Phenocryst Phases Research Articles

Heterogeneous pumice populations of the 52 ± 3 ka Maninjau caldera-forming eruption, West Sumatra, Indonesia: Evidence of multiple magma reservoirs feeding a large silicic eruption

Three color variations of grey members (including grey and banded juvenile clasts) were observed in the 52 ka ignimbrite deposits of Maninjau caldera, Indonesia; namely dark grey (DGM), pale grey (PGM), and light grey (LGM). All grey members were phenocryst-rich ( 24, 20, and 31 % for DGM, PGM, and LGM, respectively) and comprise plagioclase, pyroxene, amphibole, biotite, and oxides as the main phenocryst phases; however, apatite is exclusively present in DGM. Plagioclase with unzoned and coarsely sieved texture was observed in all grey members, while finely sieved and oscillatory zoned textures were exclusive in LGM. Notably, DGM is characterized by the highest MgO value among all grey members, followed by PGM and LGM (∼0.72, 0.46, and 0.34 wt% MgO under ∼74.0 wt% SiO2, respectively). Based on the amphibole geothermobarometer and plagioclase hygrometer, we found that all grey magmas were stored at a relatively similar range of pressure and water content, but DGM yields a higher apparent temperature than those of PGM and LGM. Such distinctive petrography and chemical characteristics, coupled with different temperature conditions strongly suggest that each grey member originated from different magma bodies, which were stored below the most voluminous white magma (the source of white pumice). The sudden decompression of the white magma via overpressure causes destabilization to the smaller grey magmatic bodies, allowing them to rise and erupt as grey and banded pumices during the final eruption stage. Our results enrich the evidence of the formation of multiple magma reservoirs in large-silicic magmatic systems, which might be a common behavior before large eruptions. Moreover, our detailed glass compositions for each juvenile type may be useful for further regional tephrochronology studies.

SO2 emissions from the Timanfaya eruption (1730–36 CE), Lanzarote, Canary Islands

The 1730–36 CE eruption of Lanzarote (Canary Islands) is the third largest basaltic fissure eruption known to have occurred in the last 1000 years, after the Icelandic events of Laki (1783–84 CE) and Eldgja (939 CE). Approximately 5.2 km3 (1.1 × 1012 kg) of alkalic basalt (sensu lato) lava and pyroclastic rocks were emplaced on land, with a further unknown amount entering the sea, The activity occurred in five eruptive phases from vents situated along a 15-km long, E-W trending fissure. Each phase began with Strombolian fire fountain activity, building large spatter and scoria cones, accompanied and followed by effusive ‘a'ā and pāhoehoe lava flow emplacement. Phases I and II were the most explosive and occurred between 1st September 1730 and June 1731. Samples of scoria and lava surfaces were collected from all phases and the major elements, S, Cl and F contents of matrix glasses and glass inclusions in olivine (the dominant phenocryst phase) were analysed by electron microprobe. Water contents of selected glasses were measured on polished wafers by Fourier Transform Infrared spectroscopy. Glass compositions mimic the range of published whole rock compositions, with Phase I being predominantly basanitic and later phases alkali basalt. Whole rock variations in incompatible trace element ratios are mimicked by ratios involving TiO2, K2O and P2O5 in glass. Matrix and glass inclusion analyses in a given sample are similar, indicating that the inclusions formed synchronously with eruption and that most phenocryst crystallization occurred during earlier evolution of the magmas. Matrix glasses in scoria and surface lava samples have 80–300 ppm S and 260–750 ppm H2O, whereas glass inclusions in olivine have 420–2650 ppm S and 0.1 to 0.5 wt% H2O. Glass inclusions with low sulfur contents were trapped during degassing-induced crystallization as the magma approached the surface. The inclusions with the highest sulfur contents, at given P2O5/TiO2 ratio, are inferred to represent un-degassed melt, and their S content and S/K2O ratios correlate very strongly with incompatible element concentrations and P2O5/TiO2 ratios. The original S contents of 131 matrix glasses were calculated from their P2O5/TiO2 ratios and compared to their measured degassed S contents. The mass of S released per unit mass of magma ranged from about 0.25% in Phase I Episode 1to 0.17% in later phases, with propagated uncertainties of about 2 to 5%. About 40 Mt. of SO2 was released overall, with ca. 25 Mt. being released during the first 5 months of the eruption (Phase I). Estimates of average magma discharge rates imply that plumes remained in the troposphere, with the possible exception of those associated with extreme peak discharge rates during Phase I Episode 1 which may have just reached the upper troposphere – lower stratosphere. Lower eruption rates during the later phases would have confined SO2 emissions to the troposphere. Although acid and sulfate peaks dated to 1730 and 1731 have been reported from some Greenland ice cores, the evidence for stratospheric impact, such as cooling at the Earth's surface and increased stratospheric turbidity, is minimal.

