REE Partition Coefficients Research Articles

REE abundances in ancient seawater inferred from marine limestone and experimental REE partition coefficients between calcite and aqueous solution

In our earlier work, we determined relative REE partition coefficients between calcite and aqueous (CaCl2 + NaCl) solution, but absolute values could not be obtained because of small overgrowths of calcite. In this study, we have made similar experiments in order to determine the absolute REE partition coefficients between calcite and aqueous solution, and have evaluated partition coefficients between calcite and seawater. Calcite-supersaturated solution doped with REEs was constantly pumped into a reactor to form calcite overgrowths on the seeds under CO2 + N2 gas. Absolute values of REE partition coefficients, Kd(REE)=(XREE/XCa)calcite/([REE,tot.]/[Ca])solution, have been obtained. The patterns of Kd(REE) show convex tetrad effects and Y/Ho fractionation, which is consistent with our earlier work. We have calculated REE abundances in Permian seawater using the partition coefficients corrected to seawater compositions and REE analyses of Permian seamount-type limestones. All the estimated REE abundances in Permian seawater indicate concave tetrad effects, large Y/Ho fractionation and negative Ce and Eu anomalies in their chondrite-normalized patterns, which are quite similar to those for present-day seawater. The estimated REE abundances in Permian seawater, however, are lower than those in present-day seawater by approximately one order of magnitude. This suggests that significant parts of marine limestones could not incorporate seawater REEs after the permeability and porosity of the original biogenic carbonate sediments with low-REE contents had been reduced to certain critical levels.

Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway): Part II. REE and the trapped liquid fraction

Rare earth elements in bulk cumulates and in separated minerals (plagioclase, apatite, Ca-poor and Ca-rich pyroxenes, ilmenite and magnetite) from the Bjerkreim–Sokndal layered intrusion (Rogaland Anorthosite Province, SW Norway) are investigated to better define the proportion of trapped liquid and its influence on bulk cumulate composition. In leuconoritic rocks (made up of plagioclase, Ca-poor pyroxene, ilmenite, ±magnetite, ±olivine), where apatite is an intercumulus phase, even a small fraction of trapped liquid significantly affects the REE pattern of the bulk cumulate, together with cumulus minerals proportion and composition. Contrastingly, in gabbronoritic cumulates characterized by the presence of cumulus Ca-rich pyroxene and apatite, cumulus apatite buffers the REE content. La/Sm and Eu/Eu* vs. P 2O 5 variations in leuconorites display mixing trends between a pure adcumulate and the composition of the trapped liquid, assumed to be similar to the parental magma. Assessment of the trapped liquid fraction in leuconorites ranges from 2 to 25% and is systematically higher in the north-eastern part of the intrusion. The likely reason for this wide range of TLF is different cooling rates in different parts of the intrusion depending on the distance to the gneissic margins. The REE patterns of liquids in equilibrium with primitive cumulates are calculated with mass balance equations. Major elements modelling ( Duchesne, J.C., Charlier, B., 2005. Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway): Part I. Constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent. Lithos. 83, 299–254) permits calculation of the REE content of melt in equilibrium with gabbronorites. Partition coefficients for REE between cumulus minerals and a jotunitic liquid are then calculated. Calculated liquids from the most primitive cumulates are similar to a primitive jotunite representing the parental magma of the intrusion, taking into account the trapped liquid fraction calculated from the P 2O 5 content. Consistent results demonstrate the reliability of liquid compositions calculated from bulk cumulates and confirm the hypothesis that the trapped liquid has crystallized as a closed-system without subsequent mobility of REE in a migrating interstitial liquid.

Geochemistry of Precambrian carbonate intercalated in pillows and its host basalt: implications for the REE composition of circa 3.4 Ga seawater

The REE composition of early Archean seawater has been estimated through analyses of dolomite intercalated in pillow basalts from the Pilbara Block, Western Australia. We have critically evaluated the possible disturbances that have affected the chemical composition of the carbonate through geologic time. Dolomite samples from the Pilbara show a chondrite-normalized LREE-enriched pattern with profound downward convex tetrad effects, suggesting that these REE signatures are related to ancient seawater from which the dolomite precipitated. Contribution of a hydrothermal component to the seawater signature appears negligible based on the small Eu anomalies observed in dolomite. Therefore, dolomite samples from the Pilbara Block appear to preserve REEs intrinsic to ancient seawater, suggesting that the REE composition of seawater could be estimated given that carbonate–seawater partition coefficients of REEs are known. We used REE partition coefficients based on the analyses of a dolomite reference sample (JDo-1 from the Geological Survey of Japan) and show that REE concentrations of ∼3.4 Ga seawater could have been one or two orders of magnitude higher than at present. Moreover, it appears that 3.4 Ga seawater was significantly enriched in HREE and had less pronounced W-type tetrad effects relative to that of present seawater. These may reflect paleo-environmental conditions particular to seawater in the Archean, such as high-salinity and [CO 3 2−].

