Open-system Magma Chamber Research Articles

Insights Into Magma Chamber Processes From the Relationship Between Fabric and Grain Shape in Troctolitic Cumulates

The strength of foliations defined by shape preferred orientation of plagioclase in troctolitic cumulates from the Layered Series of the Skaergaard intrusion, and the Rum Eastern Layered Intrusion, increases as the grains become more tabular, due either to the greater propensity of highly non-equant grains to be re-arranged by magmatic currents or tectonic disruption of poorly consolidated mush, or by the effects of a pre-existing shape preferred orientation on final grain shape in fully solidified rocks. The stratigraphic evolution of grain shape, microstructures and fabrics in the lowest 320 m of the Skaergaard Layered Series records the progressive inflation of the chamber to its final size. During the earliest stages of solidification, the extent of in situ nucleation and growth on the chamber floor decreased upward through the stratigraphy, due to the development of a thermally insulating blanket of mush on the floor. An upward increase in foliation strength as the chamber inflated to its final size was a result of the increasing strength of convection of the bulk magma and an increasing contribution to the floor mush of crystals derived from the walls of the enlarging magma chamber. Plagioclase in the troctolites in the open-system magma chamber of the Rum Eastern Layered Intrusion is generally more equant than that in the Skaergaard intrusion, perhaps related to the slower crystal growth on the margins of the continuously replenished Rum chamber. Significant sub-solidus modification of original igneous microstructures is observed in Rum troctolites from parts of the stratigraphy recording frequent replenishment events.

The Neodymium Stable Isotope Composition of the Oceanic Crust: Reconciling the Mismatch Between Erupted Mid-Ocean Ridge Basalts and Lower Crustal Gabbros

The trace element and isotopic compositions of mid-ocean ridge basalts (MORB) provide an important cornerstone for all studies seeking to understand mantle evolution. Globally there is a significant over-enrichment in the incompatible trace element concentrations of MORB relative to levels which should be generated by fractional crystallization. Thermal and geochemical constraints suggest that MORB require generation in open system magma chambers. However, the petrology of lower oceanic crustal rocks suggests instead that these enrichments maybe formed through reactive porous flow (RPF). Stable isotope compositions are process dependent and therefore provide an excellent mechanism to compare these contrasting models. This study presents the first neodymium (Nd) stable isotope compositions of Indian MORB and well characterized gabbroic rocks from the lower oceanic crust sampled at the Southwest Indian Ridge (Hole 735B). Indian MORB is extremely homogenous with a mean δ146Nd of −0.025 ±0.005‰ which is identical to the composition of Pacific MORB. Despite significant variability in the source composition of MORB globally (i.e. 143Nd/144Nd) their indistinguishable δ146Nd compositions suggests they were homogenized through the same process along the global ridge network. In stark contrast, oceanic gabbros have δ146Nd ranging from −0.026‰ to −0.127‰, doubling the natural variability in Nd stable isotopes observed in terrestrial rocks. Clinopyroxene separates possess variable δ146Nd but are isotopically heavier than the gabbroic whole rocks at the same major element compositions. These large variations in δ146Nd cannot be generated solely by the fractionation or accumulation of clinopyroxene and/or plagioclase. Hole 735B preserves widespread evidence of RPF which could induce kinetic isotopes fractionation during crystal growth. In clinopyroxene kinetic isotope fractionations will only induce ca. 0.02‰ variations therefore several cycles of dissolution and reprecipitation of isotopic signatures at grain boundaries are required to explain the range of δ146Nd observed in the gabbros. Given the large disconnect between the average composition of the lower crust (δ146Nd = −0.076‰) and MORB globally and the evidence of limited melt extraction into the upper crust at Hole 735B it is highly unlikely that the melts involved in RPF contributed in a substantial way to the Nd isotope composition of erupted MORB.

Interplay between volcanic and plutonic systems: A case study of the early Miocene Solarya Volcano-plutonic Complex in NW Anatolia (Turkey)

