Main Magma Research Articles

Geochemistry of caldera‐forming and resurgent magmas in the Pliocene Hiwada caldera, northeast Japan: An example of a monotonous intermediate system

AbstractThe 2.9‐Ma Hotokezawa Ignimbrite, which was ejected from the Aizu caldera cluster in the northeast Japan arc, is a typical monotonous intermediate ignimbrite, with 40–50 vol% crystals and an eruptive volume of >140 km3 dense‐rock equivalent. This ignimbrite filled Hiwada caldera and was deformed by post‐caldera plutonic intrusions that formed a resurgent dome. The Hotokezawa Ignimbrite is a calc‐alkaline, medium‐K dacite to rhyolite with SiO2 contents of 67.9–71.3 wt%, and has homogeneous trace‐element abundances and Sr–Nd isotopic ratios. These geochemical features suggest that the Hotokezawa magma was formed by partial melting of amphibolitic crustal rocks. This crystal‐rich magma did not appear during the post‐caldera stage. Therefore, it is plausible that the chamber of eruptible magma was emptied by the caldera‐forming eruption. In contrast, post‐caldera plutonic rocks exhibit a variety of compositions and have a clear SiO2 gap corresponding to the caldera‐forming magma: the early pluton (tonalite) and later ones (quartz porphyry, granite porphyries, and granite) contain 62.0–66.6 and 71.2–76.5 wt% SiO2, respectively. The tonalite and the Hotokezawa Ignimbrite form a continuous trend in their major‐element variations. The Sr–Nd isotopic ratios of the ignimbrite and tonalite overlap, but those of the porphyries and granite are more enriched. The early tonalite represents the more basic part of the Hiwada caldera system that was held in small pockets separate from the main magma chamber, because its trace‐element abundances are varied and distinct from those of the Hotokezawa Ignimbrite. The distinct compositional change from the Hotokezawa Ignimbrite to the late porphyries and granite indicates that the partial melting crust generating felsic magma was renewed by the subsequent intrusion of the mantle melts. The new felsic magma ascended through subsidence‐related faults into the shallow caldera system and emplaced as laccoliths forming the resurgent dome.

Apatite Crystals Reveal Melt Volatile Budgets and Magma Storage Depths at Merapi Volcano, Indonesia

Abstract Magma volatile budgets and storage depths play a key role in controlling the eruptive styles of volcanoes. Volatile concentrations in the melt can be inferred from analyses of glass inclusions, which however may not be present in the investigated rocks or may have experienced post-entrapment processes that modify their volatile records. Apatite is becoming an alternative robust tool for unraveling the information of magmatic volatiles. Here we report a comprehensive dataset for the concentrations of volatiles and major elements in apatite crystals in the rocks from two eruptions with contrasting eruptive styles: the 2006 (dome-forming) and 2010 (explosive) eruptive events at Merapi volcano (Java, Indonesia). We obtained two-dimensional compositional distributions and in situ concentrations of H2O, CO2, F, Cl and S in 50 apatite crystals occurring at various textural positions. The CO2 concentrations we report are probably the first ones from natural volcanic apatite. Using the volatile concentrations in apatite and existing thermodynamic models and geothermobarometers, we have calculated the volatile abundances of the pre-eruptive melts of the two eruptions. We find that the apatite from the 2006 and 2010 deposits have a similar compositional range of volatiles, with a bimodal distribution of F–H2O–CO2 contents. The apatite included in amphibole has higher H2O (0·9–1·0 wt %) and CO2 (≥2400 ppm), but lower F (0·9–1·4 wt %), compared to crystals included in plagioclase, clinopyroxene, or in the groundmass (H2O: 0·4–0·7 wt %; CO2: 40–900 ppm; F: 1·7–2·3 wt %). Using these volatile concentrations and apatite-melt exchange coefficients we obtained two distinct ranges of H2O–CO2–S–F–Cl concentrations in the melt. Melts in equilibrium with apatite included in amphibole had 3–8 wt % H2O, ≥8000 ppm CO2, 340–2000 ppm S, whereas melts in equilibrium with apatite included in anhydrous minerals and in the groundmass had lower H2O (1·5–4 wt %), CO2 (60–2500 ppm), and S (10–130 ppm). We calculated the melt H2O–CO2 saturation pressures and found that they correspond to two main magma storage depths. The shallow reservoir with melts stored at ≤10 km below the crater agrees with the depths constrained by melt inclusions, as well as the geodetic, geophysical and seismic tomography studies from the literature. We have also found a significantly deeper melt storage zone at ≥25–30 km recorded by the C- and H2O-rich apatite in amphibole and barometry calculations using amphibole and high-Al clinopyroxene, which matches with the depths reported in seismic tomography studies. The high CO2/H2O and CO2/SO2 concentrations of the deep melt can help to explain the sharp increase in these ratios in fumarolic gas that were sampled just before the eruption in 2010. Supply of deep volatiles to the shallower magma column before the eruption in 2010 could have increased the magma buoyancy, and thus led to higher magma ascent rates and associated eruption explosivity. Evidence for the faster pre-eruptive magma ascent in 2010 than 2006 is also found on the diffusion distance of Cl in apatite microlites.

