Sidewall Crystallization Research Articles

Open-system magmatic behaviour beneath monogenetic volcanoes revealed by the geochemistry, texture and thermobarometry of clinopyroxene, Kaikohe-Bay of Islands volcanic field (New Zealand)

The ascent and temporary crustal storage of magmas beneath basaltic monogenetic volcanoes in continental settings is poorly understood, but is important in the study of their petrogenesis, and for monitoring future volcanic unrest. The Kaikohe-Bay of Islands volcanic field in northern New Zealand comprises Quaternary-aged monogenetic basalt volcanoes in an intraplate setting. Clinopyroxene phenocrysts in the basalts provide an opportunity to investigate the ascent history of the magmas that may not be evident from studies at the scale of whole-rock samples. The phenocrysts are texturally and compositionally diverse, and demonstrate the dominance of open system processes such as magma mixing and crystal entrainment in the crust. Many phenocrysts have an Mg (Mg# ~80–83) and Cr rich rim overgrowth which is in equilibrium with the host rock, but a resorbed, core (Mg# ~70) that crystallised from a more evolved magma. These crystals record mafic recharge, presumably the trigger to eruption. Other crystals are characterised by a high-Mg (Mg# ~82–84) and Cr core surrounded by a disequilibrium, low-Mg (Mg# 70–75) rim. The rims are reversely zoned and demonstrate late-stage equilibrium with the host rock. These crystals either nucleated in the host magma or were entrained from earlier basaltic intrusions of similar composition. The rim zones record re-melting of intrusions, or interaction with more differentiated magmas, followed by mafic recharge. Subordinate crystal types include oscillatory zoned crystals (Mg# >80) that nucleated in the ascending magma, and diffusely, patchy-zoned crystals that are variously antecrystic or xenocrystic in origin. Cognate gabbro inclusions contain clinopyroxene in equilibrium with the host basalt, suggesting they originated from side-wall crystallisation. Crystal-melt equilibria indicate that the cognate clinopyroxene formed at 430 ± 170 MPa (2σ) (=16 ± 6 km depth), and a subordinate population formed at 730 ± 160 MPa (=28 ± 6 km depth). These depths coincide with major seismic velocity contrasts at a zone of partial melt (10–19 km) and the Moho (~28 km). Thus, buoyancy or rheology contrasts in the crust temporarily slowed magma ascent and promoted periods of crystallisation and the assimilation of antecrystic and xenocrystic components. Similar magmatic processes occurred at the neighbouring and contemporaneous Whangarei volcanic field. Both of these fields have been active for millions of years, and longevity could explain the formation of differentiated crustal instrusives. In systems where magmas temporarily stall, there is a greater likelihood of detecting pre-eruption geophysical phenomena that could act as signals to pending eruptions.

Copper and Li diffusion in plagioclase, pyroxenes, olivine and apatite, and consequences for the composition of melt inclusions

Silicate melt inclusions are an important tool to reconstruct original concentrations of volatiles and metals in silicate melts, as these constituents are partly lost during magma solidification. However, melt inclusions analyzed in co-precipitated minerals in volcanic rocks commonly show strongly divergent Cu contents, raising doubts about the validity of their composition. To understand the origin of this divergence a multiple approach was followed. First, the phenomenon was documented by recording petrographic details of five volcanic samples and analyzing 189 melt inclusions by laser-ablation ICP-MS. Second, diffusion experiments on gem-quality plagioclase, clinopyroxene, orthopyroxene, apatite and olivine were performed in a gas mixing furnace to constrain diffusion coefficients of Cu and to a lesser extent of Li. Third, re-equilibration experiments were performed on melt inclusions within plagioclase crystals to induce changes in their Cu and Li contents.The LA-ICP-MS analyses reveal that plagioclase-hosted and orthopyroxene-hosted melt inclusions in volcanic rocks commonly contain an order of magnitude more Cu than melt inclusions in co-precipitated clinopyroxene and olivine. Plagioclase-hosted melt inclusions in intrusive rocks, on the other hand, do not show this divergence. The diffusion experiments conducted on minerals find that Cu and Li diffusion in plagioclase is extremely fast (log D = −13.0 to −11.5 m2 s−1 at 1000 °C) and in both cases occurs via two separate diffusion mechanisms. Copper diffusion coefficients in apatite, clinopyroxene, orthopyroxene and olivine are 2–3 orders of magnitude lower but are still high compared to most other elements. Both the re-equilibration experiments on melt inclusions and quantitative modeling based on the measured diffusion coefficients demonstrate that at 1000 °C plagioclase-hosted melt inclusions can re-equilibrate their Cu and Li content with that of the surrounding magma within a few hours to a few weeks, whereas apatite-, clinopyroxene-, orthopyroxene- and olivine-hosted melt inclusions require tens of years to hundreds of years to do so. Abnormally high Cu contents of plagioclase- and orthopyroxene-hosted melt inclusions appear to result from postentrapmental Cu gain. In the case of orthopyroxene-hosted melt inclusions Cu seems to have been gained as a consequence of postentrapmental sidewall crystallization, which triggered the formation of sulfide globules that acted as a Cu sink. This process likely occurred at depth in magma chambers. Plagioclase-hosted melt inclusions, on the other hand, seem to have gained Cu in exchange for hydrogen that diffused out of the melt inclusions during or after volcanic eruption. Our results suggest that Cu (and also Li) concentrations measured in melt inclusions have to be generally treated with caution, and that particular prudence has to be used in the case of plagioclase and orthopyroxene hosts.