Linking volcanic and plutonic systems and examining them, together is critical to evaluate magma chamber processes and mantle-crust, interactions in lithospheric scale. Solarya Volcano-plutonic complex, (SVPC) is one of the major volcano-plutonic complexes located in NW, Turkey and it is remarkable for its coeval and cogenetic plutonic, hypabyssal and volcanic rocks. Solarya pluton (22 Ma) is an epizonal, pluton which consists of three main rock groups; K-feldspar, megacrystalline granodiorite, microgranodiorite and haplogranite. Plutonic rocks pass into hypabyssal association, represented by, granodiorite/quartz-diorite porphyry sheet intrusive rocks which were, injected into the ring and radial faults concurrently with the, emplacement of the pluton. Hypabyssal and volcanic rocks cut one another, and fragments of hypabyssal rocks are found in volcanic rocks. Volcanic, association, is formed from trachyandesite, andesite and dacite lavas, with associated pyroclastic rocks. New 40Ar/39Ar radiometric ages of, volcanic and hypabyssal associations display that, they have 40Ar/39Ar, biotite ages changing from 23.1 ± 0.2 to 21.0 ± 0.2 indicating that they, are coeval with Solarya Plutonic Association. Petrographical features of, SVPC display disequilibrium textures such as oscillatory zoning, sieve, and honeycomb textures in plagioclase, reverse mantled biotite and, hornblende crystals as indicatives of open system magma chamber processes, (mixing/mingling and/or AFC). Plutonic, hypabyssal and volcanic, associations of SVPC show similar major-trace and Sr-Nd-Pb-O isotopic, compositions indicating a common magma source and similar magma chamber, processes. They are medium to high-K calc-alkaline rocks and display, enrichment in LILE and depletion in Nb, Ta, P and Ti. The geochemical and, isotopic characteristics of SVPC are considered to reflect an enriched, mantle (EMII) or sub-continental lithospheric mantle source. Bulk mixing, model between ƐNd and 87Sr/86Sr isotopic data suggests that, the crustal, contribution to the enriched mantle source is about 25–45%. Evaluation of all these data suggests that, Solarya Volcano-plutonic, Complex with early Miocene age was developed under syn-convergent, extensional regime concurrent with the exhumation of other major volcanoplutonic, and core complexes (eg. Kozak, Menderes, Kazdağ, Çataldağ) in NW, Anatolia.

Chemical mass balance equations for open-system magma chamber processes that result in crystal zoning

Recent advances in micro-analysis have revealed that isotopic zoning profiles in igneous crystals reflect progressive changes in liquid composition during open-system magmatic processes. Consequently, the isotope ratio of the integrated (bulk) crystals and that of liquid are inevitably different. This study develops a mass balance model to describe the compositional relationships between the bulk crystals (total suspended crystals), liquid, magma (crystals plus liquid), and crystal rims during open-system magma chamber processes. The model incorporates the effects of concurrent magma recharge (or mixing), assimilation, magma extraction, and fractional crystallization. The mass balance differential equations for suspended crystals and liquid are solved simultaneously. The analytical solution of the equations gives a quantitative account of the evolution paths of trace elements and isotopes within total suspended crystals, liquid, magma, and crystal rims. The isotopic zoning profiles in phenocrysts vary markedly depending on the relative rates of recharge, assimilation, magma extraction, and fractional crystallization. The model can reproduce the 87Sr/86Sr zoning patterns in natural feldspars from different magma systems.

A Model for the High-temperature Origin and Paradoxical Distribution of Pegmatites in Mafic Plutons, Smartville Complex, California

The distribution of pegmatitic rocks in the zoned gabbro–diorite plutons of the late Jurassic Smartville Complex is counter-intuitive. Whereas pegmatitic gabbros are common in mafic cumulate olivine gabbros within the zoned plutons, they appear to be absent from the more evolved gabbros and diorites. We argue that this paradox is resolved by examining the crystallization and temperature history of the rocks in question. The evolved rocks in the plutons consist of two-pyroxene hornblende biotite gabbro and diorite. The amphibole in these rocks occurs as a partial replacement of pyroxene that forms as a consequence of the incongruent crystallization reaction generalized as amphibole ± quartz ± plag1 = pyroxene + hydrous melt ± plag2 [reaction (1)]. This is a vapor-absent reaction that can buffer melt water content during crystallization and conceivably preclude water saturation altogether. Experimental evidence places the onset of the incongruent formation of amphibole at c. 900°C. In contrast, amphibole and other hydrous phases occur only in trace amounts in the cumulate olivine gabbros. The water-buffering reaction is not encountered during crystallization, the intercumulus melt proceeds to water saturation, and high-temperature plagioclase–pyroxene pegmatites form. We argue that the primary role for undercooling favored for the formation of granitic pegmatites is not likely to be applicable to the Smartville gabbro pegmatites. As crystallization of the gabbro pegmatite proceeds, the amphibole-in boundary is eventually reached and incongruent crystallization of amphibole occurs. Nevertheless, once water saturation occurs, it is irreversible, and is eventually manifested by the low-temperature deuteric alteration associated with the pegmatites. In short, we argue that pegmatite formation in the Smartville gabbros, and probably many other gabbroic plutons, reflects water saturation of a magma at high temperature, estimated here at >900°C. In systems where reaction (1) is attained prior to water saturation, pegmatite formation may be delayed until late in the crystallization history or precluded altogether. MELTS modeling of basalt crystallization suggests a solution to the paradox, indicating that the difference between the two systems can reflect equilibrium versus fractional crystallization. Equilibrium crystallization of trapped, intercumulus melt favors high-temperature crystallization and water saturation and, hence, pegmatite formation. Fractional crystallization, for example in an open-system magma chamber, favors lower-temperature crystallization and permits reaction (1) to proceed, inhibiting water saturation and preventing pegmatite formation. The open-system nature of the fractionating magma chamber might also allow escape of volatiles at its margins, with the lower volatile content then communicated to the magma as a whole as the system convects or otherwise mixes.

