Wall-rock Fragments Research Articles

Origin of chromitites in the Norilsk-1 intrusion (Siberian LIP) triggered by assimilation of argillaceous rocks by Cr-rich basic magma

Сhromite-rich rocks and monomineralic chromitites occur in mafic-ultramafic layered intrusions worldwide, and are economically important as they host major resources of chromium and platinum group elements. One of the acknowledged genetic concepts for such mineralization advocates bulk chromite crystallization in a response to hybridization of Cr-rich basic magma with felsic melts, derived from partial melting of the wall rocks. However, there has been lack of direct insights into this process and its details have been remaining largely obscured. In this study, we report data on chromite-rich assemblages of the Noril'sk-1 intrusion (Siberian LIP), with a particular focus on chromite-hosted multiphase inclusions. Composition of the latter is different from the rocks of the Noril'sk-1 intrusion and inherits geochemical fingerprints of the wall rock argillites and, therefore, represent snapshots of the heavily-contaminated medium of the magma-wall rock reaction front. Mineral relationships in chromite-rich breccias with fragments of the wall rocks also provide valuable insight into chromite mineralization mechanisms. Our results, along with geological evidence and data on other Noril'sk-type intrusions, indicate that bulk crystallization of chromite, aided by partial suppression of silicate crystallization, occurred in the hybrid medium during digestion of the wall rocks by ascending and emplacing basic magma. Continuous flow of the magma around outshoots of the wall rocks or through a “magmatic karst” in the wall rocks, allowed for a continuous precipitation of chromite. Concentration of chromite grains to form dense mineralization apparently occurred due to formation of chromite-rich blobs around the wall rock fragments, and selective collection and transport of chromite by fluid bubbles, which formed via degassing of wall rocks and then carried chromite and relics of the wall rocks (xenoliths) to the upper parts of the pluton. We propose that assimilation-driven formation of chromite-rich lithologies in intrusions requires: (1) sufficient Cr contents in magma, which allow for its oversaturation in Cr-spinel only by a sudden cooling and addition of SiO2, K2O or/and H2O; (2) efficient disintegration and digestion of the host rocks, releasing considerable amounts of these components to the magma, and (3) a mechanism ensuring accumulation of excess chromite within a small volume (e.g. continuous reaction of Cr-rich magma with the products of the host rocks' assimilation or/and mechanical concentration of chromite).

Genesis of the graphite orbicules in the Huangyangshan graphite deposit, Xinjiang, China: Evidence from geochemical, isotopic and fluid inclusion data

The Huangyangshan graphite deposit in the East Junggar Orogen (China) contains intriguing graphite orbicules. This contribution is designed to present reasonably comprehensive assessment of their origin. In this deposit, the intrusive orebodies are mechanically juxtaposed against contacts with granitic wall rocks. The orbicules are typically concentrically zoned in texture with a notable binary configuration involving a central core and an outer shell. The orbicule cores possess mineralogical and compositional variations that are more or less similar to those of the granitic wall rocks. The orbicule shells are composed of various ferromagnesian minerals (clinopyroxene + hornblende) in stark contrast to the matrix, where hornblende, sanidine and albite have higher Mg#, Ab and An values, respectively. The replacement of silicates by graphite and sulfides (pyrrhotite + chalcopyrite + pyrite) is common. The graphite throughout the matrix and orbicules features a patchy distribution of major elements (C) and dispersed elements (Cr, Ni, Co, S, Cu, Zn, As, Pb, etc.). The carbon isotopic compositions of the orbicular ore are −20.9 to −20.5‰, analogous to those of the surrounding strata, i.e., −25.6 to −19.3‰. The sulfur isotopic compositions of sulfides in the orbicular ore are −4.8 to −2.6‰, which differ from those of the surrounding strata, i.e., 16.0–23.0‰. Fluid inclusions principally comprise the volatile component CH4 and solid daughter minerals (graphite + chalcopyrite) with Tm > 450 °C. Vapor-liquid two-phase fluid inclusions formed under conditions of Th = 143.6–367.5 °C, P = 3–194 bar, and fO2 = −52.76 to −31.15. The hydrogen and oxygen isotopic values of the orbicular ore are δD = −110.3 to −107.0‰ and δ18Owater = 10.1–11.7‰. Zircons from the orbicular ore have 206Pb/238U ages of 304–359 Ma, with a weighted average of 319.0 ± 6.4 Ma, overlapping the geochronological age of the granitic host rocks (305.1–321.4 Ma). The orbicular ore has SiO2 concentrations ranging from 53.30 to 72.61 wt%, falling within the compositional fields of granite, quartz monzonite, granodiorite, diorite, and monzodiorite in the TAS classification diagram. The Sr-Nd and Hf isotopic data from the orbicular ore indicate mixing between materials derived from the depleted mantle and materials derived from the crust, with a dominant mantle contribution. The petrographic, geochemical, isotopic and fluid inclusion characteristics indicate that the processes of incorporation of H2O and wall rock fragments, liquid immiscibility and exsolution of a late magmatic fluid were responsible for orbicule formation in the Huangyangshan deposit. The following genetic model is proposed: (1) In the late Carboniferous, a new orbicular magma containing organic matter-derived CH4 and deep-mantle SO2 emplaced within the Huangyangshan pluton. Contemporaneously, crystal fragments and H2O were incorporated into the orbicular magma from the granitic host rocks. (2) H2O saturation was reached in the orbicular magma and fostered segregation of a hydrous ferromagnesian melt (orbicules) with a silicic residual melt (matrix). (3) At approximately 630 °C, 2000 bar and −18 lgfO2, graphite and sulfides formed en masse through deuteric alteration: 3CH4 + 2SO2 + 2(Fe2+)7[Si4O11]2(OH)2 (hornblende) = 3C (graphite) + 2FeS (pyrrhotite) + 8SiO2 (quartz) + 2(Fe3Si4O10(OH)2 + Fe3(OH)6) (chlorite), following the expulsion of a late magmatic fluid from the ferromagnesian melt.