Partitioning of Sr, Ba, Rb, Y, and LREE between alkali feldspar and peraluminous silicic magma

Samples from well-known rhyolitic systems in the western USA were analyzed to determine trace-element partition coefficients between sanidine and coexisting melt. Both D Sr and D Eu correlate positively with An concentration in sanidine. Barium, not surprisingly, shows more correlation with Or in sanidine. All three show negative correlations with whole-rock Al 2 O 3 . Partition coefficients for REE show a decreasing trend with strong positive DEu. DLREE correlate negatively with the whole-rock Al 2 O 3 . In peraluminous rocks, Al 2 O 3 in magma seems to exert a major influence on partition coefficients of trace elements. With increasing Al 2 O 3 concentration, the melt composition becomes increasingly similar to that of feldspar. Incompatible trace elements (Rb, Y, Nb, and Zr) in sanidine show positive trends whereas compatible trace elements (Sr, Eu, and Ba) show negative correlations when plotted against whole-rock Al 2 O 3 . LREEs, which are expected to substitute for Ca in the sanidine structure, behave like Sr and Eu. Because trace-element partitioning is influenced by both crystal and magma compositions, multiple regression analysis has been used to predict trace-element partition coefficients. The equations will assist workers in the selection of partition coefficients used in petrogenetic models of silicic systems.

Petrogenesis of the Maowu pyroxenite–eclogite body from the UHP metamorphic terrane of Dabieshan: chemical and isotopic constraints

The Maowu eclogite–pyroxenite body is a small (250×50 m) layered intrusion that occurs in the ultra-high-pressure (UHP) metamorphic terrane of Dabieshan, China. Like the adjacent Bixiling complex, the Maowu intrusion was initially emplaced at a crustal level, then subducted along with the country gneisses to mantle depths and underwent UHP metamorphism during the collision of the North and South China Blocks in the Triassic. This paper presents the results of a geochemical and isotopic investigation on the metamorphosed Maowu body. The Maowu intrusion has undergone open system chemical and isotopic behavior three times. Early crustal contamination during magmatic differentiation is manifested by high initial 87Sr/ 86Sr ratios (0.707–0.708) and inhomogeneous negative ε Nd( T) values of −3 to −10 at 500 Ma (probable protolith age). Post-magmatic and pre-UHP metamorphic metasomatism is indicated by sinusoidal REE patterns of garnet orthopyroxenites, lack of whole-rock (WR) Sm–Nd isochronal relationship, low δ 18O values and an extreme enrichment of Th and REE in a clinopyroxenite. Finally, K and Rb depletion during UHP metamorphism is deduced from the high initial 87Sr/ 86Sr ratios unsupported by in situ Rb/Sr ratios. Laser ICP-MS spot analyses on mineral grains show that (1) Grt and Cpx attained chemical equilibrium during UHP metamorphism, (2) Cpx/Grt partition coefficients for REE correlate with Ca, and (3) LREE abundances in whole rocks are not balanced by that of the principal phases (Grt and Cpx), implying that the presence of LREE-rich accessory phases, such as monazite and apatite, is required to account for the REE budget. Sm–Nd isotope analyses of minerals yielded three internal isochrons with ages of 221±5 Ma and ε( T)=−5.4 for an eclogite, 231±16 Ma and ε( T)=−6.2 for a garnet websterite, and 236±19 Ma and ε( T)=−6.9 for a garnet clinopyroxenite. The Cpx/Grt chemical equilibrium and the consistent mineral isochron ages indicate that the metasomatic processes mentioned above must have occurred prior to the UHP metamorphism. These Sm–Nd ages agree with published zircon and monazite U–Pb ages and constrain the time of UHP metamorphism to 220–236 Ma. The Maowu and Bixiling layered intrusions are similar in their in situ tectonic relationship with their country gneisses, but the two bodies are distinguished by their magma-chamber processes. The Bixiling magmas were contaminated by the lower crust, whereas the Maowu magmas were contaminated by the upper crustal rocks during their emplacement and differentiation. The two complexes represent two distinct suites of magmatic rocks, which have resided in the continental crust for about 300–400 Ma before their ultimate subduction to mantle depths, UHP metamorphism and return to the crustal level.

The REE-Ti mineral chevkinite in comenditic magmas from Gran Canaria, Spain: a SYXRF-probe study

The REE-Ti silicate chevkinite has been recognised previously in Miocene ignimbrites from Gran Canaria, and in correlative offshore syn-ignimbrite turbidites. We have estimated the partition coefficients of REE, Y, Zr and Nb for chevkinite and co-existing peralkaline rhyolitic (comendite) glass using synchrotron-XRF-probe analyses (SYXRF) in order to evaluate the role of this mineral in the REE budget of felsic peralkaline magmas. The Zr/Nb ratio of the chevkinite is 1.55–1.7, strongly contrasting with Zr/Nb of 6.5 in the associated glass. Zr shows a three-fold enrichment in chevkinite relative to the residual melt, whereas Nb is enriched by a factor >10. The enrichment of Ce and La in chevkinite is even more significant, namely 19 wt(%) Ce and 12 wt(%) La, compared to 236 ppm Ce and 119 ppm La in the glass. Chevkinite/glass ratios are 988±30 for La, 806±30 for Ce, 626±30 for Pr, 615±40 for Nd, 392±50 for Sm, 225±30 for Eu, 142±25 for Gd, 72±20 for Dy. For trace elements, we derived KdTE of 74±25 for Y, >8 for Hf, >50 for Th, 15±5 for Nb and 3.55±0.4 for Zr. Mineral/glass ratios for co-existing titanite are 28±10 for La, 86±20 for Ce, 98±30 for Pr, 134±35 for Nd, 240±50 for Sm, 50±20 for Eu, 96±25 for Gd, 82±25 for Dy, 99±30 for Y, 45±10 for Nb and 3±0.5 for Zr. Based on these data, the removal of only 0.05 wt% of chevkinite from a magma with initially 300 ppm Ce would deplete the melt by 93 ppm to yield 207 ppm Ce in the residual liquid. Chevkinite thus appears, when present, to be the controlling mineral within the LREE budget of evolved peralkaline magmas.

