Partial Melting Of Eclogite Research Articles

The effect of COH fluids on partial melting of eclogite and lherzolite under moderately oxidizing and reducing conditions

Fluids rich in water and carbon (COH) are among the most reactive agents of mantle metasomatism and play a major role in the continuous degassing of the Earth. The addition of small amounts of H2O and CO2 depress the solidus of mantle peridotite by several hundred degrees, resulting in the common occurrence of highly mobile, volatile-rich melts, which are a main cause of heterogeneities in the Earth's mantle. Most experimental studies have investigated relatively oxidizing conditions in which COH fluids are mixtures of H2O + CO2, whereas experiments on the partial melting of mafic and ultramafic rocks under reducing conditions, in which CH4 + H2O mixtures occur, remain sparse. We have performed experiments on eclogite and lherzolite compositions at pressures from 1.8 to 6 GPa, and temperatures ranging from 800 to 1600 °C at different oxygen fugacity conditions (FMQ - 5.6 to FMQ - 0.3), either with 5 wt% CH4 + H2O or 5 wt% H2O added. Our results show that under reducing conditions (FMQ - 5.6 or IW - 0.7), the solidi of both eclogite and lherzolite with COH fluids are about 200 °C lower than the anhydrous solidi, but 100 °C higher than those under oxidizing conditions with 5 wt% H2O. The mineral chemistry of residual solid phases in both systems depends on oxygen fugacity (fO2). The stability of clinopyroxene at the solidus of lherzolite under reducing conditions is significantly greater than its stability under oxidizing conditions, whereas garnet is only stable during the partial melting of eclogite at low fO2. The composition of partial melts of lherzolite with 5 wt% CH4 + H2O under reducing conditions at low pressures is quartz normative rather than olivine normative. The higher-temperature solidi and phase relations under reducing conditions indicate that, if Archean mantle was more reduced than the modern mantle, komatiite could be formed under reducing conditions with no need for deep melting or the presence of abundant water. Further, our experiments indicate that low fO2 favors the precipitation of graphite or diamond, so a large amount of diamond or graphite could have accumulated in the Earth's deep mantle and the Martian mantle.

Iron Isotope Composition of Adakitic Rocks: The Shangcheng Pluton, Western Dabie Orogen, Central China

There has been little research on the metal isotopic composition of adakitic rock. The main objective of our investigation was to obtain more knowledge on the iron isotopic composition of adakitic rocks and provide new evidence for the genesis of Shangcheng pluton from an iron isotope perspective. The Dabie orogen is divided into eastern and western areas by the Shangcheng-Macheng fault, and the Shangcheng pluton is located in the western Dabie orogen area. The iron isotopic composition of these rocks ranges from 0.08‰ to 0.20‰ (2SD, n = 3). The δ56Fe values of two rocks from the SGD (Sigudun) unit are relatively low (0.11 ± 0.03‰ and 0.08 ± 0.04‰), while the δ56Fe values of the other samples are basically consistent (0.18–0.2‰). Evidence from elemental geochemical characteristics and petrogenesis defines the Shangcheng pluton as adakitic rocks. Our investigation on the elemental and isotopic compositions hints that the enrichment of heavy iron isotopes cannot be explained by weathering/alteration and fluid exsolution. Fractional crystallization of magnetite may account for the enrichment of light iron isotopes in two rocks from the SGD unit, while the fractional iron isotope trend in the other five samples can be explained by Δ56Fecrystal-melt = ~0.035‰. Two investigated rocks from SGD units may have been derived from the partial melting of amphibolite, while the other five samples may have been derived from the partial melting of eclogite containing 10–15% garnet.

Isotope fractionation during partial melting of eclogite

本文对榴辉岩部分熔融过程中不同同位素体系是否存在分馏这一当前研究热点进行了综述。榴辉岩作为研究洋陆俯冲、超高压变质以及壳幔相互作用的主要岩石类型，其部分熔融与地壳增生、板片折返过程以及俯冲隧道中元素的迁移分配等具有紧密的联系。作为典型的高压-超高压变质岩石，榴辉岩可通过俯冲带将壳源信息携带至地幔深部，影响地幔的化学组成，并可在大洋玄武岩中得以表现。近些年，随着仪器分析技术的发展，实验研究和理论计算均表明榴辉岩部分熔融过程中稳定同位素可以产生显著的分馏。作为常见的放射性成因子体同位素和传统稳定同位素Sr-Nd-Hf-O被广泛应用于源区示踪、岩浆混合以及结晶分异等过程。但目前有研究指出，在非平衡熔融过程中，熔体和源区的Sr-Nd-Hf-O同位素可发生解耦，导致二者的同位素组成不均一。另外，通过研究榴辉岩及其熔融产物的金属稳定同位素特征，发现榴辉岩部分熔融过程中，由于石榴石效应，会造成Ca、Mg、Fe、Li等金属稳定同位素的分馏。因此，当利用稳定同位素示踪榴辉岩熔体的源区时，需要考虑其分馏的影响。

