Partition Coefficients For Rare Earth Elements Research Articles

Mineral Geochemistry of Apatite in the Jiama Porphyry‐Skarn Deposit, Tibet and its Geological Significance

AbstractThe Jiama deposit, a significant porphyry‐skarn‐type copper polymetallic deposit located within the Gangdese metallogenic belt in Tibet, China, exemplifies a typical porphyry metallogenic system. However, the mineral chemistry of its accessory minerals remains under‐examined, posing challenges for resource assessment and ore prospecting. Utilizing electron microprobe analysis and LA‐ICP‐MS analysis, this study investigated the geochemical characteristics of apatite in ore‐bearing granite and monzogranite porphyries, as well as granodiorite, quartz diorite, and dark diorite porphyries in the deposit. It also delved into the diagenetic and metallogenic information from these geochemical signatures. Key findings include: (1) The SiO2 content, rare earth element (REE) contents, and REE partition coefficients of apatite indicate that the dark diorite porphyry possibly does not share a cogenetic magma source with the other four types of porphyries; (2) the volatile F and Cl contents in apatite, along with their ratio, indicate the Jiama deposit, formed in a collisional setting, demonstrates lower Cl/F ratios in apatite than the same type of deposits formed in a subduction environment; (3) compared to non‐ore‐bearing rock bodies in other deposits formed in a collisional setting, apatite in the Jiama deposit exhibits lower Ce and Ga contents. This might indicate that rock bodies in the Jiama deposit have higher oxygen fugacity. Nevertheless, the marginal variation in oxygen fugacity between ore‐bearing and non‐ore‐bearing rock bodies within the deposit suggests oxygen fugacity may not serve as the decisive factor in the ore‐hosting potential of rock bodies in the Jiama deposit.

Distribution of REE between amphibole and pyroxenes in the lithospheric mantle: An assessment from the lattice strain model

Abstract Amphibole and pyroxenes are the main reservoirs of rare earth elements (REEs) in lithospheric mantle that has been affected by hydrous metasomatism. In this study, we developed semi-empirical models for REE partitioning between orthopyroxene and amphibole and between clinopyroxene and amphibole. These models were formulated on the basis of parameterized lattice strain models of mineral–melt REE partitioning for orthopyroxene, clinopyroxene, and amphibole, and they were calibrated using major element and REE data of amphibole and pyroxenes in natural mantle samples from intraplate setting. The mineral–melt REE partitioning models suggest that amphibole is not in equilibrium with coexisting pyroxenes in the mantle samples, and that the amphibole crystallized at a lower temperature than that of the pyroxenes. We estimated the apparent amphibole crystallization temperature using major element compositions of the amphibole and established temperature- and composition-dependent models that can be used to predict apparent pyroxene–amphibole REE partition coefficients for amphibole-bearing peridotite and pyroxenite from intraplate lithospheric mantle. Apparent pyroxene–amphibole REE partition coefficients predicted by the models can be used to infer REE contents of amphibole from REE contents of coexisting pyroxenes. This is especially useful when the grain size of amphibole is too small for trace element analysis.

A REE-based zircon geothermometer based on improved lattice strain modeling of zircon-melt REE partition coefficients

Several methods for tracing geological processes based on the rare earth element (REE) concentrations in zircon have been proposed in recent years. Application of these methods requires quantitative knowledge of the partition coefficients (D) for the REE between zircon and co-existing phases, which may be melt, fluid, or other minerals. Precisely determining the DREE of zircon, however, is challenging. This is especially true for the light rare earth elements, which generally occur in very low concentrations in zircon. This difficulty can be overcome by modeling DREE using a prediction function based on the crystal lattice strain model. The prediction function is defined by three empirically derived lattice strain parameters: the apparent Young's Modulus E of the Zr site in which the REE resides; the ionic radius r0 of a (fictive) REE optimally fitting into this site; and the maximum partition coefficient D0 of this optimally fitting REE. In principle, these three parameters can be determined by fitting the prediction function to sets of measured DREE. The ionic radii of the REE, however, are all larger than r0 for zircon, commonly resulting in strain model fits yielding physically unrealistic values of r0, in particular. Here, we use a compilation of zircon-melt REE partition coefficients from the literature to establish a new empirical relationship between r0 and E, which is used as an additional constraint during lattice strain model curve fitting. Lattice strain parameters for literature DREE data sets of natural and synthetic zircon samples are calculated using our constraint resulting in a new parametrization of the inverse temperature dependence of D0. This parametrization enables quantitative estimates of REE-based crystallization temperatures of magmatic zircons, complementing estimates based on Ti-in-zircon geothermometry.