The effect of H 2O on crystal-melt partitioning of trace elements

Many experimental data demonstrate distinct differences between crystal-liquid partition coefficients D i measured under high temperature, anhydrous conditions and those determined at lower temperatures in the presence of H 2O. In this study we develop a thermodynamic method of separating the effects of H 2O from those of temperature. We then apply the method to predict partitioning of REE between clinopyroxene, garnet and silicate melt over wide ranges of temperature, pressure and H 2O content. Our initial inputs are the model of Wood and Blundy (1997) for REE partitioning between clinopyroxene and anhydrous melt and the melting temperatures of diopside on the join CaMgSi 2O 6-H 2O (Eggler and Rosenhauer, 1978). We then make the hypothesis that the effect of H 2O on the activity of REE clinopyroxene component (REEMgAlSiO 6) in the melt is the same as the measured effect on CaMgSi 2O 6 component. This leads to predictions of REE partition coefficients for clinopyroxene coexisting with hydrous melt at any P,T and H 2O content up to 45 weight %. The results agree with observed REE partition coefficients with a standard deviation which is the same as that for the anhydrous data. We conclude, therefore, that the ‘H 2O-effect’ may, in this case, be accurately predicted. We extend the approach to garnet by using the join Mg 3Al 2Si 3O 12-H 2O to estimate the effects of H 2O on all ‘garnet-like’ components in the melt. This enables calculation of garnet-melt REE partition coefficients for melts containing up to 25% H 2O. The observation that H 2O influences major and trace component activities in a similar manner enables us to make some generalisations about the combined effects on partitioning of decreasing temperature and increasing water content of the melt. The relative enthalpies of fusion ΔH f of major and trace components dictate whether trace element partition coefficients increase or decrease with H 2O addition: ΔH f trace > ΔH f major gives increasing D trace with addition of H 2O and ΔH f trace < ΔH f major leads to decreasing D trace with addition of H 2O. Note that D trace is strictly the ratio of mole fractions of charge-balanced components such as REEMgAlSiO 6 in solid to melt phases. In the cases considered here, however, D trace closely approximates D = [trace] solid [trace] melt where [trace] refers to weight concentration. For clinopyroxene ΔH f REE < ΔH f major and H 2O addition leads to substantial decreases in REE partition coefficients. For garnet, ΔH f REE ≈ ΔH f major and addition of H 2O results in little change in D REE. The consequences are illustrated with respect to Yb contents of melts of spinel lherzolite. For partial melting under anhydrous conditions, Yb is relatively compatible in clinopyroxene and the Yb contents of the product melts remain close to 5 times source values at all F (fraction of equilibrium batch melting) between 0 and 0.25. Hydrous peridotite produces melts, because of low D Yb, which are strongly enriched in Yb (up to at least 10 times source) and whose Yb concentrations depend strongly on F. This means that the low Yb concentrations observed in many island arc tholeiites cannot be due to the addition of water to fertile peridotite.

Near mantle solidus trace element partitioning at pressures up to 3.4 GPa

We present new experimental partitioning data for a range of petrogenetically important elements at pressures of up to 3.4 GPa. The experiments are designed to mimic low degrees of anhydrous melting beneath mid‐ocean ridges. The available data indicate that the partition coefficients are pressure, temperature, and composition dependent. Therefore partitioning behavior over the appropriate range of pressure, temperature, and composition must be quantified, in order to model continuous extraction of melt during the adiabatic rise of mantle material. For this purpose, we have parameterized the partitioning behavior of the REE, Hf, Zr, U, and Th based on a simple thermodynamic model. Although these parameterizations cannot be used for retrieving thermodynamic constants yet, they do yield accurate descriptions of the partitioning behavior that are useful for modeling decompression melting. Our parameterizations show that the partitioning of trace elements is strongly dependent on the Ca and Al‐content of the clinopyroxene (cpx) and REE are always incompatible in cpx on the peridotite solidus at pressures up to 3.4 GPa. For garnet the data indicate that the heavy REE partition coefficients decrease with increasing pressure. Our data also indicates that Pb is more incompatible than Ce in clinopyroxene; Ce and Pb have similar partition coefficients in garnet. Therefore the presence of a residual phase with high Pb partition coefficients is required to produce the near‐constant Ce/Pb ratios in MORB and OIB. Sulfides are the most likely phase to buffer the Pb content in the melt. Except at small porosities (&lt;0.3%), clinopyroxene on the peridotite solidus is unable to fractionate U from Th significantly (15% 230Th‐excess), whereas garnet can fractionate U from Th effectively at porosities up to 1%. Therefore if the 230Th‐excesses in mid‐ocean ridge basalts are melting phenomena, then melting with garnet residual is required in order to be compatible with physical observations on porosities and upwelling rate at mid‐ocean ridges. New model calculations that include the compositional dependent partitioning of the trace elements show that the predicted physical characteristics (depth and extent of melting, upwelling rate, porosity) of the MORB melting regime are similar for the Lu/Hf, Sm/Nd, and U‐Th systems.