Evidence for the presence of carbonate melt during the formation of cumulates in the Colli Albani Volcanic District, Italy

Fergusite and syenite xenoliths and mafic lapilli from two locations in the Villa Senni ignimbrite of the Colli Albani Volcanic District show evidence for fractionation of a silicate magma that led to exsolution of an immiscible carbonate melt. The fergusite xenoliths are divided into two groups on the basis of their clinopyroxene compositions. Group 1 clinopyroxene records the crystallisation of a silicate melt and enrichment of the melt in Al, Ti and Mn and depletion in Si as well as enrichment in incompatible trace elements. The second group of clinopyroxene compositions (group 2) comes mainly from Ba-F-phlogopite- and Ti-andradite-bearing fergusites. They have significantly higher Si and lower Al and Ti and, like the coexisting phlogopite and garnet are strongly enriched in Mn. The minerals in the fergusites containing group 2 clinopyroxene are enriched in Ba, Sr, Cs, V and Li all of which are expected to partition strongly into a carbonate melt phase relative to the coexisting silicate melt. The compositional data suggest that the group 1 fergusites record sidewall crystallisation of CO2-rich silicate melt and that once the melt reached a critical degree of fractionation, carbonate melt exsolved. The group 2 fergusites record continued crystallisation in this heterogeneous silicate – carbonate melt system. Composite xenoliths of fergusite and thermometamorphic skarn record contact times of hundreds to a few thousand years indicating that fractionation and assimilation was relatively rapid.

Geochemical homogeneity of a long-lived, large silicic system; evidence from the Cerro Galán caldera, NW Argentina

By applying a number of analytical techniques across a spectrum of spatial scales (centimeter to micrometer) in juvenile components, we show that the Cerro Galan volcanic system has repeatedly erupted magmas with nearly identical geochemistries over >3.5 Myr. The Cerro Galan system produced nine ignimbrites (∼5.6 to 2 Ma) with a cumulative volume of >1,200 km3 (DRE; dense rock equivalent) of calc-alkaline, high-K rhyodacitic magmas (68–71 wt.% SiO2). The mineralogy is broadly constant throughout the eruptive sequence, comprising plagioclase, quartz, biotite, Fe–Ti oxides, apatite, and titanite. Early ignimbrite magmas also contained amphibole, while the final eruption, the most voluminous Cerro Galan ignimbrite (CGI; 2.08 ± 0.02 Ma) erupted a magma containing rare amphibole, but significant sanidine. Each ignimbrite contains two main juvenile clast types; dominant “white” pumice and ubiquitous but subordinate “grey” pumice. Fe–Ti oxide and amphibole-plagioclase thermometry coupled with amphibole barometry suggest that the grey pumice originated from potentially hotter and deeper magmas (800–840°C, 3–5 kbar) than the more voluminous white pumice (770–810°C, 1.5–2.5 kbar). The grey pumice is interpreted to represent the parental magmas to the Galan system emplaced into the upper crust from a deeper storage zone. Most inter-ignimbrite variations can be accounted for by differences in modal mineralogy and crystal contents that vary from 40 to 55 vol.% on a vesicle-free basis. Geochemical modeling shows that subtle bulk-rock variations in Ta, Y, Nb, Dy, and Yb between the Galan ignimbrites can be reconciled with differences in amounts of crystal fractionation from the “grey” parent magma. The amount of fractionation is inversely correlated with volume; the CGI (∼630 km3) and Real Grande Ignimbrite (∼390 km3) return higher F values (proportion of liquid remaining) than the older Toconquis Group ignimbrites (<50 km3), implying less crystal fractionation took place during the upper-crustal evolution of these larger volume magmas. We attribute this relationship to variations in magma chamber geometry; the younger, largest volume ignimbrites came from flat sill-like magma chambers, reducing the relative proportion of sidewall crystallization and fractionation compared to the older, smaller-volume ignimbrite eruptions. The grey pumice clasts also show evidence of silicic recharge throughout the history of the Cerro Galan system, and recharge days prior to eruption has previously been suggested based on reversely zoned (OH and Cl) apatite phenocrysts. A rare population of plagioclase phenocrysts with thin An-rich rims in juvenile clasts in many ignimbrites supports the importance of recharge in the evolution and potential triggering of eruptions. This study extends the notion that large volumes of nearly identical silicic magmas can be generated repeatedly, producing prolonged geochemical homogeneity from a long-lived magma source in a subduction zone volcanic setting. At Cerro Galan, we propose that there is a zone between mantle magma input and upper crustal chambers, where magmas are geochemically “buffered”, producing the underlying geochemical and isotopic signatures. This produces the same parental magmas that are delivered repeatedly to the upper crust. A lower-crustal MASH (melting, assimilation, storage, and homogenization) zone is proposed to act as this buffer zone. Subsequent upper crustal magmatic processes serve only to slightly modify the geochemistry of the magmas.

Accurate quantification of melt inclusion chemistry by LA-ICPMS: a comparison with EMP and SIMS and advantages and possible limitations of these methods

Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) has recently emerged as a powerful in situ microanalytical technique for major to trace elements in heterogeneous samples such as fluid and melt inclusions. Here, a rigorous comparison of melt inclusion (MI) data acquired by electron microprobe (EMP), ion microprobe (the secondary ion mass spectrometry, SIMS) and LA-ICPMS is used to evaluate the applicability and advantages/drawbacks of these approaches. We are specifically interested in determining if LA-ICPMS data on entire, unexposed, crystallized MI that cannot be homogenized in the lab are accurate and of a useful precision. Quantification of LA-ICPMS MI signals requires the use of an internal standard, i.e., the concentration of one element, or an element ratio, at the time of MI entrapment must be known independently, in order to derive the pure MI composition from the MI plus host mixed signal. Analysis of plagioclase-hosted glassy MI of a mid-ocean ridge basalt (MORB) sample from the East Pacific Rise illustrates that melt inclusion chemistry can be accurately quantified by LA-ICPMS, including the correction for postentrapment sidewall crystallisation of the host mineral without prior reheating in the lab. The LA-ICPMS data obtained on crystallized MI demonstrate agreement with the EMP and SIMS data on exposed glassy MI at the 1 standard deviation uncertainty level except for a few elements close to their limits of detection. LA-ICPMS data reduction schemes include the quantification of analytical uncertainty on each element of single MI. Therefore, weighted average element concentrations can be obtained for MI assemblages, at precisions that compare well with those of average element concentrations obtained by EMP and SIMS. Simple sample preparation minimizing inclusion loss through polishing combined with the analytical efficiency of 50 inclusions plus neighbouring host mineral at up to 40 elements per day enable the collection of statistically relevant datasets by LA-ICPMS. These allow to recognize nonrepresentative MI (e.g., heterogeneous entrapment). Application to individual clinopyroxene crystals from the AD79 pumice horizon of Mt. Somma-Vesuvius reveals chemical variability that exceeds the analytical precision on single melt inclusions. This variability was not obvious from the limited data set obtained by SIMS and EMP. The largest source of nonquantifiable error for EMP and SIMS data stems from the requirement of reheating the melt inclusions in the lab in order to reverse postentrapment crystallisation onto inclusion walls or growth of crystallites. For LA-ICPMS analysis of unexposed MI, the reliability with which the internal standard (IS) element concentration is known determines the quality of the data. LA-ICPMS, however, cannot analyse H 2O, F, S and Cl reliably, has higher limits of detection (LODs) than SIMS for some elements for MI below ∼25 μm, has lower spatial resolution than both EMP and SIMS and consumes much more sample per analysis. Therefore, EMP, SIMS and LA-ICPMS are complementary in MI research, and the type of application will determine the analytical method or methods of choice.