Deciphering early Neoproterozoic and Cambrian high-grade metamorphic events in the Archean/Mesoproterozoic Rauer Group, East Antarctica

A U–Pb geochronological and rare earth element (REE) geochemical study of zircon, monazite and garnet was carried out on rocks of Mesoproterozoic and Archean crustal domains in the Rauer Group of East Antarctica. The zircon and monazite U–Pb age spectra define concordia intercepts mainly at ca 1200, 990–910, and 530–500 Ma, suggesting that the Mesoproterozoic crustal domain is a significant part of the Rayner Complex that also underwent early Neoproterozoic and Cambrian high-grade metamorphism. The age data, mineral inclusion assemblages in zircon, and REE features for zircon and garnet indicate that all the granulite facies mineral assemblages in this domain might have formed during early Neoproterozoic metamorphism. Some zircon and monazite grains or domains have experienced complete U–Pb isotopic resetting during Cambrian reworking, which did not result in new zircon and monazite growth. The Archean crustal domain consists mainly of Paleo–Mesoarchean orthogneisses interleaving with Neoproterozoic paragneisses that contain inherited metamorphic zircon domains with ages of ca 1330 and 970 Ma. The mineral assemblages in these gneisses formed during a single Cambrian granulite facies metamorphic event. Garnet-bearing and -free rocks cooled to solidus temperatures at ca 527 and 517 Ma, respectively, whereas the isotopic system of early-crystallized zircon was completely reset during the growth of new zircon. As such, all the zircon domains in the same sample could have the same concordant or weighted mean age. The 511 Ma monazite and 506 Ma zircon overgrowths in a paragneiss have REE contents in equilibrium with garnet, implying that later modification and isotopic resetting of zircon and monazite might have resulted in younger U–Th–Pb ages and, in this case, establishing the age–mineral assemblage relationship based on REE partition coefficients between zircon/monazite and garnet may be invalid. Overall, the available data support the notion that different crustal components of the Rauer Group were juxtaposed in the Cambrian as a consequence of the Gondwana assembly.

Partition coefficients of trace elements between carbonates and melt and suprasolidus phase relation of Ca-Mg-carbonates at 6 GPa

Abstract The presence of Ca-Mg-carbonates affects the melting and phase relations of peridotites and eclogites in the mantle, and (partial) melting of carbonates liberates carbon from the mantle to shallower depths. The onset and composition of incipient melting of carbonated peridotites and carbonated eclogites are influenced by the pure CaCO3-MgCO3-system making the understanding of the phase relations of Ca-Mg-carbonates fundamental in assessing carbon fluxes in the mantle. By performing high-pressure and high-temperature experiments, this study clarifies the suprasolidus phase relations of the nominally anhydrous CaCO3-MgCO3-system at 6 GPa showing that Ca-Mg-carbonates will (partially) melt for temperatures above ~1300 °C. A comparison with data from thermodynamic modeling confirms the experimental results. Furthermore, partition coefficients for Li, Na, K, Sr, Ba, Nb, Y, and rare earth elements between calcite and dolomitic melt, Ca-magnesite and dolomitic melt, and magnesite and dolomitic melt are established. Experiments were performed at 6 GPa and between 1350 to 1600 °C utilizing a rotating multi-anvil press. Rotation of the multi-anvil press is indispensable to establish equilibrium between solids and carbonate liquid. Major and trace elements were quantified with EPMA and LA-ICP-MS, respectively. The melting temperature and phase relations of Ca-Mg-carbonates depend on the Mg/Ca-ratio. For instance, Ca-rich carbonates with a molar Mg/(Mg+Ca)-ratio (XMg) of 0.2 will transform into a dolomitic melt (XMg = 0.33–0.31) and calcite crystals (XMg = 0.19–0.14) at 1350–1440 °C. Partial melting of Mg-rich carbonates (XMg = 0.85) will produce a dolomitic melt (XMg = 0.5–0.8) and Ca-bearing magnesite (XMg = 0.89–0.96) at 1400–1600 °C. Trace element distribution into calcite and magnesite seems to follow lattice constraints for divalent cations. For instance, the compatibility of calcite (XMg = 0.14–0.19) for Sr and Ba decreases as the cation radii increases. Ca-Mg-carbonates are incompatible for rare earth elements (REEs), whereby the distribution between carbonates and dolomitic melt depends on the Mg/Ca ratio and temperature. For instance, at 1600 °C, partition coefficients between magnesite (XMg = 0.96) and dolomitic melt (XMg = 0.8) vary by two orders of magnitudes from 0.001 to 0.1 for light-REEs to heavy-REEs. In contrast, partition coefficients of REEs (and Sr, Ba, Nb, and Y) between magnesite (XMg = 0.89) and dolomitic melt (XMg = 0.5) are more uniform scattering marginal between ~0.1–0.2 at 1400 °C.