Compositional and pressure controls on calcium and magnesium isotope fractionation in magmatic systems

Stable isotope fractionation in magmatic systems depends on equilibrium isotope fractionation between different phases. However, current equilibrium stable isotope theory mostly assumes ideal crystal structures and simple chemical compositions, but it is unclear how the pressure and complex compositional variations in natural microscopic mineral structures affect inter-mineral stable isotope fractionation and thus control stable isotope fractionation in macroscopic magmatic systems. Here, we calculate the CaO and MgO bond lengths controlled by pressure and compositional variations of coexisting garnet (Grt) and clinopyroxene (Cpx) in the Roberts Victor eclogites from the Kaapvaal Craton and use these data as a proof of concept to interpret their inter-mineral Ca and Mg isotopic compositions (Δ44/40CaGrt-Cpx and Δ26MgGrt-Cpx). Our results show that the CaO difference between Grt and Cpx (ΔCaOGrt-Cpx) shows a significant increase with CaO content from 3.4 to 13.6 wt.% in Grt and with pressure from 2.9 to 6.9 GPa. ΔCaOGrt-Cpx has an excellent negative correlation with inter-mineral Ca isotope fractionation corrected for temperature effect (Δ44/40CaGrt-Cpx × T2/106), indicating that inter-mineral Ca isotope fractionation is controlled by pressure and compositional variations of the Grt through effects on the bond lengths. Inter-mineral Mg isotope fractionation corrected for temperature effect (Δ26MgGrt-Cpx × T2/106) in these eclogites shows a negative correlation with pressure but no obvious correlations with the mineral compositions, suggesting the dominant role of pressure effect in addition to temperature. The MgO bond length of Grt increases by about 0.02 Å with increasing CaO content in Grt of these eclogites, implying a mild compositional effect on inter-mineral Mg isotope fractionation. The results suggest that pressure and compositional variations in minerals control the equilibrium stable isotope fractionation between minerals. Utilizing the Ca isotope fractionation factor controlled by crystal chemistry of garnet, our modelling indicates that partial melting of eclogite in the mantle could not significantly fractionate Ca isotopes and, therefore, that low δ44/40Ca values in previously reported basalts cannot be attributed to the involvement of eclogite in their sources. This suggests that crystal chemistry exerts major controls on isotope fractionation in magmatic systems in addition to temperature.

Critical phenomena and granatization of water-containing eclogite at P = 3,7-4,0 GPa, T = 1000-1300 °C

The phase relationships have been experimentally studied at eclogitization of basalts and the melting of H2O‑containing eclogite in the basalt-H2O system at P = 3,7-4,0 GPa, T = 1000-1300 C. It is established that the phase relationships depend on temperature. The formation of a supercritical fluid-melt occurs at T = 1000 C, P = 3,7 GPa, conversion eclogite-granatite occurs at T = 1000-1100 C, P = 3,9 GPa, partial melting of eclogite with the formation of Na-alkali silicate melt and clinopyroxenite restite at 1150 C and 1300 C. The supercritical fluid-melt has a high reactivity, resulting in the formation of megacrists of garnet, its enrichment with Ti, the replacement of garnet with clinopyroxene, the formation of ilmenite, K‑containing amphibole, the conversion of eclogite into garnetite as a result of mass crystallization of garnet.

Critical Phenomena and Garnetization of Hydrous Eclogite at P = 3.7–4.0 GPa and T = 1000–1300°C

The phase relations upon eclogitization of basalt and melting of H2O-bearing eclogite were studied experimentally for basalt–H2O in the range of P = 3.7–4.0 GPa and Т = 1000–1300°C. It is shown that the phase relations depend on temperature. The formation of a supercritical fluid/melt occurs at T = 1000°C and P = 3.7 GPa; the eclogite‒garnetite transition, at T = 1000–1100°C and P = 3.9 GPa; and partial melting of eclogite with the formation of a Na-alkaline silicate melt and clinopyroxenite restite, at 1150 and 1300°C. the supercritical fluid/melt has a high reactivity, which results in the formation of garnet megacrysts, enrichment of garnet in Ti, and replacement with clinopyroxene, the formation of ilmenite, K-bearing amphibole, and the transformation of eclogite into garnetite due to the mass crystallization of garnet.