Crystal Size Distributions and Trace Element Compositions of the Fluorapatite from the Bijigou Fe–Ti Oxide-Bearing Layered Intrusion, Central China: Insights for the Expulsion Processes of Interstitial Liquid from Crystal Mush

Abstract The Neoproterozoic Bijigou intrusion is one of the largest and well-differentiated Fe–Ti oxide-bearing layered intrusion in Central China, and hosts Fe–Ti oxide ore layers in the middle zone with a total thickness of ∼112 m. In order to examine the role of compaction and compositional convection on the solidification of a layered intrusion associated with the crystallization of large amounts of Fe–Ti oxides, we collected the samples from a drill core profile of the apatite-oxide gabbronorite unit above the main Fe–Ti oxide layer in the middle zone of the Bijigou intrusion and carried out a detailed study on the crystal size distributions (CSDs) and trace element compositions of the fluorapatite in the samples. The apatite-oxide gabbronorite unit is mainly composed of pyroxene and plagioclase with Fe–Ti oxides and fluorapatite interstitial to the silicates, and can be further divided into the lower and upper sections in terms of grain size, rare earth element (REE) concentrations of fluorapatite and stress deformation of minerals. In the lower section, the plagioclase and pyroxene of the rocks are often bent, fluorapatite crystals have grain sizes ranging from ∼0·10 × 0·30 mm to ∼1·00 × 2·50 mm and the average Ce concentration of the fluorapatite of each sample varies from 230 to 387 μg/g. In contrast, the plagioclase and pyroxene of the rocks from the upper section are sparsely bent, fluorapatite crystals range in size from ∼0·05 × 0·05 mm to ∼0·15 × 0·40 mm, and the average Ce concentration of the fluorapatite of each sample varies from 468 to 704 μg/g. Modeling results show that the fraction of trapped liquid (FTL) is ∼7% in the lower section and ∼15% in the upper section, and relatively elevated REE (e.g. Ce) concentrations of the fluorapatite of the upper section are thus likely attributed to the trapped liquid shift (TLS) effect. The TLS effect may have also enhanced the textural coarsening of the fluorapatite of the upper section, which is illustrated by a convex-upward curve for <0·1 mm crystals and a counter-clockwise rotation around a fixed point in the CSDs of the fluorapatite. The CSDs of the fluorapatite of the lower section, however, change from a steep slope for <0·25 mm crystals to a gentle slope for >0·25 mm crystals with a kinked trend akin to mixed crystal populations, which is interpreted as the exchange of interstitial liquid with the main magma body due to compositional convection. The different FTL and fluorapatite CSDs of the lower and upper sections indicate that the interstitial liquid may have been expelled from the crystal mush of the lower section more efficiently than from the upper section, which is likely controlled by both compaction and compositional convection. However, it was the compositional convection that dominated the expulsion of interstitial liquid in the whole apatite-oxide gabbronorite unit, indicating that compositional convection may prevail after the crystallization of large amounts of Fe–Ti oxides from interstitial liquid and weaken the role of compaction.

The Skaros effusive sequence at Santorini (Greece): Petrological and geochemical constraints on an interplinian cycle

Santorini volcanic complex (Greece) is the result of a long evolutionary history, marked by the alternation of explosive (Plinian) eruptions and interplinian low explosive/effusive eruptive cycles. Products emitted during the interplinian stages are well exposed along the rim of the calderic structure formed during the major Minoan (3.6 ka) Plinian eruption. We conducted a systematic sampling of the basaltic to dacitic lavas erupted by the interplinian volcanic centre of Skaros, active between 67 and 54 ka. The continuously exposed products of the Skaros lifecycle offer the tremendous opportunity to reconstruct the petrological evolution of the volcanic centre, and in turn to provide new insights on the behaviour of Santorini feeding system during interplinian stages. The combination of whole-rock, mineral chemistry and synchrotron X-ray computed microtomography analyses enabled us to decipher the main physico-chemical parameters of the Skaros plumbing system that drove ascent and differentiation processes of magmas. Results indicate that the main magma ponding zone beneath Skaros centre extended from 12 to 4 km b.s.l., where it underwent frequent refilling by basaltic melts. In the later evolutionary stage of the Skaros system, the basaltic replenishment became less frequent allowing the ultimate differentiation of magma towards dacitic composition. The shallow nature of the magma ponding zone, sited well above the H2O saturation depth, favoured the continuous degassing and inhibited the increase of vapor pressure during differentiation, preventing the onset of explosive eruptions, as in case of the major explosive events at Santorini.

