High Field Strength Elements Minerals Research Articles

HFSE and REE mineralization at Machinga prospect in the Chilwa alkaline province, southern Malawi

Peralkaline rocks from Machinga prospect, situated at the north-eastern margin of Zomba-Malosa syeno-granitic pluton, were investigated to characterize high field strength elements (HFSE) and rare earth elements (REE) mineralization and elucidate the genesis of HFSE- and REE-bearing rocks based on mineralogical and geochemical methods. The investigation utilized two sets of samples: (1) quartz syenite, granite and granitic pegmatite and (2) a second set that includes eudialyte-bearing granite and pegmatite.A range of REE and HFSE minerals was observed in the rocks including monazite, bastnäsite, synchysite, pyrochlore, zircon and eudialyte. Pseudomorphs after eudialyte, mainly comprising zircon and quartz, occur abundantly in the pegmatite and to a lesser extent in alkaline granite. HFSE and REE are variably distributed in the three rocks types (quartz syenite, granite and granitic pegmatite). However, the most elevated concentration of total REE (between 1.0 wt % and 5.2 wt %) corresponds to highest Nb + Ta + Zr concentration (between 1.5 wt % and 7.5 wt %). The elevated concentration largely occurs in metasomatized granitic pegmatite and metasomatized quartz syenite (?) (intruded by granitic pegmatite in parts) but also corresponds to relatively fresh eudialyte bearing alkaline granite.Using geochemistryand field mapping results; the study demonstrates that the rocks comprising the Machinga prospect are a product of extended fractionation of silica saturated alkaline magma. As the magma fractionation proceeded, alkaline granite was emplaced adjacent to the quartz syenite as an outer ring. Later associated granitic pegmatite lenses crystalized as final stage of emplacement of the pluton.Based on petrography and mineral chemistry, three mineral paragenetic stages associated with HFSE and REE minerals were distinguished. The first magmatic stage crystallized zircon and other igneous minerals. The second stage is characterized by precipitation of eudialyte, and the third by decomposition and replacement of eudialyte by zircon and quartz and other secondary HFSE and REE repositories. These last two stages occurred during late magmatic to early metasomatic-hydrothermal activities, which were closely associated and likely occurred concurrently.

Metasomatic High Field Strength Element, Tin, and Base Metal Enrichment Processes in Lithium Pegmatites from Southeast Ireland

Abstract Compared to average crustal abundances, high field strength elements (HFSEs) including Zr, Nb, Hf, Ta, and U are commonly enriched in rare element pegmatites. Albite-spodumene pegmatites may show economic grades of these elements, along with Sn, primarily in oxide minerals. Processes leading to enrichment and precipitation of HFSEs in these rocks are not well understood. Here, we characterize the textures and geochemistry of minerals of HFSEs, tin, and base metals in the Leinster albite-spodumene pegmatites. We use these data to infer processes for enrichment and precipitation of these metals during pegmatite crystallization, especially subsolidus processes. The Leinster albite-spodumene pegmatites are located within the East Carlow deformation zone on the eastern flank of the Caledonian S-type Leinster batholith, southeast Ireland. The final crystallization stages of these pegmatites are characterized by autometasomatism and hydrothermal overprint leading to in situ greisenization and precipitation of massive, commonly replacive, albitites. Cassiterite and HFSE minerals (columbite-tantalite and zircon) crystallized predominantly during these late stages. Crystals of HFSE minerals that precipitated during the early magmatic stages commonly exhibit evidence of resorption and additional growth during later stages. Others, such as microlite and uraninite, only crystallized during metasomatism or from hydrothermal fluids. Base metal sulfides are among the last precipitates from these fluids. We present a detailed paragenetic sequence for the Leinster albite-spodumene pegmatites and show that late-stage aqueous fluids transported HFSEs, especially after all the melt had crystallized. Tantalum enrichment seems to have been controlled by processes affecting the entire crystallizing medium, as opposed to fractional crystallization of columbite-tantalite. The textures and parageneses described in the present and our previous work are well explained by element partitioning between coexisting liquids with characteristics similar to those described in published melt-melt-fluid immiscibility models for rare element pegmatites but do not exclude other models for early-stage pegmatite evolution. The chemical and textural features of columbite-tantalite and cassiterite in the Leinster albite-spodumene pegmatites are seen in similar rare element pegmatites and rare metal granites elsewhere, suggesting wide applicability of the processes interpreted for Leinster. Late-stage processes of the type that affected the lithium pegmatites at Leinster may either enhance or reduce economic potential: ore metal tenor may be increased because late-stage columbite-tantalite is generally richer in Ta, and/or ore metals may be lost from pegmatites to country rocks. Lithium pegmatites, including the ones at Leinster, are commonly associated spatially with Sn-W veins and greisens and share some geochemical and textural features, such as evidence of widespread albitization. We propose that lithium pegmatites are transitional products regarding the interrelated dimensions time, temperature, and depth in S-type granite-related Li-Sn-W mineralizing systems.