Quantitative multi-element analysis of minerals, fluid and melt inclusions by laser-ablation inductively-coupled-plasma mass-spectrometry

Laser-ablation ICPMS has become widely accessible as a powerful and efficient multi-element microanalytical technique. One of its key strengths is the ability to analyse a wide concentration range from major (tens of wt.%) to trace (ng/g) levels in minerals and their microscopic inclusions. An ArF excimer laser system (λ = 193 nm) with imaging optics for controlled UV ablation and simultaneous petrographic viewing was designed specifically for representative sampling and quantitative multi-element analysis of microscopic fluid, melt and mineral inclusions beneath the sample surface. After a review of the requirements and recent technical developments, results are presented which together document the reliability and reproducibility of quantitative microanalysis of complex samples such as zoned crystals or fluid and melt inclusions in various host minerals. Analytical errors due to elemental fractionation are reduced to the typical precision achieved by quadrupole LA-ICPMS in multi-element mode (2–5% RSD). This progress is largely due to the small size of aerosol particles generated by the optimized UV optical system. Depth profiling yields representative and accurate concentration results at a resolution of ∼0.1 μm perpendicular to the ablation surface. Ablation is largely matrix-insensitive for different elements, such that silicate and borate glasses, silicates and oxide minerals, or direct liquid ablation can be used interchangeably for external standardization of any homogeneous or heterogeneous material. The absolute ablation rate is material dependent, however, so that quantitative LA-ICPMS analysis requires an internal standard (i.e., an independent constraint such as the absolute concentration of one element). Our approach to quantifying fluid and melt inclusion compositions is described in detail. Experiments with synthetic fluid inclusions show that accurate results are obtained by combining the LA-ICPMS analysis of element concentration ratios with a microthermometric measurement of the NaCl equivalent concentration and an empirical description of the effect of major cations on the final melting temperatures of ice, hydrohalite or halite. Expected calibration errors for NaCl-H 2O-dominated fluids are smaller than the typical analytical scatter within an assemblage of simultaneously trapped fluid inclusions. Analytical precision is limited by representative ablation of all phases in heterogeneous inclusions and the integration of transient ICPMS signals, to typically ±10 to 20% RSD. Element concentrations in devitrified and even coarsely crystallized silicate melt inclusions can be reconstituted from LA-ICPMS signals. Deconvolution of inclusion and host signals with internal standardization automatically corrects for sidewall crystallization after melt entrapment at high temperature. A test using melt inclusions in a midocean ridge basalt, a summary of published geochemical studies and a new application to REE analysis of coexisting fluids and mineral phases in carbonatite-related veins illustrate the versatility and some of the strengths and limitations of LA-ICPMS, in comparison with other microanalytical techniques.

The Oligocene Lund Tuff, Great Basin, USA: a very large volume monotonous intermediate

Unusual monotonous intermediate ignimbrites consist of phenocryst-rich dacite that occurs as very large volume (>1000 km 3) deposits that lack systematic compositional zonation, comagmatic rhyolite precursors, and underlying plinian beds. They are distinct from countless, usually smaller volume, zoned rhyolite–dacite–andesite deposits that are conventionally believed to have erupted from magma chambers in which thermal and compositional gradients were established because of sidewall crystallization and associated convective fractionation. Despite their great volume, or because of it, monotonous intermediates have received little attention. Documentation of the stratigraphy, composition, and geologic setting of the Lund Tuff – one of four monotonous intermediate tuffs in the middle-Tertiary Great Basin ignimbrite province – provides insight into its unusual origin and, by implication, the origin of other similar monotonous intermediates. The Lund Tuff is a single cooling unit with normal magnetic polarity whose volume likely exceeded 3000 km 3. It was emplaced 29.02±0.04 Ma in and around the coeval White Rock caldera which has an unextended north–south diameter of about 50 km. The tuff is monotonous in that its phenocryst assemblage is virtually uniform throughout the deposit: plagioclase>quartz≈hornblende>biotite>Fe–Ti oxides≈sanidine>titanite, zircon, and apatite. However, ratios of phenocrysts vary by as much as an order of magnitude in a manner consistent with progressive crystallization in the pre-eruption chamber. A significant range in whole-rock chemical composition (e.g., 63–71 wt% SiO 2) is poorly correlated with phenocryst abundance. These compositional attributes cannot have been caused wholly by winnowing of glass from phenocrysts during eruption, as has been suggested for the monotonous intermediate Fish Canyon Tuff. Pumice fragments are also crystal-rich, and chemically and mineralogically indistinguishable from bulk tuff. We postulate that convective mixing in a sill-like magma chamber precluded development of a zoned chamber with a rhyolitic top or of a zoned pyroclastic deposit. Chemical variations in the Lund Tuff are consistent with equilibrium crystallization of a parental dacitic magma followed by eruptive mixing of compositionally diverse crystals and high-silica rhyolite vitroclasts during evacuation and emplacement. This model contrasts with the more systematic withdrawal from a bottle-shaped chamber in which sidewall crystallization creates a marked vertical compositional gradient and a substantial volume of capping-evolved rhyolite magma. Eruption at exceptionally high discharge rates precluded development of an underlying plinian deposit. The generation of the monotonous intermediate Lund magma and others like it in the middle Tertiary of the western USA reflects an unusually high flux of mantle-derived mafic magma into unusually thick and warm crust above a subducting slab of oceanic lithosphere.