Eocene seismogenic reactivation of a Jurassic ductile shear zone at Cap de Creus, Pyrenees, NE Spain

The Cap de Creus peninsula in NE Spain exposes Variscan amphibolite-facies rocks, transected by greenschist-facies mylonitic shear zones accommodating displacements of up to 1 km. One of these shear zones contains pseudotachylytes with 1–4 mm thick fault veins and cm-scale high-angle injection veins and breccias. The pseudotachylyte matrix encloses mm-to μm-scale wallrock fragments and consists of ultrafine biotite and feldspar, often with a microporphyritic structure suggesting crystallization from a melt. Recent dating of the host shear zone yielded a Jurassic (170-160 Ma) age. Here we present 40Ar/39Ar dating of the pseudotachylytes yielding Early Eocene ages between 52.76 ± 1.64 Ma and 44.55 ± 0.77 Ma, indicating Alpine brittle faulting at times that Jurassic low-angle ductile shears had been tilted to their present vertical position following Alpine thrusting in the Pyrenees. The kinematics of the pseudotachylytes suggest strike-slip reactivation along the host shear zone, consistent with plate-kinematic analyses indicating coeval Iberia - Europe motion dominated by strike-slip. The dimensions of the pseudotachylyte-bearing zone and estimated brittle displacements suggest a moment magnitude of ~4.7–4.9. The average thicknesses of the fault veins indicate deformation at ≥4 km depth, an energy density of ~9.106 J/m2 and a frictional resistance of ~23 MPa.

Volcanic ash generation: Effects of componentry, particle size and conduit geometry on size-reduction processes

Grain size distributions (GSD) of pyroclastic materials are the product of processes ranging from primary fragmentation efficiency to tephra transport. As such, a detailed description of their physical and chemical state can provide pivotal information regarding such processes. By constraining the GSD of volcanic deposits one can thereby deliver powerful constraints on the energetics and dynamics of eruptions. In order to do so we must distinguish between two primary controls: 1) fragmentation of magma to tephra, and 2) secondary transport-related processes (such as abrasion and comminution), within the conduit and until deposition of the particles. Variations in particle interactions in the conduit together with conduit geometry, may be major factors in modifying the GSD. As in all physicochemical processes, for eruptive dynamics an experimental basis is an essential element of calibration and quantification. Here, we have conducted the first experimental investigation linking fragmentation and ash production via the influence of 1) particle-componentry and particle-size, and 2) the conduit geometry (constricted versus unconstricted) on the GSD. Rapid decompression experiments with loose tephra material from the fall deposit of Pomici Principali eruption (10.3 ka, Campi Flegrei) have been conducted in an optically transparent setup that enables the optical monitoring of particle dynamics with a high-speed camera. The samples employed can be classified into two main groups; 1) pumices, and 2) dense clasts (including crystals, lava clasts and wall rock fragments). Our results indicate that 1) a conduit diameter constriction (simulating obstacles in the conduit walls) is likely to reduce the average diameter of individual clasts and increase the generation of ash (<63 μm, fine ash); 2) the presence of larger pumice particles in the starting sample results in more efficient ash production; 3) the presence of dense particles is linked to a higher efficiency of size-reduction processes (whereby the presence of crystal-rich pumice appears to have a counterproductive effect). These findings correlate well with existing studies, enabling a general assessment of possible secondary size-reduction processes controlling the GSD of a volcanic deposit. Further, our results also suggest that secondary-generated ash should be considered for hazard assessment and modeling, since increased amounts of ash may affect, e.g., the plume dispersion dynamics, and in turn the area affected by ash deposition.

