Magmatic Dikes Research Articles

Acceleration of buoyancy-driven fractures and magmatic dikes beneath the free surface

SUMMARY By performing buoyancy-driven fracture experiments in brittle gelatin we observe that the ascent velocity of a fracture containing a finite volume of fluid increases when approaching the free surface. We theoretically describe this free-surface effect and quantify it by introducing an effective depth-dependent fracture toughness and developing an ascent model on the basis of linear fracture mechanics. We develop a successful inversion approach and resolve the actual and critical fracture length and the ascent velocity far away from the free surface from the observation of the fracture tip migration alone. Other parameters, as the fluid volume included in the fracture and the in situ fracture toughness, can be derived. Applying the model and inversion to the 1998 eruption at Piton de la Fournaise, Reunion Island, reveals estimates of the length and critical length of the feeding magma batch, the magma batch volume and the in situ fracture toughness. It further indicates that the ascent velocity of the magma batch was probably much smaller at greater depths and that the batch might have been initiated several months or years before the eruption.

Temporal evolution of a magmatic dike system inferred from the complex frequencies of very long period seismic signals

I present a detailed description of temporal variations in the complex frequencies of very long period (VLP) events observed in association with an earthquake swarm that occurred beneath Hachijo Island, Japan. The individual VLP signals show simple, decaying, harmonic oscillations with periods near 10 s, and they last for about 300 s. The VLP activity began on 20 August 2002 and continued until 2 April 2003. Spectral analysis of the VLP waveforms identifies two spectral peaks, of which the complex frequencies show systematic temporal variations at the beginning of the VLP activity. The Q values of both the peaks show increasing trends from roughly 15 to 35 in the time period between 22 and 26 August. The frequency of one peak shows a decreasing trend from 0.1 to 0.09 Hz, while the frequency of the other peak shows an increasing trend from 0.128 to 0.136 Hz during this time. After these systematic variations, the frequencies and Q values of the two peaks remain roughly constant. Waveform simulations based on a fluid‐filled crack model indicate that the observed systematic temporal variations can be reasonably explained by gradual expansion of a crack containing a basalt‐gas mixture. The roughly constant frequencies and Q values of the two peaks after these systematic variations suggest that the magmatic dike persistently existed beneath Hachijo Island for more than 6 months. Mass and heat advection is required to sustain the dike system, implying magmatic flow and convection in the dike and magma chamber beneath this island.

Volatile transport during the crystallization of anatectic melts: oxygen, boron and hydrogen stable isotope study on the metamorphic complex of Naxos, Greece

Crystallization of anatectic melts in high-temperature metamorphic terrains releases volatile-rich magmas that can be transported into adjacent lithologies. This study addresses the variations in the oxygen, boron and hydrogen isotopic composition of aplite-pegmatite dikes that formed during the crystallization of anatectic melts in regional high-temperature metamorphism on the island of Naxos, Greece, and propagated upward into the overlying sequences of metamorphic schist. The transport distance of these dikes was increased through a significant horizontal component of travel that was imposed by contemporaneous low-angle extensional shearing. Laser fluorination oxygen isotope analyses of quartz, tourmaline, garnet, and biotite mineral separates from the aplite-pegmatite dikes show a progressive rise in δ 18O values with increasing distance from the core. Oxygen isotope fractionations among quartz, tourmaline, and garnet show temperature variations from > 700°C down to ∼400°C. This range is considered to reflect isotopic fractionation beginning with crystallization at high temperatures in water-undersaturated conditions and then evolving through lower temperature crystallization and retrograde sub-solidus exchange. Two processes are examined for the cause of the progressive increase in δ 18O values: (1) heterogeneous δ 18O sources and (2) fluid-rock exchange between the aplite/pegmatite magmas and their host rock. Although the former process cannot be ruled out, there is as yet no evidence in the exposed sequences on Naxos for the presence of a suitable high δ 18O magma source. In contrast, a tendency for the δ 18O of quartz in the aplite/pegmatite dikes to approach that of the quartz in the metamorphic rock suggests that fluid-rock exchange with the host rock may potentially be an important process. Advection of fluid into the magma is examined based on Darcian fluid flow into an initially water-undersaturated buoyantly propagating aplitic dike magma. It is shown that such advective flow could only account for part of the 18O-enrichment, unless it were amplified by repeated injection of magma pulses, fluid recycling, and deformation-assisted post-crystallization exchange. The mechanism is, however, adequate to account for hydrogen isotope equilibration between dike and host rock. In contrast, variations in the δ 11B values of tourmalines suggest that 11B/ 10B fractionation during crystallization and/or magma degassing was the major control of boron geochemistry rather than fluid-rock interaction and that the boron isotopic system was decoupled from that of oxygen.