The Whakamaru group ignimbrites, Taupo Volcanic Zone, New Zealand: evidence for reverse tapping of a zoned silicic magmatic system

The Whakamaru group ignimbrites are widespread voluminous welded ignimbrites which crop out along the eastern and western margins of the Taupo Volcanic Zone (TVZ), New Zealand. The ignimbrites have a combined volume exceeding 1000 km 3, and were erupted from a large caldera in the central TVZ around 340 ka, following a c. 350 ka hiatus in caldera-forming activity in TVZ. Analysis of individual pumice clasts identifies five distinct magma types (rhyolite types A to D, and high alumina basalt) and significant gradients in temperature, water content, and Sr isotopic composition in the pre-eruptive Whakamaru magmatic system. There is a marked variation in mineral assemblage with composition; type A low-silica rhyolite pumices contain plagioclase, quartz, orthopyroxene, hornblende, biotite, and magnetite/ilmenite with distinctive large rounded quartz phenocrysts. High-silica (types B and C) pumices contain quartz (smaller, subhedral phenocrysts), plagioclase, sanidine, biotite, and magnetite/ilmenite. Type D pumices are rich in plagioclase and biotite phenocrysts, and have anomalously high Rb contents (>200 ppm) relative to all other pumice types. Rhyolite types B and C are related to type A magma by a two-stage crystal fractionation process, probably by side wall crystallisation and convective fractionation. The first stage involved 30–40% fractionation of a plagioclase-dominated (sanidine-free) assemblage to produce a type B magma, which in turn underwent fractionation of a plagioclase/quartz/sanidine assemblage to produce the highly evolved, but relatively Ba-depleted, type C magmas. Stratigraphic variations in modal proportions of mineral phases, and calculated Fe–Ti oxide equilibrium temperatures indicate that eruptions commenced with the hottest, least evolved magmas, and more evolved magmas became important at a later stage in the eruption along with a high alumina basalt component. This reverse-zoned sequence precludes simple sequential tapping of a large zoned magma chamber, and indicates a complex magma chamber configuration and/or withdrawal dynamics during eruption. Type D magma, which appears to be unrelated to either types A or B by crystal fractionation, may have formed a separate subjacent chamber that was ruptured and incorporated into the eruption. The Whakamaru magma system provides clear evidence that (less evolved) low silica rhyolites undergo significant fractionation at shallow crustal levels in central TVZ, to produce the generally more evolved rhyolites more commonly erupted at the surface, and suggests large ignimbrite eruptions may tap multiple magma chambers.

Constraints on the origin of zonation of the granite complexes in the Fichtelgebirge (Germany and Czech Republic): evidence from a gravity and geochemical study

Compositional zoning of granitic plutons is a common feature, but the spatial variation of zoning is rarely studied in detail. It is suggested that important additional information can emerge if the compositional zoning is regarded with relation to the root zones, i.e. with the granite shape at depth. A combined geochemical and gravity investigation was performed on the Hercynian Fichtelgebirge granites, which are composed of an older intrusive complex (OIC) and younger intrusive complex (YIC). The OIC and YIC are distinct from each other with respect to magma origin, differentiation pattern, shape of the granite complexes at depth and the zonation pattern in relation to the root zone. The OIC shows normal compositional zoning, whereas the YIC both reverse (YIC-1) and normal zoning (YIC-2). The general zonation pattern of the OIC and the YIC is the result of a combination of multiple injections of single magma batches and in situ differentiation during magma emplacement. In addition to the general zonation pattern, local zonation in individual granite bodies of the YIC was caused by sidewall crystallisation producing a less-differentiated marginal facies (G2) and a more-differentiated core facies (G3 and G4). It is suggested that the complex zonation patterns of the Fichtelgebirge granite complexes reflect interactions between the rate of magma output which is partly governed by their rheological properties and the rate of deformation which provides the room for the magma to be emplaced.

Origin and emplacement of a reversely zoned, Pan-African granitoid pluton from the Sinai Massif, Egypt

The Iqna granitoid pluton ( ca 580 Ma) is a post-orogenic Pan-African intrusion in the north central sector of the Sinai Massif. Three distinct zones can be recognised on the basis of field relationships and geochemistry. The pluton is reversely zoned from a margin of alkali feldspar granite through an internal zone of syenogranite to a core of monzogranite. The reverse zonation of the Iqna granitoid pluton is interpreted as an emplacement feature of partially fractionated diapirs. Fractional crystallisation modelling suggests that the early formed phases (plagioclase, K-feldspar, biotite and quartz) were produced by the side-wall crystallisation in a deeper magma chamber. The crystallisation of these early fractionates created a less dense residual melt that moved upwards in the magma chambers. Continuous upward accumulation of less dense (more silicic) melt gave rise to a vertically graded magma chamber. The higher-level emplacement of the Iqna Pluton began with the intrusion of the alkali feldspar granite diapir (which became the marginal zone) from the subjacent chamber. This was then followed by the emplacement of the syenogranite diapir mantle zone component that displaced the earlier granite to its margins and finally by the monzogranite portion of the magma chamber into the core of the pluton.

