Negative High Field Strength Elements Anomalies Research Articles

Origin of ultramafic–mafic bodies on the Isles of Lewis and Harris (Scotland, UK): Constraints on the Archean–Paleoproterozoic evolution of the Lewisian Gneiss Complex, North Atlantic Craton

The Lewisian Gneiss Complex (LGC) is a tonalite-trondhjemite-granodiorite (TTG)-dominated fragment of the North Atlantic Craton (NAC) in northwest Scotland. End-member models describe the LGC as representing either a continuous piece of Archean crust or up to 12 geologically distinct Archean terranes, with interpretations sitting on a spectrum between these end-members. There is particular uncertainty over the correlations between the Archean–Paleoproterozoic magmatic and metamorphic events recorded by mainland part of the LGC and the part exposed on the Outer Hebridean islands of Lewis and Harris. In this paper, we present the results of field mapping, petrography, and major, trace and platinum-group element (PGE) bulk-rock geochemistry for four ultramafic–mafic bodies in Lewis and Harris, namely: Maaruig, Loch Mhorsgail, Coltraiseal Mor and Beinn a’ Chuailean. We consider the effects of metamorphism and element mobility, their petrogenesis, and potential correlations with ultramafic–mafic rocks elsewhere in the LGC. Our data indicate that the studied ultramafic–mafic rocks can be subdivided into two petrologically distinct groups. Metaperidotites and metapyroxenites from Maaruig and Loch Mhorsgail are interpreted as Archean (>2.8 Ga) cumulates distinct from anything currently identified in the mainland LGC, with this interpretation based on distinctive modal layering, a discordance with surrounding TTG gneiss, fractionated PGE patterns ([Pd/Ir]N = 1.3–6.6) and negative HFSE anomalies ([Nb/La]N = 0.2–0.8). Metagabbronorites from Coltraiseal Mor and Beinn a’ Chuailean, which also exhibit negative high field strength-element (HFSE) anomalies ([Nb/La]N = 0.2–0.7) and show mildly fractionated ([Pd/Ir]N = 1.2–2.8) PGE patterns, most likely represent deformed Paleoproterozoic dykes. These occurrences could be correlatives of a suite of ca. 2.4 Ga mafic dykes exposed throughout the mainland LGC (the Scourie Dykes), with the Outer Hebridean occurrences having experienced more intense Paleoproterozoic (Laxfordian) deformation/reworking. These interpretations suggest that the LGC lithologies of Lewis and Harris were proximal to the mainland LGC’s Central Region by the early Paleoproterozoic but raises the possibility that they were distinct crustal blocks in the Mesoarchean.

Petrogenesis of Cenozoic high–Sr/Y shoshonites and associated mafic microgranular enclaves in an intracontinental setting: Implications for porphyry Cu-Au mineralization in western Yunnan, China

