Geochemical Characterization of Intraplate Magmatism from Quaternary Alkaline Volcanic Rocks on Jeju Island, South Korea

The major and trace elements of Quaternary alkaline volcanic rocks on Jeju Island were analyzed to determine their origin and formation mechanism. The samples included tephrite, trachybasalts, basaltic trachyandesites, tephriphonolites, trachytes, and mantle xenoliths in the host basalt. Although the samples exhibited diversity in SiO2 contents, the relations of Zr vs. Nb and La vs. Nb indicated that the rocks were formed from the fractional crystallization of a single parent magma with slight continental crustal contamination (r: 0–0.3 by AFC modeling), rather than by the mixing of different magma sources. The volcanic rocks had an enriched-mantle-2-like ocean island basalt signature and the basalt was formed by partial melting of the upper mantle, represented by the xenolith samples of our study. The upper mantle of Jeju was affected by arc magmatism, associated with the subduction of the Pacific Plate beneath the Eurasian Plate. Therefore, we inferred that two separate magmatic events occurred on Jeju Island: one associated with the subduction of the Pacific Plate beneath the Eurasian Plate (represented by xenoliths), and another associated with a divergent setting when intraplate magmatism occurred (represented by the host rocks). With AFC modeling, it can be proposed that the Jeju volcanic rocks were formed by the fractional crystallization of the upper mantle combined with assimilation of the continental crust. The xenoliths in this study had different geochemical patterns from previously reported xenoliths, warranting further investigations.

Pre-existing fault-controlled eruptions from the lateral tips of a laccolith in SE Iceland

<p>Volcano eruption forecasting relies on models of sub-volcanic magmatic plumbing systems that link ground deformation to sub-surface magma movement. However, many of these models typically assume that eruption sites occur directly above laccolithic reservoirs. Furthermore, many of these models assume deformation of the host rock is exclusively elastic with few studies highlighting the role inelastic deformation (e.g., faulting/fracturing). Whilst the dynamics of magma flow have previously been well constrained in ancient in sub-volcanic systems, its geometrical and kinematic relationship with the corresponding host rock deformation remains poorly understood which, is critical to volcanic hazard assessment.</p><p>Here, we examine the structure of the shallow-level (i.e. intruded <1 km below the palaeosurface), silicic Reyðarártindur laccolith in SE Iceland, and demonstrate how the underlying mechanisms of lateral magma flow coupled with pre-existing host rock structures influenced the localisation of volcanic activity. In particular, we use anisotropy of magnetic susceptibility (AMS) fabric analysis and show that the intrusion contains several laterally emplaced magma lobes, with magma flowing along a SW-NE axis, parallel to the strike of pre-existing, steeply dipping fault arrays in the host basalt lavas. Lateral magma flow and inflation of the lobes promoted upward intrusion along these pre-existing faults, which we posit acted as preferential pathways for magma to reach eruption sites that were laterally offset by tens to hundreds of metres from the underlying main intrusion.</p><p>Our interpretation provides field evidence for the reactivation of pre-existing structures as inclined magma conduits to eruptive vent sites on the outer margins of subjacent lateral magma bodies. This supports seismic observations where (i) Volcanoes overlie the lateral tips of subjacent intrusions in subvolcanic systems; (ii) ground, and host rock deformation preceding eruptions can be most prominent in areas adjacent to the volcano site; and (iii) volcanoes overlie and are aligned along fault traces suggesting that pre-existing normal faults influence the localisation of volcanic activity.</p><p> </p>

Celestine discovered in Hawaiian basalts

We report here the first occurrence of celestine (SrSO4) in recent oceanic basalts. Celestine was found in moderately altered accidental volcanic blocks from Ka‘ula Island, a rejuvenated tuff cone in the northern Hawaiian Islands. This occurrence is novel not only for the presence of celestine but also for the absence of barite, the sulfate mineral most commonly found in oceanic hydrothermal deposits. Celestine was found lining vesicles and partially fillings voids within the matrix of several high Sr (2200–6400 ppm) Ka‘ula basalts. High-quality wavelength-dispersive microprobe analyses of celestine are reported here for near end-member celestine (>90%). The Ka‘ula celestine deposits are compositionally heterogeneous with large variations in Ba content (0.9–7.5 wt%) within single mineral aggregates. The most likely source of the Sr for celestine in the Ka‘ula basalts was the host basalt, which contains ~1200 ppm. This is about 10 times higher than normally found in mid-ocean ridge basalts and 4 times greater than commonly observed in Hawaiian basalts. Hydrothermal alteration by S-bearing fluids related to the eruption that transported these accidentally fragments probably mobilized Sr in the blocks. These S-rich solutions later precipitated celestine during or following the eruption. We were unable to confirm the origin for the Sr via Sr isotope measures because the Ka‘ula celestine was too fine grained, friable, and widely dispersed to be concentrated for Sr isotope analyses. Future studies of basalts from active volcanoes on oceanic islands, especially for basalts with elevated Sr contents (>1000 ppm), should be aware of the possible presence of celestine in moderately altered lavas.