Tracing the genesis of skarn‐type iron deposit in central North China Craton: Insights from mineral zoning textures in ore‐forming intrusion

The Handan–Xingtai region in the southern Taihang Mountains in central North China Craton hosts several skarn‐type iron deposits. Here we investigate an ore‐forming intrusion associated with a typical iron skarn deposit to explore the magmatic evolution from mineral zoning textures, and to constrain the ore genesis. We report results of X‐ray mapping of the composition of amphibole and plagioclase. The amphibole thermobarometers suggest: (a) magma chamber at a depth of 17 km; (b) two types of magma with different physical and chemical properties, one representing the main magma chamber with high water content, and the other characterized by more mafic composition and higher oxygen fugacity. The final emplacement depth of magma is estimated as ca. 1.5 km. The oxygen fugacity shows an obvious increase from the primary magma chamber to its emplacement depth. The zoning texture of plagioclase and amphibole suggest that the mafic magma was injected into felsic magma chamber many times, leading to magma mixing. We construct a petrogenetic and metallogenic model which envisages the following processes: (a) during 135–120 Ma, mantle‐derived magma and crust‐derived magma underwent mixing and homogenization at the crust–mantle boundary; (b) the hybrid magma rose to the middle crust and formed a magma chamber; (c) multiple injection of mafic magma into the felsic magma chamber resulted in oscillatory zoning; (d) the magma with high water content and higher modal content of plagioclase and amphibole formed through multiple magmatic injection‐mixing‐crystallization processes; and (f) the high water content and higher oxygen fugacity enabled the hybrid magma to rise to the shallow crust, where it released iron‐rich melt and fluids, which reacted with the surrounding limestones to generate the skarn‐ type iron deposit.

The relationship between the Taihe Fe-Ti oxide ore-bearing layered intrusion and the adjacent peralkaline A-type granitic pluton in SW China: Constraints from compositions of amphiboles and apatite-hosted melt inclusions

The Taihe igneous complex, which is located in the inner zone of the Emeishan Large Igneous Province (ELIP), consists of an Fe-Ti oxide ore-bearing layered gabbroic unit and a silicic enclave-bearing peralkaline A-type granitic unit. The apatite-hosted melt inclusions in the gabbroic unit and the amphiboles in both the gabbroic unit and the granitic unit were used to investigate the relationship between the two units. The apatite-hosted melt inclusions have 32.3–68.3 wt% SiO2, 0.51–23.9 wt% FeOt, 0–7.19 wt% TiO2, 8.87–19.0 wt% Al2O3, 2.39–19.5 wt% CaO, 1.70–7.27 wt% Na2O, 0.24–5.60 wt% K2O, and 0.13–6.83 wt% P2O5. Such a large compositional range is difficult to be achieved through fractional crystallization but can be accounted for by silicate liquid immiscibility developed during the differentiation of the gabbroic unit. The weighted bulk composition of the Taihe gabbroic unit is extremely rich in Fe and poor in Si (17.2 wt% FeOt and 36.2 wt% SiO2), indicating that some Si-rich components must have been removed. A significant compositional gap in the amphiboles in the gabbroic rocks and the adjacent peralkaline A-type granitoids, i.e., calcic amphibole in the gabbros and apatite-hosted inclusions and sodic-calcic to sodic amphiboles in the granites and their silicic enclaves, was observed. This cannot be explained by the previously suggested differentiation of a common basaltic parental magma from gabbros to silicic enclaves and granites, indicating a sudden change in the physical-chemical conditions of the related magmas. We propose that the silicic enclaves enclosed in the peralkaline A-type granitic pluton represent the missing Si-rich components of the Taihe layered gabbroic unit. The continuous expulsion of immiscible Si-rich melts from the lower crystal mush into the upper main magma body caused the crystallization of large amounts of Fe-Ti oxides in the gabbroic unit. Then, the residual Si-rich magmas in the upper part of the Taihe mafic magma chamber were drawn into the subsequently intruding peralkaline granitic magmas during the magmatic transgression process, forming the silicic enclaves within the adjacent contemporaneous peralkaline A-type granitic pluton.

Gabbroides of Peterman island (West Antarctica). First information on geochemistry

Peterman Island is located in the archipelago of the Wilhelm Islands on the west coast of the Antarctic Peninsula (Graham Land). It is composed of gabbroids and granitoids of the Andean complex, which formed almost 100 million years later than the volcanic group of the Antarctic Peninsula. To clarify their genesis and geodynamic conditions of formation, gabbroids of the Andean complex are of particular interest, since the petrological models of their formation are well developed. Gabbroid intrusions comprise small bodies that are widespread along the Antarctic Peninsula. Among them stand out olivine gabbros, normal gabbros, norites and hornblende gabbros. Also are found small bodies of melanogabbro-pegmatites and intramagmatic dykes, that are associated with the manifestations of ore mineralization of magnetite, ilmenite and sulfides. For this reason, they are of interest for both the minerals search and for solving the question of their genesis. To this end, we performed geochemical studies of Peterman Island gabbroids. Gabbroids of Peterman Island are represented by amphibolized medium-grained gabbro with hypidiomorphic texture. Among them, xenoliths of thinly stratified gabbroids 3 × 8 m in size were found, which are characteristic of stratified intrusions, for example, Stillwater, Bushveld, etc. Gabbroids of Peterman Island have low content of silica and potassium and according to the petrochemical characteristics correspond to peridotite gabbro. They have low contents of Cr, Ni, V and high strength lithophilic Y and Nb elements. Gabbroids have been crystallized from basic magma, differentiated in the intermediate crustal magma chambers. Positive anomalies of Sr, Eu, and Ti in the multielement diagrams and positive anomalies of europium Eu/Eu* suggest the accumulation of plagioclase and apparently, ilmenite in the magmatic chamber. The primary magma source for gabbroids was probably the primitive mantle (PM). Gabbroids are contaminated with crustal matter. This contamination is probably due to their regressive metamorphism, caused by the introduction of later intrusions of Andean complex granitoid. Finely layered xenolithic gabbroids do not differ from other homogeneous gabbros of Peterman Island in terms of chemical composition.This xenolith most likely represents a part (fragment) of the wall of the magma chamber in which the differentiation of the initial main magma took place. According to the obtained geochemical data, a wide range of compositions of the Andean complex gabbroids formed as a result of crystallization differentiation of magma melted from rocks of the composition of the primitive mantle (PM) in crustal magma chambers, which also resulted in the accumulation of ore elements — V, Co, and Cu in the residual magmatic melts.