The role of eclogites in the redistribution of water in the subcontinental mantle of the Siberian craton: results of determination of the water content in minerals from the Udachnaya pipe eclogites

A comprehensive study of 26 mafic mantle xenoliths from the Udachnaya kimberlite pipe was carried out. The contents of major and trace elements, equilibrium temperature parameters, and water content in the rock-forming minerals were determined. The temperatures of formation of the studied rocks are estimated at 800–1300 °C. According to IR spectroscopy data, the water content in clinopyroxenes from the studied eclogites varies from values below the detection limit to 99 ppm. The IR spectra of garnets lack bands of water. The water content in clinopyroxene and orthopyroxene from garnet websterite is 72 and 8 ppm, respectively. The water content in the average rock, calculated from the ratio of the rock-forming minerals, varies from a few to 55 ppm. No relationship among the water content, equilibrium temperatures, and rock composition is established. The low water contents in the eclogites are close to the earlier determined water contents in peridotites from the same pipe and are, most likely, due to the re-equilibration of the eclogites with the rocks of the peridotitic lithospheric mantle. The dehydration of the protolith during its subduction and the partial melting of eclogites before their removal by kimberlitic magma to the surface might be an additional cause of the low water contents in the mantle eclogite xenoliths.

The petrogenesis of the Neoarchean Kukuluma Intrusive Complex, NW Tanzania

The Kukuluma Intrusive Complex (KIC) is a late Archean igneous complex, dominated by monzonite and diorite with subordinated granodiorite. The monzonite and the diorite suites have low silica content (SiO2 ≤ 62 wt%), moderate Mg# (Mg#average = 49), high Sr/Y (Sr/Yaverage = 79) and high La/Yb (La/Ybaverage = 56) ratios, and strongly fractionated (Lan/Ybn = 9–69) REE patterns. Their moderate Ni (Niaverage = 50 ppm), Cr (Craverage = 85 ppm), variable Cr/Ni ratio (0.65–3.56) and low TiO2 (TiO2average = 0.5 wt%) indicate little to no interaction with the peridotitic mantle. For most major elements (Al2O3, FeOt, Na2O, TiO2 and P2O5) the monzonite and the diorite suites display subparallel trends for the same SiO2 content indicating they represent distinct melts. Intrusions belonging to the diorite suite have high Na2O (Na2Oaverage = 4.2 wt%), Dy/Ybn (Dy/Ybn-average = 1.6), a positive Sr anomaly and uncorrelated Nb/La and Zr/Sm ratios suggesting derivation from partial melting of garnet-bearing amphibolite. Intrusions belonging to the monzonite suite have higher Na2O (Na2Oaverage = 5.61 wt%), Dy/Ybn (Dy/Ybn-average = 2.21), a negative Sr anomaly and correlated Nb/La and Zr/Sm ratios consistent with derivation from partial melting of eclogite with residual rutile. Small variations in the Th/U ratio and near chondritic/MORB average values (Th/Umonzonite = 3.65; Th/Udiorite = 2.92) are inconsistent with a subducting slab signature, and it is proposed that the monzonite and the diorite suites of the KIC formed by partial melting of garnet-bearing, lower mafic crust of an oceanic plateau. The granodiorite suite has lower Mg# (Mg#average = 41), moderately fractionated REE, low Sr/Y (Sr/Yaverage = 20), La/Yb (La/Ybaverage = 15), Dy/Ybn (DyYbn-average = 1.24) and small negative Eu anomalies suggesting derivation from partial melting of amphibolite and plagioclase fractionation. Near-MORB Th/U (Th/Uaverage = 2.92) and Zr/Sm (Zr/Smaverage = 30.21) ratios are consistent with intracrustal melting of amphibolite.Archean rocks with an “adakitic” geochemical signature have been used to argue in favour of a plate tectonics regime in the Archean. The results presented here suggest that tectonic models for the Tanzania Craton, which invoke a subduction-related setting for all greenstone belts may need revision.

Review of melting experiments on carbonated eclogite and peridotite: insights into mantle metasomatism

Experimental studies on the partial melting of eclogite and peridotite provide important clues on mantle metasomatism. Here, we review results from some of the recent experiments and show that melting of carbonated eclogite and peridotite can produce carbonatitic to carbonated silicate melt, in which carbonates melt preferentially before Ti oxides and silicates. Low-degree melting results in carbonatitic melt coexisting with Ti oxides and silicates. This process also leads to the fractionation between some high-field strength elements (Nb, Ta, Zr, Hf, and HREE) and highly incompatible elements (U and Th) in the melt. When Ti oxides are nearly exhausted in eclogite, extremely high TiO2 contents (e.g. 19 wt.%) are present in the melt with marked concentration of Nb and Ta. These results help to explain the features of carbonatitic metasomatism and the Nb–Ta spike in oceanic island basalts as identified in experimental studies. These studies also explain the reducing conditions that stabilize diamond in the deep mantle (>150 km) as well as the occurrence of diamond at different depths reported in various studies. Melting in such a reduced mantle can happen through redox reaction between diamond, pyroxene, and olivine, in which the initial liquid is a carbonated silicate melt. However, the theoretical oxygen fugacity (fO2) in the asthenosphere is much lower than that predicted by the reaction and requires elevated fO2, which can be caused by the addition of relatively oxidized materials from the lower mantle, deep asthenospheric material, and various recycled components. A combination of these processes generates locally oxidized domains in the deep mantle.

