Systematic use of trace elements in igneous processes

This paper develops an application of the inverse method proposed by Allégre et al. (1977) and Minster et al. (1977). Using a set of trace element data in a suite of primary lavas, it is possible to statistically test whether the data can be described by batch partial melting. Once this is achieved, one can inverse the problem and calculate the parameters which govern the process, that is: the degrees of melting corresponding to each lava, the initial source concentrations and partition coefficients for each element. This is a non-linear, strongly underdetermined problem, but when the set of trace elements is restricted to REE, taking advantage of the fact that their properties are smoothly related, the problem can be overconstrained by assuming a set of poorly constrained model parameters. Information contained in the data and in the external constraints can then be estimated and used to build a strategy of data acquisition. From a table of mineral-liquid partition coefficients, the mineralogies of the source and of the material entering the liquid can be calculated. The method has been tested on synthetic data sets representing natural cases, and proved to converge towards the real solution even when a very poor definition of the external constraints is introduced.The calculations have been applied to REE data on Grenada basanitoids (Shimizu and Arculus, 1976). It is confirmed that these lavas can be derived by 4 to 17% batch partial melting of a garnet lherzolite source (7% gt, 23% cpx) that is relatively enriched in light REE. This result is discussed in relation with Nd and Sr isotopic composition data.     

Systematic use of trace elements in igneous processes

Trace element concentration data can be used in a systematic way for the study of igneous processes by means of constructing models of such processes which satisfactorily account for the observations. We propose to treat the problem as an inverse problem. The concept of trace element paths (TEP) is introduced as a representation of the solution to the direct problem. The inverse problem consists of estimating, by a resolution of the equations, the various parameters of a model so as to provide a best fit to observed TEP. A detailed account of the theory is given in the case of equilibrium fractional crystallization. The estimated parameters are then those figuring in the Rayleigh distillation law, namely, 1) the initial concentrations of trace elements in the parental magma, 2) the bulk partition coefficients of the elements, and 3) the degree of crystallization corresponding to each sample of the magmatic suite analyzed.A slightly generalized maximum likelihood method is used to solve the linearized equation by a stable, iterative algorithm. Information theory is then shown to yield an account of the distribution and flow of information during the process of solving the inverse problem. The concept of Data Importances is generalized, and its use in optimizing the study justified. The technique is successfully applied to a synthetic data set, and then illustrated on a data set from Terceira (Azores). The results are used to refine the conclusions reached in part I, and permit a more detailed discussion of the model.Now at Dept. Geological and Planetary Sciences, California Institute of Technology, Pasadena, Calif., USA     

Experimental cpx/melt partitioning of 24 trace elements

Cpx/melt partition coefficients have been determined by ion probe for 24 trace elements at natural levels in an alkali basalt experimentally equilibrated at 1,380°C and 3 GPa. One goal was to intercompare Ds for both high-field-strength elements and rare earth elements (REE) in a single experiment. Relative to the REE spidergram, Hf and Ti show virtually no anomaly, whereas Zr exhibits a major negative anomaly. Other incompatible elements (Ba, K, Nb) fall in the range of published values, as do elements such as Sr, Y, Sc, Cr and V. Pb shows a value intermediate between La and Ce. Values for Be, Li and Ga are reported for the first time, and show that Be is as incompatible as the light REEs whereas Li and Ga are somewhat more compatible than the heavy REE.     

Zirconium,niobium and certain other trace elements in some alkali igneous rocks

The elements Rb, Sr, Yt, Zr, Nb and Ta have been determined in a variety of alkali igneous rocks and concentration values have been compared with those found in some of the Younger Granites of N. Nigeria. Na-rhyolites, comendites, pantellerites and related rocks from different localities show some close similarities. Nb and Ta both tend to become enriched in (these) late stage differentiates and can exceed 200 p.p.m. and 10 p.p.m. respectively; Zr is similarly high and can exceed 2000 p.p.m. There is appreciable variation in the K/Rb ratio but less in that of Nb/Ta for rocks from the same petrographic province. Data for acid rocks containing Na-amphibole are compared. Nb increases as Sr decreases and becomes conspicuous when Sr is below the detection limit of 40 p.p.m.; this tends to correspond to the appearance of riebeckite. In contrast Sr and Nb can both be high in under-saturated alkali plutonic and volcanic rocks.     