Cenozoic high–Sr/Y shoshonites in western Yunnan, China, are spatially and temporally associated with porphyry deposits and mafic volcanic rocks that formed at a post–collisional setting. However, the petrogenesis of these rocks and the origins of the associated mineralization remain unclear. Here we present new geochemical data for the high–Sr/Y shoshonites and associated mafic microgranular enclaves (MMEs) in the Jinshajiang–Ailaoshan mineralization belt, which formed as a result of India–Asia collision during the Cenozoic. Both fertile and barren shoshonitic porphyries were emplaced during the Eocene–Oligocene (32–37 Ma) and are characterized by enrichment in large–ion lithophile elements (LILE), depletion in high field strength elements (HFSE), high K2O (4–6 wt%), Sr contents (674–1370 ppm), and low Y contents (7–18 ppm). The barren Zhanhe, Yongsheng, Songgui, and Jianchuan, and fertile Machangqing (Cu–Au–Mo mineralization) porphyry intrusions have variable zircon εHf(t) values of −6 to +9 and old TDM2 ages of 1433 to 900 Ma. However, the barren Shigu porphyry intrusion yields relatively positive zircon εHf(t) values (−4 to +8) and younger TDM2 ages (903–631 Ma). These features indicate that the Shigu intrusion was derived from late Permian juvenile lower crust, and the other high Sr/Y shoshonites were derived mainly from Neoproterozoic juvenile lower crust. Fertile and barren porphyries have variable whole–rock εNd(t) values (−5 to −0.7) and (87Sr/86Sr) i ratios (0.70595–0.70788) that are within the range of coeval mafic magmas in western Yunnan. The MMEs comprise mafic enclaves and andesitic enclaves that formed during the Eocene (34–35 Ma) and have spheroidal shapes, igneous textures and contain acicular apatites. This indicates that the MMEs are globules of coeval mafic magma that was injected into and mixed with the host shoshonitic magma. The enclaves show low TiO2 contents (0.7–0.8 wt%), high MgO (6–9 wt%), Cr (184–450 ppm) and Ni contents (98–157 ppm), positive LILE anomalies, marked negative HFSE anomalies, a narrow range of εNd(t) values (−1.2 to −0.6), and (87Sr/86Sr) i values of 0.70556–0.70635. The andesitic enclaves show adakitic geochemical affinities and have similar zircon εHf(t) values (−2.7 to +2.5) to the host porphyries, whereas, the mafic enclaves have relatively negative zircon εHf(t) values (−8.7 to +1.5), similar to enriched mantle. These evidences suggest the high−Sr/Y shoshonites were produced by partial melting of juvenile lower crust mixed with ultrapotassic or potassic mafic magmas as represented by the andesitic enclaves. The mafic enclave melts were derived from enriched mantle metasomatized by slab–derived fluids, which elevated the Mg#, Cr, Ni contents of the host porphyries at Machangqing, Zhanhe and Yongsheng.Zircon saturation temperatures (TZr) of the high–Sr/Y shoshonitic rocks range from 735 to 777 °C and indicate the shoshonites were derived mainly from water–fluxed melting of lower crust. The Manchangqing fertile intrusion had a high oxidation state (zircon Ce4+/Ce3+ = 38–180, average = 91) and was derived by mixing between juvenile lower–crustal melts and voluminous coeval lamprophyres. In contrast, the barren Jianchuan, Songgui, Shigu, Yongsheng and Zhanhe high–Sr/Y shoshonites formed by partial melting of juvenile lower crust and some mixing with mafic lavas, and are characterized by low oxygen fugacity (zircon Ce4+/Ce3+ = 0.5–77, average = 20). As such, we suggest the mafic magmas triggered water–fluxed, moderate–degree partial melting of the lower crust and caused amphibole breakdown during melting. This process could not provide enough water for the formation of porphyry Cu-Au deposits in western Yunnan, and therefore formed low–fO2, relatively low–H2O, and barren high–Sr/Y magmas. However, ultrapotassic magmas (e.g., lamprophyres) with high H2O are able to trigger intense water–fluxed, partial melting of metal–fertilized lower crust, which generated high–fO2, relatively high–H2O, fertile, high–Sr/Y shoshonitic magmas in this intracontinental setting.

Assessing the Validity of Negative High Field Strength-Element Anomalies as a Proxy for Archaean Subduction: Evidence from the Ben Strome Complex, NW Scotland