Late Magmatism of the Galway Granite Batholith: I. Dacite Dikes

The Galway Granite is singular among the Irish latest Caledonian (-400Ma) batholiths for the number and variety of synand post-consolidation sheet intrusions. The most numerous and persistent are microphyric dacitic dikes, typically arranged in sets trending N-NNE, perpendicular to the long axis of the batholith. The sets cut the eastern and western blocks of the batholith, in contrast to their absence from the uplifted and eroded central block. Dacite geochemistry is similar to that of the megacrystic granodiorite that comprises the greater part of the batholith. However, the longer dike sets manifest mineralogical and chemical gradients along strike, from rhyodacite at the batholith axis to andesitic dacite outside the batholith. Hybridisation of granitoid magma with a more mafic magma is considered to have produced the dike magmas. Diking was induced by east-west stress relaxation of the batholith, coeval with a final plutonic episode involving high-level intrusion of alkali leucogranite sills. Regional ENE-WNW transcurrent faulting became active before the diking had concluded.

Late Magmatism of the Galway Granite Batholith: I. Dacite Dikes

The Galway Granite is singular among the Irish latest Caledonian (-400Ma) batholiths for the number and variety of syn- and post-consolidation sheet intrusions. The most numerous and persistent are microphyric dacitic dikes, typically arranged in sets trending N-NNE, perpendicular to the long axis of the batholith. The sets cut the eastern and western blocks of the batholith, in contrast to their absence from the uplifted and eroded central block. Dacite geochemistry is similar to that of the megacrystic granodiorite that comprises the greater part of the batholith. However, the longer dike sets manifest mineralogical and chemical gradients along strike, from rhyodacite at the batholith axis to andesitic dacite outside the batholith. Hybridisation of granitoid magma with a more mafic magma is considered to have produced the dike magmas. Diking was induced by east-west stress relaxation of the batholith, coeval with a final plutonic episode involving high-level intrusion of alkali leucogranite sills. Regional ENE-WNW transcurrent faulting became active before the diking had concluded.

Postmagmatic mineral parageneses of dikes may be used to estimate the PT data of their country rocks - An example from the Bayerischer Wald, Germany

Narrow magmatic dikes cool so fast that their cooling history is almost exclusively determined by that of the country rock. Postmagmatic hydrothermal minerals in dikes, if present, are therefore formed under the PT conditions of the country rock. This effect may be used to estimate the temperature and pressure of the country rock at the time of intrusion of such dikes. For the late Variscan dikes of the Bayerischer Wald a temperature of 200 ′ 50 °C and a pressure of 100 to 180MPa is estimated.

Experimental abiotic synthesis of methanol in seafloor hydrothermal systems during diking events