Mineral/melt partitioning of trace elements during hydrous peridotite partial melting

This experimental study examines the mineral/melt partitioning of incompatible trace elements among high-Ca clinopyroxene, garnet, and hydrous silicate melt at upper mantle pressure and temperature conditions. Experiments were performed at pressures of 1.2 and 1.6 GPa and temperatures of 1,185 to 1,370 °C. Experimentally produced silicate melts contain up to 6.3 wt% dissolved H ₂O, and are saturated with an upper mantle peridotite mineral assemblage of olivine+orthopyroxene+clinopyroxene+spinel or garnet. Clinopyroxene/melt and garnet/melt partition coefficients were measured for Li, B, K, Sr, Y, Zr, Nb, and select rare earth elements by secondary ion mass spectrometry. A comparison of our experimental results for trivalent cations (REEs and Y) with the results from calculations carried out using the Wood-Blundy partitioning model indicates that H ₂O dissolved in the silicate melt has a discernible effect on trace element partitioning. Experiments carried out at 1.2 GPa, 1,315 °C and 1.6 GPa, 1,370 °C produced clinopyroxene containing 15.0 and 13.9 wt% CaO, respectively, coexisting with silicate melts containing ~1–2 wt% H ₂O. Partition coefficients measured in these experiments are consistent with the Wood-Blundy model. However, partition coefficients determined in an experiment carried out at 1.2 GPa and 1,185 °C, which produced clinopyroxene containing 19.3 wt% CaO coexisting with a high-H ₂O (6.26±0.10 wt%) silicate melt, are significantly smaller than predicted by the Wood-Blundy model. Accounting for the depolymerized structure of the H ₂O-rich melt eliminates the mismatch between experimental result and model prediction. Therefore, the increased Ca ²⁺ content of clinopyroxene at low-temperature, hydrous conditions does not enhance compatibility to the extent indicated by results from anhydrous experiments, and models used to predict mineral/melt partition coefficients during hydrous peridotite partial melting in the sub-arc mantle must take into account the effects of H ₂O on the structure of silicate melts.     

The partitioning of trace elements between clinopyroxene and trachybasaltic melt during rapid cooling and crystal growth

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 charge-imbalanced configurations.     

The partitioning of trace transition elements in crystal structures: a provocative review with applications to mantle geochemistry

Interrelationships between chemical compositions and crystal structures of minerals pioneered by Goldschmidt have been overlooked by modern geochemists. While analytical techniques and data for trace elements have become more sophisticated, progress has been slow in relating abundance data of individual elements to their relative enrichments in specific sites in mineral structures. The concept of diadochy has degenerated into an analytical relationship between major and trace elements, instead of its original crystallographic basis of replacement of one atom by another at a specific site in a crystal structure. Future interpretations of trace element data must consider the partitioning of atoms between different coordination sites in multisite mineral structures.The well-known partitioning of nickel and chromium into the earliest minerals during magmatic crystallization is extended to magma evolution in the mantle. The strong preference of Ni²⁺ and Cr³⁺ for octahedral sites in minerals leads to their enrichment in certain Iherzolites which are refractory residua during partial fusion of the mantle.     