Interphase REE Partitioning at the Boundary between the Earth’s Transition Zone and Lower Mantle: Evidence from Experiments and Atomistic Modeling

Trace elements play a significant role in interpretation of different processes in the deep Earth. However, the systematics of interphase rare-earth element (REE) partitioning under the conditions of the uppermost lower mantle are poorly understood. We performed high-pressure experiments to study the phase relations in key solid-phase reactions CaMgSi2O6 = CaSiO3-perovskite + MgSiO3-bridgmanite and (Mg,Fe)2SiO4-ringwoodite = (Mg,Fe)SiO3-bridgmanite + (Mg,Fe)O with addition of 1 wt % of REE oxides. Atomistic modeling was used to obtain more accurate quantitative estimates of the interphase REE partitioning and displayed the ideal model for the high-pressure minerals. HREE (Er, Tm, Yb, and Lu) are mostly accumulated in bridgmanite, while LREE are predominantly redistributed into CaSiO3. On the basis of the results of experiments and atomistic modeling, REE in bridgmanite are clearly divided into two groups (from La to Gd and from Gd to Lu). Interphase REE partition coefficients in solid-state reactions were calculated at 21.5 and 24 GPa for the first time. The new data are applicable for interpretation of the trace-element composition of the lower mantle inclusions in natural diamonds from kimberlite; the experimentally determined effect of pressure on the interphase (bridgmanite/CaSiO3-perovskite) REE partition coefficients can be a potential qualitative geobarometer for mineral inclusions in super-deep diamonds.

Mineral–melt partition coefficients and the problem of multiple substitution mechanisms: insights from the rare earths in forsterite and protoenstatite

A trace element may substitute into a mineral by more than one substitution mechanism, complicating the thermodynamic description of its partition coefficients. In order to understand this phenomenon better, the mineral/melt partition coefficients for all 14 rare earth elements (REE) plus Y and Sc were measured experimentally for coexisting forsterite and protoenstatite in the system CaO–MgO–SiO2 ± Al2O3 ± TiO2 at 1406 °C and atmospheric pressure. For both phases, the results show these large trivalent cations (REE3+) replace Mg2+ on octahedral sites, but with charge-balance achieved by two different mechanisms: (1) cation vacancies (2 REE3+ + vacancy = 3 Mg2+); and (2) substitution of Al for Si (REE3+ + Al3+ = Mg2+ + Si4+). The overall REE partition coefficient is the sum of the partition coefficients for each substitution mechanism. Because the stoichiometric control is different for each mechanism, the relative importance of the mechanism varies with melt composition, including the activities of both silica and alumina in the melt ($${{a}}_{{{\text{SiO}}_{ 2} }}^{\text{melt}}$$ and $${{a}}_{{{\text{AlO}}_{ 1. 5} }}^{\text{melt}}$$). The coexistence of forsterite and protoenstatite fixes the silica activity, allowing the effect of $${{a}}_{{{\text{AlO}}_{ 1. 5} }}^{\text{melt}}$$ to be separated from that of $${{a}}_{{{\text{SiO}}_{ 2} }}^{\text{melt}}$$. The relative importance of the two mechanisms depends strongly on the identity of the REE for forsterite, but not for protoenstatite. The results are used to test the lattice strain model: the two substitution mechanisms in forsterite imply different values for the Young’s modulus in the Brice equation, despite the fact that the REE3+ cations likely occupy the same crystallographic site in both mechanisms, casting doubt on the physical basis of the lattice strain theory. Comparison with literature data confirms earlier observations that the activity coefficients of REE2O3 in silicate melts decrease with increasing SiO2 content of the melt, but the effect decreases with increasing atomic number, from La to Lu, and is almost negligible for Sc. The influence of melt composition should apply to the mineral/melt REE partition coefficients of all other minerals. Recognizing that observed mineral/melt partition coefficients are often the sums of contributions from multiple substitution mechanisms, each with its own dependence on both crystal composition and the stoichiometric control from the melt composition, will improve parameterizations of the mineral/melt partition coefficients of other rock-forming minerals. Partition coefficients for Na, Al, Ca, Ti, and Zr are also reported.

