Rim Compositions Research Articles

Fluid-assisted zircon and monazite growth within a shear zone: a case study from Finnmark, Arctic Norway

The U–Pb ages, REE content, and oxygen isotopic composition of zircon rims developed within a major shear zone in the Kalak Nappe Complex (KNC), Arctic Norway have been determined along with the age of monazite crystals. Different generations of granitic veins have been distinguished based on both field criteria and monazite ages of 446 ± 3 and 424 ± 3 Ma. Within each of these veins, inherited zircon cores are mantled by homogeneous low CL-response zircon rims which yield a range of concordant U–Pb dates of ca. 470–360 Ma. Significant numbers of zircon rims coincide with the timing of monazite crystallization. The zircon rims have moderate light REE enrichment compared to cores, distinctive (Sm/La) n values of less than 12, and La between 0.3 and 10 ppm. This indicates free elemental exchange between newly formed zircon rims and the surrounding matrix. The rims have calculated accumulated alpha-radiation dosages corresponding with a crystalline structure and δ18O values of 1‰. This implies rim crystallization directly from a zirconium-saturated hydrothermal fluid which was modified by some silicate melt. Growth of the zircon rims was prolonged and locally variable due to preferential fluid flow. A third type of zircon can be recognized, forming both rims and cores, with high alpha-radiation doses, and significant enrichment in La, Pr, and Eu. These are interpreted as low-temperature hydrothermally altered metamict zircons. The high volatile input and partial melting in the shear zone favoured prolonged zircon rim growth due to its ability to easily nucleate on inherited seeds. On the other hand, monazite, susceptible to dissolution and re-growth, crystallized in brief episodes, as has been predicted from theoretical phase diagrams. From a regional perspective, these results elucidate cryptic Ar–Ar cooling ages, providing the first record of a Late Ordovician heating and cooling phase within the KNC prior to the climactic Scandian collision.

The magmatic plumbing system beneath El Hierro (Canary Islands): constraints from phenocrysts and naturally quenched basaltic glasses in submarine rocks

A thermobarometric and petrologic study of basanites erupted from young volcanic cones along the submarine portions of the three El Hierro rift zones (NE-Rift, NW-Rift and S-Ridge) has been performed to reconstruct magma plumbing and storage beneath the island. Mineral-melt thermobarometry applied to naturally quenched glass and clinopyroxene rims yields pressures ranging from 350 to 1070 MPa with about 80% of the calculated pressures being in the range of 600–800 MPa. This corresponds to a depth range of 19–26 km, implying that the main level of final crystal fractionation is within the uppermost mantle. No systematic dependence between sample locality and fractionation pressures could be observed. Olivine and clinopyroxene crystals in the rocks are complexly zoned and have, on an inter-sample as well as on an intra-sample scale, highly variable core and rim compositions. This can best be explained by mixing of multiply saturated (olivine, magnetite, clinopyroxene, ilmenite), moderately evolved magmas with more mafic magmas being either only saturated with olivine + spinel or with olivine + spinel + clinopyroxene. The inter-sample differences indicate derivation from small, isolated magma chambers which have undergone distinct fractionation and mixing histories. This is in contrast to oceanic intraplate volcanoes situated on plumes with high melt supply rates, e.g. Kilauea Volcano (Hawaii), where magma is mainly transported through a central conduit system and stored in a shallow magma chamber prior to injection into the rift zones. The plumbing system beneath El Hierro rather resembles the magma storage systems beneath, e.g. Madeira or La Palma, indicating that small, intermittent magma chambers might be a common feature of oceanic islands fed by plumes with relatively low fluxes, which results in only limited and periodic magma supply.

Multiple Plagioclase Crystal Populations Identified by Crystal Size Distribution and in situ Chemical Data: Implications for Timescales of Magma Chamber Processes Associated with the 1915 Eruption of Lassen Peak, CA