Evidence for dyke-parallel shear during syn-intrusion fracturing

Dyke intrusion is a highly dynamic process with seismicity preceding and accompanying magma emplacement on timescales of hours to days. Recent surveys of microseismicity indicate shear failure along fault planes parallel to the orientation of intruding dykes. However, the precision of earthquake hypocentre locations is typically limited to tens or hundreds of meters and cannot resolve whether the hypocentres relate to strain of wall rock fragments within the dykes, fault damage around the intrusions or peripherally in the country rock. Here we present high-resolution three-dimensional (3D) reconstructions of outstanding coastal exposures of a swarm of 19 dolerite dykes, near Albany, Western Australia using an unmanned aerial vehicle and Structure-from-Motion photogrammetry. It is observed that the number of faults and joints increases towards the dyke swarm, which, alongside mutually overprinting relationships, indicate that dyke emplacement and faulting were coeval. The faults contain cataclasites and are parallel to the dykes. In contrast, Mohr–Coulomb theory predicts shear failure on strike-parallel faults inclined ∼30° to the dyke plane. The faults and joints form a damage zone associated with the dyke swarm, even though the dykes themselves occupy Mode I extension fractures. These results confirm the recent geophysical evidence for dyke-parallel shear failure that can occur in the host rocks around intruding dykes. We suggest that coeval dyke-parallel seismicity both reactivated existing fracture networks and nucleated new fractures. Based on the premise that host-rock fracturing induces changes in elastic properties, remote stresses can reorientate locally leading to shear failure. This model provides for the first time an explanation for the origin of double-couple failure that is parallel in 3D (strike and dip) to dykes during their emplacement.

Fractal Analysis of In Situ Host Rock Nepheline Syenite Xenoliths in a Micro-Shonkinite Dyke (The Elchuru Alkaline Complex, SE India)

Formation of the fragments of the wall-rock during dyking is one of the important manifestations of instantaneous magmatic events. This process is well documented at shallower depths of Earth’s crust but not at deeper levels. In this paper the in situ xenoliths of host rock nepheline syenite within a micro-shonkinite dyke emplaced at mid-crustal depths is described and the fractal theory applied to evaluate origin of the xenoliths. The nepheline syenite xenoliths are angular to oval shaped and sub-millimetre to ~50 cm long. The xenoliths are matrix supported with clasts and matrix being in equal proportions. Partly detached wall-rock fragments indicate incipient xenolith formation, which suggested that the model fragmentation processes is solely due to dyke emplacement. Fractal analytical techniques including clast size distribution, boundary roughness fractal dimension and clast circularity was carried out. The fractal data suggests that hydraulic (tensile) fracturing is the main process of host rock brecciation. However, the clast size and shape are further affected by postfragmentation processes including shear and thermal fracturing, and chemical erosion. The study demonstrates that dyking in an isotropic medium produces fractal size distributions of host rock xenoliths; however, post-fragmentation processes modify original fractal size distributions.

Links between deformation, chemical enrichments and Li-isotope compositions in the lithospheric mantle of the central Siberian craton

We report the concentrations ([Li]) and isotopic compositions of Li in mineral separates and bulk rocks obtained by MC-ICPMS for 14 previously studied garnet and spinel peridotite xenoliths from the Udachnaya kimberlite in the central Siberian craton as well as major and trace element compositions for a new suite of 13 deformed garnet peridotites. The deformed Udachnaya peridotites occur at >5GPa; they are metasomatized residues of melt extraction, which as a group experienced greater modal and chemical enrichments than coarse peridotites. We identify two sub-groups of the deformed peridotites: (a) mainly cryptically metasomatized (similar to coarse peridotites) with relatively low modal cpx (<6%) and garnet (<7%), low Ca and high Mg#, sinusoidal REE patterns in garnet, and chemically unequilibrated garnet and cpx; (b) modally metasomatized with more cpx and garnet, higher Ca, Fe and Ti, and equilibrated garnet and cpx. The chemical enrichments are not proportional to deformation degrees. The deformation in the lower lithosphere is caused by a combination of localized stress, heating and fluid ingress from the pathways of ascending proto-kimberlite melts, with metasomatic media evolving due to reactions with wall rocks. Mg-rich olivine in spinel and coarse garnet Udachnaya peridotites has 1.2–1.9ppm Li and δ7Li of 1.2–5.0‰, i.e. close to olivine in equilibrated fertile to depleted off-craton mantle peridotites from literature data, whereas olivine from the deformed peridotites has higher [Li] (2.4–7.5ppm) and a broader range of δ7Li (1.8–11.6‰), which we attribute to pre-eruption metasomatism. [Li] in opx is higher than in coexisting olivine while Δ7LiOl-Opx (δ7LiOl−δ7LiOpx) ranges from −6.6 to 7.8‰, indicating disequilibrium inter-mineral [Li] and Li-isotope partitioning. We relate these Li systematics to interaction of lithospheric peridotites with fluids or melts that are either precursors of kimberlite magmatism or products of their fractionation and/or reaction with host mantle. The melts rich in Na and carbonates infiltrated, heated and weakened wall-rock peridotites to facilitate their deformation as well as produce high [Li] and variable, but mainly high, δ7Li in olivine. The carbonate-rich melts preferentially reacted with the opx without achieving inter-mineral equilibrium because opx is consumed by such melts, and because of small volumes and uneven distribution of the metasomatic media, as well as short time spans between the melt infiltration and the capture of the wall-rock fragments by incoming portions of ascending kimberlite magma as xenoliths. Trapped interstitial liquid solidified as cryptic components responsible for high [Li] and the lack of δ7Li balance between olivine and opx, and bulk rocks. Unaltered δ26Mg values (0.20–0.26‰) measured in several olivine separates show no effects of the metasomatism on Mg-isotopes, apparently due to high Mg in the peridotites.