Using Rare Earth Elements (REE) to Decipher the Origin of Ore Fluids Associated with Granite Intrusions

A practical method is presented to estimate rare earth element (REE) concentrations in a magmatic fluid phase in equilibrium with water-saturated granitic melts based on empirical fluid–melt partition coefficients of REE (kPREE). The values of kPREE can be calculated from a set of new polynomial equations linking to the Cl molality (mClv) of the magmatic fluid phase associated with granitic melts, which are established via a statistical analysis of the existing experimental dataset. These equations may be applied to the entire pressure range (0.1 to 10.0 kb) within the continental crust. Also, the results indicate that light REEs (LREE) behave differently in magmatic fluids, i.e., either being fluid compatible with higher mClv or fluid incompatible with lower mClv values. In contrast, heavy REEs (HREE) are exclusively fluid incompatible, and partition favorably into granitic melts. Consequently, magmatic fluids tend to be rich in LREE relative to HREE, leading to REE fractionation during the evolution of magmatic hydrothermal systems. The maximum kPREE value for each element is predicted and presented in a REE distribution diagram constrained by the threshold value of mClv. The REE contents of the granitic melt are approximated by whole-rock analysis, so that REE concentrations in the associated magmatic fluid phase would be estimated from the value of kPREE given chemical equilibrium. Two examples are provided, which show the use of this method as a REE tracer to fingerprint the source of ore fluids responsible for the Lake George intrusion-related Au–Sb deposit in New Brunswick (Canada), and the Bakircay Cu–Au (–Mo) porphyry systems in northern Turkey.

Interphase distribution of rare and rare earth elements in silicate-fluorine systems at T = 800–1200˚C AND P = 1–2 kbar (experimental investigation)

The melting in the system of aluminosilicate (granite) melt — fluoride salt melt at T = 800–1200˚C and P = 1–2 kbar was experimentally investigated. The data on the phase composition of silicate-salt systems and the character of the interphase distribution of ore components are presented. The partition coefficients of rare-earth elements between the granite melt and fluoride phase at 900–1200˚C, P = 1–2 kbar were obtained. It is shown that REE enrich the fluoride phase with respect to the silicate melt. Elements of the (V, Nb, Ta), (Ti, Zr, Hf) groups almost always prefer a silicate melt relative to fluoride.

Tracing the evolution of a fertile REE granite by modelling amphibole-melt partitioning, the Strange Lake story

In principle, potentially economic (fertile) granites can be distinguished from uneconomic (barren) granites based on the composition of their magmas. Ideally, this composition would be evaluated by analysing melt inclusions, but such inclusions are rarely preserved and difficult to analyse. Information on the composition of magmas can also be obtained from the composition of the minerals crystallising from these magmas, if the mineral-melt partition coefficients are known. In some cases, where these partition coefficients are not known, they have been calculated from lattice strain theory. Here we report arfvedsonite-melt partition coefficients for the rare earth elements (REE) calculated from the results of analyses of arfvedsonite, bulk rock and melt inclusions in granites and pegmatites in the Strange Lake peralkaline rare metal pluton. These coefficients were fitted to the lattice strain model from which values of the ideal partition coefficient (D0), the ideal radius (r0) and the elastic response (EM) were determined. The light REE were shown to partition preferentially into the M4 site and the heavy REE into the M2 site, requiring that partition coefficients be calculated for each site. The partition coefficients for both sites were shown to decrease systematically with increasing evolution of the magma, i.e., from the early hypersolvus granite to the late pegmatites. In the case of the M4 site, D0, r0 and EM vary linearly with the Ca content of the arfvedsonite. By contrast, these parameters for the M2 site vary linearly with temperature. The two relationships form the basis of a predictive model, in which the arfvedsonite-melt REE partition coefficients for other peralkaline granites can be estimated, provided that the Ca content of the amphibole and its crystallisation temperature are known. This model was applied to a peralkaline granitic pegmatite from the Amis complex (Namibia) for which data on the composition of the amphibole and corresponding magma (melt inclusions) have been reported. The model successfully predicts the concentrations of the various REE in this magma, thereby providing confidence that it can be used to discriminate between granites that are potentially fertile and those that are barren in respect to the REE.