Systematic use of trace element in igneous process

This paper describes an extended application of the Rayleigh distillation law to trace element behavior in a fractional crystallization sequence. Using a trace element with a very low bulk partition coefficient as a reference (as suggested by Anderson and Greenland, 1969, and extended by Treuil and Varet, 1973), we can derive bulk partition coefficients for other elements and, in turn, the mineralogical composition of the cumulates. Trace elements with large D, such as Ni and Cr, further constrain the system, and we can deduce the initial composition of the magma. An example of this technique is shown for Terceira Island in the Azores.Contribution IPGNS no 229Now at Dept. of Geological Sciences, California Institute of Technology, Pasadena, California, USA     

Chemical partitioning of trace and major elements in soils contaminated by mining and smelting activities

Soils from historical Pb mining and smelting areas in Derbyshire, England have been analysed by a 5-step sequential extraction procedure, with multielement determination on extraction solutions at each step by ICP-AES. Each of the chemical fractions is operationally defined as: (i) exchangeable; (ii) bound to carbonates or specifically adsorbed; (iii) bound to Fe–Mn oxides; (iv) bound to organic matter and sulphides; (v) residual. The precision was estimated to be about 5%, and the overall recovery rates were between 85 and 110%. The carbonate/specifically adsorbed and Fe–Mn oxide phases are the largest fractions for Pb in soils contaminated by both mining and smelting. Most of the Zn is associated with Fe–Mn oxide and the residual fractions. Cadmium is concentrated in the first 3 extraction steps, particularly in the exchangeable phase. The most marked difference found between soils from the mining and smelting sites is the much higher concentrations and proportions of metals in the exchangeable fraction at the latter sites. This indicates greater mobility and potential bioavailability of Pb, Zn and Cd in soils at the smelting sites than in those in the mining area. The most important fraction for Fe and Al is the residual phase, followed by the Fe–Mn oxide forms. In contrast, the Fe–Mn oxide fraction is the dominant phase for Mn in these soils. In the mining area, most of the Ca is in the carbonate fraction (CaCO₃), while the exchangeable and residual phases are the main fractions for Ca at the smelting sites. Phosphorus is mainly in the residual and organic fractions in both areas. The exchangeable fractions of Pb, Zn and Cd in soils were found to be significantly related to the concentrations of these metals in pasture herbage.     

Silicate perovskite-melt partitioning of trace elements and geochemical signature of a deep perovskitic reservoir

We have determined the partitioning of a wide range of trace elements between silicate melts and CaSiO₃ and MgSiO₃ perovskites using both laser ablation-ICPMS and ion microprobe techniques. Our results show that, with the exception of Sc, Zr, and Hf, all trace elements we considered are incompatible in MgSiO₃ perovskite, from highly incompatible for U, Th, Ba, La, Sr and monovalent elements to slightly incompatible for heavy rare earth elements. MgSiO₃ perovskite-melt partition coefficients increase slightly with Al content in the perovskite. These observations contrast strongly with partitioning between CaSiO₃ perovskite and silicate melts. In the latter case, all rare earth elements are clearly compatible as are U and Th. Our data also suggest that, contrary to pressure and temperature, melt composition can significantly affect CaSiO₃ perovskite-melt partitioning; partition coefficients for rare earth elements and U and Th increase with decreasing CaO melt content. The presence of ∼0.4 wt% water in melt makes little difference, however. Partitioning of trace elements into the large site of both MgSiO₃ and CaSiO₃ perovskites follows the near-parabolic dependence on ionic radius predicted from the lattice strain model. The peaks of the parabolae are much higher for the CaSiO₃ phase, perhaps suggesting that the mechanisms of charge compensation for heterovalent substitution are different in the two cases. Our partitioning data have been used to assess the potential effect of perovskite fractionation into the lower mantle during early Earth history. Crystallisation of less than 8% of a mixture of CaSiO₃ and MgSiO₃ perovskites could have led to a ‘layer’ enriched in U and Th without disturbing the chondritic pattern of refractory lithophile elements in the primitive upper mantle. The resultant reservoir could have high Sm/Nd, U/Pb, Sr/Rb, Lu/Hf ratios similar to the HIMU component of ocean island basalts, but would not balance the observed depletion of the primitive upper mantle in Si and Nb.     