Magma accumulation beneath Santorini volcano, Greece, from P-wave tomography

AbstractDespite multidisciplinary evidence for crustal magma accumulation below Santorini volcano, Greece, the structure and melt content of the shallow magmatic system remain poorly constrained. We use three-dimensional (3-D) velocity models from tomographic inversions of active-source seismic P-wave travel times to identify a pronounced low-velocity anomaly (–21%) from 2.8 km to 5 km depth localized below the northern caldera basin. This anomaly is consistent with depth estimates of pre-eruptive storage and a recent inflation episode, supporting the interpretation of a shallow magma body that causes seismic attenuation and ray bending. A suite of synthetic tests shows that the geometry is well recovered while a range of melt contents (4%–13% to fully molten) are allowable. A thin mush region (2%–7% to 3%–10% melt) extends from the main magma body toward the northeast, observed as low velocities confined by tectono-magmatic lineaments. This anomaly terminates northwest of Kolumbo; little to no melt underlies the seamount from 3 to 5 km depth. These structural constraints suggest that crustal extension and edifice loads control the geometry of magma accumulation and emphasize that the shallow crust remains conducive to melt storage shortly after a caldera-forming eruption.

Emplacement mechanisms of a dyke swarm across the brittle-ductile transition and the geodynamic implications for magma-rich margins

Igneous dykes are the main magma transport pathways through the Earth's crust, and they are considered to contribute to tectonic extension in volcanic rifts. Dykes are typically considered to result from brittle fracturing, even in the ductile crust. A common assumption is that dyke orientation is controlled by tectonic stresses, such that dykes in rifts are expected to be vertical and perpendicular to extension. Here we report on detailed field observations of a spectacularly well-exposed dyke swarm to show that dykes were not systematically emplaced by purely brittle processes and that dyke orientation may differ from the dominant tectonic stress orientations. The dyke complex formed near the brittle-ductile transition during opening of the Iapetus Ocean and is now exposed in the Scandinavian Caledonides. Distinct dyke morphologies related to different emplacement mechanisms has been recognized: 1) Brittle dykes that exhibit straight contacts with the host rock, sharp tips, en-echelon segments with bridges exhibiting angular fragments; 2) Brittle-ductile dykes that exhibit undulating contacts, rounded tips, ductile folding in the host rock and contemporaneous brittle and ductile features; 3) Ductile “dykes” that exhibit rounded shapes and mingling between the soft ductile host rock and the intruding mafic magma. The brittle dykes exhibit two distinct orientations separated by c. 30° that are mutually cross-cutting, suggesting that the dyke swam did not consist of only vertical sheets perpendicular to regional extension, as expected in rifts. We were able to use the well-exposed host rock layers as markers to perform a kinematic restoration to quantify the average strain accommodating the emplacement of the dyke complex: it accommodated for >100% extension, but counter-intuitively it also accommodated for 27% crustal thickening. We infer that the magma influx rate was higher than the tectonic stretching rate, implying that magma was emplaced forcefully, as supported by field observations. Finally, our observations suggest that the fast emplacement of the dyke swarm triggered a rapid shallowing of the brittle-ductile transition, and lead to a considerable weakening of the crust. The interpretations presented here could potentially have large implications for surface topography and seismicity in active rifts and volcanic areas around the world.

Size distributions of fractures, dykes, and eruptions on Etna, Italy: Implications for magma-chamber volume and eruption potential

The main magma source for eruptions on Etna (Italy) is poorly constrained. Here we use data on the size distributions of volcanic fissures/feeder-dykes, crater cones, dyke thicknesses, and lava flows to estimate the average magma volume flowing out of the chamber during eruptions and the volume of the chamber. For the past four centuries the average magma volume leaving the chamber during each eruption is estimated at 0.064 km3. From the theory of poroelasticity the estimated chamber volume is then between 69 and 206 km3. For comparison, a sill-like, circular chamber (an oblate ellipsoid) 1 km thick and 14 km in diameter would have a volume of about 154 km3. The elastic strain energy stored in the host rock during inflation of such a chamber is about 2.8 × 1014 J. Estimating the surface energy of a typical dyke-fracture as about 107 J m−2, the results suggest that the stored strain energy is sufficient to generate a dyke-fracture with an area of about 28 km2. The average strike-dimension of volcanic fissures/feeder-dykes in Etna is about 2.7 km. It follows that the estimated strain energy is sufficient to generate a feeder-dyke with a strike-dimension of 2–3 km and with a dip-dimension as great as 10 km, agreeing with the maximum estimated depth of the magma chamber.