Petrology of the explosive deposits from the April 2021 eruption of La Soufrière volcano, St Vincent: a time-series analysis of microlites

Abstract After more than three months of lava dome extrusion, La Soufrière (St Vincent) transitioned to a series of explosive eruptions in April 2021. Here we present a time-series petrologic analysis of the phenocryst and microlite populations during the first c. 48 h of explosivity to constrain ascent conditions and processes that drove changes in behaviour. Primary eruptive products were crystal-rich (45–50 vol%) basaltic andesites with similar phenocryst phase assemblages and compositions. The change in eruptive style is consistent with overpressurization as a consequence of second boiling from anhydrous microlite crystallization. The microlites display variation between the explosive phases, with two populations: (1) ‘inherited’ − normally zoned high-An plagioclase (>An 70 ) + olivine (Fo 62–79 ) + clinopyroxene + titanomagnetite, inferred to have crystallized at depths >15 km and high water pressures; (2) ‘juvenile’ − unzoned plagioclase (An 45–65 ) + clinopyroxene + orthopyroxene + intermediate pyroxene (Wo 12–38 ) + titanomagnetite, inferred to have crystallized upon ascent due to decompression and degassing. Scoria from the first explosions featured extensive groundmass crystallization and a significant ‘inherited’ microlite population. Later explosions had a more abundant ‘juvenile’ microlite population and lower crystallinity, consistent with more rapid ascent from depth, initiated by decompression following initial blasts and destruction of the lava dome.

Interpretation of Geochemical Data of Eocene Volcanism in Eastern Sivas Province (Central Anatolia, Türkiye)

The study area covers the eastern part of Zara district (Sivas) located in East-Central Anatolia. The units found in the region from bottom to top are characterized by the Upper Creataceous-Paleocene Refahiye Complex, the Middle-Upper Eocene Akıncılar Formation, the Karataş volcanics and Kösedağ Syenite, the Lower Miocene Onarı Formation, the Upper Miocene-Pliocene İsola volcanics, the Şerefiye volcanics, the Kadıköy Formation and the uppermost Quaternary sediments. In the frame of this study, the Karataş volcanics, which were formed as a result of the second magmatic activity of the Eocene period, are studied in detail. The Karataş volcanics are greenish black on their undisturbed clean surfaces, and some parts are brown-purple in color and have distinctive crack systems. It shows a reddish-pinkish color distribution in regions where degradation is observed intensely. The SiO2 content, which is the main criterion for determining the magma character of Karataş volcanics, is between 52.09 % and 58.45 %. MgO contents of similar importance are between 0.53 % and 2.21 %. Some of the light rare earth elements, La have 22.2-40.7 ppm, Ce 44.4-67.3 ppm, Pr 5.23-8.88 ppm, Sm 4.41-7.15 ppm, Eu 1.24-1.83 ppm; one of the heavy rare earth elements, Lu have 0.20-0.45 ppm and Yb 1.88-2.98 ppm values. The Karataş volcanics consist of basaltic-trachyandesite and trachyandesite. Evidence of liquid-crystal fractional crystallization (FC) in the phenocryst phase during the formation of the Karataş volcanics is clearly observed in the geochemical diagrams. Traces of partial melting and crustal contamination were found in geochemical studies. Following the closure of the Neo-Tethys ocean in the region, a widespread crustal thickening occurred in the region with the northward divergence of the northern branch of Neo-Tethys, and a suture zone was formed along this line. The Karataş volcanics were formed as a product of active volcanism after the collision in the region.

Rock characteristics of post-caldera volcanoes in Dieng volcanic complex (DVC), Central Java, Indonesia

The Dieng volcanic complex (DVC) has one of the densest post-caldera volcanisms activity presents in Indonesia, yet its population density is considerably high. Therefore, it is important to identify the rock characteristics produced by the DVC post-caldera volcanoes to understand the risks and future hazards (i.e., eruption style). Based on lithology, we have classified DVC post-caldera volcanoes as (1) pyroclastic domain (PD; including Pagerkandang, Merdada, and Pangonan), and (2) lava domain (LD; including Prambanan, Kendil, Pakuwaja, Sikunir, Sikarim, and Seroja). PD is characterized by the domination of pyroclastic materials (mostly ash and lapilli) with oxidized scoria and volcanic lithics (fresh and/or altered) as the main components. The oxidized scoria clasts are moderately vesicular (27–41 % vesicularity; ) and phenocryst poor (<5 % phenocryst crystallinity, ), with plagioclase, pyroxene, and oxides as the main phenocryst phases. The LD is composed predominantly of lava. The observed lavas are typically dense (mostly <1 % , phenocryst rich (21–47 % ), and include plagioclase, pyroxene, biotite, amphibole, and oxides as the main phenocryst phases. Such differences in mineralogy and textures (i.e., vesicularity and crystallinity) suggest that PD and LD were likely sourced from different magmatic sources with different eruption styles (explosive and effusive styles, respectively). We have suggested that civilization settlements near PD are facing major threats from explosive magmatic, phreatomagmatic, and phreatic eruptions that could produce significant fallouts, ballistic materials, and highly destructive pyroclastic density currents. LDs pose a threat in the form of effusive magmatic eruptions such as lava flows and/or domes.