Breccia formation by particle fluidization in fault zones: Implications for transitory, rupture-controlled fluid flow regimes in hydrothermal systems

Breccias in the Rusey Fault (Cornwall, UK) and the Roamane Fault (Porgera gold deposit, Papua New Guinea) provide insights about the dynamics of fluid flow and flow velocities when fault ruptures breach overpressured reservoirs of hydrothermal fluid. These faults contain cockade-like breccias in which rock fragments are mantled by spheroidal overgrowths of hydrothermal minerals. At Rusey, the overgrowths are quartz-dominant, whereas in the Roamane Fault overgrowths are composed of calcite, quartz, or gold-rich pyrite. Although none of the rock fragment cores of accretionary spheroids are in contact with their neighbours, the spheroidal overgrowths do contact each other and are at least partially cemented together. The hydrothermal overgrowths mostly comprise either outwards coarsening crystals that radiate from the surface of the core rock fragment, or finer-grained, inequigranular to mesh-like intergrowths. Concentric textural banding and oscillatory growth zones are present in some hydrothermal mantles. The breccias occur as fault-parallel layers and lenses, each up to several tens of centimeters thick. Adjacent layers are characterized by texturally-distinct ranges of clast sizes and different proportions of clasts to hydrothermal overgrowths. In the Rusey Fault, many texturally-distinct breccia layers are present within a 3m wide fault core. Some layers truncate others and many breccia layers exhibit grainsize grading or banding. Clasts in the breccias include fragments of wall-rock, veins and various fault damage products, including fragments of earlier generations of cemented breccia. Brecciation was episodic and separated by periods of cementation. The distinctive textures of the breccias are interpreted to have formed by fluidization of fault damage products in a high fluid flux regime. Hydrothermal coatings developed while clasts were in a suspended state during fluid ascent through dilatant fault segments. Layered breccias record multiple episodes of particle fluidization and indicate that the faults provided conduits for repeated transitory fluid upflow. Particle size distributions indicate that fluid velocities during fluidization were in the range 0.1 ms−1 to 1 ms−1. The maximum flow rates correspond to fluid fluxes of 10 to 100 L.s−1 per meter strike length of fault through dilatant fault apertures up to several tens of centimeters wide. Such high flow rates characteristically induce intense swarm seismicity rather than mainshock-aftershock seismicity, and have implications for the dynamics and rates of formation of fault-related hydrothermal ore deposits.

VEINS AND HYDROTHERMAL BRECCIAS OF THE RANDU KUNING PORPHYRY Cu-Au AND EPITHERMAL Au DEPOSITS AT SELOGIRI AREA, CENTRAL JAVA INDONESIA

The Randu Kuning prospect is situated at Selogiri area, Wonogiri, Central Java, Indonesia. This location is about 40 km to the south-east from Solo city or approximately 70 km east of Yogyakarta city. Many Tertiary dioritic rocks related alterationmineralisation were found at the Randu Kuning area and its vicinity, including hornblende microdiorite, hornblende-pyroxene diorite and quartz diorite. Mineralisation type of the Randu Kuning prospect was interpreted as porphyry Cu-Au and a number epithermal Au-base metals deposits in its surrounding. The closed existing of porphyry Cu-Au and epithermal Au-base metals type deposits at the Randu Kuning area produced a very complex of veins and hydrothermal breccias crosscutting relationship. A lot of porphyry veins types were found and observed at the Randu Kuning area, and classified into at least seven types. Most of the porphyry veins were cross cut by epithermal type veins. Many epithermal veins also are found and crosscut into deeply porphyry vein types. There are genetically at least two type of hydrothermal breccias have recognized in the research area, i.e. magmatic-hydrothermal breccia and phreatomagmatic breccia. Magmatic hydrothermal breccias are mostly occured in contact between hornblende microdiorite or quartz diorite and hornblende-pyroxene diorite, characterized by angular fragments/clasts supported or infilled by silicas, carbonates and sulphides matrix derived from hydrothermal fluids precipitation. Phreatomagmatic breccias are characterized by abundant of the juvenile clasts, indicated contact between hot magma with fluid or water as well as many wall rock fragments such as altered diorites and volcaniclastic rock clasts set in clastical matrix. The juvenile clasts usually compossed by volcanic glasses and aphanitic rocks in rounded-irregular shape. Both veining and brecciation processes have an important role in gold and copper mineralisation of the Randu Kuning Porphyry Cu-Au and epithermal Au-base metals deposits, mostly related to the presence of quartz veins/veinlets containing significant sulphides, i.e., quartz with thin centre line sulphides (Abtype) veins, pyrite±chalcopyrite (C type) veinlets, pyrite+quartz± chalcopyrire±carbonate (D type) veins of porphyry types as well as epithermal environment quarts+ sulphides+carbonate veins.