High mass resolution ion microprobe analysis of rare earth elements in silicate glass, apatite and zircon: lack of matrix dependency

The concentrations of all rare earth elements (REE: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) in silicate glass, apatite and zircon were measured by an ion microprobe method with a high mass resolving power of 9300 at 1% peak height. Observed contents of REE in apatite extracted from a dacite agree well with those measured by the isotope dilution-thermal ionization mass spectrometry (ID-TIMS) method within 30% error at 2 σ. The contents of heavy REE in several zircons are consistent with those measured by ID-TIMS and instrumental neutron activation analysis (INAA), while the light REE abundances are significantly lower by ion microprobe than by the other methods. The discrepancy is partly due to the presence of 1–5-μm inclusions of apatites within zircon crystals. Secondary ion yields of REE observed in a silicate glass show a very similar trend to those in apatite and zircon. Since the chemical composition of the silicate glass is appreciably different from the apatite and zircon, relative secondary ion yields of REE are probably independent from matrix effect, which is the most important finding of this work. Calculated partition coefficients of REE between melt and apatite agree well with reported values while the coefficients between melt and zircon are discrepant from those in literature. The REE coefficients in zircon increase with increase of the ionic radius logarithmically, suggesting power law dependence of the coefficients.

Rare-earth elements in synthetic zircon: Part 2. A single-crystal X-ray study of xenotime substitution

Zircon crystals synthesized in a Li-Mo oxide melt and doped with trivalent lanthanides and Y (REE), both with and without P, were examined by single-crystal X-ray diffraction (XRD). REE are incorporated into the Zr site in the zircon structure, and some Zr appears to be displaced to the Si site. Crystals doped with middle REE (MREE, Sm to Dy) and Y, plus P follow the xenotime substitution (REE 3+ + P 5+ = Zr 4+ + Si 4+ ) rather closely, whereas crystals doped with heavy REE (HREE, Er to Lu) deviate from the xenotime substitution, having REE:P atomic ratios significantly greater than one. Xenotime substitution requires that P 5+ replace Si 4+ , but this substitution becomes limited by strain at the Si site in HREE-doped crystals. As Si sites become saturated with P 5+ , additional charge balance in synthetic zircon crystals may be provided by Mo 6+ and Li + from the flux entering interstitial sites, accounting for an additional 0.3 to 0.6 at% HREE beyond that balanced by P 5+ ions. Heavy REE are more compatible in the zircon structure than are LREE and MREE, and HREE substitution is ultimately limited by the inability of the zircon structure to further accommodate charge-compensating elements. Thus the limit on REE concentrations in zircon is not a simple function of REE 3+ ionic radii but depends in a complex way on structural strain at Zr and Si sites, which act together to limit REE and P incorporation. The mechanisms that limit the coupled xenotime substitution change from LREE to HREE. This change means that REE fractionation in zircon may vary according to the availability of charge-compensating elements. REE partition coefficients between zircon and melt must also depend in part on the availability of charge-compensating elements and their compatibility in the zircon structure.

Trace element partitioning between plagioclase and melt: investigation of dopant influence on partition behavior

We present results from an ion microprobe study of REE-doped and natural concentration plagioclase-basalt run products of Drake (1972) that carries on from our earlier study (Bindeman et al., 1998). The goals of this work are (1) to determine plagioclase/melt partition coefficients for all REE for four analyzed plagioclase compositions (An 40–80); and (2) to determine whether doping with REE influences partition coefficients of other REE and other trace elements. In combination with our analyses of Sr-doped runs (Bindeman et al., 1998), the new data allow us to compare partition coefficients ( D ι’s) of trace elements at natural concentrations with those in REE-doped and Sr-doped runs. In these comparisons, runs have the same run temperature and compositions, but different doping element(s). We find that ln( D REE) decreases as a linear function of REE atomic number, in contrast to most plagioclase-melt partition experiments. The slopes of the ln( D ι) vs. %An and RT ln( D ι) vs. %An dependencies of REE and monovalent and divalent cations increase from smaller to larger ions of each valence group. D La/ D Y and D La/ D Lu increase by more than 5 times as plagioclase composition changes from An 80 to An 40. Within each valence group, slopes on ln( D ι) vs. %An and RT ln( D ι) vs. %An plots increase linearly with ionic radius, with the trivalent REE showing the steepest slope vs. ionic radius dependence. Application of the elastic modulus model of Blundy and Wood (1994) to our results yielded good results, confirming that elastic properties determine partition behavior. We also present D ι’s for U. We find no detectable effects of trace element doping on D ι’s of 30 trace elements of varying size and charge. This implies that coupling between different trace elements is not a significant process during partitioning in natural systems, even for the case of heterovalent substitutions (such as REE). However, D REE and D Y in REE-doped runs are 30–100% higher than D REE in undoped runs and Sr-doped runs. Doping with thousands of ppm of three selected REE (it does not matter which three) affects D ι’s of Y and all REE in these runs, including those at natural concentration levels. We suggest that substituting trace cations largely compete for sites of substitution with the mineral-forming cations Na and Ca. We speculate that lower D REE in REE-doped runs are the result of a change in the REE substitution mechanisms at doped concentrations of all REE which leads to a different value of the Henry’s law constant.