Parameterized lattice strain models for REE partitioning between amphibole and silicate melt

The distribution of rare earth elements (REEs) between amphibole and silicate melt is important for understanding a wide variety of igneous and metamorphic processes in the lithosphere. In this study, we used published experimental REE and Y partitioning data between amphibole and silicate melt, the lattice strain model, and nonlinear least-squares regression method to parameterize key partitioning parameters in the lattice strain model ( D 0 , r 0 , and E ) as a function of pressure, temperature, and both amphibole and melt compositions. Two models, which give nearly identical results, are obtained in this study. In the first model, D 0 depends on temperature and amphibole composition: it positively correlates with Ti content and negative correlates with temperature and Mg, Na, and K contents in the amphibole. In the second model, D 0 depends solely on the melt composition: it positively correlates with Si content and negatively correlates with Ti and Ca contents in the melt. In both the mineral and melt composition models, r 0 negatively correlates with the ferromagnesian content in the M4 site of the amphibole, and E is a constant. The very similar coefficients in the equations for r 0 and best-fit values for E in the two models allow us to connect the two models through amphibole-melt phase equilibria. An application of our model to amphiboles in mantle xenoliths shows that observed major element compositional variations in amphibole alone can give rise to order of magnitude variations in amphibole-melt REE partition coefficients. Together with experimental data simulating fractional crystallization of arc magmas, out models suggest that: (1) REE partition coefficients between amphibole and melt can vary by an order of magnitude during arc magma crystallization due to variation in the temperature and composition of the amphibole and melt, and that (2) amphibole fractional crystallization plays a key role in depleting the middle REEs relative to heavy REEs and light REEs in arc magmas.

The experimental determination of REE partition coefficients in the water-calcite system

To better quantify the behaviour of rare earth elements (REE) in low temperature natural systems, calcite was precipitated from NaCl-bearing aqueous solutions in the presence of La and Yb using a constant addition technique at pH ~6 and 25°C under steady-state conditions. The presence of REE in the fluid significantly slows calcite growth rates. Partition coefficients (DREE), determined from analysis of the aqueous solution compositions, range from 102.8 at the lowest aqueous La concentration to 101.4 at the highest aqueous concentration of Yb. DREE was found to correlate with aqueous REE concentrations, as well as with the calcite saturation state. Experimental results, together with previous spectroscopic evidence, suggest the incorporation of REE3+ into the Ca2+ site of calcite while maintaining charge balance by the concurrent incorporation of Na+. The value of the equilibrium constants for the corresponding exchange reaction was determined for La, Eu and Yb by combining our results with previously published experimental data. Subsequently, these constants were used together with a lattice strain model to estimate corresponding values for the other REE. Application of this exchange model suggests that DREE vary significantly with the composition of natural waters including seawater. Such variations are likely the origin for the large differences of DREE values reported in the literature based on experimental and field evidence.

Low-δ13C carbonates in the Miocene basalt of the northern margin of the North China Craton: Implications for deep carbon recycling

Three types of carbonates have been found in the Miocene basalt in the Dongbahao area (Inner Mongolia), including wide veins and veinlets of carbonate in basalt and carbonates in peridotite xenoliths. Except for the dolomitic zonation in the basalt, all of the carbonates are calcite. Despite their different appearances, they share almost identical geochemical characteristics of low LILE (low large ion lithophile element), HFSE (high field strength element), and REE (rare earth elements) contents (ΣREE=0.51–137ppm); negative Ce anomalies; and low Ce/Pb ratios (0.51–74.5). Moreover, they show high δ18OSMOW values (20.95–22.61‰) and 87Sr/86Sr ratios (0.7087±0.0003 (1σ, n=17)). These characteristics indicate a sedimentary precursor for these carbonates. However, the occurrence and petrographic characteristics imply an igneous origin for the carbonates rather than a hypergene process. Further, the trace element compositions of the silicate melt and carbonate melt in the calcite-dolomite-silicate zonations fall on the same variation lines in the plots of Y-Ho, La-Yb, Li-Pb and Ba-Cu. It is suggested that these melts could have evolved from one magma system or could have been equilibrated. Given the partition coefficients of REEs and alkali elements (Cs, Rb, and K) between the carbonate melt and silicate melt, it can be inferred that these melts could have been formed from a primary H2O-Si-bearing Mg-Ca-carbonate melt by an immiscibility process at 1–3GPa. Considering the southward subduction of the Paleo-Asian ocean along the northern margin of the North China Craton (NCC), these carbonate melts could have been derived from the melting of subducted sedimentary carbonate rocks. Interestingly, these carbonates have quite depleted carbon isotopic compositions (δ13CPDB=−8.23‰ to −11.76‰) but moderate δ18OSMOW values, implying coupled H2O-CO2 degassing during subduction and/or recycling to the Earth’s surface. Low-δ13CPBD carbonates appearing at the global scale may suggest an underestimated path of CO2 emission back to the atmosphere.