Products of the 1915 Lassen Peak eruption reveal evidence for a magma recharge–magma mixing event that may have catalyzed the eruption and from which four compositional members were identified: light dacite, black dacite, andesitic inclusion, and dark andesite. Crystal size distribution, textural, and in situ chemical (major and trace element and Sr isotope) data for plagioclase from these compositional products define three crystal populations that have distinct origins: phenocrysts (long axis > 0·5 mm) that typically have core An contents between 34 and 36 mol %, microphenocrysts (long axis between 0·1 and 0·5 mm) that have core An contents of 66–69, and microlites (long axis < 0·1 mm) with variable An core contents from 64 to 52. Phenocrysts are interpreted to form in an isolated dacitic magma chamber that experienced slow cooling. Based on textural, compositional, and isotopic data for the magma represented by the dacitic component, magma recharge was not an important process until just prior to the 1915 eruption. Average residence times for phenocrysts are in the range of centuries to millennia. Microphenocrysts formed in a hybrid layer that resulted from mixing between end-member reservoir dacite and recharge magma of basaltic andesite composition. High thermal contrast between the two end-member magmas led to relatively high degrees of undercooling, which resulted in faster crystal growth rates and acicular and swallowtail crystal habits. Some plagioclase phenocrysts from the dacitic chamber were incorporated into the hybrid layer and underwent dissolution–precipitation, seen in both crystal textures and rim compositions. Average microphenocryst residence times are of the order of months. Microlites may have formed in response to decompression and/or syn-eruptive degassing as magma ascended from the chamber through the volcanic conduit. Chemical distinctions in plagioclase microlite An contents reveal that melt of the dark andesite was more mafic than the melt of the other three compositions. We suggest that mixing of an intruding basaltic andesite and reservoir dacite before magma began ascending in the conduit allowed formation of a compositionally distinct microlite population. Melt in the other three products was more evolved because it had undergone differentiation during the months following initial mixing; as a consequence, melt and microlites among these three products have similar compositions. The results of this study highlight the integrated use of crystal size distribution, textural, and in situ chemical data in identifying distinct crystal populations and linking these populations to the thermal and chemical characteristics of complex magma bodies.

Tourmaline geochemistry and δ11B variations as a guide to fluid–rock interaction in the Habachtal emerald deposit, Tauern Window, Austria

Tourmalines from the Habachtal emerald deposit in the Eastern Alps formed together with emerald in a ductile shear zone during blackwall metasomatism between pelitic country rocks and a serpentinite body. Electron microprobe and secondary ion mass spectrometric (SIMS) analyses provide a record of chemical and B-isotope variations in tourmalines which represent an idealized profile from metapelites into the blackwall sequence of biotite and chlorite schists. Tourmaline is intermediate schorl-dravite in the country rock and become increasingly dravitic in the blackwall zones, while F and Cr contents increase and Al drops. Metasomatic tourmaline from blackwall zones is typically zoned optically and chemically, with rim compositions rich in Mg, Ti, Ca and F compared with the cores. The total range in δ11B values is −13.8 to −5.1‰ and the within-sample variations are typically 3–5‰. Both of these ranges are beyond the reach of closed-system fractionation at the estimated 500–550°C conditions of formation, and at least two boron components with contrasting isotopic composition are indicated. A key observation from tourmaline core analyses is a systematic shift in δ11B from the country rock (−14 to −10‰) to the inner blackwall zones (−9 to −5‰). We suggest that two separate fluids were channeled and partially mixed in the Habachtal shear zone during blackwall alteration and tourmaline-emerald mineralization. A regional metamorphic fluid carried isotopically light boron as observed in the metapelite country rocks. The other fluid is derived from the serpentinite association and has isotopically heavier boron typical for MORB or altered oceanic crust.

A thin-section scale original inhomogeneity of bulk rock chemistry inferred from compositional zoning of garnet in the Sambagawa metamorphic rocks, central Shikoku, Japan

The EPMA analysis of a pelitic sample from the Besshi district, central Shikoku, in the Sambagawa metamorphic belt reveals that two types of garnet porphyroblasts with different sizes and chemical compositions are present within a single thin section. One type consisting of medium-sized (d ∼ < 1 mm) grains has a high Mn content at the core and shows a pyrope-rich profile. The other type is comparatively larger in size (d ∼ 2 mm). The rim compositions of both types are different. The influence of the initial bulk rock chemistry on the garnet is calculated by the forward model. A comparison of the observations with the calculated results indicates that the difference between the two types of garnet grains is attributed to the initial Fe/Mg ratio of the local bulk rocks.

Geochemistry of mafic phenocrysts from alkaline lamprophyres of the Spanish Central System: implications on crystal fractionation, magma mixing and xenoliths entrapment within deep magma chambers