The Magmatic Evolution and the Regional Context of the 1835 AD Osorno Volcano Products (41°06’S, Southern Chile)

Abstract Osorno volcano (41°06’S, 72°20’W) is a composite stratovolcano of the Central Southern Volcanic Zone of the Chilean Andes. It is the southernmost member of a NE–SW trending alignment of volcanic edifices including La Picada and Puntiagudo volcanoes and the Cordón Cenizos chain. According to contemporary descriptions recorded by Charles Darwin in 1835, two eruptive events occurred: the first during January–February, and the second during November–December 1835 and January 1836. The volcano erupted basaltic andesite lavas and tephra fall deposits (52.4 to 52.9 SiO2 wt. %), which contain phenocrysts of olivine, plagioclase, clinopyroxene, and spinel. The compositions of these phenocryst phases, together with those of olivine-hosted melt inclusions, allowed us to constrain intensive parameters for the pre-eruptive magmas. These varied from 1060°C to 1140°C, with an oxygen fugacity buffer of ~ΔQFM +1.1, dissolved water concentrations of up to 5.6 wt. % (average of ~4.2 wt. %) and maximum pressures equivalent to ~7-km depth. Textural relations, such as crystal accumulations and clots, zoning in crystals and other indications of disequilibrium, lead us to infer the involvement of a crystal mush, rich in individual crystals and clots of crystals, which underwent a degree of disaggregation and entrainment into the transiting magma prior to eruption. Comparison of trace element abundances, including rare earth elements, fluid-mobile elements, and relatively fluid-immobile elements, combined with 87Sr/86Sr and 143Nd/144Nd isotope ratios, allows us to consider variations in slab-derived fluid input and the minor role of crustal contamination on the Osorno eruptive products and those from neighboring volcanic systems. Our results suggest both a greater contribution from slab-derived fluid and a higher degree of partial melting in the systems supplying stratovolcanoes (Osorno, Calbuco, and La Picada) relative to those supplying small eruptive centers built over the major regional Liquiñe-Ofqui Fault Zone.

Development of major element proxies for magmatic H2O content in oceanic basalts

Analysis of H2O concentrations in quenched glass has enabled significant improvements in our understanding of its role in mantle melting, magma differentiation, eruption dynamics, and the origin of mantle heterogeneity. Direct measurements of dissolved H2O in glass, however, are not always possible, and we lack robust methods of constraining magmatic H2O contents in aphyric, bulk rock samples that lack glass. Here, we present a major element hygrometer for mid-ocean ridge (MOR) and back-arc basin (BAB) basalt magmas based on the sensitivity of phenocryst phase assemblages to magmatic H2O contents, which translate into resolvable differences in liquid lines of descent (LLDs) as a function of magmatic H2O concentrations. Existing hygrometers lack sufficient resolution to be useful at the low H2O concentrations typical of MOR and BAB basalts (<1.0 wt%). We develop the major element proxy, Al2O3/FeO*(7.0) (fractionation-corrected to 7 wt% MgO), for determining magmatic H2O contents using cogenetic suites of oceanic basalts with well-defined LLDs and well-constrained H2O contents. H2O(7.0) positively correlates with Al2O3/FeO*(7.0) in the mid-ocean ridge basalt dataset, and this relationship is maintained in back-arc basin basalts with a broader range of water contents (up to 2.0 wt%). The main petrological control over this covariation is the role of H2O in suppressing plagioclase crystallization, while crystallization pressure and magmatic oxygen fugacity play lesser roles. Herein, we present an empirical model that uses Al2O3/FeO*(7.0) to estimate the magmatic water content in plagioclase-saturated oceanic basaltic magmas: H2O(7.0) = 1.109Al2O3/FeO∗(7.0) − 1.111. This model enables the estimation of magmatic H2O content using whole-rock major element data, which can be readily determined for aphyric or crystalline lavas that lack quenched glassy rinds, melt inclusions, or appropriate phenocryst assemblages.

Magmatic origins and storage conditions for the historic eruption of Tseax Volcano, British Columbia, Canada

The Northern Cordilleran Volcanic Province (NCVP) is the most active volcanic belt in Canada, yet there are few detailed petrologic studies of the individual volcanoes. Tseax volcano is the southernmost volcanic centre of the NCVP, the second youngest (~1700 CE) eruption in Canada, and led to one of Canada's worst natural disasters killing up to ~2000 Nisga'a First Nation people. We present a conceptual model for the origins, evolution and pre-eruptive storage of the Tseax magma which erupted in ~0.5 km3 of volcanic material, mainly in the form of valley-filling lava flows. All Tseax products are alkali Fe- Ti- rich basanite-to-tephrite having trace element distributions similar to the other mafic NCVP lavas. Phenocrysts consist of plagioclase, olivine and titanomagnetite, often forming glomerocrysts suggesting co-crystallisation of these 3 mineral phases. Clinopyroxene is never observed as a phenocryst phase. The lavas have low Mg#’s implying they are fractionated relative to a mantle-derived ‘parent’ magma. Trace elements indicate the magma derives from melting of a fertile mantle source (i.e., (Nb/K)n ~ 1), most likely the upper asthenosphere. There is no evidence in the Tseax volcanic rocks for magma mixing or lithospheric contamination during ascent. The phenocryst assemblage suggests rapid ascent of a low viscosity magma to <5.5 km where titanomagnetite becomes the first phase on the liquidus (~ 1133 °C). There the magma stalled for a very short period of time under PT conditions where clinopyroxene did not reach saturation. Based on the size of the plagioclase phenocrysts, the magma stalled for less than a year and cooled down to 1094–1087 °C prior to eruption.