Highly depleted Hadean mantle reservoirs in the sources of early Archean arc-like rocks, Isua supracrustal belt, southern West Greenland

Growing evidence from the accessible geological record reveals that crust–mantle differentiation on Earth started as early as 4.4 Ga. In order to assess the extent of early Archean mantle depletion, we obtained 176Lu– 176Hf, 147Sm– 143Nd, and high field strength element (HFSE) concentration data for the least altered, well characterized boninite-like metabasalts and associated metasedimentary rocks from the Isua supracrustal belt (southern West Greenland). The metasediments exhibit initial εHf (3720) values from −0.7 to +1.5 and initial εNd (3720) values from +1.6 to +2.1. Initial εHf (3720) values of the least altered boninite-like metabasalts span a range from +3.5 to +12.9 and initial εNd (3720) values from −0.3 to +3.2. These initial Hf-isotope ratios display coherent trends with SiO 2, Al 2O 3/TiO 2 and other relatively immobile elements, indicating contamination via assimilation of enriched components, most likely sediments derived from the earliest crust in the region. This model is also consistent with previously reported initial γOs (3720) values for some of the samples. In addition to the positive εHf (3720) values, the least disturbed samples exhibit positive εNd (3720) values and a co-variation of εHf (3720) and εΝd (3720) values. Based on these observations, it is argued, that the most depleted samples with initial εHf (3720) values of up to +12.9 and high 176Lu/ 177Hf of ∼0.05 to ∼0.09 tap a highly depleted mantle source with a long term depletion history in the garnet stability field. High precision high field strength element (HFSE) data obtained for the Isua samples confirm the contamination trend. Even the most primitive samples display negative Nb–Ta anomalies and elevated Nb/Ta, indicating a subduction zone setting and overprint of the depleted mantle sources by felsic melts generated by partial melting of eclogite. Collectively, the data for boninite-like metabasalts support the presence of strongly depleted mantle reservoirs as previously inferred from Hf isotope data for Hadean zircons and combined 142Nd– 143Nd isotope data for early Archean rocks.

Source components of the Gran Canaria (Canary Islands) shield stage magmas: evidence from olivine composition and Sr–Nd–Pb isotopes

The Canary Island primitive basaltic magmas are thought to be derived from an HIMU-type upwelling mantle containing isotopically depleted (NMORB)-type component having interacted with an enriched (EM)-type component, the origin of which is still a subject of debate. We studied the relationships between Ni, Mn and Ca concentrations in olivine phenocrysts (85.6–90.0 mol.% Fo, 1,722–3,915 ppm Ni, 1,085–1,552 ppm Mn, 1,222–3,002 ppm Ca) from the most primitive subaerial and ODP Leg 157 high-silica (picritic to olivine basaltic) lavas with their bulk rock Sr–Nd–Pb isotope compositions (87Sr/86Sr = 0.70315–0.70331, 143Nd/144Nd = 0.51288–0.51292, 206Pb/204Pb = 19.55–19.93, 207Pb/204Pb = 15.60–15.63, 208Pb/204Pb = 39.31–39.69). Our data point toward the presence of both a peridotitic and a pyroxenitic component in the magma source. Using the model (Sobolev et al. in: Science 316:412–417, 2007) in which the reaction of Si-rich melts originated during partial melting of eclogite (a high pressure product of subducted oceanic crust) with ambient peridotitic mantle forms olivine-free reaction pyroxenite, we obtain an end member composition for peridotite with 87Sr/86Sr = 0.70337, 143Nd/144Nd = 0.51291, 206Pb/204Pb = 19.36, 207Pb/204Pb = 15.61 and 208Pb/204Pb = 39.07 (EM-type end member), and pyroxenite with 87Sr/86Sr = 0.70309, 143Nd/144Nd = 0.51289, 206Pb/204Pb = 20.03, 207Pb/204Pb = 15.62 and 208Pb/204Pb = 39.84 (HIMU-type end member). Mixing of melts from these end members in proportions ranging from 70% peridotite and 30% pyroxenite to 28% peridotite and 72% pyroxenite derived melt fractions can generate the compositions of the most primitive Gran Canaria shield stage lavas. Combining our results with those from the low-silica rocks from the western Canary Islands (Gurenko et al. EPSL 277:514–524, 2009), at least four distinct components are required. We propose that they are (1) HIMU-type pyroxenitic component (representing recycled ocean crust of intermediate age) from the plume center, (2) HIMU-type peridotitic component (ancient recycled ocean crust stirred into the ambient mantle) from the plume margin, (3) depleted, MORB-type pyroxenitic component (young recycled oceanic crust) in the upper mantle entrained by the plume, and (4) EM-type peridotitic component from the asthenosphere or lithosphere above the plume center.

