Lithium, Oxygen and Magnesium Isotope Systematics of Volcanic Rocks in the Okinawa Trough: Implications for Plate Subduction Studies

Determining the influence of subduction input on back-arc basin magmatism is important for understanding material transfer and circulation in subduction zones. Although the mantle source of Okinawa Trough (OT) magmas is widely accepted to be modified by subducted components, the role of slab-derived fluids is poorly defined. Here, major element, trace element, and Li, O and Mg isotopic compositions of volcanic lavas from the middle OT (MOT) and southern OT (SOT) were analyzed. Compared with the MOT volcanic lavas, the T9-1 basaltic andesite from the SOT exhibited positive Pb anomalies, significantly lower Nd/Pb and Ce/Pb ratios, and higher Ba/La ratios, indicating that subducted sedimentary components affected SOT magma compositions. The δ7Li, δ18O, and δ26Mg values of the SOT basaltic andesite (−5.05‰ to 4.98‰, 4.83‰ to 5.80‰ and −0.16‰ to −0.09‰, respectively) differed from those of MOT volcanic lavas. Hence, the effect of the Philippine Sea Plate subduction component, (low δ7Li and δ18O and high δ26Mg) on magmas in the SOT was clearer than that in the MOT. This contrast likely appears because the amounts of fluids and/or melts derived from altered oceanic crust (AOC, lower δ18O) and/or subducted sediment (lower δ7Li, higher δ18O and δ26Mg) injected into magmas in the SOT are larger than those in the MOT and because the injection ratio between subducted AOC and sediment is always >1 in the OT. The distance between the subducting slab and overlying magma may play a significant role in controlling the differences in subduction components injected into magmas between the MOT and SOT.

Phase relations and pre-eruptive conditions at low f(O2) in Pantelleria peralkaline rhyolites

<p>The volcanic system of Pantelleria is an example of volcanism in a continental rift basin which over the years has attracted much researcher due to the different eruptive styles it exhibits, ranging from effusive to explosive. Investigating the cooling history as well as the magma transport dynamics of peralkaline rhyolitic magma is useful to understand the eruptive behaviour of the pantelleritic magma system.</p><p>The present work seeks to obtain information on the liquidus temperature of alkali feldspar in pantellerite from the Fastuca pumice fall unit (PAN13) under water-saturated conditions. Alkali feldspar is one of the most abundant crystalline phases in peralkaline rhyolitic melts as well as in evolved, alkali-rich magma compositions (e.g., trachyte, phonolite).</p><p>A series of water-saturated isobaric single-step cooling experiments were performed at reducing conditions (graphite filler rod, water P-medium, ~NNO-2) with final temperature range of 670 °C-880 °C and water pressure of 20-150 MPa. Phase equilibria show that clinopyroxene is the first liquidus phase always appearing by 750 °C, followed by alkali feldspar over the entire pressure and temperature (P-T) range investigated, with also the presence of aenigmatite crystallizing near the liquidus at P of 50 MPa. Providing experimental constraints on pre- and syn-eruptive magma crystallization is fundamental to better understand the eruptive dynamics of peralkaline rhyolitic magmas. This is important for volcanic hazard assessments of peralkaline rhyolitic magmatic systems.</p>

Contributions of Volatiles to the Venus Atmosphere from the Observed Extrusive Volcanic Record: Implications for the History of the Venus Atmosphere