Distribution and partitioning of major and trace elements in pyrite-bearing sediments of a Mediterranean coastal lagoon

The formation of iron sulphide minerals exerts significant control on the behaviour of trace elements in sediments. In this study, three short sediment cores, retrieved from the remote Antinioti lagoon (N. Kerkyra Island, NW Greece), are investigated concerning the solid phase composition, distribution, and partitioning of major (Al, Fe) and trace elements (Cd, Cu, Mn, Pb, and Zn). According to ²¹⁰Pb, the sediments sampled correspond to depositions of the last 120 years. The high amounts of organic carbon (4.1–27.5%) result in the formation of Fe sulphides, predominantly pyrite, already at the surface sediment layers. Pyrite morphologies include monocrystals, polyframboids, and complex FeS–FeS₂ aggregates. According to synchrotron-generated micro X-ray fluorescence and X-ray absorption near-edge structure spectra, authigenically formed, Mn-containing, Fe(III) oxyhydroxides (goethite type) co-exist with pyrite in the sediments studied. Microscopic techniques evidence the formation of galena, sphalerite and CuS, whereas sequential extractions show that carbonates are important hosts for Mn, Cd, and Zn. However, significant percentages of non-lattice held elements are bound to Fe/Mn oxyhydroxides that resist reductive dissolution (on average 60% of Pb, 46% of Cd, 43% of Zn and 9% of Cu). The partitioning pattern changes drastically in the deeper part of the core that is influenced by freshwater inputs. In these sediments, the post-depositional pyritization mechanism, illustrated by overgrowths of Fe monosulphides on pre-existing pyrite grains, results in relatively high degree of pyritization that reaches 49% for Cd, 66% for Cu, 32% for Zn and 7% for Pb.     

Patterns of trace element partitioning

When a phenocryst and its lava matrix have been analysed for both major and trace elements, plots of partition coefficient (mineral/matrix) against ionic radius give a family of sub-parallel curves, one univalent, one divalent, etc. Onuma et al. (1968) demonstrated this with two analyses and concluded, from the shape of these curves, that trace element partition between phenocryst and groundmass is determined primarily by crystal structure of the phenocryst. In this study, over 50 such analyses, taken from the literature, have been plotted on modified Onuma diagrams, in order to analyse the constant and variable factors in the curves. It is demonstrated that these curves can provide important additional information in trace element studies by revealing, for example, the site or sites in a mineral which a given element is occupying, the valency state of the element and even, in some cases, the proportion of different valency states present.     

Behaviour of trace elements during thermal metamorphism and or granitization of the metasediments and basic igneous rocks

The behaviour of trace elements in various metasediments and associated basic igneous rocks around a granite pluton has been studied. From these studies it has been found that the nature of chemical changes is different, both in magnitude and order, in the different groups of rocks. It is suggested that the nature of granitization has chiefly been controlled by the original composition of the rocks which has been responsible for setting up a chemical gradient across the contact under the influence of which the movement of cations has taken place.

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Zusammenfassung Das Verhalten von Spurenelementen wurde in verschiedenen Metasedimenten und benachbarten mafitischen Magmagesteinen um einen Granitpluton herum untersucht. Es wird gezeigt, daß die chemischen Änderungen ihrem Gehalt und ihrer Anordnung nach in den unterschiedlichen Gesteinsgruppen verschieden sind. Der Granitisationsprozeß ist hauptsächlich von der ursprünglichen Zusammensetzung der Gesteine abhängig, welche das chemische Gefälle erzeugen. Die Mobilisation der Kationen geht vom Kontakt aus.