An experimental approach to examine fluid-melt interaction and mineralization in rare-metal pegmatites

AbstractNiobium and tantalum, rare metals and high field strength elements (HFSEs) that are essential to modern technologies, are concentrated among others in lithium-cesium-tantalum (LCT) pegmatites and rare metal granites. The most important hosts for Nb-Ta in these types of deposits are the columbite group minerals (columbite-tantalite), but at some ore deposits significant Ta is also contained in wodginite, microlite, and tapiolite. Previous solubility experiments of HFSE minerals have been limited to high temperatures because of the slow diffusivities of HFSEs in granitic melts. An experiment protocol is described herein that allows HFSE mineral solubilities to be determined at lower temperatures, more in line with the estimated solidus temperatures of LCT pegmatites and rare metal granites. This is achieved through the interaction of a melt that is enriched in high field strength elements (e.g., P and Nb or Ta) with a fluid enriched in a fluid-mobile element (FME, e.g., Mn). A starting glass enriched in a slow diffusing HFSE was synthesized, and HFSE mineral saturation is obtained via the diffusion of a FME into the melt via interaction with a fluid. This interaction can occur at much lower temperatures in reasonable experimental durations than for experiments that require diffusion of niobium and tantalum. The solubility product of columbite-(Mn) from the fluid-melt interaction experiment in a highly fluxed granitic melt at 700 °C is the same as those from dissolution and crystallization (reversal) experiments at the same P-T conditions. Thus, both methods produce reliable measurements of mineral solubility, and the differences in the metal concentrations in the quenched melts indicates that the solubility of columbite-(Mn) follows Henry's Law. Results show that columbite-(Mn) saturation can be reached at geologically reasonable concentrations of niobium in melts and manganese in hydrothermal fluids. This experimental protocol also allows the investigation of HFSE mineral crystallization by fluid-melt interactions in rare-metal pegmatites. Magmatic origins for columbite group minerals are well constrained, but hydrothermal Nb-Ta mineralization has also been proposed for pegmatite-hosted deposits such as Tanco, Greenbushes, and granite-hosted deposits such as Cínovec/Zinnwald, Dajishan, and Yichun. This study shows that columbite-(Mn), lithiophilite, and a Ca-Ta oxide mineral (that is likely microlite) crystallized from experiments in fluid-melt systems at temperatures as low as 650 °C at 200 MPa. It is important to note that HFSE minerals that crystallize from fluid-melt interactions texturally occur as euhedral crystals as phenocrysts in glass, i.e., are purely magmatic textures. Therefore, crystallization of HFSE minerals from fluid-melt interactions in rare metal granites and pegmatite deposits may be more widespread than previously recognized. This is significant because the formation of these deposits may require magmatic-hydrothermal interaction to explain the textures present in deposits worldwide, rather than always being the result of a single melt or fluid phase.