<p>One of the most important questions in planetary science is the origin of the current Venus atmosphere, its relationship and coupling to Venus’ geologic and geodynamic evolution, andwhy it is so different from that of the Earth. We specifically address the following question:Does the eruption of the total volume of extrusive volcanic deposits observed in the exposed geologic record of Venus contribute significantly to the current atmosphere through volatile release during emplacement of the extruded lavas? To address this question, we used the observed geologic and stratigraphic record of volcanic units and features, and their volumes, as revealed by Magellan (1; their Fig. 26 and Table 5).  We converted the volumes of the main volcanic units to lava/magma masses using a density of 3000 kg m<sup>-3</sup>. Next, we chose the upperthickness values, and added the contributions from allof the units; summing the values of the "total eruptives" gives the absolute upper limit estimate of the mass of documented volcanics that could contribute to the atmosphere, 7.335 x 10<sup>20 </sup>kg. We then compare this with the current mass of the Venus atmosphere (4.8 x 10<sup>20 </sup>kg). We find that in order to make the current atmosphere from the above volcanics, the magma would have to consist of 65.4% by mass volatiles, which is, of course, impossible. We conclude that the grand totalof the currently documented volcanics can not have produced other than a very small fraction of the current atmosphere.</p><p>Exsolution of volatiles during volcanic eruptions is significantly dependent on surface atmospheric pressure (2-3). However, the total volumeof lava erupted in the period of global volcanic resurfacingis still insufficient to produce the CO<sub>2</sub>atmosphere observed today, even if the ambient atmospheric pressure at that time was only 50% of what it is today. Therefore, a very significant part of the current CO<sub>2</sub>atmosphere must have been inherited from a time prior to the observed geologic record, sometime in the first ~80% of Venus history. Furthermore, the total volumeof lava erupted in the stratigraphically youngest period of the observed record (1) is insufficient to account for the current abundance of SO<sub>2 </sub>in the atmosphere; thus, it seems highly unlikely that current and recently ongoing volcanism could be maintaining the currently observed ‘elevated’ levels of SO<sub>2 </sub>in the atmosphere (4).  In addition, because of the fundamental effect of atmospheric pressure on the quantity of volatiles that will be degassed, varying the nature of the mantle melts over a wide range of magma compositions and mantle fO<sub>2 </sub>appears to have minimal influence on the outcome.  We conclude that the current Venus atmosphere must be a “fossil atmosphere”, largely inherited from a previous epoch in Venus history, and if so, may provide significant insight into the conditions during the first 80% of Venus history.</p><p>(1) Ivanov and Head (2013) Plan. Space Sci. 84, 66; (2) Gaillard & Scaillet, 2014, EPSL 403, 307; (3) Head & Wilson, 1986, JGR 91, 9407;(4)Esposito, 1984, Science 223, 1072.</p>

The Origin and Melt Evolution of Massif-type Anorthosite Parental Magmas: Thermodynamically Controlled Major Element Constraints

<p>The parental magmas of massif-type anorthosites are suggested to originate from either the mantle or lower crust. If the source is the mantle, the magmas are presumed to have undergone crustal assimilation prior to plagioclase crystallization, which has produced melt compositions similar to anorthosite parental magmas (high-Al gabbros/basalts). If the source is the lower crust, the produced anorthosite parental melts are presumed to be monzodioritic (jotunitic) in composition. However, many studies have suggested that the monzodioritic rocks related to massif-type anorthosites rather represent residual melt compositions left after anorthosite fractionation. In this study, we have used the most recent thermodynamic modeling tools, Magma Chamber Simulator (MCS) and Rhyolite-MELTS to conduct partial melting, assimilation-fractional crystallization (AFC), and fractional crystallization (FC) models to address the unresolved questions about the source and compositional evolution of the anorthosite parental magmas.</p><p>AFC models were conducted at high lower crustal pressures (1000 MPa) by using MCS. In the models, we used four different sublithospheric mantle partial melt compositions and 11 different assimilants with representative average lower crustal compositions compiled from literature. In addition, equilibrium partial melting of the same lower crustal compositions was modeled separately by using rhyolite-MELTS. The melt major element compositions produced by both modeling tools were compared to suggested natural anorthosite parental magma compositions. Finally, to further study the evolution of these melts after their generation, FC models were run at different crustal pressures (1000-100 MPa) by using MCS. These differentiated melt compositions were compared to a global array of monzodioritic rocks presumed to represent residual melts left after anorthosite fractionation.</p><p>The preliminary modeling results point towards the mantle being a more suitable candidate for the source of the anorthosite parental magmas and that the parental magma compositions are better represented by high-Al gabbros than monzodioritic rocks: assimilation of mafic lower crustal material by mantle-derived magmas produces melts that are the most fitting analogues. Somewhat similar melts can also be produced by directly melting the lower crust, but this requires the crust to melt completely, which we consider improbable. The models further suggest fractional crystallization of high-Al gabbroic parental magmas produce residual melt evolution trends similar to the array of anorthosite-related monzodioritic rocks.</p>

Heterogeneous Hadean crust with ambient mantle affinity recorded in detrital zircons of the Green Sandstone Bed, South Africa

The nature of Earth’s earliest crust and the processes by which it formed remain major issues in Precambrian geology. Due to the absence of a rock record older than ∼4.02 Ga, the only direct record of the Hadean is from rare detrital zircon and that largely from a single area: the Jack Hills and Mount Narryer region of Western Australia. Here, we report on the geochemistry of Hadean detrital zircons as old as 4.15 Ga from the newly discovered Green Sandstone Bed in the Barberton greenstone belt, South Africa. We demonstrate that the U-Nb-Sc-Yb systematics of the majority of these Hadean zircons show a mantle affinity as seen in zircon from modern plume-type mantle environments and do not resemble zircon from modern continental or oceanic arcs. The zircon trace element compositions furthermore suggest magma compositions ranging from higher temperature, primitive to lower temperature, and more evolved tonalite-trondhjemite-granodiorite (TTG)-like magmas that experienced some reworking of hydrated crust. We propose that the Hadean parental magmas of the Green Sandstone Bed zircons formed from remelting of mafic, mantle-derived crust that experienced some hydrous input during melting but not from the processes seen in modern arc magmatism.