Contrasting types of metasomatism in dunite, wehrlite and websterite xenoliths from Kimberley, South Africa

Dunite, wehrlite and websterite are rare members of the mantle xenolith suite in the Kimberley kimberlites of the Kaapvaal Craton in southern Africa. All three types were originally residues of extensive melt extraction and experienced varying amounts and types of melt re-enrichment. The melt depletion event, dated by Re–Os isotope systematics at 2.9 Ga or older, is evidenced by the high Mg# (Mg/(Mg + Fe)) of silicate minerals (olivine (0.89–0.93); pyroxene (0.88–0.93); garnet (0.72–0.85)), high Cr# (Cr/(Cr + Al)) of spinel (0.53–0.84) and mostly low whole-rock SiO 2, CaO and Al 2O 3 contents. Shortly after melt depletion, websterites were formed by reaction between depleted peridotites and silica-rich melt (>60 wt% SiO 2) derived by partial melting of eclogite before or during cratonization. The melt–peridotite interaction converted olivine into orthopyroxene. All three xenolith types have secondary metasomatic clinopyroxene and garnet, which occur along olivine grain boundaries and have an amoeboid texture. As indicated by the preservation of oxygen isotope disequilibrium in the minerals and trace-element concentrations in clinopyroxene and garnet, this metasomatic event is probably of Mesozoic age and was caused by percolating alkaline basaltic melts. This melt metasomatism enriched the xenoliths in CaO, Al 2O 3, FeO and high-field-strength-elements, and might correspond to the Karoo magmatism at 200 Ma. The websterite xenoliths experienced both the orthoyproxene-enrichment and clinopyroxene–garnet metasomatic events, whereas dunite and wehrlite xenoliths only saw the later basaltic melt event, and may have been situated further away from the source of melt migration channels.

A model for rutile saturation in silicate melts with applications to eclogite partial melting in subduction zones and mantle plumes

This experimental study examines the solubility of rutile in silicate melts and presents a model for rutile saturation as a function of temperature, pressure and melt composition. Rutile saturation experiments were carried out in the system SiO 2–TiO 2–Al 2O 3–MgO–CaO–Na 2O–K 2O at 1 bar to 35 kbar and 1150 to 1450 °C on model rhyodacite (∼ 69 wt.% SiO 2) and haplobasalt (∼ 54 wt.% SiO 2) base melt compositions. At rutile saturation, the concentration of TiO 2 in the model rhyodacite base melt increases at 1 bar from 3.27 ± 0.03 wt.% at 1150 °C to 13.88 ± 0.04 wt.% at 1450 °C. At 1350 °C it decreases from 8.89 ± 0.08 wt.% at 1 bar to 2.06 ± 0.13 wt.% at 35 kbar. Rutile solubility is significantly higher in the haplobasalt base melt at a given pressure and temperature. The concentration of TiO 2 in rutile-saturated haplobasalt increases at 1 bar from 20.9 ± 0.3 wt.% at 1300 °C to 39.0 ± 0.3 wt.% at 1450 °C. At 1350 °C it decreases from 25.8 ± 0.3 wt.% at 1 bar to 15.67 ± 0.16 wt.% at 15 kbar. Results from these experiments were combined with data from the literature to formulate a model for rutile saturation in silicate melts. Application of this model to partial melting of MORB-type eclogite indicates that beneath volcanic arcs low-degree, hydrous partial melts of rutile-bearing subducted oceanic crust contain only ∼ 600 ppm TiO 2. Therefore, rutile will remain as a residual phase in the eclogite and the amount of TiO 2 that will be transferred to the mantle wedge will be small because so little TiO 2 is dissolved in the melt. Partial melting of recycled oceanic crust in an upwelling mantle plume can exhaust rutile from the residual solid at moderate degrees of partial melting (∼ 22%). The retention of rutile in subducted oceanic lithosphere during dehydration and/or partial melting, combined with exhaustion of rutile during partial melting of eclogite in mantle plumes suggests that HFSE enrichments in recycled crust that were established during subduction may be detectible in OIB.