The Permian alkaline lamprophyres from the Spanish Central System (SCS) are highly porphyritic rocks which carry a heterogeneous population of clinopyroxene and kaersutite zoned phernocrysts. Clinopyroxene phenocrysts may show 1) normal zoning (Cpx-I), 2) reverse zoning with Fe-rich green cores (Cpx-II), and 3) reverse zoning with colourless Al-poor, silica-rich cores (Cpx-III). Kaersutite phenocrysts also show a slight reverse zoning. Major and trace element composition of Cpx-I suggests that their compositional variation is related to a crystal fractionation process from melts similar to the host lamprophyres. The Cpx-II cores represent crystallization from highly evolved melts (low Mg-Cr contents and incompatible element enrichment), genetically related with the SCS alkaline magmatism, and the growth or surrounding Mg-rich inner rims points to a magma mixing process. The major and trace element composition of Cpx-III cores supports derivation from a magma which has fractionated plagioclase. This characteristic, together with their similarities when compared to clinopyroxenes from charnockite xenoliths, suggests that they might be xenocrysts from deep calc-alkaline cumulates. The composition of melts in equilibrium with clinopyroxene and amphibole phenocrysts supports a model in which Cpx-II and Cpx-III cores would have been incorporated into a more primitive lamprophyric magma stagnated at lower crustal levels. The low pressure composition of all phenocryst outer rims indicates that they crystallised directly from the host alkaline magma at their subvolcanic emplacement levels.

Plagioclase in the Skaergaard intrusion. Part 1: Core and rim compositions in the layered series

The anorthite content of plagioclase grains (XAn) in 12 rocks from the layered series of the Skaergaard intrusion has been studied by electron microprobe (typically ∼30 core and ∼70 rim analyses per thin section). Mean core compositions vary continuously from An66 at the base of the layered series (LZa) to An32–30 at the top. On the other hand, crystal rims are of approximately constant composition (An50 ± 1) from the LZa to the lower Middle Zone (MZ). Above the MZ, core and rim compositions generally overlap. Profiles across individual plagioclase grains from the lower zone show that most crystals have an external zone buffered at XAn ∼50 ± 1. The simplest explanation for these features is that during postcumulus crystallization in the lower zone, interstitial liquids passed through a density maximum. This interpretation is consistent with proposed liquid lines of descent that predict silica enrichment of the liquid associated with the appearance of cumulus magnetite.

Composition and thermal evolution of the lithospheric mantle beneath the Harrat Uwayrid, eastern flank of the Red Sea rift (Saudi Arabia)

Harrat Uwayrid is one of several volcanic fields that developed along the western margin of the Arabian plate during opening of the Red Sea rift. Volcanic activity started in the late Miocene and Pliocene with the eruption of alkaline flood basalts. After a period of extensional faulting, numerous cinder cones formed in the central part of the volcanic field during the Quaternary. Xenoliths of both mantle and crustal origin are abundant. The mantle-derived suite is dominated by spinel lherzolite with X Mg of olivine and X Cr of spinel ranging from 0.868 to 0.911 and from 0.074 to 0.636, respectively. In addition to lherzolite, various types of pyroxenites and wehrlites occur. Most xenoliths contain primary and/or secondary amphibole (pargasite/kaersutite), indicating at least two enrichment events, both related to the formation of the Red Sea. Chemical zonation profiles of mineral grains constrain the thermal evolution of the xenoliths and their source regions in the shallow upper mantle. Ca contents in olivine are constant in the inner parts of the grains, but increase in the outermost zones (< 35 μm). X Mg values of spinel grains adjacent to olivine often show a rimward increase in their outer zones (< 150 μm). Both types of zonation patterns are interpreted to result from heating by the host magma during transport. Zonation patterns of Al and Ca/(1 − Na) in pyroxene grains record different thermal evolutions, such as (i) simple cooling, whereby a ‘near-equilibrium’ state was rarely achieved, (ii) cooling followed by heating, and (iii) simple heating. Diffusion modelling on measured zoning profiles suggests that cooling lasted for no more than ∼ 10 Myr. Temperature estimates for pyroxene core and rim compositions, calculated by using various pyroxene geothermometers range from ∼ 800 to 1140 °C (at an assumed pressure of 1.5 GPa). In conjunction with upper pressure limits derived from the composition of spinel, these values imply relatively high temperatures for the shallow upper mantle. This is corroborated by P– T estimates for spinel lherzolites obtained by combination of the two-pyroxene thermometer based on the solvus between enstatite and diopside and the Ca-in-olivine geothermobarometer. While the high- T state that preceded cooling was probably a regional phenomenon, renewed heating was of local nature, most probably related to advective heat transport by ascending and stagnating magmas.