The Eocene Big Timber stock, south-central Montana: Development of extensive compositional variation in an arc-related intrusion by side-wall crystallization and cumulate glomerocryst remixing

The Eocene Big Timber stock in the Crazy Mountains of south-central Montana is an elliptical, 8 by 13 km, compositionally and texturally diverse composite intrusion with a well-developed radial dike swarm. A sharp intrusive contact separates its two phases: the core of the intrusion is fine-grained quartz monzodiorite, and the volumetrically dominant remainder is composed of medium-grained diorite and gabbro. Differentiation-related major oxide variation within the stock is extensive and spatially nonsystematic. However, abundances of most trace elements were not strongly influenced by differentiation; late zircon and apatite fractionation caused moderate heavy and slight light rare earth element abundance depletions, respectively. Mineral compositions and assemblages indicate crystallization between ≈950 and 700 °C at a pressure of ≈0.8 kbar (3 km). Mixing models indicate that fractionation of varying amounts of plagioclase, orthopyroxene, clinopyroxene, magnesio-hastingsite, hornblende, biotite, titanite, apatite, and magnetite (the stock9s principal constituents, with quartz and potassium feldspar) and remixing of these minerals and residual liquids controlled compositional evolution in the reservoir. Crystals apparently nucleated at the reservoir wall while residual silicate liquid was displaced inward and remixed. Some crystals were plucked from the solidification front, as indicated by glomerocrysts present throughout the stock, and also remixed with residual liquid. Solidification of the reservoir represented by the stock involved heat loss to enclosing wall rock, side-wall crystallization, and subsequent, variably effective, crystal-liquid remixing. This process is an important variant of conventionally invoked models pertaining to solidification of intrusions and explains extensive, relatively nonsystematic compositional variation. The genesis of compositional evolution in other intrusions characterized by extensive, spatially nonsystematic variation may result from the important process documented herein. Compositional and geologic relationships are consistent with magma genesis related to subduction and magmatic-arc processes inboard from the western edge of the early Cenozoic North American plate. Arc magmatism in south-central Montana during Eocene time is consistent with models pertaining to early Cenozoic southward sweep and westward retreat of magmatism. Magmatism represented by the Big Timber stock provides significant new support for steepening subduction, westward retreat of the subduction hinge line, and development of an asthenospheric mantle wedge that fueled renewed magmatism beneath the western edge of the North American continent following early Cenozoic shallow subduction.

High-Na dacite from the Jean Charcot Trough (Vanuatu), Southwest Pacific

Rifting that produced the Jean Charcot Trough (JCT) in the Vanuatu back-arc region commenced 2–3 Ma, leading to SiO 2-bimodal magmatism in the northern half of the trough. Dacite with high Na 2O (>6.0%) and low K 2O (<1.0%) was recovered from the northern termination of the JCT, accompanied by tholeiitic N-MORB-type back-arc basin basalt similar to that erupted at the central spreading axis of the North Fiji Basin (NFB). Incompatible element ratios are very similar in the dacite and associated basalt, and also in the NFB back-arc basin basalts ( Zr Rb >10 , Nb Rb >0.5 , Y Rb >2.0 , Nb La >0.5 , and Ba Zr <1.5 ), but are quite different to lavas from the active Vanuatu volcanic arc. The dacites are compositionally close to oceanic plagiogranite produced during fractional crystallization of an N-MORB basaltic magma, and mass balance calculations show that they may be similarly derived from the associated basalts. We show that JCT basalts are derived from a mantle source compositionally very close to that beneath the central spreading axis of the NFB, with minimal contamination by a subduction component. Westward injection of diapiritic source mantle similar to that of the NFB back-arc basin basalts may have caused rifting of the JCT. During incipient back-arc rifting, high Na/K felsic magmas may have evolved from parental low-K basalts via side-wall crystallization within a large lower crustal magma chamber.

Convective fractionation during magnetite and hematite dissolution in silicate melts

AbstractSingle crystals of magnetite and of hematite have been dissolved at atmospheric pressure in superheated melts in the systems CaO-MgO-Al2O3-SiO2 and CaO-Al2O3-SiO2, and in a basalt. The crystals were suspended in alumina crucibles containing ca 3.5 cm 3 of melt. Quenched run products were examined optically and by electron probe analysis to establish the distribution of Fe in the glassy charges. There is usually a concentration of Fe at the base of a run product, consistent with flow of dissolved matter from the crystal to the floor. One or more columns of brown, Fe-rich glass may extend from the underside of a relic crystal towards the floor. In CMAS run products, such columns typically extend this entire distance, whereas in the CAS and basalt run products, the columns are either detached from the crystal or do not reach the floor. In the CMAS melt (viscosity ~1 poise) there is apparently continuous release of Fe-bearing melt from around a dissolving crystal, whereas in the CAS and basalt melts (viscosities 7000 and 300 poise, respectively) release is intermittent. In CMAS run products the Fe content is usually greatest, in glass, at the base, and declines gradually upwards; in CAS and basalt runs the bottom of a crucible is occupied by discrete, sharply bounded pillows of Fetich glass, with only slight, or no, gradation in composition. Rising gas bubbles can elevate small blobs of the denser, Fe-bearing melt from around a dissolving crystal, and trains of bubbles in the CAS melt may guide this Fe-bearing melt, against gravity, to the surface of the charge. When the bubbles burst at the surface, this dense melt is left in an unstable location and releases diapirs which descend to the bottom of the crucible. In spite of the evidence that convective fractionation occurs in these haplomagmas and in the basalt, it remains to be demonstrated that it will occur during sidewall crystallization or during the growth of minerals in a cumulus mush to cause magmatic differentiation.