The abiotic synthesis of organic compounds in seafloor hydrothermal systems is one mechanism through which the subsurface environment could be supplied with reduced carbon. A flow-through, fixed-bed laboratory reactor vessel, the Catalytic Reactor Vessel (CRV) system, has been developed to investigate mineral–surface promoted organic synthesis at temperatures up to 400°C and pressures up to 30 MPa, conditions relevant to seafloor hydrothermal systems. Here we present evidence that metastable methanol can be directly synthesized from a gas-rich CO 2–H 2–H 2O mixture in the presence of a mineral substrate. Experiments have been performed without a substrate, with quartz, and with a mixture of quartz and magnetite. Temperatures and pressures in the experiments ranged from 200°C to 350°C and from 15 to 18 MPa, respectively. Maximum conversion of 5.8×10 −4% CO 2 to CH 3OH per hour was measured using a mixture of magnetite and quartz in the reactor. After passivation of the stainless steel reactor vessel, experiments demonstrate that methanol is formed at temperatures up to 350°C in the presence of magnetite, and that the formation rate decreases over time. The experiments also show a loss of surface reactivity at 310°C and a regeneration of surface reactivity with increased temperature up to 350°C. Concentrations of CO 2 and H 2 used in the experiments simulate periodic, localized and dynamic conditions occurring within the seafloor during and immediately following magmatic diking events. High concentrations of CO 2 and H 2 may be generated by dike injection accompanied by exsolution of CO 2 and reaction of dissolved H 2O with FeO in the magma to form H 2. The experiments described here examine how the ephemeral formation of an H 2–CO 2-rich vapor phase within seafloor hydrothermal systems may supply reactants for abiotic organic synthesis reactions. These experiments show that the presence of specific minerals can promote the abiotic synthesis of simple organic molecules from common inorganic reactants such H 2O, CO 2 and H 2 under geologically realistic conditions.

Constraints on dike propagation from continuous GPS measurements

The January 1997 East Rift Zone eruption on Kilauea volcano, Hawaii, occurred within a network of continuous Global Positioning System (GPS) receivers. The GPS measurements reveal the temporal history of deformation during dike intrusion, beginning ∼8 hours prior to the onset of the eruption. The dike volume as a function of time, estimated from the GPS data using elastic Green's functions for a homogeneous half‐space, shows that only two thirds of the final dike volume accumulated prior to the eruption and the rate of volume change decreased with time. These observations are inconsistent with simple models of dike propagation, which predict accelerating dike volume up to the time of the eruption and little or no change thereafter. Deflationary tilt changes at Kilauea summit mirror the inferred dike volume history, suggesting that the rate of dike propagation is limited by flow of magma into the dike. A simple, lumped parameter model of a coupled dike magma chamber system shows that the tendency for a dike to end in an eruption (rather than intrusion) is favored by high initial dike pressures, compressional stress states, large, compressible magma reservoirs, and highly conductive conduits linking the dike and source reservoirs. Comparison of model predictions to the observed dike volume history, the ratio of erupted to intruded magma, and the deflationary history of the summit magma chamber suggest that most of the magma supplied to the growing dike came from sources near to the eruption through highly conductive conduits. Interpretation is complicated by the presence of multiple source reservoirs, magma vesiculation and cooling, as well as spatial variations in dike‐normal stress. Reinflation of the summit magma chamber following the eruption was measured by GPS and accompanied a rise in the level of the Pu'u O'o lava lake. For a spheroidal chamber these data imply a summit magma chamber volume of ∼20 km3, consistent with recent estimates from seismic tomography. Continuous deformation measurements can be used to image the spatiotemporal evolution of propagating dikes and to reveal quantitative information about the volcanic plumbing systems.