Diffusion along interphase boundaries and its effect on retrograde zoning patterns of metamorphic minerals

In this study we use two dimensional chemical patterns and numerical modeling to estimate the relative rates of chemical transport along interphase boundaries (ib) and through grain (s) interiors during retrograde Fe–Mg exchange between garnet and biotite at a garnet–biotite–quartz triple junction. We demonstrate that systematic variations in garnet–rim compositions, which are frequently observed along garnet–quartz interfaces, and deviations from concentric retrograde zoning patterns start to develop when chemical transport along the interphase boundaries becomes slow during cooling. The capacities for chemical transport along an interphase boundary depend on the product D ib K ib/s a, where D ib is the diffusion coefficient of the exchangeable components within the interphase boundary medium, K ib/s is the equilibrium partitioning coefficient between the cation exchange partners and the interphase boundary medium and a is the interphase boundary width. The model is applied to analyze the retrograde zoning patterns in garnets from the Mozambique belt (SE-Kenya), which cooled from 820°C at a rate of ca. 2°C/my. It is found that non-equilibrated compositions in garnet along garnet/quartz interphase boundaries started to develop below 700°C due to insufficient rates of chemical transport along these boundaries. The transport capacities of garnet/quartz interphase boundaries was estimated to have been between about 1E-23 cm3/s (575°C) and 1E-20 cm3/s (700°C) from modeling the observed X Fe pattern in garnet close to a garnet–quartz–biotite triple junction and relying on published data on the diffusivity of Fe2+ in garnet. Similar transport capacities are obtained; when the interphase boundary is assumed to be filled with a material that has the transport properties and chemical composition of a free melt in equilibrium with garnet, biotite and quartz at the respective conditions. In contrast, if the transport properties of the interphase boundary medium are related to the diffusivities and solubility of Fe2+ and FeOH+ within a free aqueous solution, chemical transport along the interphase boundaries would be much more efficient, and exchange equilibrium would have been maintained during the entire cooling history of the rocks. The observation of systematic deviations from local equilibrium along the garnet–quartz interphase boundaries leads us to exclude the presence of an aqueous fluid along the interphase boundary at any time during cooling.

Effect of WC particle size on microstructure and rim composition in the Ti(C 0.7N 0.3)–WC–Ni system

Ti(C 0.7 N 0.3 )– x WC–20Ni systems were studied to investigate the effect of WC particle size on the microstructure and rim composition of the systems. Variations in the content and particle size of the added WC had a negligible influence on changes in the phase and microstructure of the Ti(C 0.7 N 0.3 )– x WC–20Ni system, sintered at 1510 °C for 1 h. Based on the rim composition in the Ti(C 0.7 N 0.3 )– x WC–20Ni system, the particle size of the added WC appears to play a role in determining the composition of the rims.

Investigating the origin of anorthitic plagioclase through a combination of experiments and natural observations

The origin of anorthitic plagioclase in volcanic rocks worldwide remains an enigma. Arenal volcano, Costa Rica, provides a good example of the problem: up to An 96 plagioclase are found within moderately evolved basaltic andesites. We follow up a recent suggestion [Lundstrom, C.C., Boudreau, A., Pertermann, M., 2005. Diffusion-reaction in a thermal gradient: Implications for the genesis of anorthitic plagioclase, high alumina basalt and igneous mineral layering. Earth Planet. Sci. Lett. 237, 829–854] that diffusion-reaction processes occurring between ascending magmas and gabbroic wall rocks might provide a means of producing anorthitic plagioclase without requiring exotic, ultra-calcic melt compositions by integrating observations from three analytical methods: (1) laboratory diffusion-reaction experiments in the diopside–albite–anorthite (DAA) system; (2) transmission electron microscopy (TEM) of plagioclase grains from both a diffusion-reaction experiment and an Arenal lava; and (3) Sr isotope microstratigraphy of phenocrysts and megacrysts from natural lavas from the current eruption at Arenal. Two plagioclase saturated DAA melts, juxtaposed in experiments at either 1 bar pressure (anhydrous) or at 0.5 GPa pressure (hydrous) for 3–5 days at 1244 or 1125 °C, illustrate the dramatic effect of H 2O on facilitating plagioclase reaction. In the presence of wet melt, enrichment in anorthitic plagioclase occurs at the material interface while in the presence of the dry 1 bar melt, plagioclase shows no reaction. TEM analyses of a homogeneous anorthitic plagioclase from a diffusion-reaction experiment indicates heterogeneous columns of more anorthite-rich plagioclase at the < 100 nm scale while a homogeneous anorthitic plagioclase phenocryst from a 1968 lava from Arenal found much of the crystal to be composed of 20–50 μm diameter subgrains, having ∼ 1 μm diameter tubes filled with Fe, K, Si-rich glass. Large core–rim variations in 87Sr/ 86Sr occur within three plagioclase phenocrysts from both 1968 and 1999 Arenal lavas. Although core compositions vary widely (0.7038–0.7052), all three phenocrysts have similar 87Sr/ 86Sr rim compositions (0.7043). The variability in 87Sr/ 86Sr at the phenocryst scale but homogeneity in 87Sr/ 86Sr at the bulk scale in Arenal lavas [Ryder, C.H., Gill, J.B., Tepley III, F.J., Ramos, F., Reagan, M., this volume. Closed to open differentiation at Arenal volcano (1968–2003). J. Volcanol. Geotherm. Res.] could be explained by erupted lavas representing a near steady state balance between ascending magmas and a flux derived from diffusion-reaction processes occurring with surrounding crustal gabbros.