Differentiated rocks of the Galapagos hotspot

Abstract Virtually all the MORB-like tholeiites and differentiated rocks of the Galapagos Islands are concentrated along the central axis of the archipelago within 100 km of its leading island. Both have a remarkable symmetry. Those close to the centre are strongly tholeiitic, but towards the north and south they become increasingly alkaline. Most of the differentiated suites are readily explained by crystal fractionation at shallow depths, but voluminous, aphyric rhyolites erupted from a large active volcano immediately downstream from the hotspot are more difficult to explain. Although the geochemical criteria are consistent with crystal fractionation, these extreme differentiates are not accompanied by expected amounts of intermediate compositions, such as those found on older islands; instead, they have been erupted concurrently with basalts that have undergone only minor amounts of differentiation. The central region in which tholeiitic rocks and their differentiates have been erupted lies west of a transform fault that separates 10 million-year-old lithosphere on the west from thinner 5 million-year-old lithosphere on the east. It is also the region of thickest crust: the MOHO reaches a maximum depth of about 18 km below sea-level directly underneath the same region. Three possible mechanisms of differentiation have been considered for the voluminous rhyolites: (a) inward crystallization of a large reservoir; (b) side-wall crystallization with convective segregation of a light felsic liquid; and (c) melting of the base of the crust. Although the first two are consistent with the geochemical nature of the rhyolites, they are not compatible with the geological relations. Moreover, they do not explain why such rocks are found only in very restricted settings. In the case of the Galapagos, these extreme differentiates are found only in the region where the crust reaches a maximum thickness near the centre of the Galapagos Platform and directly downstream from the hotspot. On a global scale, oceanic rhyolites are found close to spreading ridges and nowhere else. These relations indicate that oceanic rhyolites are generated only where the lithospheric mantle is thin enough to permit the thermal effect of hotspots to reach the base of unusually thick crust. Remelting of pockets of felsic differentiates produced by earlier differentiation when the gabbroic layer was close to the ridge would result in trace element compositions consistent with crystal fractionation.

Tilting, burial, and uplift of the Guadalupe Igneous Complex, Sierra Nevada, California

Research Article| October 01, 1993 Tilting, burial, and uplift of the Guadalupe Igneous Complex, Sierra Nevada, California PETER J. HAEUSSLER; PETER J. HAEUSSLER 1Earth Sciences Board and Institute of Tectonics, University of California, Santa Cruz, California 95064 Search for other works by this author on: GSW Google Scholar SCOTT R. PATERSON SCOTT R. PATERSON 2Department of Geological Sciences, University of Southern California, Los Angeles, California 90089-0740 Search for other works by this author on: GSW Google Scholar Author and Article Information PETER J. HAEUSSLER 1Earth Sciences Board and Institute of Tectonics, University of California, Santa Cruz, California 95064 SCOTT R. PATERSON 2Department of Geological Sciences, University of Southern California, Los Angeles, California 90089-0740 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1993) 105 (10): 1310–1320. https://doi.org/10.1130/0016-7606(1993)105<1310:TBAUOT>2.3.CO;2 Article history First Online: 01 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 PETER J. HAEUSSLER, SCOTT R. PATERSON; Tilting, burial, and uplift of the Guadalupe Igneous Complex, Sierra Nevada, California. GSA Bulletin 1993;; 105 (10): 1310–1320. doi: https://doi.org/10.1130/0016-7606(1993)105<1310:TBAUOT>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 SocietyGSA Bulletin Search Advanced Search Abstract It is often incorrectly assumed that plutons have a relatively uneventful structural history after emplacement. The 151 Ma Guadalupe Igneous Complex (GIC) in the Foothills Terrane, California, was involved in three post-emplacement events: (1) ∼30° of southwestside-up tilting during ductile regional faulting and contraction, (2) burial of the pluton from ∼4 to 12 km during crustal thickening of the wall rocks, and (3) uplift with only minor tilting in the Late Cretaceous. Tilting of the pluton is indicated by (1) southwest to northeast gradational changes from layered gabbros and diorites to granites and granophyres; (2) northeastward dips of layering in gabbro, internal contacts, and bedding of overlying coeval(?) volcanic rocks; (3) northeastward decrease in wall-rock metamorphic grade; and (4) paleomagnetic data from 14 localities across the pluton. We argue that tilting occurred between 146-135 Ma during southwest-northeast-directed regional contraction. This contraction is indicated by widespread folds and cleavages and by reverse motion on the Bear Mountains fault zone (BMFZ), a large northeast-dipping shear zone that bounds the GIC on its southwest side. Burial of the GIC, which overlapped in time but outlasted tilting, is suggested by (1) post-emplacement contractional faulting, folding, and cleavage development; (2) analyses of strains associated with widespread cleavage that indicate vertical thickening of ∼100% and (3) microstructural and mineral assemblage data that indicate shallow emplacement of the GIC, in contrast to mineral assemblage and limited geobarometric data from adjacent 120-110 Ma plutons that indicate moderate emplacement levels. Late Cretaceous uplift is indicated by 95-75 Ma sedimentary rocks that unconformably overlie the 120-110 Ma plutons.This geologic history is interesting for several reasons. First, although the GIC participated in extensive post-emplacement deformation, it lacks internal structural evidence of these events, except locally along the Bear Mountains fault zone. Second, the agreement between paleomagnetic and structural evidence for tilting suggests that no large latitudinal displacement of the GIC is required. Third, the paleomagnetic data also help to define the geometry of the magma chamber now represented by the GIC. Lack of streaking of paleomagnetic site-mean directions demonstrates that the pluton acted as a single unit after cooling through the blocking temperature (450-560 °C) of low-titanium titanomagnetite; however, variations in the dip of internal layering and contacts, from 70° at the base to 30° near the top of the pluton, indicate that not all of these features were horizontal and planar when they formed. We propose that this variation in dip of layering is most consistent with sidewall crystallization of magma resulting in drape of layering along the walls of the intrusion. Therefore, internal layering within this pluton does not record paleohorizontal. 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.