Partitioning of Ni between olivine and siliceous eclogite partial melt: experimental constraints on the mantle source of Hawaiian basalts

Olivine is abundant in Earth’s upper mantle and ubiquitous in basaltic lavas, but rarely occurs in eclogite. Partial melts of eclogite are, therefore, not in equilibrium with olivine, and will react with peridotite as they migrate through the upper mantle. If such melts erupt at Earth’s surface, their compositions will be highly modified and they may be olivine-saturated. We investigated experimentally the reaction between olivine and siliceous eclogite partial melt, and determined element partitioning between olivine and the melt produced by this reaction. Our results demonstrate that mixing of reacted eclogite partial melt with primitive basalt is capable of producing the positive correlation between melt SiO2 content and olivine Ni content observed in some Hawaiian lavas. Experiments were carried out by equilibrating eclogite partial melt or basalt with San Carlos olivine at 1 bar and 1,201–1,350°C. Our results show that eclogite partial melts equilibrated with mantle olivine retain their high SiO2, low FeO and MgO characteristics. Further, olivine-melt partition coefficients for Ni measured in these experiments are significantly larger than for basalt. Mixing of these melts with primitive Hawaiian tholeiitic lavas results in crystallization of high-Ni olivines similar to those in Makapuu-stage Koolau lavas, even though the mixed magmas have only moderate Ni contents. This results from a hyperbolic increase of the Ni partition coefficient with increasing polymerization of the mixed melt. Note that while eclogite partial melt in contact with peridotite will equilibrate with pyroxene as well as olivine, this will have the effect of buffering the activity of SiO2 in the reacted melt at a higher level. Therefore, an eclogite partial melt equilibrated with harzburgite will have higher SiO2 than one equilibrated with dunite, enhancing the effects observed in our experiments. Our results demonstrate that an olivine-free “hybrid” pyroxenite source is not required to explain the presence of high-Ni olivines in Hawaiian lavas and, therefore, indicate that the proportion of eclogite in the Hawaiian plume is less than has been estimated in recent studies.

Iron/manganese ratio and manganese content in shield lavas from Ko’olau Volcano, Hawai’i

Precise Fe/Mn ratios and MnO contents have been determined for basalts from the Hawaiian shields of Ko’olau and Kilauea by inductively coupled plasma mass spectrometry. It is well known that the youngest Ko’olau (Makapu’u-stage) shield lavas define a geochemical endmember for Hawaiian lavas in terms of CaO and SiO 2 contents and isotopic ratios of O, Sr, Nd, Hf, Pb, and Os. We find that their MnO content is also distinct. Despite the small range in MnO, 0.146 to 0.176 wt%, the precision of our data is sufficient to show that among unaltered Ko’olau lavas MnO content is correlated with Nd–Hf–Pb isotopic ratios, La/Nb and Al 2O 3/CaO elemental ratios, and contents of SiO 2, MgO and Na 2O + K 2O adjusted for olivine fractionation. These trends are consistent with two-component mixing; one endmember is a SiO 2-rich, MnO-, and MgO-poor dacite or andesite melt, generated by low degree (10–20%) partial melting of eclogite. Since this low-MgO endmember (dacite or andesite melt) has very low FeO and MnO contents, mixing of high Fe/Mn dacite or andesite melt with a MgO-rich picritic melt, the other endmember, does not significantly increase the Fe/Mn in mixed magmas; consequently, Ko’olau and Kilauea lavas have similar Fe/Mn. We conclude that the high Fe/Mn in Hawaiian lavas relative to mid-ocean ridge basalt originates from the high MgO endmember in Hawaiian lavas.

238U– 230Th– 226Ra– 210Pb constraints on the genesis of high-Mg andesites at White Island, New Zealand

On White Island, New Zealand, the intensified period of strombolian–volcanian and phreatomagmatic explosive activity that commenced in March 1977 led to eruption of unusually primitive, high-Mg andesites. These are Fo 80–93 olivine-saturated rocks that have MgO contents up to 10% (Mg# = 65–71) and SiO 2 of 56–58%. They have incompatible trace element characteristics that are typical of arc rocks. 87Sr/ 86Sr, 143Nd/ 144Nd and 176Hf/ 177Hf ratios (0.7049–0.7053, 0.51282–0.51266 and 0.28301–0.28298, respectively) are consistent with subducted sediment addition and/or crustal input but there is no clear correlation of either isotope ratio with MgO. The rocks have modest (3–10%) 238U excesses at low ( 230Th/ 232Th) ratios (0.697 to 0.722). 226Ra– 230Th disequilibria are also restricted but, unusually, include both 226Ra excesses and deficits with ( 226Ra/ 230Th) = 0.94–1.07. ( 210Pb/ 226Ra) o ranges from 0.98 to 1.52 requiring gas accumulation that may increase over time and with decreasing MgO. Sr/Y and Tb/Yb ratios are both low and relatively invariant at 8 and 0.3, respectively, and along with the 238U excesses preclude an origin in which residual garnet was involved. The occurrence of some 226Ra deficits suggests the presence of residual amphibole during partial melting for some samples. Rapid magma ascent (to preserve the 226Ra disequilibria) limits the amount of possible melt–wall rock interaction that might reduce source-derived Tb/Yb ratios and in the mantle or raise 87Sr/ 86Sr in the crust. The White Island high-Mg andesites did not form by partial melting of eclogite in the subducting Pacific plate. Their primitive, olivine-saturated compositions suggest that their source was peridotitic and experimental data suggest that equilibration with peridotite at low temperatures at 0.5–1.5 GPa and in the presence of elevated alkalis can reconcile the high SiO 2 and MgO of the rocks. This may reflect stalling of the magmas beneath the continental lithosphere for a few 1000 years.