Experimental comparison of trace element partitioning between clinopyroxene and melt in carbonate and silicate systems, and implications for mantle metasomatism

Experiments in the systems diopside-albite (Di-Ab) and diopside-albite-dolomite (Di-Ab-Dmt), doped with a wide range of trace elements, have been used to characterise the difference between clinopyroxene-silicate melt and clinopyroxene-carbonate melt partitioning. Experiments in Di-Ab-Dmt yielded clinopyroxene and olivine in equilibrium with CO2-saturated dolomitic carbonate melt at 3 GPa, 1375 °C. The experiments in Di-Ab were designed to bracket those conditions (3 GPa, 1640 °C and 0.8 GPa, 1375 °C), and so minimise the contribution of differential temperature and pressure to partitioning. Partition coefficients, determined by SIMS analysis of run products, differ markedly for some elements between Di-Ab and Di-Ab-Dmt systems. Notably, in the carbonate system clinopyroxene-melt partition coefficients for Si, Al, Ga, heavy REE, Ti and Zr are higher by factors of 5 to 200 than in the silicate system. Conversely, partition coefficients for Nb, light REE, alkali metals and alkaline earths show much less fractionation (<3). The observed differences compare quantitatively with experimental data on partitioning between immiscible carbonate and silicate melts, indicating that changes in melt chemistry provide the dominant control on variation in partition coefficients in this case. The importance of melt chemistry in controlling several aspects of element partitioning is discussed in light of the energetics of the partitioning process. The compositions of clinopyroxene and carbonate melt in our experiments closely match those of near-solidus melts and crystals in CMAS-CO2 at 3 GPa, suggesting that our partition coefficients have direct relevance to melting of carbonated mantle lherzolite. Melts so produced will be characterised by elevated incompatible trace element concentrations, due to the low degrees of melting involved, but marked depletions of Ti and Zr, and fractionated REE patterns. These are common features of natural carbonatites. The different behaviour of trace elements in carbonate and silicate systems will lead to contrasted styles of trace element metasomatism in the mantle.

Trace element distribution in Central Dabie eclogites

Coesite-bearing eclogites from Dabieshan (central China) have been studied by ion microprobe to provide information on trace element distributions in meta-basaltic mineral assemblages during high-pressure metamorphism. The primary mineralogy (eclogite facies) appears to have been garnet and omphacite, usually with coesite, phengite and dolomite, together with high-alumina titanite or rutile, or both titanite and rutile; kyanite also occurs occasionally as an apparently primary phase. It is probable that there was some development of quartz, epidote and apatite whilst the rock remained in the eclogite facies. A later amphibolite facies overprint led to partial replacement of some minerals and particularly symplectitic development after omphacite. They vary from very fine-grained dusty-looking to coarser grained Am + Di + Pl symplectites. The eclogite facies minerals show consistent trace element compositions and partition coefficients indicative of mutual equilibrium. Titanite, epidote and apatite all show high concentrations of REE relative to clinopyroxene. The compositions of secondary (amphibolite facies) minerals are clearly controlled by local rather than whole-rock equilibrium, with the composition of amphibole in particular depending on whether it is replacing clinopyroxene or garnet. REE partition coefficients for Cpx/Grt show a dependence on the Ca content of the host phases, with DREECpx/Grt decreasing with decreasing DCa. This behaviour is very similar to that seen in mantle eclogites, despite differences in estimated temperatures of formation of 650–850 °C (Dabieshan) and 1000–1200 °C (mantle eclogites). With the exception of HREE in garnet, trace elements in the eclogites are strongly distributed in favour of minor or accessory phases. In particular, titanite and rutile strongly concentrate Nb and Zr, whilst LREE–MREE go largely into epidote, titanite and apatite. If these minor/accessory minerals behave in a refractory manner during melting or fluid mobilisation events and do not contribute to the melt/fluid, then the resultant melts and fluids will be strongly depleted in LREE–MREE.

Rare earth elements in Holocene reefal microbialites: a new shallow seawater proxy

The concentration of rare earth elements and yttrium (REE + Y) was determined in Holocene Mg-calcite microbialites from shallow reef framework cavities at Heron Reef, Great Barrier Reef. Shale-normalized REE + Y patterns of 52 microbialite samples show: (1) uniform heavy REE enrichment (Nd SN/Yb SN = 0.236, SD = 0.026); (2) consistent negative Ce and positive La anomalies; (3) marine Y/Ho ratios (56.17, SD = 2.66); and (4) slightly positive Gd anomalies. All of these features are consistent with the geochemistry of well-oxygenated, shallow ambient seawater. REE partition coefficients were calculated relative to shallow Coral Sea seawater. They are uniform (relative SD = 10.2%) across the entire mass range and almost two orders of magnitude higher than those between coral and seawater. Hence, terrigenous detritus-free, modern microbialites are a more reliable proxy for seawater REE chemistry than are skeletal carbonates. Ancient limestones have been considered largely problematic as sources for REE proxies owing to perceived problems with diagenesis, partly on the basis of relatively high REE concentrations in some limestones compared to modern skeletal carbonates. However, high REE concentrations in modern microbialites suggest that ancient limestones with relatively high REE concentrations, if not contaminated by terrigenous detritus, may reflect original seawater chemistry. Terrigenous contamination, if present, is readily detectable on the basis of co-occurring trace element concentrations (Sc, Hf, Th) and Y/Ho ratio. Hence, ancient, particularly reefal, limestones may provide reliable seawater REE proxies. The occurrence of microbialites in clean limestones as old as 3.5 Ga suggests the possibility of reconstructing shallow marine REE chemistry over most of Earth history with important implications for paleogeography and paleoredox studies.