Lutetium incorporation in magmas at depth: Changes in melt local environment and the influence on partitioning behaviour

The structure of two Lu doped (4000 ppm) model end member silicate liquids, a highly polymerised haplogranite (Si–Al–Na–K–O) and a less polymerised anorthite–diopside (Si–Al–Mg–Ca–O), have been studied up to 8 GPa using in situ x-ray diffraction techniques. The results are the first to identify trace rare Earth element incorporation in silicate melts at high pressure. At pressures below 5 GPa, the bonding environment of Lu–O was found to be dependent on composition with coordination number CNLu–O=8 and bond distance rLu–O=2.36 Å in the haplogranite melt, decreasing to CNLu–O=6 and rLu–O=2.29 Å in the anorthite–diopside melt. This compositional variance in coordination number at low pressure is consistent with observations made for Y–O in glasses at ambient conditions and is coincident with a dramatic increase in the partition coefficients previously observed for rare Earth elements with increasing melt polymerisation. With increasing pressure we find that CNLu–O and rLu–O remain constant in the haplogranite melt. However, an abrupt change in both Lu–O coordination and bond distance is observed at 5 GPa in the anorthite–diopside melt, with CNLu–O increasing from 6 to 8-fold and rLu–O from 2.29 to 2.39 Å. This occurs over a similar pressure range where a change in the P-dependence in the reported rare Earth element partition coefficients is observed for garnet–, clinopyroxene–, and olivine–melt systems. This work shows that standard models for predicting trace elements at depth must incorporate the effect of pressure-induced structural transformations in the melt in order to realistically predict partitioning behaviour.

An assessment of subsolidus re-equilibration on REE distribution among mantle minerals olivine, orthopyroxene, clinopyroxene, and garnet in peridotites

The distribution of rare earth elements (REEs) in mantle minerals depends on temperature, pressure, and mineral composition. We developed parameterized lattice strain models for rare earth element partitioning among mantle minerals (olivine, orthopyroxene, clinopyroxene, and garnet) using mineral–melt partitioning models that were calibrated against experimentally determined high quality mineral–melt REE partition coefficients. The validity of the mineral–mineral REE partitioning models were independently tested using measured REE abundances in minerals from well-equilibrated mantle peridotites and eclogites reported in the literature. Using the new partitioning models, we critically assessed the extent of subsolidus re-equilibration on REE distribution among major rock-forming minerals in spinel and garnet peridotites. During subsolidus re-equilibration, REEs are preferentially partitioned into clinopyroxene in peridotites. The extent of REE redistribution in clinopyroxene depends on the modal abundance of clinopyroxene in the peridotite. Subsolidus exchange with orthopyroxene and olivine can increase heavy REE abundances in clinopyroxene from spinel harzburgite and middle REE abundances in clinopyroxene from garnet harzburgite by a factor of three. The higher than expected heavy REE abundances in clinopyroxene from spinel harzburgite may give rise to a false garnet signature in residual peridotites. The extents of melting for abyssal peridotites based on REE abundances in clinopyroxene can be significantly over-estimated without proper correction for subsolidus re-equilibration. REE abundances in both clinopyroxene and orthopyroxene are needed to assess the melting processes from residual peridotites.