Evidence for episodicity in the magma supply to the large Tharsis volcanoes

The volumes and estimated total lifetimes of the Tharsis shield volcanoes on Mars imply that they were built with an average magma supply rate within a factor of 2–3 of ∼0.05 m3 s−1. The morphologies and positions of the summit calderas of these volcanoes, specifically the overlapping relationships between the calderas, are evidence that multiple magma reservoirs existed within them, centered on distinctly different locations at various times. The requirement that the magma in an older reservoir must have cooled below its solidus temperature in order to change the local stresses in such a way that a new reservoir grew at a different location argues for long time gaps (probably tens of Myr) during which the supply rate of magma from the mantle was much smaller than at other times. Conversely, the need to offset conductive cooling to the surface once an active magma reservoir was established implies that the magma supply rate was large, ∼1–10 m3 s−1, for times of order 0.1 Myr. An initial pulse involving rates of more than 150 m3 s−1 acting for at least a few weeks was needed to initiate each new reservoir. The lengths of numerous long lava flows on the flanks of the Tharsis Montes imply that lava must have been erupted at rates of ∼100–300 m3 s−1 for periods of a few years to a few tens of years. The volumes of many of these flows exceed the amounts of magma that could be released from overpressured magma reservoirs of the sizes implied by the dimensions of the summit calderas with the surrounding rocks behaving elastically. A maintained high magma supply rate from the mantle (leading to buffered eruption conditions) is almost certainly required for such flows, whereas inelastic caldera collapse events (unbuffered eruption conditions) can explain the emplacement of other flows. Taken together, the various lines of evidence suggest that the Tharsis volcanoes were built episodically with active phases lasting less than 1 Myr alternating with ∼100 Myr quiet phases. We comment on the implications of these issues for the episodicity of the generation of magma in the mantle and for the mode of transport of the magma in dikes or diapirs.

Structure and petrology of Tauride ophiolites and mafic dike intrusions (Turkey): Implications for the Neotethyan ocean

Cretaceous Neotethyan ophiolites occur in four east-west–trending subparallel zones within the Tauride tectonic belt in southern Turkey. The ophiolites of the Inner, Intermediate, and Outer zones tectonically overlie the Mesozoic platform carbonates of the Tauride belt and are commonly underlain by a Cenomanian ophiolitic melange. These ophiolites consist mainly of tectonized mantle rocks, mafic-ultramafic cumulates, and gabbros, and commonly lack sheeted dike complexes and extrusive rocks of a complete ophiolite sequence. Metamorphic soles that are several hundred meters thick occur as thrust-faulted slices beneath these ophiolites and show well-developed metamorphic field gradients. Ophiolitic units and the metamorphic soles are intruded by mafic dike swarms that are truncated at the contact with the underlying melange unit. Dike rocks are made of subalkalic basalt to andesite typical of evolved island-arc tholeiites; they display large compositional variations, with SiO 2 content between 50 and 60 wt% and MgO between 8 and 4 wt%, and contain higher Ti augite phenocrysts and significantly less calcic plagioclase than their host cumulates. The majority of the analyzed dike rocks show a slight depletion in light rare earth elements (REE) with low La/SmN ratios and are depleted in both high-field strength (HFS) and heavy REEs, while enriched in large-ion-lithophile elements (LILE) relative to normal mid-ocean ridge basalt (MORB). These characteristics suggest a mantle source that underwent previous melt extractions and subsequent metasomatism by LILE- and light REE-enriched fluids. Geochemical modeling of trace elements shows that melting occurred at relatively low pressures under hydrous conditions and that it may have required the existence of an asthenospheric window, in which the dike magmas developed through tapping and mixing of melts generated within a rising melting column starting slightly within the garnet stability field, or in a transitional zone between the garnet and spinel stability fields at about 60 km depth. This asthenospheric window was probably created during subduction of a Neotethyan ridge system; magmas ascending from the melt column within this window generated dikes that crosscut the metamorphic soles and were injected into the overlying mantle wedge and oceanic lithosphere. The new 40 Ar/ 39 Ar hornblende dates of 92–90 Ma and 90–91 Ma from the metamorphic soles and dike swarms, respectively, show that evolution of these two geologic units was closely related in time and space and that they formed at the same intraoceanic subduction zone within the Inner Tauride seaway. These data suggest that the Tauride ophiolites within the three zones to the north originated from the same root zone situated north of the Tauride carbonate platform, and that they constitute remnants of a single ophiolitic nappe sheet derived from the Inner Tauride seaway within the Neotethyan ocean.