Mineral/melt partition coefficients of oceanic alkali basalts determined on natural samples using laser ablation-inductively coupled plasma-mass spectrometry (LAM-ICP-MS)

AbstractIn order to limit the range of possible differentiation mechanisms and the impact of these processes on the trace element signatures of igneous suites, it is important to be able to predict or model the compositional evolution of the primary and/or parental magmas. Part of the problem in understanding these relationships in basalts from ocean islands is the paucity of reliable trace element mineral/melt partition coefficients, particularly for undersaturated magmas. Consequently, we have measured mineral/groundmass partition coefficients for Rb, Sr, Y, Zr, Nb, Hf, Ta, Th and REE, in situ, in clinopyroxene, Fe-Ti oxides and olivine in primitive basalts from Rarotonga, Cook Islands using LAM-ICP-MS.Analyses of these mafic rocks show high concentrations of most incompatible trace elements (e.g. Sr, Th, Y, REE, etc.) in pyroxenes relative to the other phases, suggesting that fractionation of pyroxenes was the dominant factor in the distribution of these trace elements during crystal fractionation. In such cases, the highly incompatible elements (D &lt;0.01) are Ba, Cs and Nb. Elements that could also be classified as strongly incompatible (D &lt;0.1) are Rb, Ta and Th. The remaining trace elements have bulk partition coefficient values that range from ∼.0.1 up to ∼.0.8 (Sr, Hf, Zr, Y and REE). Magnetites incorporate greater amounts of Nb and Ta than the titanaugites, and any significant fractionation of magnetite would have affected the bulk distribution of Nb and Ta. The LREE have lower Kd values than other REE, with the HREE having Kd values close to unity. Consequently, with the separation of titanaugite an overall enrichment of REE in the residual liquid with an increase in the La/Yb ratio is produced.

Trace element partitioning between phlogopite, clinopyroxene and leucite lamproite melt

Partition coefficients for Rb, Sr, Sm, Nd, U, Th, Pb, La, Ce, Ba, Nb, and Li between phlogopite, clinopyroxene and melt were determined in order to characterize the effect of metasomatically grown minerals on the isotopic evolution of the earth's lithospheric mantle. Experiments were performed in a piston–cylinder-apparatus at 15 kbar and 1040–1175°C with leucite lamproite as starting composition. Three phlogopite/glass and one clinopyroxene/glass pairs were analyzed by secondary ion mass spectrometry (SIMS). Partition coefficients ( D-values) for phlogopite/melt of Rb, Cs and Ba and Ti are ≥0.5. D Cs is significantly lower than D Rb (0.50/1.34); in contrast to these two elements, D Ba varies between 0.3 and 1.6 in phlogopites of different major element composition. A similar behavior is suggested for the elements Sr ( D=0.02–0.06) and Pb ( D≈0.02). The partition coefficients measured for REE, Th and U are in the order of 10 −4–10 −6, those for Zr and Nb are 0.015 and 0.07 respectively, and D Li for phlogopite and melt is 0.2. Partition coefficients for REE and Sr between clinopyroxene and melt are relatively high compared to other studies, presumably because of the high wollastonite component in the clinopyroxenes of this work. The data for clinopyroxene as well as the data for phlogopite were used to calculate crystal lattice-site parameters for the M2-site in clinopyroxene and the X-site in phlogopite. Parameters for clinopyroxene are in good agreement with data from other works. For the phlogopite interlayer site, an optimum ionic radius of 1.71 Å and polyhedral Young's moduli of 552 kbar (1 +-cations) and 1087 kbar (2 +-cations) were found. Recent data and the results of this study show that phlogopite is able to store significant amounts of Rb, Cs and Ba in the lithospheric mantle, thus causing a radiogenic evolution of Sr. In contrast, the influence of phlogopite on the Nd-isotope evolution is negligible because of the very low partition coefficients for Sm and Nd. The concentrations of Sr, Nd and Sm and the evolution of Nd isotopes in metasomatized lithospheric mantle are controlled by clinopyroxene, which significantly contributes to a less radiogenic evolution of Sr relative to the one caused by phlogopite. This implies that the evolution of Sr isotopes depends on the modal proportion of phlogopite and clinopyroxene.