Petrography and Mineral Chemistry of Monte Epomeo Green Tuff, Ischia Island, South Italy: Constraints for Identification of the Y-7 Tephrostratigraphic Marker in Distal Sequences of the Central Mediterranean

The 56 ka Monte Epomeo Green Tuff (MEGT) resulted from the largest volume explosive eruption from Ischia island (south Italy). Its tephra is one of the main stratigraphic markers of the central Mediterranean area. Despite its importance, a detailed characterisation of the petrography and mineral chemistry of MEGT is lacking. To fill this gap, we present detailed petrographic description and electron microprobe mineral chemistry data on samples collected on-land from the MEGT. Juvenile clasts include pumice, scoria, and obsidian fragments with porphyritic/glomeroporphyritic, vitrophyric, and fragmental textures. The porphyritic index is 13–40 vol.%, and phenocryst phases include alkali-feldspar, plagioclase, clinopyroxene, ferrian phlogopite, and titano-magnetite, in order of decreasing abundance; accessory phases include sphene, hydroxy-fluor-apatite, and rare edenite. Plagioclase varies from predominant andesine to subordinate oligoclase, whereas alkali-feldspar is more variable from sanidine to anorthoclase; quasi-pure sanidine commonly occurs as either rim or recrystallisation overgrowth of large phenocrysts due to hydrothermal alteration. Secondary minerals include veins and patches of carbonate minerals, Fe-Mn oxyhydroxides, clay minerals, and zeolites. Clinopyroxene is ferroan diopside (En45–29Fs7–27) and never reaches Na-rich compositions. This feature allows the discrimination of MEGT from aegirine-bearing, distal tephra layers erroneously attributed to MEGT, with implications for the areal distribution of Ischia explosive deposits.

Paleoproterozoic calc-alkaline lamprophyres from the Sidhi Gneissic complex, India: Implications for plate tectonic evolution of the Central Indian Tectonic Zone

Calc-alkaline lamprophyres are widely regarded as important probes for unravelling continental-scale geodynamic processes. We present petrographic, mineral chemical, bulk-rock geochemical, and Sr-Nd isotope data on eleven unmetamorphosed lamprophyre dykes intruding the ca. 2.5 Ga Sidhi Gneissic Complex in the Central Indian Tectonic Zone (CITZ) – an intercontinental suture which separates the Northern Indian (Aravalli-Bundelkhand Craton) and the Southern Indian (Dharwar-Bastar-Singhbhum Craton) blocks. The Sidhi dykes have porphyritic-panidiomorphic texture typical of lamprophyres, with phlogopite/biotite as the dominant phenocryst phase, in a orthoclase dominated groundmass. Based on mineralogy and geochemistry, the Sidhi dykes can be classified as calc-alkaline lamprophyres in general and minettes in particular. All these dykes are characterised by negative Ti, Ta and Nb anomalies typical of subduction-related global calc-alkaline lamprophyres. The trace element ratios of the Sidhi lamprophyres suggest derivation from variable degrees of partial melting of a similar magma source which experienced crustal input. Bulk-rock Rb-Sr isochron of the Sidhi lamprophyres yielded an age of ca. 2278 ± 230 Ma which is consistent with the Paleoproterozoic emplacement ages reported for other rocks from this region of CITZ. Negative ɛNdT (−3.68) values of the Sidhi lamprophyres suggest their derivation from an isotopically enriched mantle source whilst their TDM Nd model ages (~2.6 Ga) imply source enrichment that took place during the Neoarchaean. Petrogenetic modelling indicate that the Sidhi lamprophyres are generated from low degree partial melting of a mantle source metasomatised by sediment-derived melt having < 4% crustal material as an input in the E-DMM. Our study support models proposing a northward subduction of the Southern Indian Block beneath the Northern Indian Block along the CITZ.

Young Silicic Magmatism of the Greater Caucasus, Russia, with implication for its delamination origin based on zircon petrochronology and thermomechanical modeling

Several kilometers of rapid uplift in the past 2–3 million years in the Greater Caucasus in Russia has produced world-class exposures of Pliocene-Pleistocene age granites and ignimbrites at the Tyrnyauz and Chegem volcanic centers just to the east of Europe's highest mountain and active volcano, Mount Elbrus. This represents one of the world's best-preserved examples of silicic magmatism in a continental collision environment. We report results of a multi-method zircon petrochronologic (U--Pb, O--Hf isotopes, trace elements) investigation of six ignimbrites, lavas, and associated granodioritic porphyries from these localities. We observe two pulses of magmatism at 2.92 and 1.98 Ma related to Chegem and Tyrnyauz, respectively. High precision CA-ID-TIMS dating of zircons from the top and bottom of the Chegem ignimbrite and its associated porphyry yield indistinguishable age spectra recording 160 kyr of magma assembly and differentiation, with 2.9181 ± 0.0014 Ma eruption age as is constrained by the youngest dated zircon population. Together with overlapping O and Hf isotopic values, this suggests that they represent a large volume of pre-eruptively homogenized magma with ~20% crystals. Zircons have isotopically diverse cores (δ18O = +3.4–6.7‰, εHf = +0.8 to +5.6) while the rims (+5.75 ± 0.20‰, εHf = 3.3 ± 0.7) are in isotopic equilibrium with associated unaltered glass and major phenocryst phases. The neighboring and younger ignimbrites and granites of the nearby Elbrus and Tyrnyauz centers have overlapping higher δ18O and lower εHf zircon values and cluster narrowly in age around 1.98 Ma, suggesting a common source for all these ignimbrites, likely in the Tyrnyauz area. Isotopic data for Chegem, Elbrus and Tyrnyauz zircons and rocks require the contribution of a high proportion (25–55%) of Paleozoic crust to magma petrogenesis.The origin of sudden spikes of voluminous silicic magmatism is further investigated using thermomechanical modeling of the collisional environment. We show that the voluminous silicic volcanism at Chegem, Tyrnyauz and Elbrus is best explained by heating of the lower crust by asthenospheric upwelling after a relict slab detached and sank into the mantle at approximately 5 Ma. This timing is coincident with both the beginning of rapid uplift of the Greater Caucasus and initiation of major volcanism. After this initial melting, continuing delamination of the lithosphere and lower crust from the subducting plate produced broad mantle melting and basaltic volcanism, potentially explaining continued regional-scale magmatism in both the Greater and Lesser Caucasus.