Strontium isotope systematics for plagioclase of the Skaergaard intrusion (East Greenland): A window to crustal assimilation, differentiation, and magma dynamics

Author(s): Hagen-Peter, G; Tegner, C; Lesher, CE | Abstract: We present in situ laser ablation-multicollector-inductively coupled plasma-mass spectrometry Sr isotope data for plagioclase from a reference stratigraphic profile of the entire Layered Series and Upper Border Series of the Skaergaard intrusion (East Greenland). Plagioclase Sr isotope compositions and anorthite contents vary systematically from the margins of the intrusion inwards. The lowest 87Sr/86Sri (calculated at 56 Ma) values (~0.7041) occur near the base and top of the intrusion and systematically increase over several lithostratigraphic zones to a value of ~0.7044, which is uniform throughout the middle ~2000 m of intrusion. Across this same profile, anorthite content of plagioclase varies smoothly from An65-70 at the base and top to ~An25 approaching the purported Sandwich Horizon. Plagioclase near the roof and proximal to rafts of partially assimilated basement gneiss are markedly more radiogenic (87Sr/86Sri up to ~0.7046). We explain the stratigraphic relationships by progressive contamination of the magma during early stages of differentiation by basement gneiss rafts entrained during emplacement and accumulated near the top of the chamber. Contamination was transient, ceasing once the entrained gneiss was consumed or isolated from the main magma reservoir as the solidification front advanced. Modeling of fractionation-assimilation processes accounts for the observed isotopic trends with only a few percent assimilation (relative to the original magma mass). The record of contamination revealed by Sr in plagioclase supports the view that the bulk of the Skaergaard intrusion formed by closed-system differentiation with only minor in situ contamination and no magma recharge. Comparing plagioclase and bulk-rock Sr suggests that the latter may have witnessed late-stage metasomatic overprinting of phases other than plagioclase.

Shallow magma pre-charge during repeated Plinian eruptions at Sakurajima volcano

Vigorous explosive eruptions that produce continuous high eruption plumes (Plinian eruptions) are generally assumed to tap a magma reservoir. The 1914 Plinian eruption at the Sakurajima volcano located on the Aira caldera rim is one such case, where the main magma reservoir was assumed to be located approximately 10 km beneath the caldera. However, we report that estimated magma storage depths immediately prior to the eruption were much shallower (0.9–3.2 km) on the basis of pressure at which volatiles within the phenocryst melt inclusions and plagioclase rims were finally equilibrated. The same is observed for two historic Plinian eruptions in 1471 and 1779. This depth is even shallower than the shallowest magma reservoir estimated from the pressure source for geodetic deformation during recent Vulcanian explosions (4 km beneath the crater). We propose that the magmas were fed from a thick conduit pre-charged from deeper reservoirs. The ground subsidence observed after 1914 within the Aira caldera may have been caused by conduit recharge following the eruption. Voluminous conduit recharge could be key to forecasting the next possible large eruption at the Sakurajima volcano.

Reconstruction of Magma Flow in Permo–Triassic Intrusions of the Angara–Taseeva Syneclise (Siberian Platform) Based on Magnetic Susceptibility Anisotropy Data

Abstract —Based on detailed studies of the anisotropy of magnetic susceptibility (AMS), the directions of magmatic-melt propagation have been reconstructed in large dolerite sills of the Angara–Taseeva syneclise. Half the sites studied showed a magnetic fabric of “normal” type, i.e., the minimum K3 axis of the AMS ellipsoid is orthogonal to the contact, and the other two axes lie in the plane of the intrusive body. Interpreting the orientation of the maximum K1 axis as the direction of the melt flow at such sites, we compiled a scheme of the main magma flow directions. The obtained results are generally consistent with the hypothesis of magma-feeding zone in the central, most downwarped part of the Angara–Taseeva depression. The distribution of the maximum axes of the AMS ellipsoid is different in sills and points to the existence of local intrusion centers. Intrusions with an AMS ellipsoid of “reverse” type are predominant on the periphery of the depression. The sites with a “normal” magnetic fabric do not demonstrate any prevailing direction of magma propagation, because there is no general direction of the melt flow in the peripheral subsurface zones of large intrusions.

Timescales of pre-eruptive magmatic processes at Vulcano (Aeolian Islands, Italy) during the last 1000 years