Incipient metamorphism between Ufa and Beloretzk, western fold-and-thrust belt, southern Urals, Russia

The low-temperature thermal history of Paleozoic and Precambrian shales and slates was studied in detail along a NW–SE transect between Ufa and Beloretzk in the western fold-and-thrust belt of the southern Urals, Russia. The aim of the investigations was to compare four thermal parameters, namely illite and chlorite crystallinity, vitrinite reflectance, and Conodont color alteration index (CAI), in order to quantify the finite (i.e. maximum) thermal grade. The transect extends from Devonian to Permian sedimentary units of the pre-Uralian foredeep, crossing Precambrian siliciclastic and carbonate units of the Bashkirian megaanticlinorium into the Paleozoic units of the Ural-Tau antiform. In general, the four methods indicate similar metamorphic grades of the samples. The finite thermal grade ranges from lower diagenesis (Ro=0.9%, CIS-FWHMill001ad=0.770 Δ°2Θ, CIS-FWHMchl002ad=0.447 Δ°2Θ, CAI=1.5) in the pre-Uralian foredeep to epizone (CIS-FWHMill001ad=0.243 Δ°2Θ, CIS-FWHMchl002ad=0.252 Δ°2Θ, CAI=6) in the eastern part of the Bashkirian mega-anticlinorium and the Zilair synclinorium. All parameters show a sudden change in value at thrust boundaries and an increase in metamorphic grade with stratigraphic age within structural units. In comparison, the westernmost Precambrian units of the Ala-Tau-anticlinorium are characterized by thermal grades of lower diagenesis. Magmatic dikes cause a wide variation of the thermal grade in the western part of the Bashkirian megaanticlinorium. Also in areas with an intensive cleavage development (Zilair synclinorium) the finite thermal grade shows a stronger relationship towards the deformation than the stratigraphic position.

Postrifting seismicity and a model for the 1993 diking event on the Coaxial segment, Juan de Fuca Ridge

Microearthquake data from two deployments of ocean bottom seismographs made shortly after the June/July 1993 rifting episode on the CoAxial segment of the Juan de Fuca Ridge constrain the propagation path of the magmatic dike and provide insight into the effect of the dike injection on the axial stress field. The first deployment, consisting of five instruments in a small array (aperture of ∼2 km) at the eruption (Flow) site (46°30′N), was made 3 months after the episode. The second deployment, consisting of two larger arrays (apertures of ∼9 km) of five instruments each, one at the eruption site and one at a site of vigorous venting of bacterial flocculent material (Floe) site (46°18′N), was made 11 months after the eruption. Epicenters from nine posteruption earthquakes near the Floe site at the south end of the segment are confined to a narrow zone on the rift axis. Epicenters from 61 earthquakes observed near the Flow site are located throughout the rift valley and include an event cluster just south of the observed lava flows. We combine these posteruption earthquakes with T wave data recorded by SOSUS hydrophones during the rifting episode to develop a model for the diking event. In this model, magma propagated at depth along the axial rift zone until reaching the eruption site near 46°31′N. During the next few days the dike swelled as magma accumulated behind the arrested tip, elevating local stress levels and generating a swarm of seismic activity. About 3 days after the magma reached the north end of the segment, the top of the dike penetrated the seafloor surface, resulting in eruptions along the rift zone at the Flow site.

Mid‐ocean ridge eruptive vent morphology and substructure: Evidence for dike widths, eruption rates, and evolution of eruptions and axial volcanic ridges

High‐resolution side scan sonar data of Mid‐Atlantic Ridge (MAR) inner valley floor axial volcanic ridges reveal details of their architectural elements. We develop quantitative models for basaltic eruptions from dikes and compare the predicted products of these eruptions with the observed morphologic features. Inhibition of gas exsolution, lack of magma disruption, and hydrothermal effects combine to decrease the rise speed of magma in submarine dikes and enhance cooling, leading to more rapid centralization of eruptions along the widest places in the dike. Dikes are predicted to initially feed eruptions from fissure vents, producing lines of hummocky ridges; centralization of activity to several adjacent vents produces chains of hummocky bulbous mounds and can enhance the effusion rate at a single vent to produce small seamounts. The widest dikes are predicted to produce smooth flows up to several kilometers in length, which should pond in adjacent lows. Edifice sizes predicted on the basis of volume fluxes, and flow lengths implied by the widths of dikes observed on the seafloor, in Iceland and in ophiolite complexes, are in quantitative agreement with the dimensions of observed features. Each ridge is made up of the products of a variety of these individual dike‐emplacement and extrusive events involving various dike widths, cooling times, and eruption durations. On the basis of typical MAR spreading rates, about one dike emplacement event would be expected every 40 years; since not all dikes reach the surface, the actual mean interval between eruptions will exceed this. MAR dikes should solidify between average dike emplacement events. Dike emplacement events are more frequent on the East Pacific Rise (EPR) and thus new dikes are more likely to reoccupy the sites of incompletely solidified older dikes. Differences between the morphology of the MAR axial volcanic ridge and the sheet flow dominated EPR are attributed to on average wider dikes erupting with greater frequency along the EPR.