Formation of crystal-rich, mixed, intermediate lavas at Pouakai Volcano and the evolution of the Taranaki volcanic lineament, western North Island, New Zealand

Over the past ~2 Ma, volcanic activity in the western North Island of New Zealand has been focused along the NW to SE-younging Taranaki volcanic lineament (TVL). Geophysical imaging of the convergent boundary between the Pacific and Indo-Australian plates shows that the subducted slab lies at least ~200 km below the present TVL and the <200 ka andesitic Mt. Taranaki. Pouakai is the second youngest (~0.25 Ma) volcanic edifice of the TVL and lies immediately NW of Mt. Taranaki. It is deeply eroded with limited exposure, but it has been sampled by a major collapse event that generated the 0.21–0.24 Ma Maitahi debris avalanche deposit. The Pouakai lavas sampled by the Maitahi deposit are medium-K, pyroxene-bearing basalts and amphibole- and pyroxene- bearing basaltic andesites and andesites. Plagioclase, clinopyroxene and amphibole are the dominant phenocryst phases. Olivine and orthopyroxene are rare. Compared to younger Mt. Taranaki lavas, the Pouakai volcanic rocks have lower K, P, Ba, Rb, Sr and Zr and higher Sc and V abundances, and they define subtly different trends on TiO2, MgO and K2O versus SiO2 variation diagrams. Most Pouakai eruptives are similar to the older ring plain debris avalanche deposits of Mt. Taranaki, but Pouakai summit lavas are compositionally similar to younger Mt. Taranaki eruptives. For the Pouakai sample suite, the geochemical trends and patterns are broadly consistent with control by fractional crystallisation (FC), or assimilation fractional crystallisation (AFC). However, textural and geochemical evidence indicates that the rocks are not representative of simple magmas related by a common liquid line of descent. Pouakai basaltic andesites and andesites are highly porphyritic and phenocrysts show complex zoning and disequilibrium reaction textures. Mg# [mol. MgO/(MgO + FeO)] of amphibole and clinopyroxene compositions vary beyond the range expected if the minerals had equilibrated with melts having bulk whole rock compositions. Furthermore, geothermobarometry based on amphibole and pyroxene compositions indicates that mafic phenocrysts in all rock types, as well as in individual samples, equilibrated with comparatively felsic melts over a wide range of pressures. Therefore, each lava sample represents a mixed magma, comprising felsic melts derived from a range of crustal and mantle sources hosting a complex, polybaric phenocryst assemblage. The whole rock compositions were derived through varying degrees of bulk fractionation of phenocryst phases with net removal of amphibole and clinopyroxene and consequent concentration of plagioclase phenocrysts.

The composition of amphibole phenocrysts in Neogene mafic volcanic rocks from the Puna plateau: Insights on the evolution of hydrous back-arc magmas

In typical Andean arc magmas, amphibole appears as a phenocryst phase only after considerable differentiation. However, some near-primitive volcanic rocks (high-Mg andesites and basalts) from monogenetic centers in the Puna plateau of Argentina also contain amphibole phenocrysts, implying special conditions of hydrous magma generation in this back-arc setting. This study documents typical examples from Southern and Northern Puna regions and uses the major and trace-element compositions of amphibole to constrain a petrogenetic model for the hydrous magmas.There are significant differences in the nature of amphiboles and their host lavas depending on location of the volcanic centers in the Southern and the Northern Puna regions. In the Southern Puna, basaltic andesitic lavas have Sr/Y values >40 and amphiboles show skeletal forms and occur in an assemblage with olivine and pyroxene. The amphibole compositions are relatively Al- and Ti-poor compared to the Northern Puna. Thermobarometry indicates amphibole crystallization temperatures of 960–1000 °C at moderate pressure (< 5 kbar). In contrast, the mafic lavas from centers in the Northern Puna show Sr/Y ratios lower than 20 and amphiboles in these rocks coexist with a plagioclase-orthopyroxene assemblage. The Northern Puna amphiboles have higher Ti and Al contents than those in the southern region and the thermobarometry estimates imply generally higher crystallization temperatures (>1000 °C) and pressures (6–8 kbar). Furthermore, the chemical composition of amphibole phenocrysts in the Northern Puna Campo Negro center suggests an alkaline affinity of the parental magmas which, together with radiogenic isotope data from earlier studies, indicates a significant contribution of the enriched lithosphere in the magma source.The new data collectively suggest high pressure evolution of hydrous magmas in the Southern Puna, whereas the Northern Puna magmas underwent more differentiation at higher levels in the crust. This contrast in the evolution history of magmas below both regions can be connected with their position relative to partial melting zones in the mid-upper crust, which are larger and longer-lived in the north than in the south, thus favoring a slower ascent of magmas in that region.