An experimental study of trace element partitioning between augite and Fe-rich basalts

Aluminous titanium-rich hedenbergite and Fe-rich augite were experimentally produced at 1050–1220°C and 0.8–2.2GPa. Major element compositions are analogous to clinopyroxene from layered intrusions, angrites, nakhlites, and late-stage lunar magma ocean cumulates. Trace element concentrations in pyroxene and coexisting melt were measured by laser ablation inductively coupled plasma mass spectrometry for rare earth elements (REE), high field strength elements (HFSE), transition metals, and large-ion lithophile elements. REE, HFSE, and transition metal partition coefficients (Ds) are tightly correlated with the cation abundance of Fe on the pyroxene M1 site (XFeM1), and also correlated with Al on the tetrahedral site (XAlT). Parameterized lattice-strain models were developed to predict REE and HFSE partition coefficients as functions of temperature and pyroxene composition (XAlT or XFeM1). The parameterized models can be used to calculate REE and HFSE partition coefficients for Fe-rich high-Ca pyroxene. We calculated partition coefficients for clinopyroxene derived from modeled end-stage lunar magma ocean cumulate compositions and observe a factor of 3 variation in clinopyroxene-melt REE partition coefficients. Using representative nakhlite clinopyroxene core and rim compositions we calculated partition coefficients and observed a factor of 4 variability in clinopyroxene-melt REE partition coefficients. HFSE partition coefficients are even more sensitive to composition, showing variations of a factor of 4 for lunar late cumulates and an order of magnitude for nakhlites. The strong dependence of REE and HFSE partitioning on composition necessitates careful selection of appropriate partition coefficients for Fe-rich systems.

The importance of crystal chemistry on REE partitioning between mantle minerals (garnet, clinopyroxene, orthopyroxene, and olivine) and basaltic melts

Partitioning of rare earth elements (REEs) between mantle minerals and basaltic melts is fundamental to understanding crystal-melt fractionation processes and can be quantitatively described by the lattice strain model. We analyzed published REE and Y partitioning data between garnet and basaltic melt and REE, Y, and Sc partitioning data between olivine and basaltic melt using the nonlinear regression method, and parameterized key partitioning parameters in the lattice strain model (D0, r0 and E) as functions of temperature, pressure, and mineral and melt compositions. We show that REE and Y partition coefficients between garnet and basaltic melt are inversely correlated with temperature and pressure, and that the correlation between REE partition coefficients and Ca content in garnet (XCa) is convoluted by the inverse relationship between D0 and XCa and the positive correlation between r0 and XCa. REE, Y, and Sc partition coefficients between olivine and basaltic melt are positively correlated with Al content in olivine and inversely correlated with forsterite content in olivine and pressure. To test the validity of the assumptions and simplifications used in the model development, we combined the garnet and olivine models with a recent model for REE partitioning between clinopyroxene and basaltic melt to obtain two mineral–mineral partitioning models. We found that the model-derived garnet-clinopyroxene and olivine-clinopyroxene REE partition coefficients are consistent with the measured data from well-equilibrated eclogite and peridotite xenoliths at subsolidus conditions. This demonstrates the internal consistency of our parameterized lattice strain models for REE partitioning in garnet, olivine, clinopyroxene and orthopyroxene. Taken collectively, the partitioning models for garnet, olivine, clinopyroxene and orthopyroxene all suggest the importance of crystal chemistry, temperature and pressure in determining REE partitioning between mantle minerals and basaltic melts, and that melt composition has only a secondary or indirect effect.

Distribution of REE and HFSE between low-Ca pyroxene and lunar picritic melts around multiple saturation points

The abundances and distribution of incompatible trace elements in low-Ca pyroxene are important to the interpretation of the thermal and chemical evolution of the lunar magma ocean and cumulate mantle. We conducted high pressure (2GPa) and high temperature (1340–1450°C) experiments to constrain trace elements partitioning between low-Ca pyroxene and a wide range of lunar picritic melt compositions (TiO2=1.6–16.6wt%) under reducing conditions imposed by graphite-lined molybdenum capsules. We show that addition of TiO2 in the melt decreases rare earth element (REE) and Y partition coefficients in low-Ca pyroxene and quantified this effect through a parameterized lattice strain model. Together with published high quality partitioning data of tetravalent high field strength elements (HFSEs) in the literature, we also developed a parameterized lattice strain model for Ti, Hf and Zr partitioning in low-Ca pyroxene. Partition coefficients of REE and HFSE are positively correlated with Al and Ca abundances in low-Ca pyroxene and negatively correlated with temperature. HFSE partition coefficients show an inverse correlation with Fe content in low-Ca pyroxene. As examples of lunar applications, we estimated REE and HFSE partition coefficients in low-Ca pyroxene close to the multiple saturation points of lunar picritic melts, and found small variations of these partition coefficients in low-Ca pyroxene from individual lunar picritic melts. Constant REE and HFSE partition coefficients can be used in melting models exploring the petrogenesis of individual picritic melts. On the other hand, large variations in estimated REE and HFSE partition coefficients for low-Ca pyroxene are expected during lunar magma ocean crystallization, underscoring the significance of temperature-composition dependent REE and HFSE partition coefficients for understanding lunar magma ocean evolution.