An integrated petrological and geochemical study on plagioclase crystals erupted at Vulcano Island (Aeolian Islands, Southern Italy) over the past 1000 years allowed us to draw a detailed model of the internal structure of the volcanic system, in which modes of magma interaction and timescales of storage at crustal depth are shown. The integration of compositional, textural and temporal record preserved in plagioclase crystals provides evidence for an articulated plumbing system constituted by several reservoirs connected at the crust-mantle boundary. Here, a basaltic-shoshonitic magma resides and is thought to feed the shallow magma reservoir of both La Fossa and Vulcanello centers, finally triggering eruptions throughout injections from depth. Textural and micro-compositional data on plagioclase crystals suggest the presence of three main magma levels located between ca. 17 and 2 km of depth beneath La Fossa Cone, which were intermittently reactivated over the whole period of activity considered. Plagioclase textures and compositional zoning indicate that the shallow (<11 km bsl) portions of the La Fossa plumbing system were particularly active over the last 1000 years, as crystals record the ascent and continuous episodes of magma recharge and mixing that affect the shallower reservoirs. The first stage of activity at Vulcanello (i.e. Vulcanello I eruption) was fed by slightly differentiated melts that directly rose from the deep basaltic-shoshonitic reservoir, residing for a short period of time into the crust before the eruption. Indeed, diffusion modeling calculations on Sr zoning in plagioclase indicate shorter timescales of residence (<2 years) for crystals erupted at Vulcanello compared to those of the La Fossa Cone eruptions (ca. 2–10 years). If compared with the time span between eruptions occurred at La Fossa, our time estimations may suggest that magma feeding the activity at La Fossa resides most of the time in reservoirs located below the plagioclase nucleation depth (~11 km of depth), finally rising up only few years before the eruption onset. According to our model, magmatic eruptions at Vulcano Island are related to the ascent of deep basic (basaltic/shoshonitic) magmas that trigger a sort of “reaction chain” through subsequent episodes of recharge and mixing toward the upper magmatic reservoirs.

Trace Element Constraints on the Differentiation and Crystal Mush Solidification in the Skaergaard Intrusion, Greenland

New major and trace element analyses of plagioclase and clinopyroxene in gabbros from the Skaergaard intrusion are used to understand the mechanisms of crystallization in the main magma body and in the crystal mush. Crystal cores show a continuous chemostratigraphic evolution in the Layered Series, with compatible elements (e.g. Cr and Ni in clinopyroxene) being progressively depleted from the bottom up, whereas incompatible elements (e.g. Sr, Ba, REE in plagioclase and clinopyroxene) become progressively enriched. We performed numerical models and showed that these trends can be explained by a simple process of fractional crystallization, except for the upper 20% of the intrusion where plagioclase and clinopyroxene trace element compositions depart from fractional crystallization trends. At this stage of magma chamber solidification, fractional crystallization becomes less efficient and is replaced by a major proportion of in situ, equilibrium crystallization. Trace elements also show significant variations in interstitial overgrowths on plagioclase and clinopyroxene cumulus crystals. They result from crystallization of the interstitial liquid in the liquid + crystal mush. However, incompatible elements, and especially REE, show a degree of enrichment in plagioclase rims (> 20 ppm Ce) that strongly exceeds the highest concentrations observed in plagioclase cores (up to 4 ppm Ce) at the top of the Layered Series. Such a strong enrichment is difficult to reconcile with a simple process of fractional crystallization of the interstitial liquid in the crystal mush, but may be related to the development of silicate liquid immiscibility in the crystal mush or to delayed nucleation of apatite. After the crystallization of plagioclase and clinopyroxene overgrowths, diffusive re-equilibration during a period of 0.1-0.25 Myr significantly changed the original zoning profiles. Ce flux from clinopyroxene into the plagioclase lattice of adjacent crystals could also have contributed to the strong Ce enrichment observed in plagioclase rims.

A combined geochronological approach to investigating long lived granite magmatism, the Shap granite, UK

With the advent of more precise dating methods, it has become apparent that zircon dates from granite plutons frequently indicate older emplacement ages than other dating methods. Here we attempt to reconcile a number of dating methods from the c. 5 km2 Caledonian Shap granite, Northern England. The results reveal a more complex and protracted evolution than indicated by application of any single dating method. Zircon U-Pb dates give a weighted mean age of 415.6 ± 1.4 (2σ) Ma. A mafic enclave, dated at 412 ± 2 (2σ) Ma (revised Rb-Sr feldspar age from Davidson et al., 2005), contains resorbed K-feldspar and zircon crystals scavenged from the host crystal mush. These ages are at odds with field relations in the thermal aureole that suggest final emplacement at approximately 404 Ma or later during Acadian deformation. Previously reported Re-Os ages on molybdenites associated with magmatic fluids, have given ages of 405.2 ± 1.8 (2σ) Ma (Selby et al., 2008) and confirm the overlap of at least some magmatic activity with Acadian deformation. A similar emplacement age is supported by Rb-Sr whole-rock-mineral and biotite K-Ar dates when adjusted for revised decay constants (402 ± 3 Ma and 401 ± 7 Ma, respectively, Wadge et al., 1978). The lower closure temperatures of these systems relative to the U-Pb system in zircon means that they are more likely to record the timing of final granite emplacement. These data suggest that most zircons grew before final granite emplacement, by about 10 Ma on average. We suggest that the majority of zircon crystals record pre-emplacement magmatic activity within a deeper part of the system. Mafic enclaves and their scavenged cargo of crystals record the assembly of a mid-crustal batholith where crystals remained at least locally mobile at 412 Ma. Gravity data support the existence of an extensive, 1500 km2 intrusive body, originally at about 15 km depth beneath Shap. This batholith is likely to have remained below the granite solidus for much of its existence due to conductive heat loss, but episodic influxes of silicic magma between c. 412 and 405 Ma are thought to have enabled periods of rejuvenation. These influxes are recorded by complex compositional zoning patterns within K-feldspar megacrysts. The Shap granite itself is likely to represent a rejuvenated crystal slurry, emplaced as a cylindrical cupola above the main magma body during Acadian transpression. This study highlights the importance of integrating different dating techniques and that final emplacement of granites can only be indicated by the youngest zircon ages.