Incremental Construction of the Unit 10 Peridotite, Rum Eastern Layered Intrusion, NW Scotland

The Rum Eastern Layered Intrusion (ELI) is the product of part of an ∼60 Ma open-system magma chamber. The 16 coupled peridotite–troctolite ± gabbro macro-rhythmic units it contains represent crystallization of multiple batches of basaltic and picritic magma. Within the ELI, Unit 10 has been considered the type example of batch fractionation of magma on Rum for more than 50 years, successively producing peridotite, troctolite and olivine gabbro. Detailed field observations and logs of the Unit 10 peridotite cumulate are presented here, together with mineralogical and textural analyses of Cr-spinel seams and their peridotite host-rocks. Numerous harrisite layers are commonly associated with diffuse, laterally discontinuous, platinum-group element (PGE) enriched Cr-spinel seams. Multiple millimetre- to centimetre-thick Cr-spinel seams occur at the bases and tops and within harrisite layers. These relationships are inconsistent with simple batch fractionation of magma. Critically, the harrisite layers also exhibit centimetre- to metre-scale, upward oriented apophyses that point to injection of magma into the overlying cumulate, indicating an intrusive origin for the harrisite. Quantitative textural and chemical analysis suggests that the Cr-spinel seams formed via in situ crystallization within the crystal mush together with intrusive peridotites from an assimilation reaction between the replenishing magma and peridotitic crystal mush. Intrusive magma replenishment in Unit 10 caused significant compositional disequilibrium between the crystallizing phases in response to the postcumulus migration of reactive liquid, resulting in chemical zoning of intercumulus plagioclase crystals. We propose that the Unit 10 peridotite is intrusive and that repeated small-volume magma replenishments are responsible for incremental construction of a large proportion of the peridotite body, similar to recent interpretations of parts of Unit 12 and Unit 14. Moreover, it is suggested that some or all of the injections of magma occurred into the crystal mush, rather than at the magma chamber floor. This new model of intra-mush Cr-spinel formation may have significant economic implications for PGE enrichment in other layered intrusions, such as the peridotite-hosted chromitites of the Stillwater Complex Ultramafic Series (Montana, USA). It is also worth noting that thin platiniferous chromitite seams considered to have formed in situ occur below the Merensky Reef of the Bushveld Complex (South Africa).

Geology, petrology and tectonomagmatic evolution of the plutonic crustal rocks of the Sabzevar ophiolite, NE Iran

AbstractThe plutonic crustal sequence exposed northeast of Sabzevar is part of the ophiolitic belt of Sabzevar that occurs along the northern margin of the Central Iran micro-continent. The sequence includes olivine and pyroxene gabbro with cumulate characteristics, isotropic gabbro, foliated gabbro and a diabase sheeted dyke complex cut by wehrlite and olivine websterite intrusions, and pegmatite gabbro and plagiogranite as small intrusions and dykes. The sequence is comparable to gabbros in known ophiolite complexes. Microscopic studies show an abundance of the mesocumulate and heteradcumulate textures that represent open system magma chambers, which are common in supra-subduction zones. The olivine → plagioclase → clinopyroxene → ± orthopyroxene → amphibole trend of mineralization in the gabbros, similar to mid-ocean ridge basalt (MORB), and olivine → clinopyroxene → ± orthopyroxene → plagioclase → amphibole, similar to arc rocks, indicate the diversity in the formation of these rocks, and represent petrographic evidence of their formation in a supra-subduction zone. The rocks have calc-alkaline to tholeiitic affinities, and niobium depletion in the spider diagrams of diabase that matches the patterns of island arc magma. These patterns, and the light rare earth element enrichment of the diabase and plagiogranite, suggest the effect and introduction of the fluids, originating from the subducting slab, beneath the mantle wedge. The low titanium compositions, matching those of arc diabase and plagiogranite, plot in the island arc to MORB tectonomagmatic fields, and suggest formation of the Sabzevar ophiolitic plutonic crustal sequence in a supra-subduction zone during Late Cretaceous time.

Parental magma composition of the syntectonic Dawros Peridotite chromitites, NW Connemara, Ireland

AbstractChromium-spinels have been widely used as petrogenetic indicators to infer parent melt compositions and the tectonic setting of their formation. This study integrates petrographic, quantitative textural and geochemical analyses of Cr-spinel seams within the Dawros Peridotite, NW Connemara, Ireland to determine the composition of their parental magmas. Calculation of Cr no. (Cr/(Cr + Al)) (0.50–0.77) values and TiO2 (0.18–0.36 wt%) contents of the Cr-spinel seams, coupled with an estimation of the Al2O3 and TiO2 contents (~11.86 wt% and ~0.39 wt%, respectively) of their parental melts, indicates that they probably formed from boninitic melts sourced from a highly depleted mantle. This implies that the Cr-spinel seams formed in a supra-subduction zone undergoing high degrees of partial melting. The Cr-spinel data support tectonic models for the formation of the Dawros Peridotite (and Connemara Metagabbro-Gneiss Complex) during island arc collision, immediately prior to Grampian orogenesis at ~470 Ma. The occurrence of the Dawros chromitite seams at the approximate transition between the lower harzburgite sequence and the upper lherzolite sequence bears marked similarities to the positions of such seams in larger anorogenic layered mafic-ultramafic intrusions, and implies that the Dawros Peridotite behaved as an open-system magma chamber.