Phase equilibrium constraints on angrite petrogenesis

Parameterizations of liquidus boundaries and solid solution in the CMAS + Fe system (Shi, 1992) have been employed to depict the liquidus equilibria relevant to the petrogenesis of angrites. Angrites are basaltic achondrites characterized by highly aluminous augite (fassaite), intermediate Mg-Fe olivine, and late-stage CaFe-olivine (kirschsteinite). Two important features of the equilibria on the olivine liquidus surface relevant to angrite petrogenesis are: 1) the presence of a thermal divide on the ol + aug + plag + liq boundary curve, which separates the compositions of source materials that produce low-silica angritic melts that crystallize highly aluminous augite from those that produce higher silica melts with tholeiitic to eucritic crystallization patterns; and 2) the change in the pseudo-invariant point on the low-silica side of the thermal divide from a plagioclase-peritectic involving spinel ( ol + aug + plag + sp + liq) at high to intermediate Mg′ (Mg/[Mg + Fe]) to two pseudo-eutectics involving kirschsteinite ( ol + aug + plag + kir + liq and ol + kir + plag + sp + liq) at low Mg′. The fassaitic (aluminous augite) pyroxene composition in Angra Dos Reis (ADOR), the presence of minor green spinel, and the absence of primary kirschsteinite (Prinz et al., 1977) indicate that crystallization of the ADOR parental liquid was governed by the intermediate-Mg′ set of equilibria such that, following crystallization of ol + aug + plag, the plagioclase reacted completely at the plagioclase-peritectic with the interstitial liquid, which subsequently crystallized beyond the plagioclase-peritectic onto the ol + aug + sp liquidus boundary curve. The ADOR bulk composition is consistent with trapping ∼10% of the parental liquid in a cumulate with cotectic proportions of fassaite and olivine. Lewis Cliff (LEW)86010 crystallized from a liquid with Mg′ similar to that of ADOR, but on the ol + plag cotectic closer to the thermal divide such that the first pyroxene to crystallize had much lower Al content than that of ADOR. In the late stages of crystallization the 86010 residual liquid (and that of LEW87051) encountered the low Mg′ set of equilibria involving kirschsteinite. These relationships require either a higher degree of melting for the 86010 parent magma or source region different than ADOR’s. These relationships are also consistent with compositionally dependent REE partition coefficients between fassaite and the ADOR liquid being as much as 1.5–2 times higher than those for the 86010 liquid at the onset of pyroxene crystallization. The combination of a trapped liquid component, higher partition coefficients, and smaller degrees of melting help to explain the observation that ADOR, an apparent cumulate, has REE concentrations twice as high as those in 86010 (Mittlefehdlt and Lindstrom, 1990), an apparent chilled liquid. The absence of a strong negative Eu-anomaly in the ADOR parent liquid, however, requires relatively high degrees of partial melting to eliminate plagioclase in the source region (resorption of plagioclase at the peritectic eliminates the Eu-anomaly that develops during crystallization), so ultimately different source regions are required. Progressive iron loss from devolatilized primitive chondrites (Allende, Murchison) produces source regions capable of producing a wide range of melt compositions with angritic to eucritic crystallization behavior. The compositions of carbonaceous and ordinary chondrite provide a similar range of potential source region compositions. However, primitive chondrite(±Fe) source regions that produce angrite-like melts have Mg′ that is too low, whereas chondrite(±Fe) sources that have Mg′ sufficiently high to yield the Mg′ in angrite minerals have too much silica (or orthopyroxene) component to yield angrite-like liquids. No single group of meteorites ± Fe simultaneously satisfies the constraints of Mg′ and silica component. However, mixtures of Fe-depleted chondrite plus a low-silica component similar to Ca-Al–rich inclusions (CAIs) can satisfy the constraints. The absence in angrites of 48Ca and 50Ti anomalies, typical of CAIs (Lugmair and Galer, 1992), suggests that the low-silica component was not simply an enrichment of CAIs, but was the result of direct accretion of high-temperature condensate (Grossman, 1972) into sizable, thermally shielded planetesimals. Thus angrites cryptically record mixing of planetesimal-sized heterogeneities in the early solar system.

Partitioning of REE, Y, Sr, Zr and Ti between clinopyroxene and silicate melts in the mantle under La Palma (Canary Islands): implications for the nature of the metasomatic agents

Ion microprobe analyses of clinopyroxenes in equilibrium with glasses (SiO 2 from 42 to 69 wt%) formed as the result of AFC-type reactions between infiltrating basaltic melt and peridotite wall-rock in the upper mantle under La Palma (Canary Islands) reveal trace element signatures usually ascribed to carbonatite metasomatism (i.e. strong REE-enrichment and Zr,Ti-depletion). Cpx/glass partition coefficients for REE, Y and Sr progressively increase with increasing SiO 2 in response to liquid composition/structure effects, approaching unity for La, Ce and Sr, and exceeding unity for the other REE in the most silicic glasses. Partition coefficients for Zr and Ti remain constant or decrease, probably as the combined effect of melt composition/structure and difficulty of charge-balancing 4 + cations in the crystal. Incorporation of the higher-charged HFSE in the cpx lattice requires complex coupled substitutions (e.g. M1Zr +1 TAl +2 M1R 2+ −1 TSi −2 or M2Na +1 M1Zr +1 TAl +1 M2Ca −1 M1R 2+ −1 TSi −1). Our results demonstrate that REE-enrichment and REE/HFSE fractionation of clinopyroxene do not necessarily reflect these characteristics in the metasomatic agent and that cpx/basalt D cannot confidently be used to infer the geochemical nature of equilibrium melts under upper mantle conditions. Moreover, we provide evidence that the strong REE-enrichment and Zr,Ti-depletion of some clinopyroxene from La Palma xenoliths is a feature commonly observed in clinopyroxenes from various mantle occurrences often interpreted as reflecting chemical interaction with metasomatic fluids and melts of different composition (either alkaline or carbonatitic) and provenance (from either the enriched asthenospheric mantle or the subducted slab). Therefore, claims of carbonatite metasomatism as opposed to silicate melt infiltration based on trace element signatures of investigated clinopyroxene should be regarded with caution unless the role of liquid composition/structure and of crystal chemical control is investigated.