The relative depletion of high field strength elements (HFSE), such as Nb, Ta and Ti, on normalised trace-element plots is a geochemical proxy routinely used to fingerprint magmatic processes linked to Phanerozoic subduction. This proxy has increasingly been applied to ultramafic-mafic units in Archaean cratons, but as these assemblages have commonly been affected by high-grade metamorphism and hydrothermal alteration/metasomatism, the likelihood of element mobility is high relative to Phanerozoic examples. To assess the validity of HFSE anomalies as a reliable proxy for Archaean subduction, we here investigate their origin in ultramafic rocks from the Ben Strome Complex, which is a 7 km2 ultramafic-mafic complex in the Lewisian Gneiss Complex of NW Scotland. Recently interpreted as a deformed layered intrusion, the Ben Strome Complex has been subject to multiple phases of high-grade metamorphism, including separate granulite- and amphibolite-facies deformation events. Additional to bulk-rock geochemistry, we present detailed petrography, and major- and trace-element mineral chemistry for 35 ultramafic samples, of which 15 display negative HFSE anomalies. Our data indicate that the magnitude of HFSE anomalies in the Ben Strome Complex are correlated with light rare earth-element (LREE) enrichment likely generated during interaction with H2O and CO2-rich hydrothermal fluids associated with amphibolitisation, rather than primary magmatic (subduction-related) processes. Consequently, we consider bulk-rock HFSE anomalies alone to be an unreliable proxy for Archaean subduction in Archaean terranes that have experienced multiple phases of high-grade metamorphism, with a comprehensive assessment of element mobility and petrography a minimum requirement prior to assigning geodynamic interpretations to bulk-rock geochemical data.

Sub-lithospheric origin of Na-alkaline and calc-alkaline magmas in a post-collisional tectonic regime: Sr-Nd-Pb isotopes in recent monogenetic volcanism of Cappadocia, Central Turkey

Intense Cenozoic magmatism accompanied the convergence and collision of the Eurasian, African and Arabian plates. In the Cappadocia Region of Central Anatolia, widespread and abundant volcanism developed from the late Miocene to the Quaternary. Calc-alkaline pyroclastic products and lava erupted diffusely during the late Miocene and Pliocene. Both calc-alkaline and Na-alkaline volcanic rocks were emplaced in close spatial and temporal proximity during the Plio-Pleistocene. Here we focus on the Plio-Pleistocene monogenetic activity, which produced two types of rocks: calc-alkaline volcanic rocks ranging in composition from basalts to rhyolites and occurring around the Acıgöl caldera, the Göllü Dağ dome and the Hasan Dağ and Erciyes Dağ composite volcanoes; Na-alkaline volcanic rocks ranging from alkali basalts to mugearites and cropping out in monogenetic centres in Karapınar, along the WSW flank of Hasan Dağ, in Acıgöl and near the Kızılırmak River, northwest of Nevşehir.The coeval occurrence of calc-alkaline and Na-alkaline volcanism observed here is in striking contrast with activity in the surrounding Western and Eastern Anatolia regions, where the calc-alkaline volcanic rocks are distinctly older than the Na-alkaline ones. The Sr and Nd isotopic compositions of the mantle sources of both rock types show a narrow, overlapping range of values (87Sr/86Sr~0.70395–0.70474 and 143Nd/144Nd~0.51268–0.51281 for the calc-alkaline products, 0.70334–0.70524 and 0.51268–0.51293 for the Na-alkaline ones). In addition, most of the Na-alkaline products, like the calc-alkaline rocks, show an arc-type distribution of incompatible trace elements, with marked enrichments in LILE (Large-Ion Lithophile Elements) and negative HFSE (High Field Strength Elements) anomalies. True intraplate magmas are absent in Cappadocia, where the genesis of Na-alkaline basalts is ascribed to mixing between different percentages of within-plate (OIB) and calc-alkaline magmas. Modelling reveals that the addition of 13% and 30% OIB-type melt to a calc-alkaline magma is enough to change a SiO2-oversaturated magma into an ol-hy-normative and ne-normative one, respectively, whereas the addition of 15% calc-alkaline magma to an OIB-type magma results in a negative NbTa anomaly in the primitive mantle-normalized spider diagrams.The various types of products of recent volcanism in Cappadocia reflect the complex tectonic setting of the Central Anatolian Block: characterised by strike-slip tectonics and local extensional pull-apart basins, different magma types formed contemporaneously through decompression melting at different depths.In this scenario, the calc-alkaline magmas derived from partial melting of a mantle wedge modified by a subduction component, whereas the within-plate (OIB) magmas derived from a deeper, unmodified sub-slab mantle source. Mixing between calc-alkaline and within-plate magmas during their rise to the surface produced the Na-alkaline Cappadocia basalts. The very low rate of extension allowed within-plate magmas (OIB) to reach the surface after having interacted with the overlying calc-alkaline magmas in the late stages of Cappadocia volcanism.