Rutile stability and rutile/melt HFSE partitioning during partial melting of hydrous basalt: Implications for TTG genesis

Synthesis experiments were conducted on a natural basalt (with 2 or 5 wt.% H 2O added) at 1.0–2.5 GPa and 900–1100 °C to investigate the stability field of rutile and rutile/liquid HFSE partitioning during partial melting of hydrous basalt. The basalt chosen has TiO 2 content close to average N-MORB. 100 ppm of Ta, Nb, Hf, Zr, etc., were added to the starting composition in order to improve analytical precision with the LAM-ICP-MS and the electron microprobe. Rutile occurs in the partial melting field of hydrated basalt at pressures higher than approximate 1.5 GPa, depending on H 2O content and bulk composition (especially TiO 2 and K 2O). Its stability increases with increasing pressure and decreasing temperature. H 2O helps produce a more mafic melt and so results in dissolution of rutile and shrinkage of the P– T field of rutile crystallization. The rutile/melt partitioning results confirm previous observations [ Green, T.H., Pearson, N.J., 1986. Ti-rich accessory phase saturation in hydrous mafic–felsic compositions at high P, T. Chem. Geol. 54, 185–201; Jenner, G.A., Foley, S.F., Jackson, S.E., Green, T.H., Fryer, B.J., Longerich, H.P., 1993. Determination of partition coefficients for trace elements in high pressure–temperature experimental run products by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM–ICP–MS). Geochim. Cosmochim. Acta 57, 5099–5103; Foley, S.F., Barth, M.G., and Jenner, G.A., 2000. Rutile/melt partition coefficients for trace elements and assessment of the influence of rutile on the trace element characteristics of subduction zone magmas. Geochim. Cosmochim. Acta 64, 933–938; Schmidt, M.W., Dardon, A., Chazot, G., and Vannucci, R., 2004. The dependence of Nb and Ta rutile–melt partitioning on melt composition and Nb/Ta fractionation during subduction processes. Earth Planet. Sci. Lett. 226, 415–432 ], including that rutile is a dominant carrier for Nb and Ta, and that rutile favours Ta over Nb with D Nb always lower than D Ta for each rutile/melt pair. In addition our experiments demonstrate that both D Nb and D Ta decrease with increasing H 2O content but increase with decreasing temperature. Rutile is a necessary residual phase during the generation of Archean tonalite– trondhjemite–granodiorite (TTG) magmas to account for the negative Nb–Ta anomaly of the magmas. The depth for TTG production via melting of subducted oceanic crust must be more than 45–50 km based on the approximate 1.5 GPa minimum pressure for rutile appearance. Rutile fractionates Nb from Ta and will result in slightly higher Nb/Ta in coexisting liquids. Archean TTG magmas with subchondritic Nb/Ta must, therefore, have been derived from low Nb/Ta source regions [cf. Rapp, R.P., Shimizu, N., Norman, M.D., 2003. Growth of early continental crust by partial melting of eclogite. Nature 425, 605–609] unless alternative magmatic processes have lowered the Nb/Ta ratio. Also rutile-bearing residues should display lower Nb/Ta after TTG liquids are extracted. Hence, the present data do not support the view that subducted rutile-bearing eclogitic oceanic crust is a superchondritic Nb/Ta reservoir on Earth.

Recycling lower continental crust in the North China craton

Foundering of mafic lower continental crust into underlying convecting mantle has been proposed as one means to explain the unusually evolved chemical composition of Earth's continental crust, yet direct evidence of this process has been scarce. Here we report that Late Jurassic high-magnesium andesites, dacites and adakites (siliceous lavas with high strontium and low heavy-rare-earth element and yttrium contents) from the North China craton have chemical and petrographic features consistent with their origin as partial melts of eclogite that subsequently interacted with mantle peridotite. Similar features observed in adakites and some Archaean sodium-rich granitoids of the tonalite-trondhjemite-granodiorite series have been interpreted to result from interaction of slab melts with the mantle wedge. Unlike their arc-related counterparts, however, the Chinese magmas carry inherited Archaean zircons and have neodymium and strontium isotopic compositions overlapping those of eclogite xenoliths derived from the lower crust of the North China craton. Such features cannot be produced by crustal assimilation of slab melts, given the high Mg#, nickel and chromium contents of the lavas. We infer that the Chinese lavas derive from ancient mafic lower crust that foundered into the convecting mantle and subsequently melted and interacted with peridotite. We suggest that lower crustal foundering occurred within the North China craton during the Late Jurassic, and thus provides constraints on the timing of lithosphere removal beneath the North China craton.