Reply [to “Comment on ‘On the relationship between dike width and magma viscosity’ by Yutaka Wada”

Reply [to “Comment on ‘On the relationship between dike width and magma viscosity’ by Yutaka Wada”

Dike rock generation and magma interactions in the Bir Safsaf igneous complex, south-west Egypt: Implications for the Pan-African evolution in north-east Africa

Dike rock generation and magma interactions in the Bir Safsaf igneous complex, south-west Egypt: Implications for the Pan-African evolution in north-east Africa

On the relationship between dike width and magma viscosity

On the basis of measurements of bulk rock and phenocryst chemical compositions, phenocryst content, and estimation of water content and magma temperature, the magma viscosity of 44 dikes from Oki‐Dozen and Tango, southwest Japan, and Ocros, Peruvian Andes, was estimated to examine a relationship between viscosity and dike width. The results show that magma viscosity increases with dike width: mafic magmas with viscosities of 101–102 Pa s form dikes l m wide, while felsic magmas with viscosities of 106–107 Pa s form dikes 100 m wide. This indicates that magma viscosity as well as magma driving pressure and host rock stiffness are important factors in explaining the variation of dike width. Because magma viscosity is a measure of internal friction, the magmatic force required to widen the fracture during hydrofracturing is proportional to viscosity. On the basis of the range of chemical composition of the studied samples, dike widths formed by highly viscous felsic magma should be of the order of 100 m maximum. Furthermore, the observed basaltic dike width variations from various tectonic settings indicate that the effective factor on driving pressure of dike magma may be not magma pressure but compressive stress. On the other hand, dikes derived from low‐viscosity basaltic magma and exceeding 100 m width, may be formed by extremely high magma pressure associated with flood basalt volcanism. This is supported by (1) the observation that a rifting event with decreased compressive stress was not always associated with the volcanism and (2) the inferred effusion rates for this type of volcanism are anomalously high, suggesting increased magma pressure. In addition, the active magma generation and the decreased host rock stiffness by locally anomalous thermal conditions like a hotspot phenomenon may assist flood basalt volcanism or emplacement of anomalously wide mafic dikes.

Mineralogy and geothermobarometry of magmatic epidote-bearing dikes, Front Range, Colorado