Long-term dynamics across a volcanic rift: 21 years of microgravity and GPS observations on the southern flank of Mt. Etna volcano

In this paper, an integrated analysis of microgravity and ground deformation measurements at Mt. Etna is performed over a 21-years time span (1994–2014), a period encompassing several flank and summit eruptions. Data were collected along a common gravity and ground deformation network located on the volcano's southern flank to monitor the dynamics of the main volcanic and tectonic processes. Gravity variations have first been corrected for the free-air effect using the vertical deformation and the experimental vertical gravity gradients measured at each station. The free-air corrected gravity changes were then reduced for seasonal fluctuations, mainly imputable to water-table fluctuations, and for high frequency variations (noise).This long-term dataset constitutes a unique opportunity to examine the behavior of Mt. Etna in a period in which the volcano exhibited different styles of activity characterized by recharging, flank eruptions and fountaining episodes at summit craters. The joint analysis highlights common periods in which gravity and deformation underwent contemporaneous changes occurring mainly in the central and eastern stations of the profile. Specifically, it was possible to distinguish three different sectors, showing significant deformation and gravity patterns: a western sector, lying west of the South Rift, a central one between the South Rift and the 1989 fracture zone, and an eastern one lying east of the 1989 fracture system. The data shows that the deflation dynamics accompanying the eruptive phases mainly affect the western portion of the profile, gradually disappearing toward East, where the more constant sliding dynamics dominates. This pattern defines a transition zone corresponding to the sector comprised between the South Rift and the 1989 fracture. Furthermore, it was also possible to detect at least three main magma charging phases in 1996, 2000 and 2011 which preceded and accompanied the onset of intense flank eruptions and lava fountaining activity at the summit craters.

Near-surface geophysical mapping of an Upper Cretaceous submarine volcanic vent in Austin, Texas, USA

Geophysical surveys were conducted at the Williamson Creek site south of Austin, Texas, to determine the structural relation of the Upper Cretaceous volcanic rocks (lava and tuff) with the associated Austin Chalk limestone. At this site, resistivity and magnetic methods were performed over the exposed volcanic and limestone rocks. Geophysical results indicate an excellent correlation between high-magnetic and low-resistivity anomalies. The pseudo-3D resistivity data show a steeply dipping funnel-shaped vent formation over the high-magnetic anomaly (up to 3000 nT). Magnetic anomalies are consistent with a uniformly magnetized body, like a volcanic vent, and the anomaly's dimensions are consistent with eroded volcanic vents in other distributed volcanic fields in the United States. Magnetic data has been integrated with resistivity data and geologic observations and subjected to 2.5D forward potential-field modeling. Modeling has revealed a perfect fit with three magnetic zones: (1) the central part corresponds to the main magma feeder (vent); (2) the surrounding zone corresponds to undifferentiated interbedded tuffs and lavas; and (3) the low-magnetization zone. Geophysical results show that additional resistivity surveys, in conjunction with magnetic surveys, could offer useful information on the shallow volcanic plugs (serpentines), which are potential oil and gas traps in Texas, and their adjacent sedimentary rocks. The procedures developed here may have applications in other areas with comparable geologic conditions.

Age constraints and geochemical evolution of the Neoarchean mafic–ultramafic Wabassi Intrusive Complex in the Miminiska–Fort Hope greenstone belt, Superior Province, Canada

