Stephen F. Foley

Vein-plus-wall-rock melting mechanisms in the lithosphere and the origin of potassic alkaline magmas

Abstract A model is developed for the origin of ultrapotassic melts by melting of veined lithosphere; the veins are rich in clinopyroxene and mica, whereas the wall-rocks consist principally of peridotites. The veins originate by solidification of low-degree melts which are themselves the results of earlier, deeper, multistage processes ultimately due to the presence of a transition zone between large-scale channelled and porous flow regimes. The melting event producing the ultrapotassic magma begins in the veins due to the concentration of hydrous phases and incompatible elements, but spreads to include the surrounding wall-rocks by a combination of two mechanisms. The alkaline magma composition is thus a hybrid of vein (V) and wall-rock (W) components. The melt hybridization mechanisms are: (i) Solid-solution melting: Minerals which from extensive solid-solutions are abundant in the vein assemblages (Cr/Al spinel, F/OH mica, amphibole and apatite). The breakdown of these phases take place over a temperature range between the solidus of the vein assemblage and the elimination of the more refractory end-members. This process bridges the temperature gap between the solid of vein and wall-rock, so that a melt component from the wall-rock is added to that from the vein before elimination of all vein minerals. Phlogopite forms the most effective of these sliding reactions, resulting in its stability at near-liquids temperatures in experiments. (ii) Dissolution of wall-rock minerals: The initial melt fraction in the vein infiltrates the surrounding wall-rock due to the dominance of surface energy minimization on melt flow at the intergranular scale. Following infiltration, dissolution of wall-rock minerals occurs at temperatures lower than their melting temperatures, thus imparting a refractory wall-rock component to the melt composition. Dissolution of olivine and/or orthopyroxene occurs preferentially, since these minerals are furthest from equilibrium with the strongly alkaline, vein-derived melt. Remobilisation of several generations of veins explains the occurrence within a restricted space and time of rocks bearing chemical characteristics which are generally thought to indicate contrasting tectonic settings (e.g. central Italy). The ultrapotassic rocks are explained as being dominanyly vein-derived (i.e. high V/W ratio): further dilution of the V-component by wall-rock, supplemented by asthenospheric melt in advanced cases, leads to the production of more voluminous basaltic rocks bearing incompatible element signatures reminiscent of those of ultrapotassic rocks.

Rutile/melt partition coefficients for trace elements and an assessment of the influence of rutile on the trace element characteristics of subduction zone magmas

Fractionation of some or all of the high field strength elements (HFSE) Nb, Ta, Zr, Hf, and Ti relative to other trace elements occurs in igneous rocks from convergent margins and in the average continental crust, and is generally attributed to a process occurring during subduction. The experimental partitioning of an extensive array of trace elements between rutile/melt pairs is presented which enables the effect of rutile during melting in subduction zones to be directly assessed. DNb and DTa are in the range 100–500, DZr and DHf are about 5, whereas all other trace elements analyzed have Drutile/melt less than 0.1. Published D patterns for Nb and Ta between rutile and water-rich fluids are similar to those for melt, whereas the values for Zr and Hf are significantly higher. DNb and DTa values for clinopyroxene and garnet are much lower than for rutile, and cannot cause the fractionation of HFSE from other elements seen in island arcs. The presence of rutile in the subducted slab residue during dehydration may be essential in the production of the geochemical signatures of arc magmas, whereas that of the continental crust, including higher Zr/Sm, may be produced by melting of eclogite.

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). n nThe 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. n nFluid/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.

Petrological characterization of the source components of potassic magmas: geochemical and experimental constraints

Abstract The source mineralogy and conditions of origin of the three main groups of ultrapotassic rocks are outlined by combining experimental constraints and an abstraction of evidence from whole-rock chemistry (including volatiles), tectonic setting and xenolith contents. Lamproites originate from a depleted source rock which was strongly re-enriched at a later stage, thus producing mica-harzburgite. Melting conditions are H2O-rich and in most cases strongly reducing. Kamafugites originate from a clinopyroxene-reich source, also with abundant mica, in more oxidizing, CO2-rich conditions. Members of the third group form in a relatively fertile spinel-peridotite also containing abundant clinopyroxene and mica. Contrasting effects of variation in (i) pressure of melting and (ii) oxygen fugacity, emphasize the importance of these parameters in the sources of ultrapotassic rocks. Currently popular models for the origin of ultrapotassic melts by partial melting of phlogopite-bearing lherzolite are inconsistent with the now extensive array of liquidus experimental results on ultapotassic rock compositions. The discrepancy between partial melting models and liquidus results is attributed to the implicit, invalid assumption in the partial melting models that incompatible elements are homegeneously distributed on a large scale. Non-peridotitic assemblages rich in mica and pyroxenes which may be completely free of olivine must have an important role in the genesis of potassic rocks as spatially restricted components of inhomogeneous source regions.