Nb and REE Distribution in the Monte Verde Carbonatite–Alkaline–Agpaitic Complex (Angola)

The Angolan alkaline–carbonatite complex of Monte Verde has a semi-circular shape and is comprised of a central intrusion of foidolite rocks surrounded by concentrically arranged minor bodies of other alkaline rocks and carbonatite magmatic breccias. This rock association is hosted by fenitized Eburnean granites. Concentric swarms of alkaline dykes of late formation, mostly of nepheline trachyte composition, crosscut the previous units. Most high-field strength elements (HFSE) and rare earth elements (REE) are concentrated in pyrochlore crystals in the carbonatite and alkaline breccias. Magmatic fluornatropyrochlore is replaced and overgrown by five secondary generations of pyrochlore formed during subsolidus stages and have higher Th, REE, Si, U, Sr, Ba, Zr, and Ti contents. The second, third, and fourth pyrochlore generations are associated with late fluids also producing quartz and REE rich minerals; whereas fifth and sixth pyrochlore generations are linked to the fenitization process. On the other hand, minerals of the rinkite, rosenbuschite, wöhlerite, eudialyte groups, as well as loparite-(Ce), occur in accessory amounts in nepheline trachyte, recording low to moderate agpaicity. In addition, minor REE-bearing carbonates, silicates, and phosphates crystallize as late minor secondary minerals into carbonatite breccia and alkaline dykes. In conclusion, the scarcity of HFSE and REE minerals at the Monte Verde carbonatite-alkaline-agpaitic complex suggests low metallogenetic interest and economic potential for the outcrops analysed in this study. However, the potential for buried resources should not be neglected.

Partial melting, fractional crystallisation, liquid immiscibility and hydrothermal mobilisation – A ‘recipe’ for the formation of economic A-type granite-hosted HFSE deposits

A number of A-type granites host resources of the high field strength elements (HFSE), including the rare earth elements (REE), that have been considered for economic exploitation. The best known of them is the Strange Lake pluton (Canada) with a resource of 20 Mt grading 1.44 wt% REE2O3, of which ~50% are heavy rare-earth oxides. In this paper, we examine the various processes that are likely to be responsible for the hyper-enrichment of the HFSE, starting with the generation of a parental magma through a very low degree of partial melting of a mantle plume or a metasomatically enriched mantle, in both cases, below a continental crust (continental rift). We trace the evolution of this magma (olivine nephelinite) through fractional crystallisation of olivine and clinopyroxene followed by plagioclase, K-rich alkali feldspar and arfvedsonite to the final residues of hyper-enriched HFSE granitic pegmatites. In so doing, we show that at least two additional steps are essential to the process of HFSE enrichment. The first of these is the abrupt termination of plagioclase crystallisation prior to the final consumption of Ca or assimilation of a Ca source and the resulting exsolution of an immiscible Ca-fluoride melt that sequesters the LREE and Y. The second of these steps is prolonged crystallisation of arfvedsonite, which delays saturation of the magma with an aqueous phase and ensures the development of an agpaitic assemblage of Na-bearing HFSE minerals (e.g., Na-zirconosilicates and Na-titanosilicates). Finally, we show that saturation of the magma with an aqueous‑carbonic fluid, which evolves to a very acidic composition, is an essential ingredient in the ‘recipe’ for the formation of an economic A-type granite-hosted HFSE deposit.

Closed system fluid-mineral-mediated trace element behaviour in peralkaline rare metal pegmatites: Evidence from Strange Lake