Crystallization and Segregation of Syenite in Shallow Mafic Sills: Insights from the San Rafael Subvolcanic Field, Utah

Exposed plumbing systems provide important insight into crystallization and differentiation in shallow sills beneath volcanic fields. We use whole rock major element, trace element and radiogenic isotopic compositions, along with mineral geochemical data on 125 samples to examine the conditions of melt differentiation in shallow sills from the exposed 4-Ma-old San Rafael subvolcanic field (SRVF), Utah. The field consists of ∼2000 dikes, 12 sills and 63 well preserved volcanic conduits. Intrusive rocks consist of mainly fine-grained trachybasalts and coarse-grained syenites, which are alkaline, comagmatic and enriched in Ba, Sr and LREE. Within sills, syenite is found as veins, lenses, and sheets totally enveloped by the basalt. The SRVF intrusions have geochemical signatures of both enriched sub-continental lithospheric and asthenospheric mantle sources. We estimate partial melting occurred between 1·2 and 1·9 GPa (50–70 km), with mantle potential temperatures in the range 1260–1326 ± 25°C, consistent with those estimated for volcanic rocks erupted on the Colorado Plateau. Geobarometry results based on clinopyroxene chemistry indicate that (1) basalt crystallized during ascent from at least 40 km deep with limited lithospheric storage, and (2) syenites crystallized only in the sills, ∼1 km below the surface. San Rafael mafic magma was emplaced in sills and started to crystallize inward from the sill margins. Densities of basalt and syenite at solidus temperatures are 2·6 and 2·4 g/cc, respectively, with similar viscosities of ∼150 Pa s. Petrographic observations and physical properties suggest that syenite can be physically separated from basalt by crystal compaction and segregation of the tephrophonolitic residual liquid out of the basaltic crystal mush after reaching 30–45% of crystallization. Each individual sill is 10–50 m thick and would have solidified fairly rapidly (1–30 years), the same order of magnitude as the duration of common monogenetic eruptions. Our estimates imply that differentiation in individual shallow sills may occur during the course of an eruption whose style may vary from effusive to explosive by tapping different magma compositions. Our study shows that basaltic magmas have the potential to differentiate to volatile-rich magma in shallow intrusive systems, which may increase explosivity.

Pseudoleucite syenites at Loch Borralan, Scotland: Petrology and a genetic model

ABSTRACT The alkaline Loch Borralan intrusion (Assynt Region, NW Highlands of Scotland) consists of a composite arrangement of several ultramafic to felsic plutonic rock bodies which were emplaced around 430 Ma into the Moine Thrust Zone during the Caledonian Orogeny. Some of the Loch Borralan rocks are ultrapotassic and contain pseudoleucite, i.e., a pseudomorph of alkali feldspar and nepheline after leucite. In total, 25 samples have been investigated, representing garnet-bearing pseudoleucite syenites and accompanying rock types such as nepheline-garnet-bearing syenites, alkali feldspar syenites, an amphibole syenite, a biotite-clinopyroxene syenite, and calcite-bearing glimmerites. Pseudoleucite is always associated with garnet, biotite, orthoclase, and minor clinopyroxene and titanite. Mineral chemical data indicate rather primitive magma compositions with no major differences between the various investigated main rock units. The abundant occurrence of up to 2 cm large, mostly euhedral pseudoleucite crystals and petrological phase considerations suggest that magmatic leucite physically separated from its host magma as a flotation cumulate. Based on our data and a comparison with previous field-based and experimental work, K-rich basanitic to tephriphonolitic melts that originated from a K-enriched mantle source may be parental to these rocks. The high liquidus temperatures at low pressures (e.g., ∼1100 °C at 1 bar PH2O) required to crystallize leucite could have resulted from the ascent of successive melt batches in a composite intrusion. Later melt batches would increase the temperature in earlier, already partially cooled batches, causing an increase in temperature and a decrease in pressure during ascent. The subsequent decomposition of leucite to pseudoleucite is interpreted to result from either dry breakdown or autometasomatism, i.e., involvement of late-magmatic fluids.