Geochemistry and tectonic setting of the Chah-Bazargan sub-volcanic mafic dykes, south Sanandaj–Sirjan Zone (SSZ), Iran

The Chah-Bazargan sub-volcanic mafic dykes (trachybasalt and basaltic trachyandesite) are located in the south of the Sanandaj–Sirjan Zone (SSZ), Iran. The dyke mineralogy mostly comprises amphibole, clinopyroxene, olivine, orthopyroxene, and plagioclase as phenocrysts and fine-grained plagioclase and some ferromagnesian minerals in the matrix. The rocks are alkaline and shoshonitic in composition. The mafic melts relate to Neotethys subduction activity beneath the southern SSZ in the ~Eocene–Miocene interval. Markedly positive Ba, U, K, Pb, and Sr and negative HFSE (high field strength elements: Nb, Ta, Zr, Hf, P, and Ti) anomalies demonstrate this subduction. The sub-volcanic mafic dykes were produced from a metasomatized upper lithospheric mantle wedge at a depth consistent with the stability field of phlogopite-spinel (or -spinel/garnet) lherzolite. Geochemical studies on the basis of the rare earth elements (REE) and HFSE, and large ion lithophile elements (LILE) display that the mantle wedge underwent degrees of partial melting averaging between 5 and 15% to form the Chah-Bazargan sub-volcanic mafic dykes. It is possible that the chemical composition of the rocks was changed due to fractional crystallization and crustal contamination during emplacement.

Origin of the Eocene porphyries and mafic microgranular enclaves from the Beiya porphyry Au polymetallic deposit, western Yunnan, China: Implications for magma mixing/mingling and mineralization

The origin of magmas with high H2O, S and metals that are linked to porphyry economic mineralization in continental collisional belts is controversial. In this paper, we studied the host porphyries and associated microgranular enclaves (MMEs) including gabbroic enclaves and dioritic enclaves from the large Beiya porphyry Au deposit in Jinshajiang-Ailaoshan mineralization belt, which provide first hand evidence of crust–mantle hybridization in the formation of fertile magma for porphyry deposit. Zircon LA-ICPMS U–Pb ages indicate coeval formation for the MMEs and the felsic porphyries in Eocene time (~36Ma). The MMEs show spheroidal shapes, acicular apatites, and oscillatory zoning with repeated resorption surfaces in plagioclases, which suggest that the MMEs are globules of a more mafic magma that was injected into and mingled with the host felsic magma. One gabbroic enclave sample exhibits high MgO (5.6wt.%), high TiO2 (1.7wt.%), enriched in large-ion lithophile element (LILE) and depleted in high field strength element (HFSE), with negative Nb, Ta and Ti anomalies. This indicates that the gabbroic enclave is similar to those of the coeval potassic mafic rocks in the Western Yunnan, which originated from the enriched mantle metasomatized by the slab-derived fluids. The dioritic enclave exhibit both potassic and adakitic-like features, and are characterized by high contents of MgO (4.4–4.9wt.%), K2O (6.4–7.4wt.%), and compatible trace elements (e.g. Ni: 115–142ppm; Cr: 214–291ppm), as well as by high Sr/Y ratios. The dioritic enclave samples yield εHf (t) values (−1.9 to +3.6) similar to those of the host porphyries. Petrographic, elemental, and isotopic evidence suggests that the Beiya dioritic enclave were formed by mixing between potassic mafic and adakitc-like melts, derived from metasomatized lithospheric mantle and lower crust, respectively. In contrast, the Beiya host porphyries are characterized by high SiO2 (66.9–71.9wt.%) and Al2O3 (13.8–15.9wt.%) contents, high Sr/Y ratios (35–79), low compatible element contents (e.g. Ni=0.9–7.2ppm; Cr =1.0–8.2ppm), low Mg# values (0.19–0.35), positive LILE anomalies, marked negative high HFSE anomalies, negative εNd(t) values (−6 to −2), and high (87Sr/86Sr)i values (0.7071–0.7081). These features indicate that the host porphyries were mainly formed by partial melting of subduction-modified lower crust with a certain assimilation of mafic melts represented by MMEs. MMEs hosted in the Beiya porphyries, formed from a hydrous, mafic magma although they formed in a non-arc setting. High magmatic water content is probably related to an enriched mantle metasomatized by fluids from a Proterozoic subducted oceanic slab. High Cu (341–626ppm) and Au contents of the MMEs suggest they crystallized from a metal-rich mafic melt. Complex assemblages of pyrite–chalcopyrite–apatite–rutile with hornblende are observed in the least altered MME. We suggest that the mafic melt probably supplied part of water, and metal to the Beiya porphyry Au system.