Large peralkaline complexes are ‘factories’ that have produced a variety of ‘exotic’ minerals including high field strength element minerals. In most cases, these minerals are secondary and crystallise in a hydrothermal paragenesis that is extremely difficult to decipher due to the complexity of the textural relationships. The Strange Lake pluton is one of these complexes, and contains 37 exotic minerals, most of which are secondary. Adding to the difficulty in establishing a comprehensive paragenesis for these minerals and an alteration/precipitation path for the pluton is the fact that there were several stages of crystallisation of the same exotic and common secondary minerals, e.g., bastnäsite, fluocerite, gadolinite, aegirine, fluorite, and zircon.In this paper, we present a model, which describes a detailed path for the alteration and precipitation of minerals in the closed hydrothermal system of a peralkaline granitic pegmatite, based on direct measurements of the evolving composition of the aqueous fluid that exsolved from the late-stage magma crystallising rare-metal pegmatites in the Strange Lake pluton. The driving force for this evolution was cooling-induced oxidation that ultimately transformed the CH4-H2 gas in this fluid to CO2. This led to a large drop in the pH, which was a major control on the composition of the fluid and the crystallisation of secondary minerals.Although large numbers of minerals formed and were replaced during the different stages of fluid evolution, the changing chemistry of the fluid was largely a response to the alteration of four minerals, namely arfvedsonite, elpidite, narsarsukite and fluorite. The earliest stage of alteration, which took place at ~360 °C, was marked by the replacement of arfvedsonite by aegirine. This alteration decreased salinity and released K, Li, and Rb to the fluid, causing K-metasomatism. At ~300 °C, CH4 and higher hydrocarbons reacted to produce CO2. This caused a massive drop in pH from a value > 10 to a value of ~3 and intense alteration, which included the dissolution of fluorite, the breakdown of elpidite to zircon and quartz and the replacement of narsarsukite by titanite. With ongoing dissolution of fluorite, Ca activity reached a level sufficient to promote the alteration of elpidite to armstrongite or gittinsite. This was accompanied by alteration of arfvedsonite to ferroceladonite and microcline to Al-phyllosilicates, enriching the fluid in Na, Fe and F. Soon after, there was a near total loss of CO2 (at ~230 °C). This loss was catastrophic and was focused along conical fractures (these developed as a result of the collapse of the roof of the pluton), with resultant fragmentation of the rocks along the fluid path. Alteration to phyllosilicates continued after the loss of CO2, as the system cooled to ~190 °C. This marked the beginning of the final stage of alteration, which involved the replacement of arfvedsonite by aegirine and hematite. It also coincided with large scale hematisation within the pluton. Finally, it led to the cementation of the fragments along the fluid path to form the fluorite-hematite ring breccia that is now evident at the margins of the pluton.The model of fluid evolution presented here is potentially applicable to many other peralkaline complexes. The only requirements are that the system was closed until a relatively late stage and that the exsolved fluid was saline and contained a reduced carbonic component. This is a feature of many peralkaline complexes, most notably, the Khibiny and Lovozero complexes in Russia, and Ilímaussaq in Greenland.

Magmatic evolution and controls on rare metal-enrichment of the Strange Lake A-type peralkaline granitic pluton, Québec-Labrador

Although it is well known that A-type granites are enriched in the rare earth elements (REE) and other high field strength elements (HFSE), the magmatic processes that concentrate these elements are still poorly understood. The 1.24 Ga Strange Lake pluton in northern Québec-Labrador provides an extraordinary example of hyper-enrichment in the REE, Zr, and Nb in a peralkaline A-type granite. The pluton consists of two hypersolvus granite units (southern and northern) and a transsolvus granite, all of which contain perthitic alkali feldspar as the earliest major mineral; the transsolvus granite also contains separate albite and microcline crystals. Arfvedsonite, a sodic amphibole, occurs exclusively as phenocrysts in the transsolvus granite, whereas in the hypersolvus granite it is present as a late, interstitial phase. The primary HFSE minerals are zircon, monazite-(Ce), gagarinite-(Ce) and the pyrochlore group minerals.Magma evolution was monitored by the alumina content in the bulk rock, which decreases from the southern to the northern hypersolvus granite and is lowest in the transsolvus granite. Alkalinity indices and bulk Si, Fe, Rb, REE, Zr, Nb concentrations show the opposite trend. Alkali feldspar compositions mirror the trend shown by the bulk rock, i.e., decreasing Al contents are accompanied by increasing Si, Fe3+, REE, Zr and Nb contents.The major driving forces for the evolution of the hypersolvus magma prior to emplacement were the early separation of a fluoride melt from the silicate melt and the crystallization of alkali feldspar and HFSE-rich phases (zircon, monazite-(Ce), pyrochlore group). An alkali feldspar-rich crystal-mush containing LREE-fluoride melt droplets was emplaced as the least evolved southern hypersolvus granite. Massive fractionation of alkali feldspar led to a sharp increase in ƒH2O and F− activity in the magma chamber that triggered the crystallization of arfvedsonite and was followed by emplacement of the northern hypersolvus granite, which contained a higher proportion of LREE-fluoride melt droplets. Further evolution in the magma chamber led to a transition from a miaskitic to an agpaitic composition. The transsolvus granite was intruded in the form of a low viscosity crystal mush of alkali feldspar, quartz, arfvedsonite (after appreciable crystallization of arfvedsonite) and LREE-fluoride melt droplets. Upon emplacement, arfvedsonite (and gagarinite-(Ce)) crystals segregated as cumulates in response to a combination of flow differentiation and gravity settling. The immiscible fluoride melt accumulated in a volatile-rich residual silicate magma, which migrated to the top of the pluton where it formed the F-REE-rich cores of highly mineralized pegmatites.