Zircon Xenocrysts from Cenozoic Alkaline Basalts of the Ratanakiri Volcanic Province (Cambodia), Southeast Asia—Trace Element Geochemistry, O-Hf Isotopic Composition, U-Pb and (U-Th)/He Geochronology—Revelations into the Underlying Lithospheric Mantle

Zircon xenocrysts from alkali basalts in Ratanakiri Province, Cambodia represent a unique low-Hf zircon within a 12,000 km long Indo-Pacific megacryst zone. Colorless, yellow, brown, and red crystals ({100}, {101}, subordinate {211}, {1103}), with hopper growth and corrosion features range up to 20 cm in size. Zircon chemistry indicates juvenile, Zr-saturated, mantle-derived alkaline melt (Hf 0.6–0.7 wt %, Y <0.2 wt %, U + Th + REE (Rare-Earth Elements) < 600 ppm, Zr/Hf 66–92, Eu/Eu*N ~1, positive Ce/Ce*N, HREE (Heavy REE) enrichment). Incompatible element depletion with increasing Yb/SmN from core to rim at ~ constant Hf suggests single stage growth. Ti-in-zircon temperatures (~570–740 °C) are lower than predicted by crystal morphology (800–900 °C) and decrease from core to rim (ΔT = 10–50 °C). The δ18O values (4.88 to 5.01‰ VSMOW (Vienna Standard Mean Ocean Water)) are relatively low for xenocrysts from the zircon Indo-Pacific zone (ZIP). The 176Hf/177Hf values (+ εHf 4.5–10.2) give TDepleted Mantle model source ages of 260–462 Ma and TCrustal ages of 391–754 Ma. The source magmas reflect variably depleted lithospheric mantle with little supracrustal input. Zircon U-Pb (0.88–1.56 Ma) and (U-Th)/He (0.86–1.02 Ma) ages are older than host basalt ages (~0.7 Ma), which suggests limited residence before transport. Zircon genesis suggests Zr-saturated, Al-undersaturated, carbonatitic-influenced, low-degree partial melting (<1%) of peridotitic mantle at ~60 km beneath the Indochina terrane.

Zircon Xenocrysts from Cenozoic Alkaline Basalts of the Ratanakiri Volcanic Province (Cambodia), Southeast Asia—Trace Element Geochemistry, O-Hf Isotopic Composition, U-Pb and (U-Th)/He Geochronology—Revelations into the Underlying Lithospheric Mantle

Zircon xenocrysts from alkali basalts in Ratanakiri Province, Cambodia, represent a unique low-Hf zircon, within a 12,000 km long Indo-Pacific megacryst zone. Colourless, yellow, brown and red crystals ({100}, {101}, subordinate {211}, {1103}), with hopper growth and corrosion features, range up to 20 cm in size. Zircon chemistry implicates juvenile, Zr-saturated, mantle-derived alkaline melt (Hf 0.6&ndash;0.7 wt%, Y &lt;0.2 wt%, U+Th+REE &lt;600 ppm, Zr/Hf 66&ndash;92, Eu/Eu*N ~ 1, positive Ce/Ce*N, HREE enrichment). Incompatible element depletion with increasing Yb/SmN from core to rim at ~ constant Hf suggests single stage growth. Ti-in-zircon temperatures (~570&ndash;740 &deg;C) are lower than predicted by crystal morphology (800&ndash;900 &deg;C) and decrease from core to rim (DT = 10&ndash;50 &deg;C). The d18O values (4.88 to 5.01&permil; VSMOW) are relatively low for xenocrysts from the zircon Indo-Pacific zone (ZIP). The 176Hf/177Hf values (+ &epsilon;Hf 4.5&ndash;10.2) give TDepleted Mantle model source ages of 260&ndash;462 Ma and TCrustal ages of 391&ndash;754 Ma. The source magmas reflect variably depleted lithospheric mantle with little supracrustal input. Zircon U-Pb (0.88&ndash;1.56 Ma) and (U-Th)/He (0.86&ndash;1.02 Ma) ages are older than host basalt ages (~0.7 Ma), suggesting limited residence before transport. Zircon genesis suggests Zr-saturated, Al-undersaturated, carbonatitic-influenced, low-degree partial melting (&lt;1%) of peridotitic mantle at ~60 km beneath the Indochina terrane.