Nb and Ta incorporation and fractionation in titanian pargasite and kaersutite: crystal–chemical constraints and implications for natural systems

New partition coefficients between liquid and pargasitic/kaersutitic amphiboles (Amph/LDNb,Ta) experimentally determined for Nb and Ta at upper-mantle conditions, combined with single-crystal structure refinement of the synthesised amphiboles, show that Amph/LDNb,Ta are strongly dependent on the structure and composition of both amphibole and melt. The correlation of the Amph/LDNb,Ta with the amphibole oxy-component is explained by the ordering of Nb and Ta at the M1 site, contributing with the fraction of Ti at M1 to locally balance the O3O2−↔O3(OH)− substitution. In our set of dehydrogenated amphiboles, variations in the SiO2 content of the melt from 41.5 to 54.6 correspond to a six-fold increase of the Amph/LDNb,Ta, in which Amph/LDNb varies from 0.14 to 0.71 and Amph/LDTa from 0.11 to 0.54. Partition coefficients for Nb and Ta abruptly increase in Ti-depleted compositions (Amph/LDNb up to 1.63 and Amph/LDTa to 1.00). The ratio of DNb to DTa (Amph/LDNb/Ta) varies from 0.71 to 1.63, and is a function of the M1 site dimension, which in turn depends on its Fe, Mg and Ti contents. The observed variations can be explained by assuming that the ionic radius of Nb is (∼0.01–0.02 A) larger than that of Ta, contrary to the common assumption that they are both equal to 0.64 A. We calibrated a simplified model for the prediction of Amph/LDNb/Ta values shown to be negatively related mainly to mg# [Mg/(Mg+Fe)] and to Ti content. High-mg# amphiboles have Amph/LDNb/Ta close to unity, so the low Nb/Ta found in convergent margin volcanics and in the continental crust cannot be explained by the involvement of amphibole in the mantle wedge. Amphibole in the subducting slab may have lower mg# and consequently high Nb/Ta values, and thus may give rise to subchondritic Nb/Ta values in coexisting melts. Nb/La is also negatively correlated with mg#, and strongly increases in Ti-depleted compositions.

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)

Abstract This paper reports the first trace element partition coefficients measured on experimentally produced products (clinopyroxene, garnet, rutile, and glass) by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS). A 266 nm (UV) laser microprobe was used to improve ablation characteristics and to achieve a fourfold reduction in ablation pit diameter compared to the previously used 1064 nm beam. Results are compared with PIXE analyses on the same experimental products, and literature values, where available, for similar systems, and include the first simultaneously measured partition coefficients for Zr, Nb, and Ta between rutile and glass. Advantages of the LAM technique include rapid results and simultaneous determination of a wide range of major and trace elements, thus ensuring sampling integrity through time-resolved analysis of the sampled material.

Trace element abundances in rutiles from eclogites and associated garnet mica schists

Abstract We present electron microprobe and laser ablation microprobe (LAM) data for a range of high field strength (Zr, Nb, Mo, Sn, Sb, Hf, Ta, W) and other trace elements (Al, Si, Ca, V, Cr, Mn, Fe, Pb, Th, U) in rutile from eclogites and garnet mica schists from Trescolmen, Central Alps. Most analysed rutiles are homogeneous (at least for Nb, Cr, W, Zr, V and Fe), both on a single grain scale and between grains from a single thin section. Concentrations of V, Zr, Nb, Sb and W determined by both electron and laser ablation microprobe techniques yield similar results and confirm the reliability of the analytical methods within estimated precision. Measurements of trace element contents of coexisting phases in eclogites and their modal abundances show that rutile is the dominant carrier (>90% of whole rock content) for Ti, Nb, Sb, Ta and W as well as an important carrier (5–45% of the whole rock content) for V, Cr, Mo and Sn. The crystallographic implications are that, for relatively rigid crystal sites such as in rutile, trace elements with a similar ionic radius are preferred over trace elements with the same charge but deviating size. Our results demonstrate the utility of rutile chemistry in the following applications: (1) By using a combination of the measured TiO2 content of the whole rock and the trace element concentration of rutile, precise whole rock data on elements that are either difficult to analyze by conventional techniques such as XRF or solution ICP-MS (Nb, Sb, Ta, W) or may be susceptible to late stage alteration (Sb) can be estimated. (2) Trace element contents of detrital rutile grains are a potentially powerful tool for sedimentary provenance studies since they reflect key element ratios (e.g., Nb/TiO2 and Cr/TiO2) of their source rocks. In addition, measurements of trace elements in detrital rutiles might help distinguish possible source rocks, e.g., high-grade metamorphic rocks such as eclogites and high-pressure granulites from hydrothermal ore deposits and kimberlites. In view of the dominance of rutile in the Sb budget of subducting oceanic crust, and the enrichment of Sb in the slab component of subduction zones, additional experimental studies on Sb-partitioning between rutile and fluid are needed in order to understand the behaviour of Sb in subduction zones.