Epidote phenocrysts (Ps 19-24 ) in Laramide-age, porphyritic dacite dikes in the Front Range of Colorado prove that epidote can crystallize directly from a magma. The dikes are high-K, calc-alkaline dacites and rhyodacites (SiO 2 = 64%-70%). Phenocryst assemblages consist of combinations of epidote, plagioclase (An 53-19 ), biotite [Fe 3+ / Fe total = 0.25; Mg/(Mg + Fe 2+ ) = 0.42-0.55], quartz, aluminous amphibole (Al total = 2.4-3.0 per formula unit), igneous garnet (And 2-6 Gros 13-27 Spess 2-13 Pyr 7-26 Alm 51-60 ), igneous muscovite, and sanidine. Holocrystalline, aphanitic groundmasses consisting mainly of quartz, plagioclase, and alkali feldspar comprise 60%-75% of the dike rocks and require that epidote grew early in the crystallization sequence. Petrographic and microprobe data distinguish three types of magmatic epidote phenocrysts, with different petrogenetic connotations implied by each type. Type I epidote occurs as euhedral phenocrysts, up to 8 mm long, with low content of allanite component ( Thermobarometric calculations provide empirical evidence that epidote is stable in silicic, calc-alkaline magmas at high total pressure, high water content, and high oxygen fugacity. The dacitic magmas originated by partial melting at temperatures above 800 °C. Temperatures (±50 °C) ranged from 800 to 880 °C during early phenocryst growth down to 620 to 670 °C during groundmass crystallization. Phenocrysts formed at pressures between 7.2 ± 1.0 and 12 ± 2.0 kbar. Oxygen fugacity was within the magnetite stability field at two log units (±1.0) below the hematite-magnetite buffer. H 2 O fugacity calculations give values ranging from 3.4 to 16 kbar, depending on how annite activity in biotite is modeled. The occurrences of types I and III epidote combined with geobarometry give a minimum pressure of epidote stability in the dike magmas of 8.0 ± 1.0 kbar. These results agree with experimental evidence and with conditions inferred for the crystallization of magmatic epidote in large, Cordilleran plutons. Phenocrysts in the dikes were preserved in conditions outside their stability limits by rapid emplacement and quenching in shallow-level dikes.

Metasomatic oxidation of upper mantle periodotite

Examination of Fe3+ in metasomatized spinel peridotite xenoliths reveals new information about metasomatic redox processes. Composite xenoliths from Dish Hill, California possess remnants of magmatic dikes which were the sources of the silicate fluids responsible for metasomatism of the peridotite part of the same xenoliths. Mossbauer spectra of mineral separates taken at several distances from the dike remnants provide data on Fe3+ contents of minerals in the metasomatized peridotite. Clinopyroxenes contain 33% of total iron (FeT) as Fe3+ (Fe3+/FeT=0.33); orthopyroxenes contain 0.06–0.09 Fe3+/FeT; spinels contain 0.30–0.40 Fe3+/FeT; olivines contain 0.01–0.06 Fe3+/FeT; and metasomatic amphibole in the peridotite contains 0.85–0.90 Fe3+/FeT. In each mineral, Fe3+ and Fe2+ cations per formula unit (p.f.u.) decrease with distance from the dike, but the Fe3+/FeT ratios of each mineral do not vary. Clinopyroxene, spinel, and olivine Fe3+/FeT ratios are significantly higher than in unmetasomatized spinel peridotites. Metasomatic changes in Fe3+/FeT ratios in each mineral are controlled by the oxygen fugacity of the system, but the mechanism by which each phase accommodates this ratio is affected by crystal chemistry, kinetics, rock mode, fluid composition, fluid/rock ratio, and fluid-mineral partition coefficients. Ratio increases in pyroxene and spinel occur by exchange reactions involving diffusion of Fe3+ into existing mineral grains rather than by oxidation of existing Fe2+ in peridotite mineral grains. The very high Fe3+/FeT ratio in the metasomatic amphibole may be a function of the high Fe3+/FeT of the metasomatic fluid, crystal chemical limitations on the amount of Fe3+ that could be accommodated by the pyroxene, spinel, and olivine of the peridotite, and the ability of the amphibole structure to accommodate large amounts of 3 + valence cations. In the samples studied, metasomatic amphibole accounts for half of the bulk-rock Fe2O3. This suggests that patent metasomatism may produce a greater change in the redox state of mantle peridotite than cryptic metasomatism. Comparison of the metasomatized samples with unmetasomatized peridotites reveals that both Fe2+ and Fe3+ cations p.f.u. were increased during metasomatism and 50% or more of iron added was Fe3+. With increasing distance from the dike, the ratio of added Fe3+ to added Fe2+ increases. The high Fe3+/FeT of amphibole and phlogopite in the dikes and in the peridotite, and the high ratios of added Fe3+/added Fe2+ in pyroxenes and spinel suggest that the Fe3+/FeT ratio of the metasomatic silicate fluid was high. As the fluid perolated through and reacted with the peridotite, Fe3+ and C−O−H volatile species were concentrated in the fluid, increasing the fluid Fe3+/FeT.

