Mobility Of High Field Strength Elements Research Articles

Mobilization and fractionation of Ti-Nb-Ta at the giant Qulong porphyry Cu-Mo mineralization system, Xizang

High field strength elements (HFSE), especially Nb, Ta, and Ti, are commonly considered to be immobile during fluid-rock interaction in ore-forming systems. Hence the mechanisms behind their migration and fractionation associated with mineralization are not often studied. Here, we present a research study of different generations of Nb-Ta-bearing hydrothermal rutile at the giant Qulong porphyry Cu-Mo deposits, Gangdese magmatic belt, Xizang. Based on mineral assemblages and elemental compositions, two distinct groups of rutile can be identified: (1) coarse-grained Rt-1 is found in quartz-biotite-anhydrite veins. It is characterized by high Nb, Fe, Cr, and W contents and formed from the mobilization of HFSE in a relatively oxidized, metal-rich, hypersaline brine during the early high-temperature potassic alteration stage. These rutile grains exhibit suprachondritic Nb/Ta ratios from 20 to 80, that are higher than those of primitive mantle. This may be attributed to the high solubility of HFSE in hypersaline brines, which favors a fractionation of Nb over Ta. (2) Abundant, disseminated, blocky Rt-2 is hosted in monzogranite and monzonitic porphyry, where it forms intergrowths with sericite, chlorite, pyrite, and chalcopyrite. It has relatively low Nb/Ta ratios and formed by alteration of precursor Ti minerals, such as biotite and titanite, during sericite and chlorite alteration stages. This indicates that this generation of rutile was precipitated, together with the ore-forming elements, from moderate-temperature, aqueous fluids. The Nb/Ta variations between the two generations of rutile, reflect the evolution of the mineralizing fluids from an early relatively oxidized, halogen-alkaline-rich composition to late less saline, moderate-temperature fluids. Both Rt-1 and Rt-2 crystallized in these different fluid settings yield 206Pb/238U ages of 16.50 ± 0.32 Ma to 14.70 ± 0.30 Ma, that record the timing of rutile growth and reflect the extended duration of HFSE mobility and final Cu-Mo mineralization. These results suggest that the mobilization and fractionation of HFSE is more common during mineralization in magmatic-hydrothermal metallogenetic systems than often observed, and may thus help elucidate the fluid evolution of magmatic-hydrothermal metallogenetic systems and aid mineral exploration.

Zircon solubility and zirconium complexation in H2O+Na2O+SiO2±Al2O3 fluids at high pressure and temperature