Experimental determination of trace element partitioning between garnet and silica‐rich liquid during anhydrous partial melting of MORB‐like eclogite

We present new experimentally determined trace element partition coefficients for nine garnet/melt and two clinopyroxene/melt pairs at 2.9–3.1 GPa and 1325°–1390°C, applicable to anhydrous partial melting of MORB‐like eclogite in the upper mantle. Phase compositions are similar to those documented in partial melting experiments of eclogite at these conditions: garnets with 16–25% grossular component and 0.4–2.0 wt % TiO2 coexist with siliceous partial melts having 52.4–57.1 wt % SiO2 and 1.1–6.7 wt % TiO2. Observed garnet/melt partitioning depends on TiO2 concentrations in garnet. Among the high‐TiO2 garnets (1.4–2.0 wt % TiO2), partition coefficients (Dgt/melt) increase with increasing grossular content of garnet (16–24%), but for the low‐TiO2 garnets (0.4–0.6 wt % TiO2) there is no difference in partitioning behavior at 19–24% grossular. In general, partition coefficients for the low‐TiO2 garnets tend to be higher than for the high‐TiO2 garnets. DZr and DHf increase with higher grossular content, as does DZr/DHf, but DZr and DHf always remain smaller than unity, and TiO2 in garnet appears to have little effect on DZr/DHf. DTh and DU have average values of 0.0012 and 0.0111 for the high‐TiO2 garnets and 0.0074 and 0.0376 for the low‐TiO2 garnets (DU/DTh of 9.1 and 5.1, respectively), affirming that partial melting of eclogite in the upper mantle can produce liquids with significant (230Th)/(238U) excess. D values of highly charged cations (Nb, Ta, Th, U) are lower in high‐TiO2 garnets. This is likely because Ti4+ occupying the garnet Y site requires charge balance by 2+ cations in Y or 3+ cations in Z, thereby limiting the availability of these charge‐balancing substitutions to accommodate other highly charged cations. The overall range of our new garnet/melt partition coefficients, regardless of Ti and grossular content, is similar to literature garnet/melt partitioning data applicable to garnet peridotite melting, and therefore bulk partition coefficients for eclogite and peridotite depend largely on mineral modes and choice of partitioning data for clinopyroxene. Using our new data and previously published data from the literature, we calculate bulk partition coefficients appropriate for two MORB‐like eclogites (82% clinopyroxene + 18% garnet and 75% clinopyroxene + 25% garnet) and a garnet peridotite (60% olivine + 17% orthopyroxene + 13% clinopyroxene + 10% garnet). Bulk partition coefficients are generally much higher for MORB‐like eclogite when compared to garnet peridotite, with the exception of Th and U. However, the ratios of bulk partition coefficients of eclogite and garnet peridotite are similar for pairs such as DSm/DYb, DZr/DHf and DU/DTh, indicating that both lithologies can induce similar trace element fractionations during partial melting in the presence of garnet.

Growth of early continental crust by partial melting of eclogite.

The tectonic setting in which the first continental crust formed, and the extent to which modern processes of arc magmatism at convergent plate margins were operative on the early Earth, are matters of debate. Geochemical studies have shown that felsic rocks in both Archaean high-grade metamorphic ('grey gneiss') and low-grade granite-greenstone terranes are comprised dominantly of sodium-rich granitoids of the tonalite-trondhjemite-granodiorite (TTG) suite of rocks. Here we present direct experimental evidence showing that partial melting of hydrous basalt in the eclogite facies produces granitoid liquids with major- and trace-element compositions equivalent to Archaean TTG, including the low Nb/Ta and high Zr/Sm ratios of 'average' Archaean TTG, but from a source with initially subchondritic Nb/Ta. In modern environments, basalts with low Nb/Ta form by partial melting of subduction-modified depleted mantle, notably in intraoceanic arc settings in the forearc and back-arc regimes. These observations suggest that TTG magmatism may have taken place beneath granite-greenstone complexes developing along Archaean intraoceanic island arcs by imbricate thrust-stacking and tectonic accretion of a diversity of subduction-related terranes. Partial melting accompanying dehydration of these generally basaltic source materials at the base of thickened, 'arc-like' crust would produce compositionally appropriate TTG granitoids in equilibrium with eclogite residues.