Neoarchaean crustal growth by combined arc–plume action: evidence from the Kadiri Greenstone Belt, eastern Dharwar craton, India

AbstractField and geochemical studies combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating set important constraints on the timing and petrogenesis of volcanic rocks of the Neoarchaean Kadiri greenstone belt and the mechanism of crust formation in the eastern Dharwar craton (EDC). The volcanic rocks are divided into three suites: tholeiitic basalts, calc-alkaline high-Mg# andesites and dominant dacites–rhyolites. The basalts (pillowed in places) show flat rare earth element (REE) and primordial mantle-normalized trace element patterns, but have minor negative Nb and Ta anomalies. They are interpreted as mantle plume-related oceanic plateau basalts whose source contained minor continental crustal input. The andesites are characterized by high Mg# (0.66–0.52), Cr and Ni, with depletion of high-field strength elements (HFSE) and enrichment of light REE (LREE) and large-ion lithophile elements (LILE). They were probably derived from a metasomatized mantle wedge overlying a subducted slab in a continental margin subduction zone. The dacites–rhyolites are silicic rocks (SiO2=61–72 wt%) with low Cr and Ni, K2O/Na2O mostly 0.5–1.1, highly fractionated REE patterns, enrichments of LILE and distinctly negative HFSE anomalies. One rhyolite sample yielded a zircon U–Pb age of 2353±32 Ma. This suite is similar to potassic adakites and is explained as the product of deep melting of thickened crust in the arc with a significant older crustal component. Collision between a continental margin arc with an oceanic plateau followed by slab break-off, upwelling of hot asthenosphere and extensive crustal reworking in a sustained compressional regime is proposed for the geodynamic evolution of the area. This is in corroboration with the scenario of EDC as a Neoarchaean hot orogen as suggested recently by some workers.Supplementary material:Details of whole-rock major and trace element determination, Nd isotope analysis and zircon U–Pb dating and trace element analysis, the geographical coordinates of the samples and the values of the international rock standards analysed are available at http://www.geolsoc.org.uk/SUP18660

Small-scale coexistence of island-arc- and enriched-MORB-type basalts in the central Vanuatu arc