The ca. 2.75–2.71GaMiminiska–Fort Hope greenstone belt (MFHGB) of the eastern Uchi domain of the Superior Province (northern Ontario) contains numerous mafic and ultramafic intrusions hosting Ni–Cu–(PGE) occurrences. The most significant, the mafic–ultramafic Wabassi Intrusive Complex and the nearby mafic Oxtoby Lake intrusion, formed between 2728 and 2727Ma and at ca. 2717Ma, respectively. The Max, Wabassi Main, and Wabassi South parts of the Wabassi Intrusive Complex and the Oxtoby Lake intrusion have been studied to establish their petrogenesis and metallogenesis, and to compare them to the highly prospective 2734Ma mafic–ultramafic intrusions in the McFaulds Lake greenstone belt∼120km to the north that host world-class Cr-(PGE), significant Ni–Cu–(PGE), and potentially significant Fe–Ti–V mineralization.The ca. 2728Ma ultramafic-dominated Max intrusion is relatively homogeneous in composition, comprising mainly harzburgite, lherzolite, and wherlite with minor olivine orthopyroxenite, orthopyroxenite, websterite, and mesocratic gabbroic rocks. It also contains disseminated to patchy net-textured Fe–Ni–Cu–(PGE) sulfides. In contrast, the ca. 2727Ma Wabassi Main intrusion is a layered mafic-dominated intrusion that consists of: (1) a basal zone containing leucocratic to mesocratic olivine gabbronorite and olivine norite, troctolite, lherzolite, and harzburgite with high Cr contents, olivine with Mg#78–72, orthopyroxene with Mg#84–67, clinopyroxene with Mg#95–91, and plagioclase with An96–37, (2) a lower zone containing mesocratic olivine gabbronorite-gabbro-norite and up to 10% Fe–Ti oxides, olivine with Mg#64–19, orthopyroxene with Mg#67–58, clinopyroxene with Mg#80–56, and plagioclase with An79–46, (3) a middle zone containing leuco- to mesocratic gabbronorite with up to 8% Fe–Ti oxides, orthopyroxene with Mg#57–56, clinopyroxene with Mg#73–66, and plagioclase with An71–63, and (4) an upper zone containing mesocratic gabbronorite, gabbro, and ferrogabbroic rocks with up to 50% Fe–Ti oxides, up to 8% apatite, low Cr and high P contents, orthopyroxene with Mg#44, clinopyroxene with Mg#60, and plagioclase with An63–52. The ultramafic-dominated Wabassi South intrusion is homogeneous in composition and is composed of peridotite. All three intrusions are enriched in Th>LREE>MREE relative to Nb–Ta and Ti, suggesting that they incorporated significant amounts of upper crustal material. This, together with their overlapping petrographic, geochemical, and mineralogical characteristics, suggests that they are co-genetic. Their formation appears to have begun with the accumulation of olivine, pyroxene, and minor chromite from a contaminated high-Mg basaltic parental magma (∼12% MgO) in the Max and Wabassi South magmatic conduits. Later, this magma crystallized cumulates in the Wabassi Main magma chamber with the following order of appearance of cumulus phases: olivine+chromite→orthopyroxene/clinopyroxene→plagioclase→magnetite+ilmenite→apatite. The parental magma composition is more evolved than magmas responsible for the generation of the Cr-(PGE) and Ni–Cu–(PGE) mineralization in the McFaulds Lake greenstone belt, but they may be more contaminated equivalents. The ca. 2717Ma mafic Oxtoby Lake intrusion is more mesocratic with significantly different trace element signatures, suggesting that it is unrelated or only indirectly related to the Wabassi Intrusive Complex.The Max, Wabassi Main, Wabassi South, and Oxtoby Lake intrusions are coeval with different volcanic assemblages of the MFHGB. Together with the mafic–ultramafic intrusions of the McFaulds Lake greenstone belt, they probably represent the remnants of an Archean large igneous complex that was emplaced within the northern Superior Province between ca. 2.88 and 2.70Ga. Among these four intrusions, Max may be the most prospective for Ni–Cu–(PGE) mineralization. The high abundance of primitive ultramafic rocks suggests that it was an open, dynamic system that accumulated olivine and pyroxene, and the trace element signature is consistent with incorporation of significant amounts of upper crustal material, permissive to the formation of Ni–Cu–(PGE) mineralization.

Prolonged ascent and episodic venting of discrete magma batches at the onset of the Huckleberry Ridge supereruption, Yellowstone

How exceedingly large, volcanic supereruptions begin provides crucial information on the storage, ascent and release of silica-rich magma in catastrophic events. Initial fall deposits of the 2.08 Ma, 2500 km3 Huckleberry Ridge eruption are multiply bedded and in several places contain reworked intervals, indicating time breaks in the opening phases of the eruption. A 2.5 m section of these fall deposits was sampled at nine levels below the earliest ignimbrite (member A) at Mount Everts (Mammoth, Wyoming). We analyzed major and trace elements and volatiles in quartz-hosted melt inclusions (MIs), reentrants (REs; unsealed melt inclusions) and associated obsidian pyroclasts (thick-walled shards) to establish quartz crystallization and storage depths and melt compositional groupings. Systematic relationships between Rb and other incompatible elements (U, Cl, B) indicate ∼55% fractional crystallization between the least and most evolved glass compositions. In contrast, H2O concentrations in MIs show scattered relationships with trace elements and are interpreted to reflect variable loss of H2O by diffusion through the quartz host during magma ascent. The wide H2O variations (1.0–4.7 wt.%) in MIs from individual fall horizons imply as much as ∼14 days of diffusive loss, reflecting highly variable and surprisingly slow decompression conditions. Water and CO2 gradients in reentrants, however, are consistent with final ascent times of <1 to 4 h (ascent rates of ∼0.3–1.5 m/s), similar to those represented by MIs that we infer to have experienced little to no diffusive H2O loss. The wide range of ascent rates for co-erupted crystals mirrors that of intermittent explosive activity at Mount St. Helens in summer 1980, and implies that the Huckleberry Ridge magma body was not strongly overpressured at eruption onset. Restored entrapment pressures and geochemical data for MIs provide evidence for six distinct populations of quartz that originally crystallized in geochemically distinct magma domains. The compositions of REs and obsidian pyroclasts, by comparison, show that by the onset of eruption, the quartz had been brought together into three discrete magma bodies, which we interpret to have been cupolas on the roof of the main magma body. These cupolas were erupted sequentially and episodically from separate vents to generate the fall deposits before escalating activity led to generation of voluminous pyroclastic flows, and this pattern of activity suggests that tectonic triggering may have destabilized multiple magma bodies. Supereruptions as large as the Huckleberry Ridge event may start hesitatingly if the parental magma bodies are not strongly overpressured, with small-scale episodic activity that is modulated by external controls that may leave no other geological evidence for their presence.