Magmatic Evolution of the Giant El Teniente Cu–Mo Deposit, Central Chile

ElTeniente, the world’s largest Cu deposit, is hosted in Late Miocene and Pliocene plutons that intrude the older Teniente Volcanic Complex (or Farellones Fm; 14·2^6·5 Ma). The Late Miocene and Pliocene plutonic host rocks of the deposit include, sequentially, the relatively large (450 km) Teniente Mafic Complex laccolith (8·9 1·4 Ma), the smaller ( 30 km) Sewell equigranular tonalite complex (7·05 0·14 Ma) and associated andesitic sills (8·2 0·5 to 6·6 0·4 Ma), small dacitic porphyry stocks (51km; 6·09 0·18 Ma), the unusual Cuand S-rich ‘Porphyry A’anhydrite-bearing granitoid stock (51km; 5·67 0·19 Ma), the Teniente Dacite Porphyry dike (51km; 5·28 0·10 Ma), minor latite dikes (4·82 0·09 Ma), and finally a small dacite intrusion (4·58 0·10 Ma).These plutonic rocks are all isotopically similar to each other (Sr/Sr1⁄4 0·7039^0·7042; eNd1⁄4þ2·5 to þ3·5) and also to the Teniente Volcanic Complex extrusive rocks, but distinct from both older Late Oligocene to Early Miocene volcanic rocks (Sr/Sr1⁄4 0·7033^0·7039; eNd1⁄4þ3·8 toþ 6·2) and younger Pliocene post-mineralization mafic dikes and lavas (Sr/Sr1⁄4 0·7041^0·7049; eNd1⁄4þ1·1 to 1·1). Multiple Cu-mineralized magmatic^hydrothermal breccia pipes were emplaced into these plutonic rocks during the same time period as the felsic porphyry intrusions, between at least 6·31 0·03 and 4·42 0·02 Ma.These mineralized breccia pipes, which formed by exsolution of magmatic fluids from cooling plutons, have their roots below the deepest level of mining and exploration drilling and were derived from the same magma chamber as the felsic porphyries, 44 km below the paleosurface.To produce the 100 10 tonnes of Cu in the deposit requires a batholith-size (4600 km) amount of magma with 100 ppm Cu. We suggest that both the mineralized magmatic^hydrothermal breccias and the progressively smaller volumes of more fractionated, but isotopically equivalent, Late Miocene and Pliocene felsic plutonic rocks that host the deposit were derived from the roof of a large, long-lived, thermally and chemically stratified, open-system magma chamber, or magmatic plumbing system, recharged from below by mantle-derived magmas. Only when this system fully solidified did post-mineralization mafic olivine-hornblende-lamprophyre dikes (3·85 0·18 to 2·9 0·6 Ma) pass through the system from the mantle to the surface. The significant progressive temporal isotopic evolution, to higher Sr/Sr (from 0·7033 to 0·7049) and lower eNd (from þ6·2 to 1·1), that occurred between the Late Oligocene and Pliocene in the vicinity of El Teniente for mafic mantle-derived magmas, and by implication their sub-arc mantle-source region, was due in part to increased mantle-source region contamination by subducted crust tectonically eroded off the continental margin. The post-mineralization olivine-hornblende-lamprophyres also imply extensive hydration of the mantle below this portion of the Andean arc by the Pliocene, which may have played a role in producing oxidized volatile-rich magmas and mineralization at ElTeniente.

Rhenium–osmium isotopes and platinum-group elements in the Rum Layered Suite, Scotland: Implications for Cr-spinel seam formation and the composition of the Iceland mantle anomaly

The Rum Layered Suite is a layered mafic–ultramafic body that was emplaced during Palaeogene North Atlantic margin rifting. It is a classic open-system magma chamber, constructed of 16 repeated coupled peridotite–troctolite units, some of which have laterally extensive ~ 2 mm-thick platinum-group element (PGE) enriched (~ 2 µg g − 1 ) Cr-spinel seams at their bases. In order to investigate Cr-spinel seam petrogenesis and enrichment of the PGE, abundances of these elements and Re–Os isotopes have been determined at three stratigraphic levels of the Rum Layered Suite that represent major magma replenishment events. Individual units preserve a range of initial 187Os/ 188Os ratios, demonstrating heterogeneity in the composition of replenishing magmas. Data for both the Cr-spinel seams and overlying silicates reveal that the processes that formed the Cr-spinel also concentrated the PGE, following magma replenishment. There is no evidence for structurally-bound PGE in Cr-spinel. Instead, the PGE budget of the Rum Layered Suite is linked to base metal sulphides, especially pentlandite, and to PGE alloys contained within the Cr-spinel seams, but which exist as separate phases at Cr-spinel grain boundaries. The range in initial Os isotope compositions ( γ Os = 3.4 to 36) in the Rum Layered Suite can be successfully modelled by 5–8% assimilation of Lewisian gneiss coupled with changing PGE contents in the replenishing magmas associated with sulphide removal. Initial 187Os/ 188Os ratios for Rum rocks range from 0.1305 to 0.1349 and are atypical of the convecting upper mantle, but are within the range for recently erupted picrites and basalts from Iceland and Palaeogene picrites and basalts from Baffin Island, Greenland and Scotland. Thus, the Os isotope data suggest that the North Atlantic Igneous Province magmas were collectively produced from a mantle source with components that remained relatively unchanged in Os isotopic composition over the past 60 Ma, and that likely contain a recycled lithospheric component.