Dynamics of an open basaltic magma system: The 2008 activity of the Halema‘uma‘u Overlook vent, Kīlauea Caldera

On March 19, 2008 a small explosive event accompanied the opening of a 35-m-wide vent (Overlook vent) on the southeast wall of Halema‘uma‘u Crater in Kīlauea Caldera, initiating an eruptive period that extends to the time of writing. The peak of activity, in 2008, consisted of alternating background open-system outgassing and spattering punctuated by sudden, short-lived weak explosions, triggered by collapses of the walls of the vent and conduit. Near-daily sampling of the tephra from this open system, along with exceptionally detailed observations, allow us to study the dynamics of the activity during two eruptive sequences in late 2008. Each sequence includes background activity preceding and following one or more explosions in September and October 2008 respectively. Componentry analyses were performed for daily samples to characterise the diversity of the ejecta. Nine categories of pyroclasts were identified in all the samples, including wall-rock fragments. The six categories of juvenile clasts can be grouped in three classes based on vesicularity: (1) poorly, (2) uniformly highly to extremely, and (3) heterogeneously highly vesicular. The wall-rock and juvenile clasts show dissimilar grainsize distributions, reflecting different fragmentation mechanisms. The wall-rock particles formed by failure of the vent and conduit walls above the magma free surface and were then passively entrained in the eruptive plume. The juvenile componentry reveals consistent contrasts in degassing and fragmentation processes before, during and after the explosive events. We infer a crude ‘layering’ developed in the shallow melt, in terms of both rheology and bubble and volatile contents, beneath a convecting free surface during background activity. A tens-of-centimetres thick viscoelastic surface layer was effectively outgassed and relatively cool, while at depths of less than 100 m, the melt remained slightly supersaturated in volatiles and actively vesiculating. Decoupled metre-sized bubbles rising through the column burst through the free surface frequently, ejecting fragments of the outgassed upper layer. When the surface was abruptly perturbed by the rock-falls, existing mm-sized bubbles expanded, leading to the acceleration of adjacent melt upward and consecutive explosions, while renewed nucleation created a minor population of 10-micron-sized bubbles. After each explosive event in September–October 2008, this layering was re-established but with decreasing vigour, suggesting that the magma batch as a whole was becoming progressively depleted in dissolved volatiles.

K2O‐rich trapped melt in olivine in the Nakhla meteorite: Implications for petrogenesis of nakhlites and evolution of the Martian mantle

AbstractWe used new analytical and theoretical methods to determine the major and minor element compositions of the primary trapped liquid (PTLs) represented by melt inclusions in olivine and augite in the Martian clinopyroxenite, Nakhla, for comparison with previously proposed compositions for the Nakhla (or nakhlite) parent magma. We particularly focused on obtaining accurate K2O contents, and on testing whether high K2O contents and K2O/Na2O ratios obtained in previous studies of melt inclusions in olivine in Nakhla could have been due to unrepresentative sampling, systematic errors arising from electron microprobe techniques, late alteration of the inclusions, and/or boundary layer effects. Based on analyses of 35 melt inclusions in olivine cores, the PTL in olivine, PTLoliv, contained (by wt) approximately 47% SiO2, 6.3% Al2O3, 9.6% CaO, 1.8% K2O, and 0.9% Na2O, with K2O/Na2O = 2.0. We infer that the high K2O content of PTLoliv is not due to boundary layer effects and represents a real property of the melt from which the host olivine crystallized. This melt was cosaturated with olivine and augite. Its mg# is model‐dependent and is constrained only to be ≥19 (equilibrium Fo = 40). Based on analyses of 91 melt inclusions in augite cores, the PTL in augite, PTLaug, contained (by wt) 53–54% SiO2, 7–8% Al2O3, 0.8–1.1% K2O, and 1.1–1.4% Na2O, with K2O/Na2O = 0.7–0.8. This K2O content and K2O/Na2O ratio are significantly higher than inferred in studies of melt inclusions in augite in Nakhla by experimental rehomogenization. PTLaug was saturated only with augite, and in equilibrium with augite cores of mg# 62. PTLaug represents the Nakhla parent magma, and does not evolve to PTLoliv by fractional crystallization. We therefore conclude that olivine cores in Nakhla (and, by extension, other nakhlites) are xenocrystic. We propose that PTLoliv and PTLaug were generated from the same source region. PTLoliv was generated first and emplaced to form olivine‐rich cumulate rocks. Shortly thereafter, PTLaug was generated and ascended through these olivine‐rich cumulates, incorporating fragments of wallrock that became the xenocrystic olivine cores in Nakhla. The Nakhla (nakhlite) mantle source region was pyroxenitic with some olivine, and could have become enriched in K relative to Na via metasomatism. A high degree of melting of this source produced the silica‐poor, alkali‐rich magma PTLoliv. Further ascension and decompression of the source led to generation of the silica‐rich, relatively alkali‐poor magma PTLaug. Potassium‐rich magmas like those involved in the formation of the nakhlites represent an important part of the diversity of Martian igneous rocks.