Zircon is an important host mineral for many high-field strength elements (HFSE), particularly Zr and Hf. Thus, its solubility in geologic fluids at high pressure and temperature plays an important role in terrestrial cycling of these elements during processes in the Earth's crust and mantle. We performed in-situ high-pressure, high-temperature measurements of zircon solubility in H2O–Na2Si3O7, H2O–Na2Si3O7+Al2O3, H2O–Na2Si2O5, H2O–NaAlSi3O8 fluids, as well as of baddeleyite solubility in H2O–NaOH fluids, by in-situ synchrotron radiation X-ray fluorescence analysis using hydrothermal diamond-anvil cells. Zirconium complexation in fluids in equilibrium with zircon was constrained by in-situ X-ray absorption near-edge structure (XANES) spectroscopy. Zircon solubility is strongly enhanced by addition of Na2Si3O7 to H2O. The degree of enhancement increases with Na2Si3O7 concentration. The Zr content of fluids containing 10wt% Na2Si3O7 reached up to 86±2ppm Zr at 550°C and 400MPa. At 30wt% Na2Si3O7, the maximum Zr concentration was 997±6ppm at 600°C and 440MPa. Zircon solubility in these fluids decreases considerably with pressure and increases slightly with temperature. Addition of Al2O3 decreases the zircon solubility. In H2O–NaAlSi3O8 fluids, the Zr concentrations are in the sub-ppm to ppm range. Zr concentrations in NaOH solutions in equilibrium with baddeleyite reached up to 390±2ppm at 600°C and 930MPa and increase with pressure and temperature. In-situ XANES spectra collected on Zr in H2O–Na2Si3O7, H2O–Na2Si3O7+Al2O3, H2O–NaOH, and H2O–HCl fluids in equilibrium with zircon provide evidence for strong differences in the Zr complexation between these fluids. Comparison of XANES spectra to those of model compounds and ab-initio simulation of XANES spectra revealed [8]Zr for the HCl solution, [7]Zr for the NaOH solution, and [6]Zr for the Na–Al-silicate-bearing solutions. For the latter solutions, formation of alkali zircono-silicate complexes is indicated by the strong dependence of zircon solubility on Na/Al and the similarity of the XANES spectra to spectra simulated based on the local structure around Zr in the alkali zircono-silicates vlasovite and catapleite. Alkali zircono-silicate complexes are responsible for the enhancement of Zr concentrations in Na–Al-silicate-bearing solutions and very likely play an important role for mobilization of HFSE during fluid–rock interaction. Because high alkali/Al can be expected in aqueous fluids at high pressure and temperature due to incongruent dissolution of feldspar and mica, the increase of zircon solubility along the NaAlSi3O8–Na2Si3O7 join points to potentially considerable Zr or HFSE transport by silicate-bearing aqueous fluids in the lower crust and upper mantle.

The solubility of fluorite in H 2O and H 2O–NaCl at high pressure and temperature

The solubility of fluorite in H 2O and H 2O–NaCl has been measured at 600–1000 °C, 0.5–2.0 GPa in a piston–cylinder apparatus. Fluorite dissolves congruently at all conditions investigated. The data indicate that the concentration of fluorite dissolved in H 2O is low at 600 °C and 0.5 GPa (0.0014 mol/kg H 2O) but increases strongly with temperature ( T) and pressure ( P) to 0.247 mol/kg H 2O at 1000 °C, 2 GPa. The data were fit with the equation log m CaF 2 = − 6.587 + 2.774 P 1/2 + (2.266 × 10 − 3 − 4.699 × 10 − 4 P) T, where m CaF 2 is molality of CaF 2, T is in Kelvin, and P is in GPa. Fluorite solubility was also measured in H 2O–NaCl fluids at 800 °C and 1.0 GPa. Results indicate a strong increase in dissolved fluorite concentration with increasing NaCl content, from 0.028 mol/kg H 2O at X NaCl = 0 to 1.673 mol/kg H 2O at X NaCl = 0.486, where X NaCl is the mole fraction of NaCl in the fluid. The data were fit to m CaF 2 = 0.01035 + 1.141 X NaCl + 4.694 X NaCl 2. The results indicate that F mobility will be further enhanced by saline brines in high P–T igneous and metamorphic environments. High F solubility in H 2O and H 2O–NaCl in the deep crust and upper mantle can promote mobilization of high field-strength elements, rare earth elements, U, and Th through transport as F-bearing complexes.

Mobility of high field strength elements (HFSE) in magmatic-, metamorphic-, and submarine-hydrothermal systems

High field strength elements (HFSE) are normally considered as ‘conservative’ elements in most geological settings. However, field and experimental evidence has suggested that these elements are actually mobile in some cases and can be transported by magmatic-, metamorphic-, and submarine-hydrothermal solutions, in particular in those extreme cases such as highly altered hydrothermal ore deposits and highly evolved products of crystal fractionation such as pegmatites or granitic bodies bordering to metamorphic rocks. In this paper, we combine our own data with those from the literature and focus our discussion on HFSE mobility at crustal fluid systems. We suggest that the mobility of HFSE is controlled by a number of factors, including P– T conditions, pH, and solution chemistry. The HFSE mobility can occur in a wide range of P– T conditions, and their solubility is generally enhanced at high pH conditions. The high F contents in the fluids also largely enhance the mobility of HFSE. The mobility of HFSE and formation of hydrothermal minerals such as zircon and titanite offer us a useful tool to directly date related hydrothermal events by U–Pb methods, which otherwise lack suitable minerals for isotope dating.