Igneous rims on micrometeorites

Melting of micrometeorites (MMs) due to atmospheric entry heating causes significant changes in the textures, mineralogies and compositions of particles that obscure their primary properties and greatly complicate interpretation of these extraterrestrial materials. Despite the abundance of melted MMs, the nature of melting processes in these materials is poorly constrained. In this study, mineralogical, textural, and compositional data on 77 MMs with igneous rims are presented, which suggest that fusion of micrometeorites during atmospheric entry occurs initially by surface melting. Textural and mineralogical evidence are presented that demonstrate unequivocally that igneous rims crystallized from a melt surrounding a largely unmelted core and establish melting as a gradational process. The compositions of igneous rims on fine-grained MMs (fgMMs) are broadly similar to those of the unmelted core except for depletions in volatile and moderately volatile elements produced by partial evaporation and suggest the formation of the rims by melting of the fine-grained matrix core. Enrichments in Fe/Si, Ni/Si, and Mn/Si in igneous rims compared with unmelted cores within fgMMs are suggested to occur due to the migration of Fe–S eutectic liquids from the core of the particle into the surface melt layer. The sulphide liquids are probably generated in the core of the particle under reducing conditions resulting by the pyrolysis of carbonaceous materials. The presence of igneous rims on fgMMs is enabled by the thermal decomposition of phyllosilicates and, therefore, indicates that fgMMs were hydrated particles prior to atmospheric entry. In contrast the compositions of igneous rims on coarse-grained MMs (cgMMs) are difficult to reconcile with those of the unmelted core and instead closely resemble those of fgMMs. The igneous rims of cgMMs are, therefore, suggested to form by melting of fine-grained matrix that was present on the exterior of these particles prior to atmospheric entry. Coarse-grained MMs with igneous rims thus were originally composite micrometeoroids, which consist of both fine-grained and coarse-grained materials, and are thought to be samples of chondrule-like objects. The abundance of composite MMs to cgMMs allows a first estimate of the mean chondrule radius within the parent bodies of MMs of ∼625 μm to be made.

Energetics of Small Molecule and Water Complexation in Hydrophobic Calixarene Cavities

Calixarenes grafted on silica are energetically uniform hosts that bind aromatic guests with 1:1 stoichiometry, as shown by binding energies that depend upon the calixarene upper rim composition but not on their grafted surface density (0.02-0.23 nm(-2)). These materials are unique in maintaining a hydrophilic silica surface, as probed by H2O physisorption measurements, while possessing a high density of hydrophobic binding sites that are orthogonal to the silica surface below them. The covalently enforced cone-shaped cavities and complete accessibility of these rigidly grafted calixarenes allow the first unambiguous measurements of the thermodynamics of guest interaction with the same calixarene cavities in aqueous solution and vapor phase. Similar to adsorption into nonpolar protein cavities, adsorption into these hydrophobic cavities from aqueous solution is enthalpy-driven, which is in contrast to entropy-driven adsorption into water-soluble hydrophobic hosts such as beta cyclodextrin. The adsorption thermodynamics of several substituted aromatics from vapor and liquid are compared by (i) describing guest chemical potentials relative to pure guest, which removes differences among guests because of aqueous solvation and van der Waals contacts in the pure condensed phase, and (ii) passivating residual guest binding sites on exposed silica, titrated by water during adsorption from aqueous solution, using inorganic salts before vapor adsorption. Adsorption isotherms depend only upon the saturation vapor pressure of each guest, indicating that guest binding from aqueous or vapor media is controlled by van der Waals contacts with hydrophobic calixarene cavities acting as covalently assembled condensation nuclei, without apparent contributions from CH-pi or other directional interactions. These data also provide the first direct quantification of free energies for interactions of water with the calixarene cavity interior. The calixarene-water interface is stabilized by approximately 20 kJ/mol relative to the water-vapor interface, indicating that water significantly competes with the aromatic guests for adsorption at these ostensibly hydrophobic cavities. This result is useful for understanding models of water interactions with other concave hydrophobic surfaces, including those commonly observed within proteins.