Short-scale variability of the SCLM beneath the extra-Andean back-arc (Paso de Indios, Argentina): Evidence from spinel-facies mantle xenoliths

Cenozoic basalts carrying ultramafic mantle xenoliths occur in the Matilde, León and Chenque hills in the Paso de Indios region, Argentina. The mantle xenoliths from the Chenque and León hills mainly present porphyroclastic textures, whereas the Matilde hill xenoliths have coarse-grained to porphyroclastic textures. The equilibrium temperatures are in the range of 780 to 940ºC, indicating a provenance from shallow sectors of the lithospheric mantle column that were subjected to a relatively low heat ffiux at Cenozoic Era.According to the modal compositions of xenoliths, the mantle beneath Matilde and León hills was affected by greater than 22% partial melting, while less depleted peridotites occur in the Chenque suite (starting from 10% partial melting). Such an observation is confirmed by the partial melting estimates based on Cr#Sp, which vary from 8 to 14% for the selected Chenque samples and from 14 to 18% for the Matilde ones.The common melting trend is overlapped by small-scale cross cutting local trends that may have been generated by open-system processes, such as open-system partial melting and/or post partial-melting metasomatic migration of exotic Na-Cr-rich melts.The two main mineralogical reaction schemes are: i) the dissolution of pyroxenes and the segregation of new olivine in olivine-rich peridotites, and ii) the replacement of primary olivine by orthopyroxene±clinopyroxene in orthopyroxene-rich peridotites. These were produced by channelled and/or pervasive melt extraction/ migration. Enhanced pyroxene dissolution is attributed to channelling of silica- undersaturated melts, whereas the replacement of primary olivine by orthopyroxene±clinopyroxene points to reaction with silica-saturated melts.Late disequilibrium reactions identified in the xenoliths comprise: the breakdown of orthopyroxene in contact with the host basalt, and (rarely) reaction coronae on orthopyroxene, clinopyroxene and spinel linked to glassy veins. Such features are apparently related to the injection of melt, likely during entrainment into the host basalts and ascent to the surface.

The mineralogy and crystal chemistry of alkaline pegmatites in the Larvik Plutonic Complex, Oslo rift valley, Norway. Part 1. Magmatic and secondary zircon: implications for petrogenesis from trace-element geochemistry

A detailed electron microprobe (EMP) and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) study of zircon from six types of miaskitic and agpaitic alkaline pegmatite from the Larvik Plutonic Complex, Oslo rift valley, Norway, was undertaken to shed light on the pegmatite petrogenesis. Detailed rare earth element (REE) analyses indicate important differences between the zircon from each type of pegmatite. Primary zircon from miaskitic Stavern-, Tvedalen- and Stålaker-type pegmatites has a mean ΣREE = 704 ppm, is depleted in LREE and has a significant positive Ce anomaly (Ce/Ce* = 44–67) and negative Eu anomaly (Eu/Eu* = 0.15–0.18). Secondary Tvedalen-type zircon is REE-enriched (ΣREE = 5035 ppm), with a flatter REE pattern, Ce/Ce* = 0.97 and a Eu anomaly similar to primary Tvedalen-type zircon (Eu/Eu* = 0.21). Secondary zircon from agpaitic Langesundsfjord-type pegmatites display a distinctive flat REE pattern characterized by overall REE enrichment (ΣREE = 967), Ce/Ce* = 1.92, and a minor negative Eu anomaly (Eu/Eu* = 0.37). Zircon from agpaitic Bratthagen-type pegmatites occurs as both altered primary and secondary phases and is strongly enriched in REE relative to other zircon (ΣREE = 4178 and 8388, respectively). Primary Bratthagen-type zircon has a similar REE pattern to miaskitic zircon, with a steeper HREE profile and smaller Ce and Eu anomalies (Eu/Eu* = 0.73; Ce/Ce* = 6.22). Secondary Bratthagen-type zircon is strongly enriched in LREE compared to primary zircon, does not display a positive Ce anomaly and has Eu/Eu* = 0.56. The altered primary and secondary Bratthagen-type zircons have elevated Th/UN ratios, suggesting a different melt source for Bratthagen-type agpaitic pegmatites. Zircon from external pegmatites has trace-element signatures similar to Stavern-, Tvedalen- and Staålaker-type primary zircon with Ce/Ce* = 214 and Nb/Ta and Th/U ratios that are similar to those of secondary Langesundsfjord- and Bratthagen-type zircon. It is suggested that the parental melt of the external pegmatites is the same as the miaskitic pegmatites, but that it has undergone alteration by hydrothermal fluids derived from the host basalt, or by post-magmatic F-rich fluids which mobilize Nb and Th. On the basis of texture, morphology and geochemistry, two populations of zircon can be recognized: (1) primary zircon from miaskitic pegmatites; and (2) secondary zircon from post-magmatic, hydrothermal assemblages. The U–Th–Pb isotope analyses indicate that the secondary and altered zircon are depleted in 238U, and enriched in LREE. Interaction of a post-magmatic hydrothermal fluid with an externally derived meteoric fluid is suggested to have influenced the REE signatures, and in particular the Eu and Ce anomalies of the late-stage zircons.