The Unit 12 allivalite, Eastern Layered Intrusion, Isle of Rum: a textural and geochemical study of an open-system magma chamber

AbstractThe Eastern Layered Intrusion of the Rum Central Complex, NW Scotland, comprises 16 macro-units broadly defined by a basal peridotite body overlain by gabbros and troctolites. The local term for the upper plagioclase-bearing cumulates is ‘allivalite’. The Unit 12 allivalite contains five subsidiary peridotite horizons, each continuous on the kilometre scale. Field, textural and geochemical evidence points to the Unit 12 allivalite forming by progressive accumulation of crystals from up to seven separate magma batches, varying from picrite to basalt. The thickness of the magma lens in the sill-like chamber was as low as 30 m during a significant part of the development of the Unit 12 allivalite, and evidence supports one eruption from the chamber during its accumulation.

Early Cretaceous alkaline volcanism of the Sierra Chica de Córdoba (Argentina): Mineralogy, geochemistry and petrogenesis

Distinct groups of volcanic rocks, mainly of potassic character, were recognized from the different localities of the Sierra Chica de Córdoba (SCC): (1) alkali basalt to trachyte suite, (2) transitional basalt to latibasalt suite and (3) basanites. Magma evolution would have taken place at crustal level(s) from distinct parental magmas, mainly through fractional crystallization in an open system magma chamber. Source composition supports garnet and phlogopite as residual phases in the mantle. Melting degrees (5–7%) from an enriched mantle source are in accordance with the peripheral position related to the great thermal anomaly that triggered the voluminous Paraná flood tholeiites. SCC volcanism is of high-Ti, with multi-elemental patterns similar to those from Alto Paranaíba (Brazil) and clearly distinct from the ones of low-Ti provinces, as Eastern Paraguay. Though associated with a mobile belt of the Pampean Orogeny, volcanism of SCC does not reveal compositional features that can be related to slab-derived mantle metasomatism, so that the small-scale heterogeneity of the SCC lithospheric mantle source must be related to enrichment events involving volatile-rich small-volume melts from the asthenosphere.

Bulk-rock geochemistry and plagioclase zoning in lavas exposed along the northern flank of the Western Blanco Depression (Northeast Pacific): Insight into open-system magma chamber processes

The present study is based on a set of lavas and crosscutting dikes collected by dives along detailed vertical transects on the northern flank of the western part of the Blanco Transform Fault, Northeast Pacific. The studied area consists of a small basin, the Western Blanco Depression (WBD), extending from the southern end of the Juan de Fuca ridge to a pseudofault trace 60 km eastward. The Northern Scarp of the WBD comprises a volcanic unit overlying a sheeted-dike complex. Major and trace element data, coupled with Sr–Nd isotope ratios, reveal a two-component mantle source, composed by an isotopically depleted matrix variably veined by more enriched material. One chemical group (NS2), indistinguishable from the other Northern Scarp samples on the basis of trace element data, has an unusually depleted isotopic composition typical of a nearly pure mantle end-member. Some cogenetic samples of the Northern Scarp have been used to constrain the differentiation modalities. Anorthite and MgO content profiles in plagioclase xenocrysts and phenocrysts reveal (i) the existence of H 2O-bearing evolved melts in the mushy zones and (ii) the occurrence of mixing process between these melts and anhydrous mafic liquids. The hydration is supported by other petrographic features such as high magmatic fO 2 values, calculated from Fe–Ti oxide pairs, and the presence of pyroxene inclusions in plagioclase phenocrysts. Mixing, consistent with the existence of Ni-rich ferrobasalts, is interpreted to be the consequence of the reservoir refilling by mafic liquids (Mg# = 70). These petrological and geochemical evidences are combined with the evolution of Mg# with depth to suggest a periodic open-system magma chamber evolution beneath the southern end of the Juan de Fuca ridge.

Information about open-system magma chambers derived from textures in magmatic enclaves: the Kameni Islands, Santorini, Greece

Post-caldera eruptions of Santorini, Greece, over the past 3000 years resulted in the formation of the Kameni Islands, which comprise a series of compositionally similar dacitic lava flows. Each lava flow has a distinct population of partially-crystalline mafic enclaves, which we propose were derived from the break-up of a layer of replenishing magma responsible for triggering the eruption. Five of the recent flows (erupted in 1570, 1939, 1940, 1941 and 1950) include enclaves of essentially identical andesitic bulk compositions, which formed by crystallization of originally aphyric melts prior to eruption. Detailed examination of angles subtended at the junctions between pairs of plagioclase grains demonstrates that enclaves from each flow have a characteristic textural signature, with distinct differences in the extent of quench-related modification of the original population of dihedral angles formed by impingement of growing grains. These variations suggest that the temperature difference between the host dacite and the replenishing andesite at the time of layer overturn and eruption differed between flows. The uniformity of major element compositions of both the replenishing magma and the host dacitic lava flows demonstrates that the critical parameter in determining the timing of layer-overturn is the pre-eruptive H2O content of both host dacite and replenishing magma. We suggest that the replenishing magma responsible for two of the three eruptions in the period 1939–1941 was significantly wetter than that responsible for the later 1950 eruption. The enclaves with the least amount of quench-related modification occur in the 1570 flow. We suggest that in this case the intruding magma was relatively dry.