Behaviour of high field strength elements in subduction zones: constraints from Kamchatka–Aleutian arc lavas

Models explaining the characteristic depletion of High Field Strength Elements (HFSE) relative to elements of similar compatibility in subduction zone magmas invoke either (1) the presence of HFSE-rich minerals in the subduction regime or (2) a selectively lower mobility of HFSE during subduction metasomatism of the mantle. In order to investigate the properties of HFSE in subduction regimes closer, we performed high precision measurements of Nb/Ta, Zr/Hf, and Lu/Hf ratios together with 176Hf/ 177Hf analyses on arc rocks from Kamchatka and the western Aleutians. The volcanic rocks of the Kamchatka region comprise compositional end members for both fluid and slab melt controlled mantle regimes, thus enabling systematic studies on the HFSE mobility at different conditions in the subarc mantle. Hf–Nd isotope and systematic Zr/Hf and Lu/Hf covariations illustrate that Zr–Hf and Lu are immobile in fluid-dominated regimes. Hf–Nd isotope compositions furthermore indicate the presence of “Indian” type depleted mantle beneath Kamchatka, as previously shown for the Mariana and Izu–Bonin arcs. In addition to a depleted mantle component, Hf–Nd isotope compositions enable identification of a more enriched mantle wedge component in the back-arc (Sredinny Ridge) that most likely consists of mantle lithosphere. The ratios of Nb/Ta and Zr/Hf are decoupled in rocks from fluid-dominated sources, indicating that Nb and Ta can be enriched in the mantle by subduction fluids to a small extent. In contrast to the fluid-dominated regime in Central Kamchatka, the budget of HFSE and Lu in rocks from the Northern Kamchatka Depression and in adakitic rocks from the western Aleutians is significantly affected by slab melts that originate from subducted oceanic crust. Compositions of the rocks with the highest slab melt components in their source (Sr/Y>30) provide no evidence that either Nb/Ta or Zr/Hf ratios are fractionated at a globally significant scale during melting of subducted oceanic crust. Subduction processes are therefore an unlikely candidate to explain the terrestrial Nb–Ta paradox (i.e., the subchondritic Nb/Ta ratios in all accessible terrestrial silicate reservoirs).

HFSE Mobility at the Slab-Wedge Interface: Hafnium-Isotope Evidence from the East Sunda-Banda Arc

Models describing trace-element behaviour in the petrogenesis of arc volcanics generally assume that high-field-strength elements (HFSE) are relatively immobile during element transfer between the subducted slab and the overlying mantle wedge. However, recent studies in the East Sunda-Banda Arcs (Vroon et al., t993; Hoogewerff et al., 1997) modify this view, and suggest that conditions in subduction settings can be favourable for the mobility of a wide range of slab-derived elements, including the HFSE-group. Sr-Nd-Pb isotopic signals in the ESA-BA are strongly dominated by subducted sediments of continental origin (SCM). Correlations between radiogenic isotopes and HFSE ratios provide indications for (1) an SCM-derived contribution to the HFSE budgets, and (2) increased mobility of HFSE down the dip of the subduction zone as a result of changes of the transfer medium from hydrous fluid to siliceous melt. Here, we present Hf isotope data for volcanic centres distributed along and across the ESA-BA, and use the results to trace a slab-derived origin of HFSE in the magmas.