Unbedded diatreme deposits reveal maar-diatreme-forming eruptive processes: Standing Rocks West, Hopi Buttes, Navajo Nation, USA

Diatremes provide partial records of how dyke-delivered magma, periodically interacting with water, produces the largest known cylindrical conduit structures of any volcano type. We address how pre-eruptive country rock is disrupted and redistributed to form the diatreme structure during an eruption by establishing the internal architecture of the unbedded diatreme at Standing Rocks West, and the volumes and sources of wall-rock within the diatreme and in a complementary tephra ring at Teshim, both in the Hopi Buttes volcanic field. The unbedded diatreme is dominantly exposed as a massif comprising multiple subvertical columns, interpreted as ephemeral-conduit deposits of well-mixed, poorly sorted, composite and juvenile pyroclast-rich deposits that truncate marginal layered deposits, plus a peripheral heterolithic country rock breccia. Wall-rock clasts are scarce in the massif and were mainly sourced from ~75 to 245 m depth below the paleosurface. By contrast, the tephra ring has abundant wall-rock fragments, dominantly from the upper 75 m of the pre-eruptive sedimentary sequence. The diatreme structure is interpreted to have been formed by many small-volume explosions fed by a low flux of basaltic magma. Late stage explosive activity was rooted mostly within pyroclastic debris at shallow to intermediate depths within the diatreme structure. This resulted in damped and shifting intra-diatreme explosions and jets that facilitated gradual mixing, recycling and remobilization of debris in the diatreme, with incremental addition of juvenile material and possibly a local rise in the crater floor.

Emplacement history of Granophyre dikes in the Vredefort Impact Structure, South Africa, inferred from geochemical evidence

The central Vredefort Impact Structure is characterised by impact melt rocks, known as the Vredefort Granophyre dikes, the mode of emplacement of which is not well known. Whole-rock and petrographic analyses of two dikes were conducted and compared to published geochemical data to elucidate the mode and timing of dike formation. The dikes are characterised by compositional and textural heterogeneity between, and within, individual dikes. Specifically, central dike portions are felsic and rich in wall rock fragments, whereas marginal dike phases are more mafic and fragment-poor. Collectively, this suggests that melt was derived from compositionally different parental melts and emplaced in at least two pulses. In addition, the chemical heterogeneity between fragment-rich and fragment-poor dike zones can be explained by variable assimilation of a mafic component, notably Ventersdorp basalt, at the base of the impact melt sheet, from which melt of the Granophyre dikes is derived. This scenario accounts for the mafic and fragment-poor character of melt emplaced first in the dikes and the more felsic and fragment-rich nature of melts of the following emplacement pulse, i.e., when the impact melt was less hot and thus less capable of digesting large quantities of (mafic) wall rock fragments. Differences in geometrical, textural, chemical and fragment characteristics between the Granophyre dikes and pseudotachylite bodies can be explained by the same process, i.e., impact melt drainage, but operating at different times after impact.

The exhumed Eocene Sultepec-Goleta Volcanic Center of southern Mexico: record of partial collapse and ignimbritic volcanism fed by wide pyroclastic dike complexes

The Sultepec-Goleta area in southern Mexico hosts one of the major Eocene silicic volcanic centers that make up the Cenozoic volcanic record of the north-central part of the Sierra Madre del Sur. This center is represented by a partially exhumed NNE-SSW trending ignimbritic field that covers an area of ∼400 km2 with a preserved volume of ∼200 km3. A remarkable feature of volcanic center is the exposure of pyroclastic dike complexes up to 1 km wide that extend almost continuously along the western and southern flanks of the volcanic field. The ignimbritic record comprises four units with different textural features as well as variable proportions of the main components (i.e. phenocrysts, pumice and lithic clasts). The most extensive unit is the basal Goleta Ignimbrite, of which the middle and upper members are richer in phenocrysts than the lower member. It has a thickness that ranges from ∼200 m in the north to at least 600 m in the south of the volcanic center. The overlying ignimbrite units (Cienega, Lobera and Potrero ignimbrites) are phenocryst-poor and are distributed in the northern sector of the volcanic field. The semi-curvilinear trend of the pyroclastic dikes along the southern flank of the Goleta Range, coupled with the greater thickness of the Goleta Ignimbrite encircled by the dikes, is indicative of the development of a partial-collapse caldera. Based on spatial relationships as well as analogies in the nature and abundances of the components, it has been recognized that the central and southern dike complexes fed the relatively phenocryst-rich Goleta Ignimbrite, whereas the phenocryst-poor units of the northern sector were extruded through the pyroclastic conduits distributed around Sultepec. We suggest that the stirring produced by the partial collapse of the magma chamber roof beneath the southern sector of the study area triggered the tapping of phenocryst-rich portions of the zoned magma chamber, resulting in an increase of phenocryst content in the upper members of the Goleta Ignimbrite. The presence of breccias and lithic-rich facies dominated by wallrock fragments at the borders of the pyroclastic dikes, including those of the northern extra-caldera sector, indicate that erosion of the conduit walls above the fragmentation level contributed to widening of the conduits and the construction of wide dike complexes.