We report here major, trace element and Sr–Nd–Pb isotopic data for a new set of basaltic lavas and melt inclusions hosted in Mg-rich olivines (Fo86–91) from Mota Lava, in the Banks islands of the Vanuatu island arc. The results reveal the small-scale coexistence of typical island-arc basalts (IAB) and a distinct type of Nb-enriched basalts (NEB) characterized by primitive mantle-normalized trace element patterns without high-field-strength element (HFSE) depletion. The IAB show trace element patterns with prominent negative HFSE anomalies acquired during melting of mantle sources enriched with slab-derived, H2O-rich components during subduction. In contrast, the NEB display trace element features that compare favourably with enriched-mid-ocean ridge basalt (MORB) and the most enriched basalts from the Vanuatu back-arc troughs. Both their trace element and Nd–Sr isotopic compositions require partial melting of an enriched-MORB-type mantle source, almost negligibly contaminated by slab-derived fluids (~0.2 wt%). The coexistence of these two distinct types of primitive magma, at the scale of one volcanic island and within a relatively short span of time, would reflect a heterogeneous mantle source and/or tapping of distinct mantle sources. Direct ascent of such distinct magmas could be favoured by the extensive tectonic setting of Mota Lava Island, allowing decompression melting and sampling of variable mantle sources. Significantly, this island is located at the junction of the N–S back-arc troughs and the E–W Hazel Home extensional zone, where the plate motion diverges in both direction and rate. More broadly, this study indicates that crustal faulting in arc contexts would permit basaltic magmas to reach Earth’s surface, while preserving the geochemical heterogeneity of their mantle sources.

Dunites from Isua, Greenland: A ca. 3720 Ma window into subcrustal metasomatism of depleted mantle

AbstractThe chemistry of surviving pieces of Eoarchaean mantle together with related crust helps us determine early crust-forming mechanisms. Two lenses of high-Mg, low-Al dunite within a ca. 3720 Ma part of the Isua supracrustal belt in Greenland are interpreted as relicts of Eoarchaean mantle with minimal crustal disturbance. The lenses are within altered, higher Al, Ca ultramafic schists and are intercalated with amphibolitized pillow basalts and gabbros with island arc chemical signatures, all intruded by 3715–3710 Ma tonalites. One variety of dunite is dominated by forsterite (Fo90–92) olivine with accessory chromite and rare clinopyroxene, which does not show high field strength element (HFSE) anomalies. Another variety contains olivine (Fo96–98), some intergrown with Ti-humite group minerals with strong positive HFSE anomalies that are complementary to the negative HFSE anomalies of the adjacent amphibolites. We propose that these dunites are tectonic slivers of ca. 3720 Ma subarc mantle that preserve evidence for varying interaction with mafic magmas in a ≥850 °C, 1.7–2.0 GPa subcrustal environment. These are by far the oldest direct geochemical link between coeval mantle and crustal rocks, and are new evidence for subduction zone–like environments on the early Earth.

Mantle amphibole trace-element and isotopic signatures trace multiple metasomatic episodes in lithospheric mantle, western Victoria, Australia

Peridotite xenoliths from western Victoria, Australia, contain varying proportions of amphibole (±apatite). The xenoliths also exhibit varying degrees of metasomatic enrichment as indicated by the rare-earth element (REE) patterns of clinopyroxene and amphibole analysed in situ by laser-ablation microprobe ICP–MS and by whole-rock trace-element patterns. Trace-element patterns allow the samples to be divided into three groups. Group A is characterised by flat REE patterns for whole-rock, clinopyroxene and amphibole, and small negative or absent high-field-strength element (HFSE) anomalies; Group B features high levels of light-REE enrichment, pronounced negative HFSE anomalies and low Ti/Eu in whole-rock trace-element patterns; and Group C shows positive HFSE anomalies in whole-rock patterns. The different trace-element patterns are consistent with metasomatism by different agents or combinations of agents, which have acted upon the already modally metasomatised sub-continental lithospheric mantle (SCLM) beneath western Victoria. Geochemical signatures suggest that Group A samples have undergone silicate melt metasomatism, Group B samples have undergone metasomatism by a carbonate-rich or carbonatitic fluid and Group C samples may record metasomatism by a fractionated silicate fluid with hydrous component.

What Is the "Calc-alkaline Rock Series"?