Primary Platinum Mineralization in the Nizhny Tagil and Kachkanar UltramaficComplexes, Urals, Russia: A Genetic Model for PGE Concentration in Chromite-RichZones

The Nizhny Tagil Uralian-(Alaskan)-type Ultramafic Complex is composed of a dunite core with a rim of wehrlite and pyroxenite. The dunite unit, which has a uniform mineral composition (olivine Fo 89.8–92.1 , avg 0.25 wt % CaO), represents an accumulation of olivine and minor chromite from a mafic magma in an open-system magma chamber. After solidification of the dunite, differentiation began with the formation of the wehrlite (olivine Fo 77.9–88.8 ) and pyroxenite, corresponding to the closure of the magmatic system. Two main types of chromitite occurrence are observed in the dunite body: small (100 ∞ 5 cm) scattered schlieren of chromite crystals within the ultramafic unit and much larger (up to 100 ∞ 5 m) concentrations. The chromite composition in both types is similar and uniform [Cr/(Cr + Al + Fe 3+ ) = 66.02, Mg/(Mg + Fe 2+ ) = 49.05 and Cr/(Cr + Al + Fe 3+ ) = 69.14, Mg/(Mg + Fe 2+ ) = 49.81, respectively]. The larger concentrations of chromite include brecciated and net-textured ore or massive veins that reflect the dynamic accumulation of chromite crystals in cavities along magma conduits inside the dunite body. These cavities are analogous to small magma chambers within the consolidated part of the dunite body and, similar to podiform chromitites in ophiolites, were created by continuous feeding of magma to the system. The platinum-group element (PGE) content of the chromite-free facies of the dunite is relatively low, Pt being the only significant PGE (typically between 0.3–35 ppb but as much as 98 ppb in one sample). In contrast, both types of chromite occurrence are enriched in PGE. Two PGE mineralizing episodes are distinguished: an Ir-Ru-(Rh ± Os)-rich, Pt-Pd-poor episode in which the platinum-group minerals (PGM) are mainly Ir alloys, and a Pt-(Pd)-rich episode in which the PGM are mainly Pt alloys (i.e., Pt 2 (Fe,Ni,Cu), PtFeNi 2 , and tetraferroplatinum- and tulameenite-like compositions, along with rare hollingworthite and an unidentified RhNiAs mineral). The primary PGM have been affected locally by a serpentinization-related low-temperature alteration that is characterized by addition of copper, with alteration to complex Pt-(Fe,Ni,Cu) alloys of varied stoichiometry, including tulameenite and platinian copper. Placer concentrations derived from the Nizhny Tagil Complex have a slightly different assemblage of PGM, with abundant isoferroplatinum, the presence of gold commonly aggregating the PGM, and unusual Au-Sn and Au-Pb alloys. The Kachkanar Complex, situated 120 km north of the Nizhny Tagil Complex, has essentially the same characteristics, apart from a slightly different composition of the ultramafic cumulates suggesting that they were derived from a magma of different composition. The PGM distribution also differs in that it contains more abundant isoferroplatinum than other PGM. The high Pt concentrations are the result of the affinity of platinum minerals for chromite and the mode of chromite deposit formation. In the cavities in which chromite crystallized, PGM tended to be incorporated in chromite or remained attached to chromite crystals. To explain the very high concentrations of chromite-associated PGM we define a CR factor, which is the ratio of the mass of silicate magma to the mass of chromite in contact with the ascending magma. Where the CR is high (i.e., a relatively low proportion of chromite in contact with a large amount of magma crystallizing the Pt minerals), the efficiency of the mechanical collection of Pt minerals is at a maximum, and the Pt concentration (i.e., amount of Pt minerals) will be high. Possible variations due to the collection of PGE by a sulfide liquid have not been established. A model is proposed whereby the observed chromite and associated Pt mineralization can result from the normal magmatic evolution of the complex, provided that a structural (hydrodynamic) trap is available to concentrate the chromite.