Origin of large-volume pseudotachylite in terrestrial impact structures

Large-volume pseudotachylite bodies in impact structures are dike like and consist of angular and rounded wall-rock fragments enveloped by a microcrystalline and sporadically glassy matrix that crystallized from a melt. Knowledge of the formation of pseudotachylite bodies is important for understanding mechanics of complex crater formation. Most current hypotheses of pseudotachylite formation inherently assume that fragmentation and melt generation occur during a single process. Based on the structure of pseudotachylite bodies at Sudbury (Canada) and Vredefort (South Africa), we show that these processes differ in time and space. We demonstrate that the centimeter- to kilometer-scale bodies are effectively fragment- and melt-fi lled tension fractures that formed by differential rotation of target rock during cratering. Highly variable pseudotachylite characteristics can be accounted for by a single process, i.e., drainage of initially superheated impact melt into tension fractures of the crater fl oor.

The role of dilation and cementation in the formation of cataclasite in low temperature deformation of well cemented quartz-rich rocks

An important textural component of cataclasites in quartz-rich rocks deformed at low temperatures is a fine-grained (<20μm) matrix composed of anhedral quartz. As cataclasites form during shearing, this component would typically be interpreted to result from extreme grain size reduction through comminution. Tabular bands of fine-grained quartz that texturally resemble cataclasite found in faulted quartz arenites and other quartz-rich, well cemented sandstones have characteristics that are incompatible with a shearing origin. These characteristics include a lack of shear displacement and wall rock fragments, gradational contacts with the wall rock, and a fine-grained quartz fill that has a different cathodoluminescence, water content, oxygen isotope chemistry, and in some cases, mineralogy, than the wall rocks. Rather than being the result of cataclasis, the fine-grained quartz in these tabular bands was precipitated in a dilating fracture. In essence, these structures are veins.In fault zones, where cataclasite is well developed and shearing is unequivocal, the fine-grained quartz matrix representing up to 50% of the fault rock volume has characteristics similar to those in the tabular bands. We interpret these volumes to be in many cases a result of cement precipitation rather than as a product of comminution. If correct, then the brittle deformation of well cemented, quartz-rich rocks deformed at low temperatures involves much more dilation and cementation than previously recognized.

Volcanology of Tuzo pipe (Gahcho Kué cluster) — Root–diatreme processes re-interpreted

The Middle Cambrian (~ 540 Ma) Gahcho Kué Kimberlite Field is situated about 275 km ENE of Yellowknife, NWT, Canada. The kimberlites were emplaced into 2.6 Ga Archean granitic rocks of the Yellowknife Supergroup. Four larger kimberlite bodies (5034, Tesla, Tuzo, and Hearne) as well as a number of smaller pipes and associated sheets occur in the field. In plan view, the Tuzo pipe has a circular outline at the surface, and it widens towards deeper levels. The pipe infill consists of several types of coherent and fragmental kimberlite facies. Coherent or apparent coherent (possibly welded) kimberlite facies dominate at depth, but also occur at shallow levels, as dikes intruded late in the eruptive sequence or individual coherent kimberlite clasts. The central and shallower portions of the pipe consist of several fragmental kimberlite varieties that are texturally classified as Tuffisitic Kimberlites. The definition, geometry and extent of the geological units are complex and zones controlled by vertical elements are most significant. The fluidal outlines of some of the coherent kimberlite clasts suggest that at least some are the product of disruption of magma that was in a semi-plastic state or even of welded material. Ragged clasts at low levels are inferred to form part of a complex peperite-like system that intrudes the base of the root zone. A variable, often high abundance of local wall-rock xenoliths between and within the kimberlite phases is observed, varying in size from sub-millimeter to several tens of meters. Wall-rock fragments are common at all locations within the pipe but are especially frequent in a domain with a belt-like geometry between 120 and 200 m depth in the pipe. Steeply outward-dipping bedded deposits made up of wall-rock fragments occur in deep levels of the pipe and are especially common under the downward-widening roof segments. The gradational contact relationships of these deposits with the surrounding kimberlite-bearing rocks as well as their location suggest that they formed more-or-less in situ. Different breccia facies inside the pipe suggest an origin by slumping, grain flows, rock fall or pyroclastic deposition. The shape and facies architecture of the Tuzo pipe suggests that the studied section of the pipe lies at a root zone–diatreme transitional structural level. Composite coherent kimberlite clasts imply that recycling processes were active over time, while reworked wall-rock rich deposits and ductily-deformed clasts of welded kimberlite point to the presence of temporary cavities in the root zone. The emplacement of the Tuzo pipe did not occur in a single, violent explosion, but involved repetitive volcanic explosions alternating with periods of relative quiescence. The observed features are typical of phreatomagmatic processes, which may include phases of less-explosive magmatic activity.