The "calc-alkaline" series of rocks was originally defined in the early 1930s on the basis of the "alkali-lime index" on a combined SiO2 versus (Na2O + K2O) and SiO2 versus CaO plot. The usage of the term has evolved considerably since, and today it is used variably for the subalkalic basaltandesite-dacite-rhyolite suite, or any rock suite containing andesite, or island-arc rocks, or rocks with high ratios of large-ion-lithophile elements (LILE) to high-field-strength elements (HFSE), or simply rocks with negative HFSE anomalies (e.g., Nb-Ta) in primitive mantle-normalized multielement diagrams. Although such variable usage is normal in science, the use of certain geochemical variation diagrams to define and depict the calc-alkaline series is not strictly appropriate. Two of these widely used diagrams are the total alkalies-silica (TAS) diagram and the (Na2O + K2O)-FeO*-MgO (AFM) triangular diagram, neither of which has calcium as one of the plotting parameters. The TAS diagram can be used to depict "alkaline," "subalkaline," and probably "transitional" rocks, but not "calc-alkaline" or "high-alumina" rocks. Care must be taken while using such diagrams for rock classification, inasmuch as some of them are inherently unsuitable and several together may classify even a single rock into widely different associations. "Association" or "suite" are probably better terms than "series," because they imply neither comagmatic relationships nor linear trends. The calc-alkaline suite of rocks is abundant along destructive plate margins, but calc-alkaline geochemistry is not a 100% foolproof indicator of subduction processes, inasmuch as calc-alkaline rocks are also known from regions undergoing extension, such as the Basin and Range province and the Gulf of California. Therefore, caution must be exercised in interpreting ancient terrains with complicated geology and calc-alkaline rocks as former subduction zones. Not all orogenic andesites are calc-alkaline, and not all calc-alkaline andesites are orogenic.

Hf-Nd Element and Isotope Perspective on the Nature and Provenance of Mantle and Subduction Components in Western Pacific Arc-Basin Systems

This paper develops methods for using the integrated study of Hf-Nd element and isotope covariations to define the nature and provenance of the mantle and subduction inputs to subduction systems. In particular, it can be demonstrated that (1) Hf-Nd isotope space permits discrimination between mantle of Pacific and Indian provenance, (2) displacements from mantle arrays on Hf-Nd isotope and trace element projections can be related to the magnitude, source and composition of the subduction input, and (3) Hf-Nd isotope and trace element covariations can be used to interpret high field strength element (HFSE) anomalies [specifically, Hf anomalies on extended rare earth element (REE) patterns] in subduction-related magmas. These methods are tested using published volcanic arc data coupled with new data from the many components of the Izu-Bonin-Mariana (IBM) subduction system, namely the pre-subduction marginal basins, the Eocene to Recent volcanic arcs, and the crust, volcanogenic sediments and pelagic sediments of the subducting Pacific plate. The results of the IBM study show that the mantle that fed the IBM system was always of Indian provenance and that Pacific volcanogenic sediments make the most significant, though variable, contribution to the subduction component. Modelling demonstrates that the Nd/Hf ratio of the subduction component probably lay between 40 and infinity and thus was probably the main cause of the negative HFSE anomalies that characterize much of the Recent arc. This result may further indicate that the subducting sediment lost elements to the mantle wedge mostly by dehydration rather than fusion. In contrast, the data also show that the positive Hf anomalies that characterize much of the Protoarc cannot be attributed directly to subduction. One option consistent with Hf-Nd systematics is that the positive Hf anomalies in the Protoarc boninites were caused by fusion of mafic veins in their shallow mantle sources. Comparison with published data on other arcs shows significant inter-arc variations. For example, the subduction components in near-continent arcs (Banda, Lesser Antilles) appear to have lower Nd/Hf ratios more consistent with sediment fusion, and at least one arc (Tonga-Fiji) carries evidence of temporal variations in mantle provenance.