Retrieval of P–T information from shear zones: thermobarometric consequences of changes in plagioclase deformation mechanisms

Changes in deformation mechanism coupled with spatial and temporal variations in reaction rates can result in preservation of disequilibrium mineral compositions in rocks affected by synmetamorphic shearing. Thermobarometric calculations on such rocks may thus yield meaningless results. We use Garbenschiefer samples from a shear zone in the Eastern Alps to study the effects of different deformational processes on calculated pressures and temperatures in samples that experienced the same overall PTt history. We focus on plagioclase, which accommodates strain by a variety of deformation mechanisms and is a key mineral in many thermobarometers. Plagioclase that deformed largely via dislocation creep mechanisms shows concentric chemical zoning, whereas plagioclase that experienced dissolution-precipitation creep preserves complex zoning. Rim compositions in the latter domains are not necessarily the youngest compositions, nor did they typically equilibrate with other phases in the assemblage. The timing of hornblende breakdown reactions relative to changes in plagioclase deformation mechanism also affected chemical zoning. Samples that escaped shear strain while near the thermal maximum yield internally consistent thermobarometric results, whereas samples that experienced shearing near the thermal maximum yield scattered results. Some of the variability in the results likely represents real differences in the P–T conditions at which equilibration occurred during deformation. However, much of the variability represents spurious results obtained by pairing mineral compositions that were never in equilibrium with one another. Extraction of useful P–T information from samples that experienced synmetamorphic deformation requires careful documentation of the relationships between deformation mechanisms and chemical zoning in order to select appropriate mineral compositions for thermobarometric calculations.

Origin of garnet crystals in calc-alkaline volcanic rocks from the Setouchi volcanic belt, Japan

AbstractCalc-alkaline dacites from the Setouchi volcanic belt contain garnet crystals that are classified petrographically and chemically into two types: type I and type M. Type-M garnets are characterized by acicular sillimanite inclusions or dissolution textures, and may be accompanied by xenolith fragments. They exhibit extensive compositional zoning with an increase in MgO/FeO and decrease in MnO content towards the margin. These petrographical and compositional features are identical to those of garnets from metamorphic xenoliths entrained in the Setouchi volcanic rocks, suggesting a xenocrystic origin for the type-M garnets. In contrast, type-I garnets lack sillimanite inclusions and have different rim compositions from the type-M garnets. Transmission electron microscope analysis has revealed the presence of minute glass inclusions in the type-I garnets, which indicate conclusively that these garnets grew in the presence of a melt. Type-I garnets have oscillatory zoning characterized by an antipathetic variation between FeO and MgO. This zoning was probably caused by magma heterogeneity within magma batch. Differences in rim compositions between the two types of garnets, and the presence of reaction rims indicate that the xenocrystic type-M garnets were incorporated into the magma after phenocrystic type-I garnet became unstable due to decompression during magma ascent.

Mantle Xenoliths and Their Host Magmas in the Eastern Alkaline Province, Northeast Mexico

