primitive mantle values, consistent with the highly fractionated character of the host rocks. High field strength element (HFSE; Ti, Zr, Hf, Sn, Nb, Ta) abundances appear to be mainly controlled by the major element composition of the host crystal, which in turn determines the crystal site parameters. A crystal-chemical control is also indicated for the REE, since clinopyroxenes and amphiboles show continuous change from LREE-enriched patterns in the calcic minerals via wave-shaped pattern in the Ca-Na minerals towards a more pronounced HREE enrichment in the most Na-rich minerals. The low absolute abundances of large ion lithophile elements (Ba, Sr, Pb, Eu 2+ ) are interpreted to reflect both a crystal-chemical aversion to incorporate these elements and the effects of prolonged feldspar fractionation on the melt composition. Eu and Pb abundances are also affected by oxygen fugacity and crustal assimilation, respectively.
2007
Olivine/melt and orthopyroxene/melt rare-earth element (REE) partition coefficients consistent with clinopyroxene/melt partition coefficients were determined indirectly from subsolidus partitioning between olivine, orthopyroxene, and clinopyroxene after suitable correction for temperature. Heavy-and middle-REE ratios for olivine/clinopyroxene and orthopyroxene/clinopyroxene pairs correlate negatively with effective cationic radius, whereas those for the light REEs correlate positively with cationic radius, generating a U-shaped pattern in apparent mineral/clinopyroxene partition coefficients versus cationic radius. Lattice strain models of partitioning modified for subsolidus conditions yield negative correlations of olivine/clinopyroxene and orthopyroxene/clinopyroxene with respect to cationic radii, predicting well the measured partitioning behaviors of the heavy and middle REEs but not that of the light REEs. The light-REE systematics cannot be explained with lattice strain theory and, instead, can be explained by disequilibrium enrichment of the light REEs in melt inclusions or on the rims of olivine and orthopyroxene. Realistic light-REE partition coefficients were thus extrapolated from the measured heavy-and middle-REE partition coefficients using the lattice strain model. Light REE olivine/melt and orthopyroxene/melt partition coefficients calculated in this manner are lower than most published values, but agree reasonably well with partitioning experiments using the most recent in situ analytical techniques (secondary-ionization mass spectrometry and laser ablation inductively coupled plasma mass spectrometry). These new olivine/melt and orthopyroxene/melt partition coefficients are useful for accurate modeling of the REE contents of clinopyroxene-poor to -free lithologies, such as harzburgitic residues of melting. Finally, the application of the lattice strain theory to subsolidus conditions represents a framework for assessing the degree of REE disequilibrium in a rock.
Geochimica et Cosmochimica Acta, 1998
The partitioning of fifteen trace elements (Rb, and Th) between clinopyroxene and synthetic melt has been studied in two compositions along an isotherm in the diopside-albite-anorthite ternary at 1 bar pressure. The two compositions correspond to ϳDi 65 An 35 and ϳDi 55 Ab 45 and produce clinopyroxenes distinct in chemistry while melt compositions range from 49 wt% SiO 2 to 61 wt% SiO 2 . The partition coefficients of high field strength elements (HFSE) increase by factors of 2-8 in Di-An experiments relative to Di-Ab experiments while other elements show very little change (Ϯ20%) between compositions. The change in HFSE partitioning correlates with increases in tetrahedral Al 2 0 3 ( IV Al) content of clinopyroxenes in the anorthite-bearing experiments. Changes in D Ta /D Nb also correlate with IV Al based on a survey of previously published determinations.
Chemical Geology, 1993
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 1 a): 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.
Contributions to Mineralogy and Petrology, 1985
Experimental determination of over seventy sets of clinopyroxene/silicate liquid (glass) partitition coefficients (D) for four rare earth elements (REE -La, Sm, Ho, Lu) in a range of REE-enriched natural rock compositions (basalt, basaltic andesite, andesite and rhyodacite) demonstrate a convex upward pattern, favouring the heavy REE (Ho, Lu) and markedly discriminating against the light REE (La). These patterns are consistent with previously documented clinopyroxene D values reported from natural phenocryst/matrix pairs and from experimental work using either REE-enriched compositions and electron microprobe analytical techniques (as in the present study) or natural or synthetic undoped compositions and mass spectrometric, ion probe or X-ray autoradiographic analytical techniques. However, the large data base in the present study allows evaluation of the effect of compositional and physical parameters on REE partitioning relationships. Considering Dno, it is shown that (1) D increases 6-fold with increasing SiO2 content of the coexisting liquid from 50 to ~ 70 wt% SiO2 (2) D increases 4-fold with decreasing temperature from 1,120~ C to 900~ (3) D increases 2-fold with increasing pressure from ~2.5 to 20 kb. (4) D increases ~2-fold as fO 2 increases from approximately that of the MW buffer to the HM buffer (5) D remains unchanged within experimental error as the water content of the melt changes from ~0.3 to > 10% by weight H20.
High-field-strength element partitioning between pyroxene and basaltic to dacitic magmas
Chemical Geology, 1994
The effects of composition and temperature on pyroxene-melt partitioning of the high-field-strength elements (HFSE)-Ti, Zr, Nb and Ta-were evaluated from doped experiments on natural mafic to intermediate composition lavas at pressures from 0.1 MPa to 0.9 GPa (0.001 to 9 kbar). The HFSE partition coefficients (D) maintain similar relative relationships: Dxi > Dzr > DTa > DNb, but vary absolutely as a function of composition and temperature, often exhibiting a range of over a factor of 5 at a single temperature. For example, Dzr ranges from 0.1 in a tholeiitic melt to 0.6 for a dacitic melt at 1100°C, 0.1 MPa. Dzr, DTa and DNb for high-and low-Ca pyroxene can be described as linear functions of Dx,. For high-Ca pyroxenes, the functions are Dzr=0.64Dvi-0.13, Dva=0.14Dv~-0.02 and DNb=0.04DTi-0.01. The low-Ca pyroxene expressions are Dzr=O.6ODTi-O.06, DTa=O.27Dvi-O.O05 and DNb=O.O8Dxi-0.005. This linear relationship suggests similar substitution mechanisms for Ti and the other HFSE in both pyroxene and silicate melts. An expression was derived to calculate the Ti content ofpyroxene based on the melt composition, Ca content of the pyroxene, temperature and pressure. This expression uses an approximation of the equilibrium constant for an exchange reaction of a Ti/Al-bearing component with a Ca-bearing component in the pyroxene. Over the experimental temperature range (1170-1070 °C), the clinopyroxene Ti contents can be reproduced with a precision of +20% (1~).