Experimental determination of partition coefficients for rare earth and high-field-strength elements between clinopyroxene, garnet, and basaltic melt at high pressures

Clinopyroxene/melt and garnet/melt partition coefficients have been determined for Ti, Sr, Y, Zr, Nb, Hf, and rare earth elements from 19 doped experiments on 1921 Kilauea basalt. The experiments were carried out from 2.0 to 3.0 GPa and 1310° to 1470 °C. The purpose was to derive a set of partition coefficients for high-field-strength elements (HFSE) and rare earth elements (REE) in a systematic, linked set of experiments at P and T conditions relevant to basalt petrogenesis. These data are used in melting models to understand the development of negative HFSE anomalies observed in many abyssal peridotite clinopyroxenes. It is shown that melting can account for the observed trace element patterns in some residual peridotites, but that other processes may also be needed to account for most residual mantle compositions in mid-ocean ridge systems. It is also shown that REE are more strongly fractionated by garnet at these P-T conditions than previously thought.

Mineral-aqueous fluid partitioning of trace elements at 900–1200°C and 3.0–5.7 GPa: new experimental data for garnet, clinopyroxene, and rutile, and implications for mantle metasomatism

In order to constrain the role of fluid phases during metasomatic processes in the upper mantle, trace element partition coefficients for Ba, Sr, Pb, Nb, Ta, Zr, Hf, Ti, La, Ce, Sm, Tb, and Yb between aqueous fluids and eclogite assemblage minerals (garnet, clinopyroxene, and rutile) have been determined experimentally at 900–1200°C and 3.0–5.7 GPa. Using a new experimental technique in which diamond aggregates are added to the experimental capsule set-up, the fluid was separated from the solid residue so that both quenched solute and residual minerals could be analysed directly. Trace element concentrations were determined in situ by laser ablation microprobe (LAM). The partitioning behaviour is controlled by temperature, pressure, and crystal chemistry; whereas fluid composition is not as crucial. Neither addition of hydrochloric acid nor high silica concentrations in the fluid have strong effects on trace element partitioning. Results indicate that in the presence of garnet or clinopyroxene, Nb and Ta are highly soluble in aqueous fluids, whereas Zr and Hf show variable solubilities. Low field strength elements (LFSE) and light rare earth elements (LREE) are always enriched in the fluid (D (fluid/Min) > 1). Generally, D (fluid/Cpx) is positively correlated with temperature only for high field strength elements (HFSE), but positively correlated with pressure for all other elements. Therefore, the lowest Nb/La is achieved at high pressures and low temperatures. However, even the highest pressures and lowest temperatures examined did not exhibit strong negative HFSE anomalies in the fluid. Garnet retains compatible trace elements at 3 GPa and 1000°C much more effectively (D (fluid/gt)Yb= 0.002) than at 5.7 GPa at the same temperature (D (fluid/gt) Yb = 0.04). Decreasing temperature results in a lowered D (fluid/gt) , particularly for Zr, Hf, and heavy rare earth elements (HREE). At 5 GPa and 900°C a strong intra-REE fractionation is observed (D (fluid/gt)Sm/Yb around 100) and significantly negative anomalies for Hf and Zr, but not for Nb and Ta, are developed. Only residual rutile fractionates all HFSE from all other trace elements. Tantalum and niobium are retained most effectively by rutile, as is the case for rutile/melt partitioning. Fluid/mineral trace element partitioning has important implications for mantle metasomatism in subarc regions. A model is proposed in which HFSE depletions, as observed in island arc volcanic rocks, could originate from a selective enrichment of the mantle wedge in LFSE and LREE by aqueous fluids derived from a rutile-bearing subducted slab. It is shown that melting of the enriched mantle wedge, which had previously been depleted by melt extraction (depleted MORB mantle) can produce magmas with trace element patterns similar to those of subduction-related volcanic rocks.