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Résumé Nous avons étudié le comportement des éléments différents dans les métasédiments différents et dans les roches pyrogènes et basiques associeés autor d'un pluton granite. Nos études montrent que la nature des changements chimiques diffère en magnitude et en ordre dans les groupes différents des roches. On a suggéré que la nature de granitisation est principalement controlée par la composition originale des roches qui a produit un gradient chimique le long du contact. Le mouvement des cations a été influencé par ce contact.

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Aluminum-dependent trace element partitioning in clinopyroxene

As technical advances have dramatically increased our ability to analyze trace elements, the need for more reliable data on the compositional dependence of trace element partitioning between minerals and melt has become increasingly important. The late-Cretaceous Carmacks Group of south central Yukon comprises a succession of primitive high-Mg ankaramitic lavas characterized by shoshonitic chemical affinities and containing large complexly zoned clinopyroxene phenocrysts. The compositional zonation of the clinopyroxene phenocrysts is characterized by relatively Fe-rich (Mg^# = Mg/(Mg + Fe) = 0.85), but mottled, cores surrounded by mantles of cyclically-zoned clinopyroxene whose Mg^# varies repeatedly between 0.9 and 0.80. These cyclically zoned clinopyroxene mantles appear to record the repeated influx and mixing of batches of primitive with more evolved magma in a deep sub-crustal (∼1.2 GPa) magma chamber(s). Laser ablation ICP-MS was used to analyze the trace element variation in these zoned clinopyroxenes. The results indicate more than a threefold variation in the absolute concentrations of Th, Zr, rare earth elements (REE), and Y within individual clinopyroxene phenocrysts, with no apparent change in the degree of REE or high field strength element (HFSE) fractionation. The variation in absolute abundances of trace elements correlates closely with the major element composition of the clinopyroxene, with the most enriched clinopyroxene having the lowest Mg^# and highest Al contents. The problem is that the amount of crystal fractionation required to explain the major element variation (∼20%) in these clinopyroxene phenocrysts cannot explain the increase in the abundance of the incompatible trace elements, which would require more than 70% crystal fractionation, if constant partition coefficients are assumed. The anomalous increase in incompatible trace elements appears to reflect an increase in their partition coefficients with increasing Al^IV in the clinopyroxene; with an increase in Al₂O₃ from 1.5 to 4.0 wt.% during ∼20% crystal fractionation over a temperature decrease of ∼100°C being associated with more that a threefold increase in the partition coefficients of Th, Zr, REE, and Y. The magnitude of these increases may indicate that the substitution of these trace elements into clinopyroxene is better modeled in some natural systems by a local charge balance model, rather than the distributed charge model that better replicates the results of annealed experiments. These findings indicate that the effect of Al on the partition coefficients of incompatible trace elements in clinopyroxene may be under appreciated in natural magmatic systems and that the application of experimentally determined clinopyroxene partition coefficients to natural systems must be done with caution.     

Kinetically induced compositional zoning in titanite: implications for accessory-phase/melt partitioning of trace elements

Usually it is assumed that the partitioning of trace elements into titanite in metaluminous granitoid plutonic environments takes place under equilibrium conditions and that compositional zoning is due solely to progressive changes in melt chemistry and/or mineral/melt partition coefficients. Examination of titanites from a variety of Caledonian metaluminous granitoids and related rocks has revealed that sector zoning is present, indicating disequilibrium partitioning. The sector zoning in titanites is defined principally by the distribution of the rare earth elements (REE), Y, Nb, Al and Fe. The REE, Y and Nb preferentially occur within the minor (100) sectors relative to the morphologically important (111) sectors. The reverse is true of Al and Fe which preferentially occur within the (111) sectors relative to the (100) sectors. The patterns of sector zoning are complicated by the fact that the relative growth rates of the various crystal faces fluctuated during growth. Sector zoning indicates that crystal-interface kinetics are responsible for the observed patterns of element partitioning. It is concluded that differences in the lateral-layerspreading rates of crystal faces bring about the sector zoning. The results have implications for the use of trace element partition coefficients in the modelling of fractionation processes.     