Pre-eruption compositional gradients and mixing of andesite and dacite magma erupted from Nevado del Ruiz Volcano, Colombia in 1985

A small volume of mixed andesite and dacite magma was ejected as pumice fall, pyroclastic flow and pyroclastic surge during the 13 November 1985 explosive eruption of Nevada del Ruiz volcano in Colombia. Whole-rock compositions range from 59 to 63% SiO 2 and are classified as medium- to high-K andesites and dacites. Three types of pumices occur in the fall deposit: (1) homogeneous, crystal-rich dacite; (2) hetereogeneously mixed dacite and andesite; and (3) homogeneous andesite with abundant microlites (volumetrically dominant). Matrix glasses show a distinct compositional gap between 68 and 71% SiO 2. The gap is similar within heterogeneous pumices of type (2) and between homogeneous pumices of types (1) and (3). In contrast, melt inclusions in phenocrysts preserve a continuous compositional spectrum between the two end-member matrix glass compositions of the homogeneous andesite and dacite. Modelling of fractionational crystallization involving removal of plagioclase, clinopyroxene, orthopyroxene and Fe-Ti oxides can produce the liquid trend defined by melt inclusions. A compositional gap in the Ruiz pumice matrix glasses is attributed to the establishment of a two-layer zonation of the magma chamber prior to eruption. Collection of a fractionated liquid (dacite) at top of the chamber resulted from side-wall crystallization and fractionation of an andesitic magma body. Independent convection in each layer led to two thermal regimes which stabilized two dominant melt compositions. The large contrast in SiO 2 of these liquids reflects the shallow slope of cotectic boundaries in composition/temperature space of calc-alkaline magmas of these compositions. Intermediate liquids trapped by melt inclusions represent volumetrically small, random sampling of the fractionation process occuring at the margin of the andesite magma. Mixing of andesite and dacite magma occurred during the explosive eruption as draw-up of denser andesite magma through the overlying dacite took place. Fluid-dynamic calculations indicate that discharge conditions of the Ruiz eruption and the viscosity ratio of the magmas was suitable for the production of mixed pumices even though the flow was in the laminar regime. These observations corroborate experimental work by Blake and Campbell (1986) on the mixing of magmas with contrasting viscosites. Sulfur yield from the eruption, manifested by a large SO 2 plume detected by satellite, and high adsorbed sulfur content on tephra fallout, indicate total sulfur emission of 4.7 × 10 8 kg S, or about an order of magnitude larger than can be accounted for by degassing of the erupted magma. If a separate sulfur-rich vapor phase existed in the magma reservoir, as implied by our results, then its pre-eruption volume was of the order of 4 to 7 vol. % of the erupted magma.

Andesites from northeastern Kanaga Island, Aleutians

Kanaga island is located in the central Aleutian island arc. Northeastern Kanaga is a currently active late Tertiary to Recent calc-alkaline volcanic complex. Basaltic andesite to andesite lavas record three episodes (series) of volcanic activity. Series I and Series II lavas are all andesite while Series III lavas are basaltic andesite to andesite. Four Series II andesites contain abundant quenched magmatic inclusions ranging in composition from high-MgO low-alumina basalt to low-MgO highalumina basalt. The spectrum of lava compositions is due primarily to fractional crystallization of a parental low-MgO high-alumina basalt but with variable degrees of crustal contamination and magma mixing. The earliest Series I lavas represent mixing between high-alumina basalt and silicic andesite with maximum SiO2 contents of 65–67 wt %. Later Series I and all Series II lavas are due to mixing of andesite magmas of similar composition. The maximum SiO2 content of the pre-mixed andesites magmas is estimated at 60–63 wt %. The youngest lavas (Series III) are all non-mixed and have maximum estimated SiO2 contents of 59 wt %. The earliest Series I lavas contain a significant crustal component while all later lavas do not. It is concluded that the maximum SiO2 contents of silicic magmas, the contribution of crustal material to silicic magma generation, and the role of magma mixing all decrease with time. Furthermore, silicic magmas generated by fractional crystallization at this volcanic center have a maximum SiO2 content of 63 wt %. All of these features have also been documented at the central Aleutian Cold Bay Volcanic Center (Brophy 1987). Based on data from these two centers a model of Aleutian calc-alkaline magma chamber development is proposed. The main features are: (1) a single low pressure magma chamber is continuously supplied by primitive low-alumina basalt; (2) non-primary high-alumina basalt is formed along the chamber margins by selective gravitational settling of olivine and clinopyroxene and retention of plagioclase; (3) sidewall crystallization accompanied by crustal melting produces buoyant silicic (>63 wt % SiO2) liquids that pond at the top of the chamber, and; (4) continued sidewall crystallization, now isolated from the chamber wall, produces silicic liquids with ≤63 wt % SiO2 that increase the thickness and lowers the overall SiO2 content of the upper silicic zone. It is suggested that the maximum SiO2 content of 63% imposed on fractionation-generated magmas is due to a rheological barrier that prohibits the extraction of more silicic liquids from a crystal-liquid mush along the chamber wall.

The origin of alkaline magmas in an intraplate setting near a subduction zone: the Ngatutura Basalts, North Island, New Zealand

Abstract The Ngatutura Basalts are one of a series of Pliocene-Quaternary alkalic basalt volcanic fields in North Island, New Zealand. They are situated in an intraplate tectonic setting behind the currently active Taupo Volcanic Zone, and 300 km above the subducting slab. The volcanic field consists of 16 small-volume monogenetic volcanic centres composed mainly of eroded scoria cones and lava flows, that occupy an extensional tectonic environment characterized by NE-striking block faulting. In some cases the faults have controlled the localization of volcanic vents. The lavas have restricted compositions, ranging from hawaiites to nepheline hawaiites, and are characterized by enriched LILE, LREE, and HFS elements, with particularly high Nb and Ta, low Ba/Nb, and high Zr/Y and Ce N /Yb N ratios. Nepheline hawaiites are slightly more differentiated than hawaiites and have higher Ce N /Yb N ratios. Petrogenetic modelling suggests that the range in composition was mainly controlled by fractional crystallization of olivine, clinopyroxene, and minor plagioclase and titanomagnetite, which is consistent with the modal phenocryst abundances. Fractionation is explained by side-wall crystallization and flowage differentiation during rapid ascent, rather than gravitative settling in a magma chamber. Ngatutura magmas were probably derived from an enriched garnet lherzolite source within the low-velocity mantle. The process of source enrichment is speculative but our preferred model calls on metasomatizing fluids in the low-velocity zone. There is no geochemical evidence for any influence of the subducted slab on their composition, even though they overlie the Pacific plate subduction zone. This implies that the extent of subduction-related contamination in the mantle wedge is not pervasive, but is confined to a limited region overlying the subducted slab. Also, the “deep mantle plume” responsible for alkalic magmatism must have originated above the slab, because it seems unlikely that such a plume could have occurred at a deeper level and penetrate the slab without some evidence. This therefore limits the depth of origin of these “deep mantle plumes” to less than 300 km.