Spatial Constraints on Magma Chamber Replenishment Events from Textural Observations of Cumulates: the Rum Layered Intrusion, Scotland

The Eastern Layered Series of the Rum Layered Suite is formed of 16 macro-units each comprising a lower peridotite and an overlying feldspar-rich layer (the local term is allivalite). The origin of the peridotite layers is unresolved, with two contrasting models. The earlier of the two is based on repeated replenishment of an opensystem magma chamber with deposition of fractionated material on the chamber floor. The second is based on the early formation of a troctolitic complex, which is then repeatedly intruded by sills of replenishing picritic magma to form the peridotite horizons. The lack of resolution of this fundamental problem is a consequence of the reliance of previously published studies on field observations. I present evidence to show that the clinopyroxene in the allivalites preserves information about the distribution of the last melt to solidify, permitting determination of not only the extent of supersolidus textural equilibration but also the sub-solidus history of the allivalite horizons. Comparison of profiles of clinopyroxene– plagioclase–plagioclase median dihedral angle across allivalite units demonstrates that it is possible to distinguish between those that were intruded by later picrite sills and those adjacent to peridotite horizons formed by replenishment and subsequent deposition of fractionated crystals above the pre-existing pile. In the region studied, only the main peridotite body of Unit 9 was intruded into a pre-existing allivalitic mush.

Open-System Magma Chamber Evolution: an Energy-constrained Geochemical Model Incorporating the Effects of Concurrent Eruption, Recharge, Variable Assimilation and Fractional Crystallization (EC-E'RA FC)

Significant petrogenetic processes governing the geochemical evolution of magma bodies include magma Recharge (including formation of ‘quenched inclusions’ or enclaves), heating and concomitant partial melting of country rock with possible ‘contamination’ of the evolving magma body (Assimilation), and formation and separation of cumulates by F ractional Crystallization (RAFC). Although the importance of modeling such open-system magma chambers subject to energy conservation has been demonstrated, the effects of concurrent removal of magma by eruption and/or variable assimilation (involving imperfect extraction of anatectic melt from wall rock) have not been considered. In this study, we extend the EC-RAFC model to include the effects of Eruption and variable amounts of assimilation, Ax. This model, called EC-E 0RAcFC, tracks the compositions (trace elements and isotopes), temperatures, and masses of magma body liquid (melt), eruptive magma, cumulates and enclaves within a composite magmatic system undergoing simultaneous eruption, recharge, assimilation and fractional crystallization. The model is formulated as a set of 4 þ t þ i þ s coupled nonlinear differential equations, where the number of trace elements, radiogenic and stable isotope ratios modeled are t, i and s, respectively. Solution of the ECE 0RAcFC equations provides values for the average temperature of wall rock (Ta), mass of melt within the magma body (Mm), masses of cumulates (Mct), enclaves (Men) and wall rock (M o a) and the masses of anatectic melt generated (M a ) and assimilated (cM a ). In addition, t trace element concentrations and i þ s isotopic ratios in melt and eruptive magma (Cm, em, dm), cumulates (Cct, em, dm), enclaves (Cen, er , d o r ) and anatectic melt (Ca, ea, d o a) as a function of magma temperature (Tm) are also computed. Input parameters include the (user-defined) equilibration temperature (Teq), a factor describing the efficiency of addition of anatectic melt (c) from country rock to host magma, the initial temperature and composition of pristine host melt (Tom, C o m, em, d o m), recharge melt (T o r , C o r , er , dor ) and wall rock (T o a, C o a, ea, d o a), distribution coefficients (Dm, Dr, Da) and their temperature dependences (DHm, DHr, DHa), latent heats of transition (melting or crystallization) for wall rock (Dha), pristine magma (Dhm) and recharge magma (Dhr) as well as the isobaric specific heat capacity of assimilant (Cp,a), pristine (Cp,m) and recharge (Cp,r) melts. The magma recharge mass and eruptive magma mass functions,Mr(Tm) andMe(Tm), respectively, are specified a priori. Mr(Tm) and Me(Tm) are modeled as either continuous or episodic (step-like) processes. Melt productivity functions, which prescribe the relationship between melt mass fraction and temperature, are defined for end-member bulk compositions characterizing the local geologic site. EC-E 0RAcFC has potential for addressing fundamental questions in igneous petrology such as: What are intrusive to extrusive ratios (I/E) for particular magmatic systems, and how does this factor relate to rates of crustal growth? How does I/E vary temporally at single, long-lived magmatic centers? What system characteristics are most profoundly influenced by eruption? What is the quantitative relationship between recharge and assimilation? In cases where the extraction efficiency can be shown to be less than unity, what geologic criteria are important and

The north?south propagating spreading center of the North Fiji Basin. Modeling of the geochemical evolution in periodically replenished and tapped magma chambers