Geochemistry and origin of cordierite-orthoamphibole gneiss and associated rocks at an Archaean volcanogenic massive sulphide camp: Manitouwadge, Ontario, Canada

Cordierite-orthoamphibole gneiss, amphibolites, quartz-biotite schist and calc-silicate rocks occur in spatial association with Archaean metamorphosed volcanogenic massive sulphide (VMS) deposits at Manitouwadge, Ontario. Trace-element characteristics, particularly those of the high field strength elements (HFSE), support field relationships that most of the cordierite-orthoamphibole gneisses represent metamorphosed equivalents of chlorite-rich rocks that were derived hydrothermally from tholeiitic basalts as an integral part of the syngenetic ore-forming processes on the Archaean seafloor. However, a garnet-rich cordierite-orthoamphibole dike, that is characterized by low TiO 2 content and high concentrations of Zr, Y, Nb and REE, and has single-zircon Pb-evaporation 207 Pb 206 Pb ages of 2715–2743 Ma, may have been derived from a synvolcanic felsic dike. Evidently, the seafloor hydrothermal alteration at Manitouwadge resulted in similar alteration products from contrasting protoliths and, therefore, was a fluid-dominant system. The amphibolite lenses within the cordierite-orthoamphibole gneiss also have been affected by the seafloor hydrothermal alteration, and some of them most likely represent metamorphosed equivalents of calcsilicate alteration products formed on the seafloor. Mass-balance calculations indicate that HFSE and REE were mobilized during the seafloor hydrothermal alteration. The mobility of HFSE and REE was controlled by both their host minerals and the hydrothermal fluids. In particular, a significant portion of igneous zircon in the garnet-rich dike has survived the seafloor hydrothermal alteration and the subsequent high-grade regional metamorphism. The regional metamorphism did not change the bulk-rock REE characteristics but was important in redefining the mineralogy of REE. Trace-element geochemistry of mafic metavolcanic rocks at Manitouwadge are similar to those of the adjacent Schreiber-Hemlo greenstone belt and are consistent with a volcanic island arc setting. However, further work is required to determine whether the Manitouwadge and adjacent greenstone belts of the Wawa Subprovince were originally a larger, continuous terrane.

Contrasting secondary mobility of Ti, P, Zr, Nb, and Y in two metabasaltic suites in the Appalachians

Substantial mobility of high-field-strength (HFS) elements (Ti, Y, P, Zr, and Nb) during greenschist facies metamorphism of two metabasaltic suites from the Appalachians, the Ascot Formation of southeastern Quebec and the Arbuckle Brook Formation of Nova Scotia, is attributed to the presence of CO2 in the fluid phase during metamorphism. The style of the mobility exhibited by these two suites is different. In the Ascot Formation, metasomatism affected rocks rich and poor in carbonates; it led to enrichment in some HFS elements and depletion in others. In contrast, in the Arbuckle Brook Formation there was depletion in CO2-rich rocks of all HFS elements and consequently much less fractionation between HFS elements.These differences may be explained by the relative effectiveness of [Formula: see text] buffering in the two suites. In the Ascot Formation, buffering was ineffective, and several different mineral assemblages were developed, reflecting a range in [Formula: see text]. Mobility of HFS elements was controlled by chemical potential differences between these mineral assemblages. In the Arbuckle Brook Formation, [Formula: see text], was externally buffered, and all rocks have the same mineral assemblage, varying only in modal calcite. Mobility of the HFS elements was controlled by the partitioning of these elements between the mineral assemblage and the fluid.Depletion of HFS elements in the Arbuckle Brook Formation is well correlated with the amount of calcite in the rocks. The systematics of the depletion allow an estimate of the relative proportions of HFS elements removed in the fluid. They are different from those in the rocks and lead to changes in the interelement ratios with progressive carbonation. However, low-carbonate rocks provide reasonably reliable estimates of primary concentrations. In certain circumstances, low-carbonate rocks (i.e., rocks that appear to be relatively fresh) do not provide reliable estimates of primary concentrations. A careful examination of the systematics of carbonation is required in metabasaltic suites for which HFS-element concentrations are the major indicator of magmatic and (or) tectonic affinity.