Alkaline magmas along the periphery of Sierra de San Carlos-Cruillas and Sierra de Tamaulipas (Eastern Alkaline Province, northeast Mexico) contain ultramafic xenoliths. Most of the host rocks are mafic (e.g., basalts, basanites, trachybasalts, phonotephrites), and have geochemical characteristics of nearly primary magmas generated in subcontinental zones (SiO2 = 42.6-48.4%, MgO = 6.3-11.2%, Mg# = 59.2-69.7). MORB-normalized patterns are similar to those displayed by extensionrelated mafic magmas. This hypothesis is supported by an enrichment in light REE ([La/Yb]N = 10.8-27.1; chondrite-normalized ratios) and the behavior of relatively immobile trace elements. Using a partial melting model for REE, the source for the mafic magmas apparently was in the garnet P-T stability field. On the other hand, peralkaline phonolites and tephriphonolites (SiO2 = 52.9-54.1%, MgO = 1.5-1.8%, Mg# = 33.1-39.8) also contain mantle xenoliths. These evolved magmas show MORB-normalized multi-element diagrams characterized by enrichment in highly incompatible elements (e.g., Rb, Sr, Ba) and positive anomalies for HFSE. Ultramafic xenoliths are predominantly protogranular spinel-lherzolites as well as harzburgites and rare dunites, websterites, and wherlites. A few samples display transitional and porphyroclastic textures, indicating that the xenoliths were carried to the surface from stable mantle zones with little or no deformation. The mineralogy (olivine + orthopyroxene + clinopyroxene ± spinel) of xenoliths contained in mafic magmas is typical of unaltered mantle nodules. However, small garnet crystals exhibiting disequilibrium textures occur in one sample. The core and rim compositions in olivine range from Fo90 to Fo94, whereas orthopyroxenes are characterized by En88-93 and clinopyroxenes by En44-51Fs3-10Wo43-50. Chromiferous spinels have Mg/(Mg + Fe+2) = 0.76-0.83 and Cr/(Cr + Al) = 0.10-0.25. In comparison to the nodules contained in mafic magmas, mantle xenoliths included in phonolitic liquids show some differences: (1) smaller size (diameter < 1 cm); (2) clinopyroxene is less common (< 5% volume) and spinel is absent; and (3) partial alteration to micaceous minerals. Equilibrium temperatures for mantle xenoliths contained in mafic magmas range from 850 to 1170°C, as calculated applying different geothermometers, whereas mantle nodules sampled by peralkaline liquids show only lower equilibrium temperatures (<900°C). A first approximation to equilibrium pressure, based on mineralogical constrains, indicates values from 10 to 25 kbar. Summing up, we consider that the mafic magmas were generated within the spinel-garnet domain of the lithospheric mantle, having little or no interaction with their wall rocks. The ultramafic xenoliths were probably sampled during magma ascent above its source. In contrast, the geochemistry of phonolitic rocks and their altered mantle xenoliths reveal that such magmas cannot be explained by direct mantle melting. The petrogenesis of these peralkaline magmas can be described by a two-step model: (1) partial melting of metasomatized mantle which produced an alkaline magma enriched in LILE and HFSE; and (2) subsequent fractional crystallization of this magma at upper mantle pressures, producing phonolites. While ascending, the peralkaline magmas sampled altered nodules in a shallow level of the subcontinental mantle. The rise and eruption of the mantle-bearing mafic and evolved magmas were facilitated by the post-Laramide extensional regime established during Tertiary time in northeast Mexico.

Petrogenesis of Mafic Inclusions in Rhyolitic Lavas from Narugo Volcano, Northeastern Japan

Mafic inclusions present in the rhyolitic lavas of Narugo volcano, Japan, are vesiculated andesites with diktytaxitic textures mainly composed of quenched acicular plagioclase, pyroxenes, and interstitial glass. When the mafic magma was incorporated into the silica-rich host magma, the cores of pyroxenes and plagioclase began to crystallize (>1000°C) in a boundary layer between the mafic and felsic magmas. Phenocryst rim compositions and interstitial glass compositions (average 78 wt % SiO2) in the mafic inclusions are the same as those of the phenocrysts and groundmass glass in the host rhyolite. This suggests that the host felsic melt infiltrated into the incompletely solidified mafic inclusion, and that the interstitial melt composition in the inclusions became close to that of the host melt (c. 850°C). Infiltration was enhanced by the vesiculation of the mafic magma. Finally, hybridized and density-reduced portions of the mafic magma floated up from the boundary layer into the host rhyolite. We conclude that the ascent of mafic magma triggered the eruption of the host rhyolitic magma.

The coexistence of jadeite and omphacite in an eclogite-facies metaquartz diorite from the southern Sesia Zone, Western Alps, Italy

An association of jadeite and omphacite has been newly found in an eclogite-facies metaquartz diorite collected from the Orco Valley area, southern part of the Eclogitic Micaschist Complex of the Sesia Zone, Western Alps, Italy. Both the jadeite and omphacite occur as idiomorphic to subidiomorphic grains in the matrix, and some of them are in contact with each other with a sharp grain boundary. Most of the jadeite shows faint and irregular zoning, with a chemical variation of Xjd (AlVIjd /(Na + Ca)) = 0.75-0.90. Half of the omphacite grains is homogeneous, and the remainder shows various zoning patterns. Some omphacite grains exhibit prograde zoning with an increase in Xjd from 0.31 to 0.55 and a decrease in Xaeg (= Fe3+/(Na + Ca)) from 0.14 to 0.04 from the core to the rim. The rim composition of the zoned omphacite is similar to that of homogeneous omphacite (Xjd = 0.40-0.56). The average rim composition of the jadeite-omphacite pairs in direct contact shows an apparent miscibility gap between Xjd = 0.50 ± 0.06, Xaeg = 0.09 ± 0.03, and Xaug (= Ca/(Na + Ca)) = 0.41 ± 0.05 in omphacite, and Xjd = 0.79 ± 0.04, Xaeg = 0.08 ± 0.03, and Xaug = 0.13 ± 0.03 in jadeite. Application of Powell (1985) garnet-clinopyroxene geothermometer gives T = 470 ± 30°C at P = 12 kbar and application of Waters and Martin (1993) garnet-omphacite-phengite geobarometer gives P = 12.4 kbar at T = 440°C and P = 12.0 kbar at T = 500°C as peak metamorphic conditions. Composition data of the pyroxene P2/n and C2/c compositional fields obtained from both this study and the literature suggest that: (1) the shape of two miscibility gaps between jadeite and omphacite and between omphacite and augite in the jadeite-augite-aegirine phase diagram of Carpenter (1983) is more concordant with the natural data than that of Holland (1990), (2) the miscibility gap between omphacite and augite closes at T ∼ 500°C and P = 15 kbar, but the miscibility gap between jadeite and omphacite may still exist, and (3) the miscibility gap between jadeite and omphacite closes at T = 700-850°C and P =15-45 kbar.