Two main numerical approaches have been previously used to model the behavior of replenished and tapped magma chambers from geochemical data: 1) iterative computations, in which the magma evolution within steady-state reservoirs is modeled cyclically (P-RTF models); each cycle involves adding recharge magma, mixing the remaining liquid together, crystallizing the mixed product, then expelling part of the residual liquid (Model A); the expulsion can also take place after the mixing event but before crystallization (Model B); 2) continuous models (C-RTF models): in the corresponding time-dependent equations, the magma undergoes fractional crystallization with simultaneous replenishment of fresh liquid (Models C and D). A pertinent test of these models requires a cogenetic magmatic series having geochemical data that are not consistent with closed-system fractional crystallization. The northern tip of the north–south propagating spreading center, located in the North Fiji Basin between 18° and 19° S (NS-PSC 18–19° S), responds to this requirement. The lava ages range from 0 to 1 Ma. The dredged volcanic rocks studied are cogenetic in a broad sense (constant isotopic and incompatible trace elements ratios). While no petrographic indications of wall-rock assimilation have been found, evidence of magma mixing has been observed in one basaltic sample (ribbon structures). The lavas, which are normal mid-ocean ridge basalts (N-MORBs), are distributed between three homogeneous compositional groups spatially ordered. The most differentiated lavas have a Fe-Ti basalt composition. We find that one version of open-system fractionation in a periodically replenished reservoir (Model B) is consistent with both the petrologic and geochemical data in explaining the formation of the two most mafic lava groups (Group 1, 64≥mg#≥61; Group 2, 59≥mg#≥52). In our model, the liquids expelled from a first magma chamber at the end of each cycle (Group 1 magmas) feed a second reservoir, which in its turn expels cyclically Group 2 liquids. A part of these expelled liquids are then stocked in a third closed-system magma chamber, where the Fe-Ti basalts (Group 3 lavas: 50≥mg#≥46) are generated through additional crystallization. Thus, the NS-PSC 18–19° S lavas seem to have been produced by three magma chambers interconnected by a sill (and/or pipe) network, ending in the last 18 km of the northern tip. Consequently, only a small fraction of magma expelled from each open-system magma chamber reaches the surface as lava flows, because a fraction of it migrates from one reservoir to another. The off-axis sampling provides evidence for the persistence of open-system fractionation over time.

Periodic behavior in lava dome eruptions

Lava dome eruptions commonly display fairly regular alternations between periods of high activity and periods of low or no activity. The time scale for these alternations is typically months to several years. Here we develop a generic model of magma discharge through a conduit from an open-system magma chamber with continuous replenishment. The model takes account of the principal controls on flow, namely the replenishment rate, magma chamber size, elastic deformation of the chamber walls, conduit resistance, and variations of magma viscosity, which are controlled by degassing during ascent and kinetics of crystallization. The analysis indicates a rich diversity of behavior with periodic patterns similar to those observed. Magma chamber size can be estimated from the period with longer periods implying larger chambers. Many features observed in volcanic eruptions such as alternations between periodic behaviors and continuous discharge, sharp changes in discharge rate, and transitions from effusive to catastrophic explosive eruption can be understood in terms of the non-linear dynamics of conduit flows from open-system magma chambers. The dynamics of lava dome growth at Mount St. Helens (1980–1987) and Santiaguito (1922–2000) was analyzed with the help of the model. The best-fit models give magma chamber volumes of ∼0.6 km3 for Mount St. Helens and ∼65 km3 for Santiaguito. The larger magma chamber volume is the major factor in explaining why Santiaguito is a long-lived eruption with a longer periodicity of pulsations in comparison with Mount St. Helens.

A Gold- and Platinum-Mineralized Layer in Gabbros of The Kap Edvard Holm Complex: Field, Petrologic, and Geochemical Relations

The Lower Layered Series a cumulates of the Kap Edvard Holm Complex contain a stratiform layer, 1 to 20 m thick and > 6 km long, that is mineralized with gold and platinum-group elements (PGEs). The Lower Layered Series a cumulates (>650 m thick) are dominated by modally layered gabbro with lesser volumes of granular anorthosite and troctolite that together form 2- to 30-m-thick layers that are most abundant above the mineralized layer. Minor volumes of poikilitic wehrlite form a few 1- to 2-m-thick layers throughout the section. Field and petrographic relations as well as cryptic variations in cumulus mineral compositions indicate that the Lower Layered Series a formed in an open system magma chamber from repeated magma injections. Within the mineralized layer, average concentrations over 3 m thickness are 250 ppb Pt, 40 ppb Pd, and 50 ppb Au, with individual samples containing up to 5 ppm Pt and 6 ppm Au. The concentrations of Ir, Os, Ru, and Rh are uniformly low (<20 ppb). Stratigraphic zoning of Pt, Pd, and Au is consistent along strike with peak Au concentrations overlying Pt and Pd peaks. Platinum minerals are primary Pt-Fe alloy, sperrylite, and moncheite, with secondary Pt sulfide overgrowths. Gold occurs as alloys with Ag and Cu. These minerals are found in contact with primary and secondary silicates and oxides and with intercumulus base metal sulfides that consist of the assemblage bornite + chalcopyrite + digenite. Deposition of the mineralized layer is associated in space and time with injections of plagioclase-saturated, primitive magma that mixed with more evolved, resident magma. These events caused efficient partitioning of Au and PGE into sulfides derived from a large volume of magma, deposition of these sulfides to form the mineralized layer, and subsequent deposition of the overlying anorthosite layers. The wehrlite layers, however, were formed by postcumulus reactions between gabbro cumulates and hydrous, intercumulus melts, probably derived from the compacting cumulus pile. Subsolidus reactions between primary minerals and meteoric hydrothermal solutions caused extensive alteration of the mineralized gabbro, oxidation and desulfurization of Cu-Fe sulfides and local recrystallization of PGE minerals.