Neogene volcanism in Elazig-Tunceli area (eastern Anatolia): geochronological and petrological constraints

The Elazig and Tunceli provinces in eastern Anatolia host a complex succession of Miocene-Pleistocene effusive and explosive volcanic rocks, divided into four distinct volcanic phases. The most abundant and widespread products are the calcalkaline Mazgirt volcanic rocks, characterized by wide Sr isotope variations (87Sr/86Sr ~0.7054-0.7077) and narrower 143Nd/144Nd (~0.51246-0.51260) and Pb isotopes (e.g., 206Pb/204Pb ~18.89-19.13). New 40Ar-39Ar ages indicate that Mazgirt volcanic activity occurred between ~16.3 and 15.1 Ma. The other three volcanic phases are represented by the Tunceli mildly alkaline basaltic lavas (~11.4-11.0 Ma), the Pliocene Karakocan (~4.1 Ma) and Pleistocene Elazig (~1.9-1.6 Ma) Na-alkali basaltic lavas with clear OIB-like geochemical signature.Mazgirt volcanics can be subdivided on the base of mode of emplacement into lava flows and lava domes units characterized by petrographic, chemical and isotopic differences: lava flows are calcalkaline, whereas lava domes mostly belong to a high-K calcalkaline series and are, on average, more LREE- and 87Sr-enriched. Lava domes are more porphyritic, with a phenocryst assemblage dominated by amphibole, whereas plagioclase and clinopyroxene are the most abundant phenocryst phases in lava flows, pointing out that evolution of dome magmas occurred in conditions of slightly higher pressure, favouring the crystallization of hydrous phases.The Karabakir Formation, previously reported as late Miocene- Pliocene, encloses Mazgirt volcanics and is capped by Tunceli basalts. These new age data constrain the Karabakir Formation emplacement from early to late Miocene.The evolution of this igneous activity mirrors the geodynamic framework of the region: the early-middle Miocene Mazgirt volcanics represent arc volcanism related to Eurasia-Arabia convergence. The late Miocene Tunceli basalts postdate the onset of post-collisional tectonics in Eastern Anatolia, whereas the Karakocan and Elazig volcanic rocks were emplaced after the initiation of strike-slip motion on the North Anatolian and East Anatolian Fault systems.

Geology and Petrogenesis of Igneous Rocks from Batur Paleovolcano, Gunungkidul, Yogyakarta: Evidence from their Textures, Mineralogy, and Major Elements Geochemistry

Batur paleovolcano is located in Wediombo Beach area, Gunungkidul Regency, Yogyakarta and is being part of Wuni Formation. Several volcanic products including lava flow, autoclastic breccia and volcanic breccia can be found associated with diorite intrusions. This research is aimed to characterize geological, mineralogical andgeochemical variations of igneous rocks from Batur paleovolcano to understand its petrogenesis. Detailed geological mapping with scale of 1:12,500 is conducted to identify geological aspects and delineate igneous rocks distributions. Igneous rocks and selected wall rocks samples were prepared for laboratory analysis including 8 samples for petrography and 5 samples for ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) analysis. Several geochemical data from previous study are also added to investigate the geochemical variations. Geological condition of the research area consists of four rock units including colluvial deposit, limestone, andesite lava and diorite intrusion. Geological structures found are normal fault and shear joint where the main stress direction is north–south. Petrography analysis showed that igneous rocks in this research area consist of diorite intrusion and andesite lava with phorphyritic texture. Plagioclase become the most abundant minerals found both as phenocryst phase and groundmass. Hornblende only occur as phenocryst phase in minor amounts as accesory mineral. Major elementsgeochemistry analysis showed the rocks are characterized by intermediate silica with low alkali content. They are can be categorized as calc-alkaline series. However, some samples are fall into tholeiitic series. Major elements variation and textural study also indicate the magma is experienced differentiation process by fractional crystallization mechanism. This study suggests that igneous rocks from Batur paleovolcano is formed by two phases of formation. Earlier phase is the formation of andesite lava in island arc tholeiitic tectonic setting then at the later phase is formation of diorite intrusion in the calc-alkaline basalts tectonic setting.