REPLACEMENT PROCESSES INVOLVING HIGH FIELD STRENGTH ELEMENTS IN THE T ZONE, THOR LAKE RARE-METAL DEPOSIT

The Thor Lake rare-element (Y-REE-Nb-Ta-Zr-Be) deposit, Northwest Territories, Canada, is a world-class deposit hosted by alkaline syenite and granite and has two main mineralized zones, namely the Nechalacho and T Zones deposit. The T Zone is shown to have originally been a pegmatite, but has experienced a strong hydrothermal overprint. Coarse-grained to megacrystic, elongate crystals defining the pegmatitic texture are completely pseudomorphed and are interpreted to have originally been nepheline. The T Zone underwent three main alteration stages, which are (from earliest to latest): (I) silicification of the principal phases of the pegmatite and the associated replacement of igneous aegirine and arfvedsonite by quartz + magnetite + high field strength element (HFSE)-bearing minerals; (II) polylithionite [KLi 2 AlSi 4 O 10 (F,OH) 2 ] alteration (Li metasomatism) that caused pervasive replacement of albite by polylithionite; and (III) phenakite (Be 2 SiO 4 ) alteration (Be metasomatism). Replacement of the primary mineralogy resulted in the formation of abundant pseudomorphs that are characterized by various HFSE minerals [typically, bastnasite-(Ce), zircon, rutile, xenotime-(Y), and columbite] and phenakite (the predominant Be mineral). The habit and mineralogy of the pseudomorphs, as well as textural relationships in partially altered crystals, make aegirine and mica-group minerals the most likely precursors of the HFSE-rich and Be-rich pseudomorphs, respectively. Although aegirine and arfvedsonite from alkaline rocks can contain HFSE, LA-ICP-MS analysis of aegirine and arfvedsonite from the T Zone indicates that the HFSE concentrations in these two minerals are insufficient to account for the bulk HFSE concentrations of the HFSE-rich pseudomorphs. Therefore, HFSE must have been added to the pseudomorphs during fluid-mineral interaction, requiring hydrothermal mobility of HFSE. An open-space filling process may have been involved in the precipitation of HFSE minerals, including zircon, rutile, anatase, and REE fluorcarbonates. Initial replacement of aegirine by quartz + magnetite increased porosity in some of the T Zone rocks, which may have facilitated HFSE precipitation. Our study also demonstrates that most hydrothermal zircon shows no association with fluorite and that the fluorite in REE-rich pseudomorphs commonly postdates the REE fluorcarbonate minerals [ e.g. , bastnasite-(Ce)]. These relationships indicate that HFSE precipitation in the T Zone requires a model that does not rely on the mixing of HFSE-, F-bearing fluids with Ca-rich fluids, as has been proposed for other HFSE deposits hosted by alkaline igneous rocks. As the precipitation of HFSE appears to be restricted to pseudomorphs where aegirine and micas have been replaced, the degree of hydrothermal enrichment of HFSE in the T Zone, and possibly in other deposits containing such minerals, may be dictated by the primary abundance of these minerals.