Low-18O Scourie dike magmas from the Lewisian complex, northwestern Scotland

Research Article| June 01, 1991 Low-18O Scourie dike magmas from the Lewisian complex, northwestern Scotland Ian Cartwright; Ian Cartwright 1Department of Earth Sciences and VIEPS, Monash University, Clayton, Victoria 3168, Australia Search for other works by this author on: GSW Google Scholar John W. Valley John W. Valley 2Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 Search for other works by this author on: GSW Google Scholar Author and Article Information Ian Cartwright 1Department of Earth Sciences and VIEPS, Monash University, Clayton, Victoria 3168, Australia John W. Valley 2Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1991) 19 (6): 578–581. https://doi.org/10.1130/0091-7613(1991)019<0578:LOSDMF>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Ian Cartwright, John W. Valley; Low-18O Scourie dike magmas from the Lewisian complex, northwestern Scotland. Geology 1991;; 19 (6): 578–581. doi: https://doi.org/10.1130/0091-7613(1991)019<0578:LOSDMF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Scourie dike swarm of the Lewisian complex, northwestern Scotland, comprises a dominant suite of quartz tholeiites and lesser volumes of norites, olivine gabbros, and bronzite picrites that were emplaced at depths of >14km in two episodes at 2.0 and 2.4 Ga.Unaltered quartz tholeiite dikes have low and homogeneous whole-rock δ18O values (2.04‰ ±0.14‰ relative to Standard Mean Ocean Water) and concordant mineral δ18O values (plagioclase = 3.1‰ to 4.1‰, clinopyroxene = 1.5‰ to 1.9‰, magnetite = -0.3‰ to 0.6‰). The dikes intrude granulite facies gneisses that have heterogeneous δ18O whole-rock values (5.8‰ to 9.7‰) and show no evidence of 18O depletion. Low-δ18O intrusive igneous rocks are generally believed to have exchanged with high-temperature meteoric or oceanic fluids after emplacement. The heterogeneous high δ18O values of the country rocks suggest that the Lewisian complex has not been pervasively infiltrated by such fluids. It is also unlikely that fluids were channeled along the dikes because: (1) the gneisses immediately adjacent to the dikes have unaltered mineralogies and stable-isotope ratios; (2) the dikes preserve unaltered igneous textures and mineralogies; (3) the dike minerals do not show open-system oxygen-isotope resetting; and (4) surface-derived fluids generally do not penetrate to the depths at which the dikes were intruded. It is more likely that these dikes were emplaced as primary low-δ18O magmas derived from the subcontinental lithosphere. The geochemistry of the Scourie dikes suggests that they have not assimilated large volumes of continental crust, and the best explanation of the data is that the dikes were derived from a low-δ18O mantle source that possibly contained the remains of subducted, hydrothermally altered oceanic crust. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Petrology and petrogenesis of the Ministers Island dike, southwest New Brunswick, Canada

The Ministers Island dike in southwest New Brunswick is a quartz tholeiitic member of the Late Triassic to Early Jurassic dike suite that outcrops along the east coast of North America. The dike's phenocryst assemblage is orthopyroxene + augite + plagioclase. The combination of petrographic, chemical (both phase and whole rock), isotopic and experimental work on representative samples from the dike places important constraints on the petrogenesis of the Ministers Island dike and by analogy on the other members of the dike suite. The petrographic, analytical and experimental results show that the Ministers Island dike magma underwent high pressure (0.8 to 1.0 GPa) fractionation of augite, followed by augite + orthopyroxene, and finally augite + plagioclase. The absence of olivine as either a phenocryst or an experimentally observed high pressure liquidus phase indicates that the magma evolved away from the Ol-Cpx-Opx-Plag pseudo-invariant point while still at high pressure and there was sufficient time for any olivine to settle out of the magma prior to emplacement of the dike. The Sr and Nd isotopic results support a metasomatised mantle source similar to EMI but with slightly more radiogenic Nd.