High-resolution Fe–Ti oxide thermometry applied to single-clast pumices from the Bishop Tuff: a re-examination of compositional variations in phenocryst phases with temperature

Numerous analyses of multiple ilmenite, titanomagnetite, sanidine, plagioclase, biotite, orthopyroxene, and clinopyroxene crystals (when present) in each of 24 single-clast pumices from the Bishop Tuff (BT), CA are presented, together with whole-rock compositions on the same single clasts. Data from multiple Fe–Ti oxide grains in each sample, coupled with an assessment of analytical uncertainty, show that there is a single population of ilmenite and titanomagnetite in each of 20 samples and no more than two populations of ilmenite and/or titanomagnetite in the remaining four. A statistical evaluation of microprobe uncertainty (for beam conditions of 15 kV and 10 nA) constrains the minimum number of ilmenite (≥ 21) and titanomagnetite (≥ 56) analyses required to obtain Fe–Ti two-oxide temperatures with propagated uncertainties ≤ ± 10 °C. Thermometry results confirm a continuous range (~ 700–800 °C) across the BT deposit. Plagioclase displays a bimodal compositional pattern, where a predominantly sodic (An14–15) vs. calcic (An22–23) population is found in single clasts where pyroxene is absent vs. present; the latter coincides with geochemical evidence for mixing between high-SiO2 rhyolite and less differentiated melts as previously documented in the literature. When sanidine rims are paired with plagioclase, calculated two-feldspar temperatures (~ 690–780 °C) are strongly correlated (R2 = 0.94) with Fe–Ti two-oxide temperatures. Application of plagioclase–liquid hygrometry reveals a systematic decrease in melt water content (~ 7.2–4.1 wt%) with increasing temperature across the BT deposit, consistent with experiments from the literature that show cotectic crystallization of quartz–sanidine–plagioclase in rhyolite melt requires a concomitant decrease of ~ 3 wt% H2O between 700 and 800 °C at 200 MPa. Single-clast, whole-rock concentrations of some elements (e.g., Rb) correlate strongly with Fe–Ti oxide temperature (R2 = 0.95). The Mg# in biotite also correlates strongly with temperature (R2 = 0.97), as do the Mg# values in orthopyroxene and clinopyroxene with whole-rock Mg# (R2 = 0.6 and 0.8, respectively) and melt water content (R2 = 0.7 for both). Evidence points to biotite crystallization in all BT samples from high-SiO2 rhyolite melt, which was already continuously zoned in composition and temperature prior to mixing with less differentiated melts, as previously noted in the literature. In contrast, several lines of evidence, including hydrous phase-equilibrium experiments on rhyolites from the literature, show that both pyroxenes in BT pumices co-crystallized with calcic plagioclase (~ An21–29), in equilibrium with both titanomagnetite and ilmenite, after mixing between less differentiated melt(s) and relatively hot (≥ 755 °C) high-SiO2 rhyolite.

Dating Clinopyroxene Phenocrysts in Submarine Basalts Using 40Ar/39Ar Geochronology

AbstractDating submarine basalts using 40Ar/39Ar geochronology is often hindered by a lack of potassium‐bearing phenocrystic phases and severe alteration in the groundmass. Clinopyroxene is a common phenocrystic phase in seafloor basalts and is highly resistive to low‐temperature alteration. Here we show that clinopyroxene phenocrysts separated from marine basalts are a viable phase for 40Ar/39Ar incremental heating age determinations. We provide results from a pilot study comprising 16 age experiments from nine clinopyroxene separates, five of which from samples with dated coeval phases. The clinopyroxene ages range from 11.5 to 112 Ma with relatively high uncertainties (ranging from 0.8% to 7.1%; median of 1.9%) compared to more traditional phases. The clinopyroxene age plateaus form at low to moderate temperature steps and are characterized by relatively elevated K/Ca of 0.002–0.4, suggesting that other K‐bearing phases hosted within the clinopyroxene are likely degassing to yield the 40Ar/39Ar age information. There are three possible origins for the K and corresponding 40Ar* including films of trapped melt/nanomineral inclusions along grain defects, secondary melt inclusion bands, or variations in degassing behaviors between lower and higher crystalline Ca pyroxene phases. Regardless of the source of the K, the age determinations are successful with 75% of the experiments producing long plateaus (&gt;60% 39Ar released) with mean square of the weighted deviations ranging from 0.6 to 1.5 and probability of fit values &gt;0.05. We conclude that clinopyroxene dating by the 40Ar/39Ar method has the potential to provide a wealth of information for previously undated, altered seafloor lithologies and continental equivalents.

Evolution in magma storage conditions beneath Kick-'em-Jenny and Kick-'em-Jack submarine volcanoes, Lesser Antilles arc