Temporal variation of mineralogy and petrology in cognate gabbroic enclaves at Arenal volcano, Costa Rica

Gabbroic enclaves ejected during the current eruption phase (A-1) and during the latest prehistoric eruption phase (A-2) of Arenal Volcano show systematic variations in texture, mineralogy and composition as a function of host rock chemistry and timing of eruption. The most differentiated enclaves occur in the more differentiated A-2 lavas. Enclaves in the A-1 volcanics are consistently less evolved. Within the current A-1 eruption, the most mafic enclaves are amphibole-bearing rocks that were erupted during the first 2–3 years of activity (1968–1970). These enclaves occur in the most differentiated A-1 volcanics and are not in equilibrium with their host rocks. They crystallized from a hydrous melt that was slightly more mafic than anything erupted during the current cycle. We interpret the enclaves as sidewall crystallization products of a melt, possibly a high-alumina basalt, that was immediately parental to the A-1 lavas. Enclaves that occur in A-1 rocks erupted after 1970 and all of the A-2 enclaves are amphibole-free and less mafic than the early A-1 enclaves. Their chemistry suggests that they formed during the early to intermediate crystallization of their host lavas. None of the enclaves contain minerals that might have equilibrated with a primary, mantle-derived melt. Geothermometry is consistent with geochemistry, with amphibole-bearing A-1 enclaves yielding the highest pyroxene temperatures (ave. 1090° C) and A-2 enclaves the lowest (ave. 1030° C). Geobarometry suggests mid- to upper crustal depths for the crystallization of all enclaves. The enclaves are cognate and reflect pre-eruptive crystallization of Arenal magmas. They record evolution from a hydrous, basaltic magma to the drier basaltic andesites that characterize the current eruption. Volatiles appear to have been lost due to depressurization during the slow ascent of the magmas through the upper levels of the crust following the initial explosive eruption. Volatile loss and depressurization resulted in the destabilization and the progressive resorption of amphibole. The A-2 lavas may represent the long-term fractionation products of basaltic andesite magmas similar in composition to the A-1 lavas. Anorthitic plagioclase, commonly thought of as a phase stabilized by high Ca/Na and high water pressure, continued to crystallize in a system with relatively low Ca/Na and which had dehydrated and/or depressurized to the point at which amphibole was no longer stable. This suggests that compositional characteristics other than high Ca/Na or high water content may have stabilized the anorthite in the basaltic and basaltic andesite melts at Arenal. We speculate that the high-alumina content of the Arenal magmas may be the stabilizing factor.

Petrogenesis of Valle Grande Member rhyolites, Valles Caldera, New Mexico: Implications for evolution of the Jemez Mountains Mgmatic System

The Valles Caldera, located near the center of the Jemez Mountains Volcanic Field in north‐central New Mexico, was formed at ∼1.12 Ma upon eruption of the upper Bandelier Tuff. The Valle Grande Member (VGM) consists of 8 rhyolite domes or dome complexes erupted from vents which define the ring fracture within the caldera. They range in age from the time of caldera formation to ∼0.45 Ma and are volumetrically and temporally the dominant post‐caldera eruptives. VGM rhyolites are high‐silica (&gt;75% SiO2 anhydrous), high‐K, are characterized by low abundances of Fe, Mg, Mn, Ti, and Ca, and are metaluminous to weakly peraluminous. Phenocryst assemblages include sanidine (Or44–61) + quartz + plagioclase (An7–20) + biotite + hornblende + Fe‐Ti oxides ± zircon ± allanite ± apatite ± clinopyroxene ± orthopyroxene. Many phenocryst phases show progressive changes in composition, size, and abundance with decreasing age. Whole rock chemistry, phenocryst chemistry, and isotopic dating indicate that VGM rhyolites can be divided into 3 groups: group 1 ranging in age from 1.18 to 0.71 Ma, group 2 from 0.55 to 0.51 Ma, and group 3 at 0.45 Ma. Groups 1 and 2 define differentiation trends in which trace elements such as Rb, Cs, Y, Nb, HREE, Ta, Th, and U increase with decreasing age, whereas Sr, Ba, Zr, LREE, and Eu decrease. Abundances of trace elements may increase or decrease 2–3 fold within eruptive groups; major elements remain relatively constant. The observed differentiation trends can be accounted for by crystal‐liquid fractionation at a minimum or eutectic using modal phenocryst phases. Fractionation may have occurred by roof and side wall crystallization with the collection of bouyant differentiated liquids at the roof of the magma chamber. Two‐feldspar geothermometry, geobarometry using hornblende rim Al contents, and plots of normative compositions on the Q‐Ab‐Or diagram indicate that fractionation took place at depths of 2.5–7.5 km with pre‐eruptive temperatures ranging from ∼720–810°C. Rhyolites erupted in the Jemez Mountains over the last 2–3 m.y. appear to be the products of many separate magma batches and were not derived from one large long‐lived magma chamber. VGM rhyolites represent 3 separate magma batches (groups 1, 2, and 3), two of which remained closed systems for sufficient periods of time to generate highly evolved differentiates (groups 1 and 2).