ENDOGENOUS AND EXOGENOUS PLUTONS: THE INFLUENCE OF EMPLACEMENT STYLE ON CONTAMINATION OF GRANITIC MAGMA

Whether granitic magmas can detach and digest fragments of wallrock or mingle with other magmas depends in large part on the style of magma emplacement. Many composite plutons grow by relatively small pulses of magma, some of which may reach their solidus before injection of the next pulse. Composite “nested” plutons may grow as new pulses of magma invade and displace their predecessors, or “sheeted” plutons may grow as new magma follows contacts between wallrock and older intrusions. Contrary to a common assumption, the innermost parts of a composite intrusion are not necessarily the last to be emplaced or the most fractionated. Emplacement kinematics of some plutons and lava domes are similar. Lava domes grow in two ways. Endogenous domes inflate as new lava ascends and ponds within a cooler lava crust, and exogenous domes accrete as new lava ascends, breaches the lava crust, and spreads over the exterior. Exogenous growth of sheeted plutons offers less opportunity for magma mingling and mixing, but allows more wallrock assimilation (contamination) than endogenous growth of nested plutons.

Ammonium sulfate on Titan: Possible origin and role in cryovolcanism

We model the chemical evolution of Titan, wherein primordial NH 3 reacts with sulfate-rich brines leached from the silicate core during its hydration. The resulting differentiated body consists of a serpentinite core overlain by a high-pressure ice VI mantle, a liquid layer of aqueous ammonium sulfate, and a heterogeneous shell of methane clathrate, low-pressure ice Ih and solid ammonium sulfate. Cooling of the subsurface ocean results in underplating of the outer shell with ice Ih; this gravitationally unstable system can produce compositional plumes as ice Ih ascends buoyantly. Ice plumes may aid in advection of melt pockets through the shell and, in combination with surface topography, provide the necessary hydraulic pressure gradients to drive such melts to the surface. Moreover, contact between the magma and wall rock (methane clathrate) will allow some methane to dissolve in the magma, as well as eroding fragments of wall rock that can be transported as xenoliths. Upon rising to the clathrate decomposition depth ( ∼ 2 MPa , or 1700 m), the entrained xenoliths will break down to ice + methane gas, powering highly explosive eruptions with lava fountains up to several kilometers high. Hence we predict that Titan is being resurfaced by cryoclastic ash consisting of ice and ammonium sulfate (or its tetrahydrate), providing an abundance of sedimentary grains, a potential source of bedload for fluvial transport and erosion, and of sand-sized material for aeolian transport and dune-building. The infrared reflectance spectrum of ammonium sulfate makes it a plausible candidate for the 5 μm-bright material on Titan's surface.

Complex bombs of phreatomagmatic eruptions: Role of agglomeration and welding in vents of the 1886 Rotomahana eruption, Tarawera, New Zealand

Basaltic bombs ejected from a Rotomahana crater during the 1886 Tarawera eruption in New Zealand are of two fundamental types: cored bombs, which have single large fragments, generally of rhyolite wall rock material, as their cores, and “loaded” bombs, which contain abundant small fragments dispersed through the bombs. Similar lithic‐bearing bombs are common products of other, phreatomagmatic eruptions worldwide. Cored bombs originate when a large wall rock fragment is entrained in basaltic melt and then ejected with an adhering skin of basalt. In contrast, loaded bombs are constructed by agglomeration of multiple fragments of basalt, among which smaller rhyolitic wall rock fragments are dispersed. Different styles and rates of bomb “loading” are reflected in the bombs' textures and compositions. These textures provide insight into the nature of fragmentation during the eruption, the processes of recycling that produce multiply ejected clasts versus “recapitulated” ones (those where early formed clasts were reingested in magma and then formed the cores of new bombs), the degree and timescale of interaction of magma with vent walls, and the timescales over which lithic material from the vent walls is incorporated into complex bombs prior to their ejection. Because fragments of rhyolitic pumice are readily refused, their condition constrains estimates of the timing of fragmentation and agglomeration. The presence of nonfused rhyolite pumice within many loaded bombs implies that the processes forming and quenching the bombs cannot have exceeded several seconds. In contrast, other bombs contain patches of compositionally modified basalt with isolated rhyolite‐derived feldspar xenocrysts that indicate complete fusing, melting and assimilation of entrained rhyolitic debris. The bomb population overall indicates that conditions within this Rotomahana vent were thermally and physically heterogeneous and that recycling of debris was ubiquitous, both as recycling of large individual clasts and by fallback and aggregation of material which was reingested in magma before being reejected as new bombs.