Kick-'em-Jenny and Kick-'em-Jack are two submarine volcanoes located in the southern Lesser Antilles arc. We present data on the temporal and spatial compositional variations in lavas and melt inclusions from Kick-'em-Jenny and four lavas from Kick-'em-Jack. Analyses of crater samples from Kick-'em-Jenny show an evolution in magma compositions between 1972 and 2013/14. Rock types have progressed from predominantly basalts in 1972 to basaltic andesites in 2013/14, while melt inclusions have also become more silicic over time, ranging basaltic to andesitic in 1972 and basaltic andesite to dacitic in 2013/14. Hornblende-phyric basalts are unusually rich in MgO (≤13 wt%) and Ni (≤230 ppm) and contain rare olivine phenocryst cores of Fo90–92, suggestive of a primitive character. Other phenocryst phases include spinel, clinopyroxene and plagioclase. Conversely, 2013/14 landslide deposit samples contain orthopyroxene as an additional phenocryst phase and are distinct from crater deposits in terms of phenocryst and bulk rock chemistry. We propose that 1972 and 2013/14 crater deposits sample Kick-'em-Jenny's current edifice, whereas 2013/14 landslide deposits likely represent “proto Kick-'em-Jenny” (an edifice existing pre 43.5 kyr BP). Geochemical data for Kick-'em-Jack support the notion that it is part of the same volcanic complex as Kick-'em-Jenny and probably represents part of the now collapsed proto Kick-'em-Jenny.Low eruption temperatures (~1000 °C), estimated from geothermometry, coupled with whole-rock magma chemistry suggest an origin of the most mafic lavas of Kick-'em-Jenny by dehydration melting of hornblende-peridotite just below the Moho. Subsequent injection of these cool, oxidised, wet primitive basalts into the deep crust was accompanied early fractionation of hornblende-dominated assemblages. Large, zoned hornblende phenocrysts track rapid ascent of magmas through the crust along a trajectory sub-parallel to the hornblende liquidus curve. Melt inclusion data provide clear evidence of volatile-rich (≤5 wt% H2O and <3200 ppm CO2) pre-eruptive magma storage 7–12 km below Kick-'em-Jenny. This new understanding of the architecture of Kick-'em-Jenny's sub-volcanic system will be useful for interpreting future seismic signals and mitigating volcanic hazards.

Crystal-poor rhyolites and rhyodacites from Volcán Tepetiltic, Mexico: Evidence for melt formation, crystallization and eruption over short timescales

A detailed petrologic study is presented for six rhyolite and rhyodacite pumices that erupted from Volcán Tepetiltic in Western Mexico to determine the pre-eruptive conditions of the magma reservoir, as the volcano exited a ~180 kya hiatus of activity. Rhyolite pumices are crystal poor (≤6.7%) and minimally saturated in seven mineral phases (plagioclase + titanomagnetite + ilmenite + apatite ± quartz ± orthopyroxene ± clinopyroxene ± hornblende ± zircon). In all samples, plagioclase is the most abundant phenocryst phase and spans a range of composition of ~20–30 mol% An in each sample. Pre-eruptive temperatures, determined from two Fe-Ti oxide geo-thermometry, range from 703 to 857 °C. Maximum pre-eruptive H2O contents, determined from plagioclase hygrometry, range from 5.0–8.9 wt%, where the most calcic plagioclase compositions correspond to the highest water contents. Comparison of compositions, textures, and Sr and Ba concentrations in plagioclase are used to determine phenocrysts from xenocrysts through comparison with experiments; xenocrysts are identified in one sample (COMP-48). Comparison of plagioclase abundances, textures and sizes with those grown in the run products of experiments from the literature reveals that (1) the pumices were all likely super-heated prior to crystallization, (2) they experienced a low degree of undercooling induced by either degassing or cooling, and (3) four of the pumices (those with <1.5% plagioclase) did not reach an equilibrium abundance of plagioclase, which constrains the amount of time spent beneath the plagioclase-in curve to less than ten days. Collectively, the data suggests that the crystal-poor pumices were partial melts, generated by an influx of basaltic andesite, that experienced super-liquidus conditions, followed by crystallization and eruption on a time-scale of weeks.

Geological, petrological and geochemical framework of Miravalles-Guayabo caldera and related lavas, NW Costa Rica

The Miravalles-Guayabo caldera is located in the Guanacaste Cordillera and its genesis and evolution is related with the geodynamic setting of northwestern Costa Rica. The development of a N-S graben, and related NE-trending fracture zones, was essentially coeval with caldera forming eruptions and the multiple collapses which dismembered earlier volcanoes.The lavas of the Miravalles-Guayabo Complex (MGC) are two-pyroxene calc-alkaline andesites and basaltic andesites with subordinate high alumina basalts and rare dacites. Interstitial matrix glasses span from andesite (in basalts) to dacite (in andesite s.l.) and rhyolite (in dacite) that overlap the bulk chemical compositions of the lavas. REE patterns and spider diagrams show typical features of a subduction-related volcanism at convergent plate boundaries with spiked incompatible-element patterns, negative anomalies for Nb and Ti in spider diagrams, enrichment in LILE (with the exclusion of Sr) and LREE, and relative depletion in HFSE. The geochemical signatures of the MGC volcanic rocks are essentially consistent with recent petrogenetic models.Thermobarometric estimates suggest that basaltic and magmas equilibrated with their phenocrystic phases at 1080–1120 °C, and pressures ranges of 380–430 MPa, whereas andesitic melts are confined at 200–230 MPa and temperatures of 1050–1070 °C. In turn, amphibole-biotite dacites equilibrated at 240–290 MPa and temperatures of 920–950 °C approaching water saturated conditions (with H2O contents up to 5.7–7.2 wt%). Thermobarometry indicate that contiguous andesitic-dacitic magma lenses are stored within the subvolcanic region at depths below 10 km, whereas basaltic magmas are stored and degas at slightly higher depths (up to ~15 km). Single eruptions associated with the emplacement of lava flows seem to be related to the tapping of portions of single magma lenses during minor unrest episodes. Conversely, caldera forming eruptions involved the ejection of differentiated rhyolitic pumices (of La Ese member) and likely occurred during recharge and pressurization of this heterogeneous reservoir under critical stress conditions.