The importance of melt TiO2 in affecting major and trace element partitioning between Fe–Ti oxides and lunar picritic glass melts

Lunar mare basalts and picritic glasses have TiO2 abundances ranging from less than 1wt.% to over 16%. Any high-Ti mare basalt or picritic glass petrogenetic model must include Fe–Ti oxides in the mantle source or invoke Fe–Ti oxide assimilation during magma ascent to the lunar surface. We conducted partitioning experiments to investigate high field strength element (HFSE), rare earth element (REE), and transition metal distribution between Fe–Ti oxides and lunar picritic glass melts over a range of melt compositions. Our results suggest that ilmenite–melt and armalcolite–melt HFSE, Cr, and V partition coefficients (DHFSE, DCr, DV) are strongly dependent on melt TiO2 content, whereas ilmenite–melt REE partition coefficients appear to be insensitive to melt composition. As TiO2 increases in picritic glass melts, HFSE, Cr, and V activities in melt also increase and Fe–Ti oxide–melt DHFSE, DCr and DV decrease. The effect of Ti on partitioning behavior can be attributed to the formation of Fe–O–Ti melt species in high-Ti melts. Ilmenite DHFSE range from compatible in oxides in equilibrium with low-Ti melts to incompatible in oxides in equilibrium with depolymerized high-Ti picritic glass melts. DHFSE are inversely correlated with TiO2 abundance in the melt and become nearly constant for melts with more than 6.8% TiO2. We present simple partitioning models that utilize the solubility of ilmenite and armalcolite in melt to effectively predict HFSE partition coefficients across a wide range of picritic glass melt compositions. The HFSE budget of ilmenite cumulates that crystallize from the lunar magma ocean strongly depends on the composition of the magma ocean. Low-Ti and high-Ti lunar basalts can be produced by an ilmenite or armalcolite bearing hybridized mantle source, or by assimilation of late-stage magma ocean cumulates. The dependence of DHFSE and DCr on melt TiO2 is consistent with the formation of lunar Type 1 armalcolite from high TiO2 picritic glass melt, however, lunar Type 2 and 3 armalcolite have a more complicated (yet undetermined) history of formation.

Distribution of REE between clinopyroxene and basaltic melt along a mantle adiabat: effects of major element composition, water, and temperature

The distribution of rare earth elements (REE) between clinopyroxene (cpx) and basaltic melt is important in deciphering the processes of mantle melting. REE and Y partition coefficients from a given cpx-melt partitioning experiment can be quantitatively described by the lattice strain model. We analyzed published REE and Y partitioning data between cpx and basaltic melts using the nonlinear regression method and parameterized key partitioning parameters in the lattice strain model (D 0, r 0 and E) as functions of pressure, temperature, and compositions of cpx and melt. D 0 is found to positively correlate with Al in tetrahedral site (Al T ) and Mg in the M2 site (MgM2) of cpx and negatively correlate with temperature and water content in the melt. r 0 is negatively correlated with Al in M1 site (AlM1) and MgM2 in cpx. And E is positively correlated with r 0. During adiabatic melting of spinel lherzolite, temperature, Al T , and MgM2 in cpx all decrease systematically as a function of pressure or degree of melting. The competing effects between temperature and cpx composition result in very small variations in REE partition coefficients along a mantle adiabat. A higher potential temperature (1,400°C) gives rise to REE partition coefficients slightly lower than those at a lower potential temperature (1,300°C) because the temperature effect overwhelms the compositional effect. A set of constant REE partition coefficients therefore may be used to accurately model REE fractionation during partial melting of spinel lherzolite along a mantle adiabat. As cpx has low Al and Mg abundances at high temperature during melting in the garnet stability field, REE are more incompatible in cpx. Heavy REE depletion in the melt may imply deep melting of a hydrous garnet lherzolite. Water-dependent cpx partition coefficients need to be considered for modeling low-degree hydrous melting.