Parallels in the origin of the geochemical signatures of island arc volcanics and continental potassic igneous rocks: The role of residual titanates

Abstract The extent of control of Ti, Nb and Ta concentrations by residual titanate minerals in island arc and subcontinental mantle enrichment processes is assessed with reference to recent reviews of the geochemistry of Indonesian Sunda island arc volcanics and of continental ultrapotassic rocks, and a reinterpretation of experimental results on TiO2 saturation levels in arc volcanic compositions. It is considered likely that the low-Ti-Nb signature of potassic eastern Sunda arc rocks originates in at least two distinct mantle sources, both of which contain residual titanates during partial melting. A low-Nb-Ta component, which is also characteristic of island arcs in which potassic magma types are absent, is probably due to melting of the rutile-bearing hybridized product of reaction between silicic partial melts of subducted oceanic crust and the peridotite mantle wedge immediately above the subduction zone. A low-Ti component containing moderate amounts of Nb is assigned to K-rich low-degree partial melts of peridotite which have solidified at shallower levels, creating a vein system with incompatible-element-enriched composition. The source of potassic arc volcanics is thus a composite of peridotite wedge plus hybridized mantle and K-rich veins. Five plausible models with two residual titanates are presented, in which the second residual titanate must be active either during melting of the peridotite wedge or K-rich veins, or during melting of peridotite at deeper levels producing the K-rich melt which later crystallised as veins. Titanate saturation in basic melt compositions is promoted by a combination of the effects of high pressure, low temperature assisted by high H2O content, high fO2 and high contents of incompatible elements. The effects of high pressures and high fO2 of reducing TiO2 contents of melts in equilibrium with titanate minerals have probably been underestimated in previous studies. Continental mantle enrichment events may be analogous to the K-rich sub-arc component due to comparably low heat flow enabling H2O assisted low-degree melting of mantle material in conditions which do not occur in other tectonic environments. Continental ultrapotassic rocks show a gradation of HFSE abundances which overlap with island arc potassic rock characteristics, implying a continuum of varying environments and physicochemical conditions such as H2O content, fO2 and source mineralogy. These variables may also determine which, if any, titanate mineral is present in individual sub-continental regions.

Trace element partition coefficients for clinopyroxene and phlogopite in an alkaline lamprophyre from Newfoundland by LAM-ICP-MS

Mineral/rock matrix partition coefficients have been determined for clinopyroxene (Cpx) and phlogopite from a Mesozoic alkaline lamprophyre from Newfoundland, Canada, by Laser Ablation Microprobe (LAM-ICP-MS). Values for twenty-one elements were obtained for Cpx, whereas only eleven were possible for phlogopite due to REE abundances below detection limits ( 1 ppm). Ablation pits of 40–50 μm diameter enabled investigation of zonation in phenocryst phases. In general, phenocrysts exhibit little trace element zonation except in the outermost overgrowths of Cpx. In these, a fourfold to fivefold increase in many trace element abundances correlates strongly with increasing Ti and Al contents, in agreement with recent experimental studies. Only Li shows appreciable zonation in phlogopite, being enriched in the rims. Comparison of the partition coefficients determined by in-situ laser analysis with those obtained from apparently pure mineral separates by solution ICP-MS indicates that, for several elements, considerable differences exist, e.g., DBa 0.0006 vs. 0.0255. These differences are attributable to the inclusion of trace element-rich overgrowths, and zones of trace element enrichment and micro-inclusions, in addition to the possibility of small amounts of matrix or glass (1–2%) in the nominally clean clinopyroxene mineral separates. n nPartition coefficients for Cpx are lower than most published values for basaltic rocks, but are comparable to experimental values for basalt determined by SIMS. Comparison with experimental values from lamproite discounts a strong bulk compositional effect, so that the discrepancy with earlier values for basalt should probably be attributed to problems with mineral separates, emphasizing the need for high quality partitioning determinations with in-situ microbeam methods. The phlogopite data extend considerably the published range; the new values are also generally lower than published values, although the discrepancy here may be due to bulk compositional effects, as many published values are from more silicic systems. The lamprophyre values are within the range of sparse experimentally determined values.

Experimentally determined partitioning of high field strength- and selected transition elements between spinel and basaltic melt

Partition coefficients for the elements Ta, Nb, Hf, Zr, Sc, V, Ga, Zn and Co have been determined by laser ablation ICP-MS and/or electron microprobe between spinel and melt using an alkali olivine basalt at 1 atm. The Dsplq for high field strength elements (HFSE) are uniform (DsplqNb = 0.08, DsplqTa = 0.06, DsplqHf = 0.05, DsplqZr = 0.06), negating the possibility of intra-HFSE fractionation during partial melting or fractional crystallization processes. Results for DsplqV continue an approximately linear trend of decreasing DsplqV with increasing fO2 from DsplqV = 68 at IW from previous studies to values of DsplqV = 0.09 at fO2 = air. DsplqSc is also fO2 dependent (0.24–0.56 with increasing fO2), whereas DsplqGa is constant at 3.2. A compositional dependence of partitioning behaviour was found for the Ti-poor solid-solution series between chromite-and magnetite-rich spinels in the log fO2 range from air to FMQ − 1 for Co, Zn and Sc. Zn and Co showed deviation from Henrys law behaviour. An approximate value for DsplqZn of 4.5 agrees well with the observed partitioning in natural peridotites, but is much larger at lower temperatures. Cobalt partitioning shows a strong negative correlation with temperature and is complicated by fO2 effects.