Clinopyroxene/Orthopyroxene Trace Element Partition Coefficients in Spinel Peridotite Xenoliths

The distribution of incompatible trace elements between clinoand ortho-pyroxenes from spinel peridotite xenoliths has been investigated. The xenoliths came from three localities within the Pannonian Basin, Eastern Central Europe, namely Szigliget, Szentbekalla and Bondoro-hegy (EmbeyIsztin et al., 1989). All upper mantle xenoliths selected for this study are anhydrous and include the following textural types; protogranular, porphyroclastic, equigranular and poikilitic. Both metasomatised and unmetasomatised xenoliths were analysed. However, there were no significant interor intragranular trace element variations in the clinoand orthopyroxenes, and the homogeneity of the major elements in the mineral phases also suggest equilibrium. The four major mineral phases (clinoand orthopyroxene, olivine and spinel) were analysed by laser ablation microprobe inductively coupled plasma mass spectrometry (LAM-ICP-MS) at Memorial University of Newfoundland; for analysis details see Jenner et al. (1993). REE, Sr, Nb, Zr, Hf and Ti are above detection limits for the clinoand orthopyroxenes; however, these elements are generally too low in the olivines and spinels for quantitative analysis. The U, Th and Ta content of orthopyroxene was also below detection limits. The averages and standard deviations of the calculated orthopyroxene/clinopyroxene partition coefficients are given in Fig 1. This figure which is based on the averages of 37 xenoliths strongly confirms the observation of Rampone et al. (1991), McDonough et al. (1992) and Sun and Kerrich (1995) that the orthopyroxene/clinopyroxene partition coefficients of the HFSE elements (which substitute in the M1 sites in the pyroxenes) are significantly higher than those of the neighbouring REE (substituting in the M2 sites). The figure also illustrates orthopyroxene/clinopyroxene partition coefficients from some spinel peridotite xenoliths from British Columbia (Sun and Kerrich, 1995). These later data were calculated using solution ICPMS data on separated clinoand ortho-pyroxenes. The U-shaped pattern for the REE partition coefficients (i.e. the increase of the partition coefficients towards the LREE) is probably the result of impurities in the orthopyroxene separate. Highly incompatible trace elements reside not only in the pyroxene lattice but in fluid inclusions and along the grain boundaries. The REE partition coefficients based on in situ analysis show regular variation with ionic radii, e.g. Fig. 2, where there is illustrated an almost linear trend of the logarithm of the partition coefficients in the D (opx/cpx) versus ionic radii. There is increased scatter of the D values as lower trace element abundances in orthopyroxene are approached. However, the higher scattering in the case of some LREE and Sr (all have relatively large ionic radii) is not simply a result of analytical uncertainty; the orthopyroxene/clinopyroxene partition coefficents of these elements tend to increase with increasing equilibrium temperatures (Fig. 3.). This variation is intrepreted to be the result of a temperature dependence of Ca partitioning between

Partitioning of trace elements between clinopyroxene and garnet: data from mantle eclogites

Concentrations of REE, Ba, Sr, Y and Zr have been measured by ion microprobe (SIMS) for garnet and clinopyroxene in a suite of eclogite xenoliths from the Roberts Victor kimberlite pipe in South Africa. The xenoliths have a restricted set of temperatures of formation estimated at 1100 ± 100°C at 5 GPa, but span a wide range of compositions; garnet Ca/(R 2+) ratios range from 0.08 to 0.50, whilst the associated clinopyroxenes also show increasing jadeite solid solution as their Ca/(R 2+) ratios increase. The cpx/grt partition coefficients ( D i ) for REE, Sr and Y decrease progressively as Ca/(R 2+ in the minerals increase; in the LREE this change amounts to approximately three orders of magnitude. Evidence for such large changes in partition coefficients as a function of Ca content is also seen in other data for mantle eclogites from South Africa and Siberia. Cpx and grit with low Ca/(R 2+ ratios have ( D i ) for HREE which are similar to those implied by mineral/liquid data for basic-ultrabasic melts. Poor correlations of Ba, La and Lu partition coefficients with cpx-grt Ca contents in the present dataset, are the result of late-stage partial alteration and relatively large errors of measurement associated with low concentrations. Zr partition coefficients show only a weak correlation with mineral Ca/(R 2+). Strong correlations are demonstrated between cpx/grt D i (calculated on a ppm or molar basis) and molar cpx/grt D Ca for most REE, Sr and Y. These may be expressed in the form: In D i ∗ cpx grt =A ln D Ca ∗ cpx grt + B B (Eq. (1)), where A and B are empirical constants for each element and vary with ionic radius in the REE. The partitioning of a given trace element progressively favours garnet rather than pyroxene as increasing amounts of Ca are more strongly partitioned into garnet than clinopyroxene; and the magnitude of this effect increases with ionic radius in the REE. The empirical linear calibrations (Eq. (1)) given for REE, Y and Sr allow calculation of the ( D i ) of a cpx-grt pair from their Ca contents (for 1100 ± 100°C, ∼ 5 GPa). Using the well-documented R 3+ Al [4] Ca −1Si −1 (YAG-type) substitution relationship for REE and Y in garnets as the basis of an exchange reaction, it is suggested that the A term in empirical Eq. (1) may be related to variations in activity coefficients as the major element (Ca) composition changes, whilst term B is largely dependent on the standard state free energy.