The timing of mantle and crustal events in South Namibia, as defined by SHRIMP-dating of zircon domains from a garnet peridotite xenolith of the Gibeon Kimberlite Province

The Gibeon Kimberlite Province, southern Namibia, comprises numerous, 70–80 Ma old kimberlite pipes and dykes commonly containing abundant mantle and crustal xenoliths. We dated by SHRIMP II zircons from a garnet peridotite xenolith (Pipe Hanaus 1). The xenolith consists of garnet, orthopyroxene, clinopyroxene and, almost completely serpentinised, olivine. Crystallisation P– T conditions range between ca. 3.3 GPa, 910 °C (core composition of minerals) and ca. 3.5 GPa, 1000 °C (rim composition). Fifteen ca. 150–40 μm long and 70–10 μm wide zircon crystals show more or less pronounced oscillatory zoning indicating precipitation in a melt or fluid. The SHRIMP results define three age groups: 1013 ± 22 Ma, 751 ± 28 Ma and 627 ± 24 Ma. These ages are clearly correlated with large-scale crustal events in the broad area of Namibia, namely with the granulite-facies metamorphism during the Namaqua collisional event and post-collisional magmatism (ca. 1000–1100 Ma), with the intense magmatic activity and initial rifting stages in the Damara belt (ca. 730–770 Ma) and with the Early Pan-African orogeny, responsible for the formation of the Damara orogen (ca. 650 Ma), respectively. Formation of zircon in the xenoliths is related to the generation and migration of fluids/melts in the mantle via e.g. asthenospheric mantle upwelling or dehydration of subducting lithosphere, associated with crustal large-scale events.

Tracing Lithosphere Evolution through the Analysis of Heterogeneous G9-G10 Garnets in Peridotite Xenoliths, II: REE Chemistry

Following previous publication of major---minor element data, this paper presents rare earth element (REE) data for heterogeneous (chemically zoned) garnets belonging to the peridotite suite of mantle xenoliths from the Jagersfontein kimberlite pipe, South Africa. The rim compositions of the garnets in the highest temperature---pressure (deepest) deformed peridotites show a typical megacryst-like pattern, of very low light REE (LREE) increasing through the middle REE (MREE) to a plateau of heavy REE (HREE) at c. 20 times chondrite; these compositions would be in equilibrium with small-volume melts of the mid-ocean ridge basalt (MORB) source (asthenosphere). With decreasing depth the garnet rims show increasing LREE and decreasing HREE, eventually resulting in humped relative abundance patterns. A set of compositions is calculated for melts that would be in equilibrium with the garnet rims at different depths. These show decreasing relative abundance of each REE from La to Lu, and the La/Lu ratio of the melts increases with decreasing depth of formation. Modelling of the effects of crystal fractionation shows that this process could largely generate the sequence of garnet rim and melt compositions found with decreasing depth, including the humped REE patterns in high-level garnets. Considering the behaviour of major---minor elements as well as REE, a process of percolative fractional crystallization is advocated in which megacryst source melts percolate upwards through peridotites and undergo fractionation in conjunction with exchange with the peridotite minerals. The initial megacryst melt probably includes melt of lithospheric origin as well as melt from the MORB source, and it is suggested that the process of percolative fractional crystallization may form a variety of metasomatic and kimberlitic melts from initial megacryst melts. Repeated metasomatism of the lower lithosphere by such differentiating melts is suggested by consideration of garnet core compositions. Such metasomatism would progressively convert harzburgites to lherzolites by increasing their CaO content, and this may account for the fact that the Cr-rich diamond---garnet harzburgite paragenesis is commonly preserved only where it has been encapsulated in diamonds.