Effects of fluorine on the solubilities of Nb, Ta, Zr and Hf minerals in highly fluxed water-saturated haplogranitic melts

The effect of fluorine on the solubilities of Mn-columbite (MnNb2O6), Mn-tantalite (MnTa2O6), zircon (ZrSiO4) and hafnon (HfSiO4) were determined in highly fluxed, water-saturated haplogranitic melts at 800 to 1000°C and 2kbar. The melt composition corresponds to the intersection of the granite minimum with the albite–orthoclase tieline (Ab72Or28) in the quartz–albite–orthoclase system (Q–Ab–Or), which is representative of a highly fluxed melt, from which high field strength element minerals may crystallize. The melt contains 1.7wt.% P2O5, 1.05wt.% Li2O and 1.83wt.% B2O3. The main purpose of this study is to examine the effect of F on columbite, tantalite, zircon and hafnon solubility for a melt with this composition. Up to 6wt.% fluorine was added as AgF in order to keep the aluminum saturation index (ASI, molar Al/[Na+K]) of the melt constant. In an additional experiment F was added as AlF3 to make a glass peraluminous. The nominal ASI of the melts are close to 1 for the minimum composition and approximately 1.32 in peraluminous glasses, but if Li is considered as an alkali, the molar ratio Al/[Na+K+Li] of the melts are alkaline (0.87) and subaluminous (1.09), respectively.The molar solubility products [MnO]∗[Nb2O5] and [MnO]∗[Ta2O5] are nearly independent of the F content of the melt, at approximately 18.19±1.2 and 43.65±2.5×10−4 (mol2/kg2), respectively for the minimum composition. By contrast, there is a positive dependence of zircon and hafnon solubilities on the fluorine content in the minimum composition, which increases from 2.03±0.03×10−4 (mol/kg) ZrO2 and 4.04±0.2×10−4 (mol/kg) HfO2 for melts with 0wt.%F to 3.81±0.3×10−4 (mol/kg) ZrO2 and 6.18±0.04×10−4 (mol/kg) HfO2 for melts with 8wt.%F. Comparison of the data from this work and previous studies indicates that ASI of the melt seems to have a stronger effect than the contents of fluxing elements in the melt and the overall conclusion is that fluorine is less important (relative to melt compositions) than previously thought for the control on the behavior of high field strength elements in highly evolved granitic melts. Moreover, this study confirms that although Nb, Ta, Zr and Hf are all high field strength elements, Nb–Ta and Zr–Hf are complexed differently in the melt.

Mineralogy of high-field-strength elements (Y, Nb, REE) in the world-class Vergenoeg fluorite deposit, South Africa