Effect of melt composition on the partitioning of trace elements between titanite and silicate melt

Isobaric and isothermal experiments were performed to investigate the effect of melt composition on the partitioning of trace elements between titanite (CaTiSiO₅) and a range of different silicate melts. Titanite-melt partition coefficients for 18 trace elements were determined by secondary ion mass spectrometry (SIMS) analyses of experimental run products. The partition coefficients for the rare earth elements and for Th, Nb, and Ta reveal a strong influence of melt composition on partition coefficients, whereas partition coefficients for other studied monovalent, divalent and most quadrivalent (i.e., Zr, Hf) cations are not significantly affected by melt composition. The present data show that the influence of melt composition may not be neglected when modelling trace element partitioning.It is argued that it is mainly the change of coordination number and the regularity of the coordination space of trace elements in the melt structure that controls partition coefficients in our experiments. Furthermore, our data also show that the substitution mechanism by which trace elements are incorporated into titanite crystals may be of additional importance in this context.     

Matrix isolation technique for the study of some factors affecting the partitioning of trace elements

Some of the factors that affect the preferred positions of cations in ionic-solid solutions were investigated utilizing vibrational spectroscopy. Solid solutions of the sulfate and chromate ions codoped with La³⁺ and Ca²⁺ in a KBr host lattice were examined as a function of the polyvalent-cation concentration. The cation—anion pairing process was found to be random for Ca²⁺ whereas the formation of La³⁺SO₄^2? ion-pairs with a C_2v bonding geometry is highly preferential to any type of La³⁺CrO₄^2? ion-pair formation. The relative populations of ion-pair site configurations are discussed in terms of an energy—entropy competition which can be applied to the partition of trace elements during magmatic processes.     

Minor elements in igneous and metamorphic apatite

Analyses of forty-six apatites from various igneous and metamorphic environments have shown enrichment relative to the lithosphere in Ce, Y, La and Sr; apatite from many granite pegmatites is also enriched in Mn. These elements appear to be substituting for Ca in the structure.The P position is far less amenable to impurity incorporation and As⁵⁺ and V⁵⁺ seldom substitute to any degree for P⁵⁺; this is partly due to environmental and partly to structural control.The ratio of Y to Ce + La is higher and total Ce + La + Y content lower, in pegmatite apatites than in apatite from regional metamorphic and plutonic environments, because of selective incorporation of the Ce-earths in K-felspars and monazite during crystallization of granites and pegmatites and enrichment of the Y-earths in residual solutions.The highest Sr content occurs in apatite from ultrabasic and alkalic rocks. Ba is also enriched in apatite from alkalic complexes and carbonatites.     

The influence of carbon on trace element partitioning behavior

Carbon has been proposed as a potential light element in planetary cores, included in models of planetary core formation, and found in meteoritic samples and minerals. To better understand the effect of C on the partitioning behavior of elements, solid/liquid partition coefficients (D = (solid metal)/(liquid metal)) were determined for 17 elements (As, Au, Co, Cr, Cu, Ga, Ge, Ir, Ni, Os, Pd, Pt, Re, Ru, Sb, Sn, and W) over a range of C contents in the Fe-Ni-C system at 1 atm. The partition coefficients for the majority of the elements increased as the C content of the liquid increased, an effect analogous to that of S for many of the elements. In contrast, three of the elements, Cr, Re, and W, were found to have anthracophile (C-loving) preferences, partitioning more strongly into the metallic liquid as the C content increased, resulting in decreases to their partition coefficients. For half of the elements examined, the prediction that partitioning in the Fe-Ni-S and Fe-Ni-C systems could be parameterized using a single set of variables was not supported. The effects of S and C on elemental partitioning behavior can be quite different; consequently, the presence of different non-metals can result in different fractionation patterns, and that uniqueness offers the opportunity to gain insight into the evolution of planetary bodies.