The Vergenoeg fluorite deposit in the Bushveld Complex in South Africa is hosted by a volcanic pipe-like body. The distribution characteristics, composition and formation conditions of high-field-strength element (HFSE)-rich minerals in different lithological units of the deposit were investigated by optical and cathodoluminescence microscopy, scanning electron microscopy, X-ray fluorescence, inductively-coupled plasma mass-spectrometry and electron-probe microanalysis. The Vergenoeg host rocks comprise a diverse silica-undersaturated assemblage of fayalite–magnetite–fluorite with variably subordinate apatite and mineral phases enriched in rare-earth elements (REEs). The Sm–Nd isotope systematics of the fluorite from the various lithological units of the pipe support the model that the HFSE budget of the Vergenoeg pipe was likely derived from a Lebowa-type granitic magma. Isotopically, there is no evidence for other REE sources. Formation of the pipe, including development of the fluorite mineralization, occurred within the same time frame as the emplacement of other magmatic rock units of the Bushveld Complex (Sm–Nd isochron age for fluorite separates: 2040±46Ma). Hydrothermal alteration is manifested in strongly disturbed Rb–Sr isotope systematics of the Vergenoeg deposit, but did not affect its HFSE and REE budget. Whole-rock chondrite-normalized REE+Y distribution patterns of two types were observed: (i) flat patterns characteristic of magnetite–fluorite unit, gossan, metallurgical-grade fluorite (“metspar”) plugs and siderite lenses, and (ii) U-shaped patterns showing enrichment towards the heaviest REE (Tm–Lu) observed in the fayalite-rich units. Common HFSE minerals are complex Nb-rich oxides (samarskite, fergusonite), REE phosphates and fluorocarbonates. Additionally, fluocerite and REE silicates, whose identification requires further work, were found. Most of the HFSE-rich minerals are spatially associated with Fe-rich phases (e.g., pyrite, magnetite, greenalite and hematite). To a smaller extent, they are found finely disseminated or healing micro-fractures in fluorite. The whole-rock REE+Y distribution patterns of the individual lithological units are mainly controlled by the distribution of Yb-rich and Y-rich xenotime in these rocks. The common occurrence of bastnäsite-(Ce) in the gossan, “metspar” plugs and especially in the rhyolitic carapace at the pipe–wall-rock contact, controls the REE+Y distribution patterns of these rocks. HFSE minerals in the Vergenoeg pipe rocks have formed in several stages. Samarskite and coarse fluorapatite belong to the primary mineral assemblage. Fergusonite and Yb-rich xenotime formed during high- to moderate-temperature hydrothermal activity. Significant remobilization of the HFSE from the early-crystallized minerals (breakdown of fluorapatite and possibly allanite with release of REE+Y) and subsequent partial redistribution of these elements into near surface rocks are inferred. The late-stage assemblages are characterized by the presence of fine-grained REE fluorocarbonates, monazite-(Ce), monazite-(La) and xenotime-(Y).

Fractionation of Nb and Ta from Zr and Hf at Mantle Depths: the Role of Titanian Pargasite and Kaersutite

Selective enrichment or depletion in either Zr and Hf (HFSE) INTRODUCTION or Nb and Ta (HFSE) is a feature commonly observed in many Because of their similar crystal-chemical properties (i.e. mantle-derived melts and amphiboles occurring as either disseminated high charge and ionic radius), high field strength elements minerals in mantle xenoliths and peridotite massifs or in vein (HFSE: Ti, Zr, Hf, Nb and Ta) are considered to behave assemblages cutting these rocks. The fractionation of Nb from Zr as a coherent group of substituents in rock-forming silicate seen in natural mantle amphiboles suggests that their incorporation minerals, and thus are not expected to fractionate greatly is governed by different crystal-chemical mechanisms. An extensive during partial melting and crystallization processes (Sun set of new partitioning experiments between pargasite–kaersutite and & McDonough, 1989). Nevertheless, fractionation of melt under upper-mantle conditions shows that HFSE incorporation HFSE from HFSE is a common feature in many and fractionation depends on amphibole major-element composition primitive mantle melts such as island-arc volcanics and and the presence or absence of dehydrogenation. Multiple regression intra-plate alkaline melts, suggesting either the presence analysis shows that DNb/Zr is strongly dependent on the mgof a solid phase with DNb,Ta significantly different from number of the amphibole as a result of a combination of amphibole DZr,Hf, or of two distinct phases controlling these element and melt structure effects, so that the following generalizations pairs. Experimental investigations of HFSE decoupling apply: (1) high-mg-number amphiboles crystallized from unmodified have been so far focused on Ti minerals (rutile and mantle melts more easily incorporate Zr relative to Nb leading to ilmenite), because of their high compatibility for HFSE an increase of the Nb/Zr ratio in the residual melt; (2) low-mgand their consequent capability of altering HFSE signumber amphiboles, such as those found in veins cutting peridotites, natures of equilibrium melts. Rutile has been shown to may strongly deplete the residual melt in Nb and cause very low have partition coefficients for Ta and Nb exceeding that Nb/Zr in residual melts. Implications and applications to mantle for Zr by one to two orders of magnitude, with Zr a environments are discussed. further one to two orders higher than other trace elements ( Jenner et al., 1993; Foley et al., 2000). Rutile could thus cause significant decoupling between Nb and Zr, and has the potential to produce the pattern typical of islandarc basalts (Foley & Wheller, 1990; Foley et al., 2000), in