Plagioclase as a witness of syn-eruptive degassing in rhyolitic magmas

2020 ◽  
Author(s):  
Ben Ellis ◽  
Julia Neukampf ◽  
Oscar Laurent ◽  
Lena Steinmann ◽  
Stefan Weyer ◽  
...  
Keyword(s):  
Melt Inclusions ◽  
Sudden Change ◽  
Isotopic Fractionation ◽  
Explosive Volcanism ◽  
Volcanic Field ◽  
Major And Trace Elements ◽  
Groundmass Glass ◽  
Strong Gradient ◽  
Anhydrous Mineral ◽  
Equilibrium Fractionation

<p>Lithium (Li) is one of the fastest diffusing elements in most geological media and so has the potential to provide information about processes occurring on timescales too short to be captured by other proxies.  These processes may be of fundamental importance both in terms of understanding what happens during explosive volcanism and for defining where lithium, an element of increasing economic importance, ends up.  To investigate the fate of Li, we studied in detail the 1.30 Ma Mesa Falls Tuff (MFT) from the Yellowstone volcanic field (USA).  MFT is a typical rhyolite of the Yellowstone system containing an anhydrous mineral assemblage of sanidine, quartz, plagioclase, clinopyroxene, fayalite, orthopyroxene and accessory phases.  We focussed on plagioclase crystals that have a strong gradient in Li contents from cores at ~25 ppm to rims with ~ 5 ppm.  This notable decrease in Li abundance is decoupled from changes in other major and trace elements.  δ<sup>7</sup>Li values measured by fs-LA-MC-ICPMS in the plagioclase crystals reveal that cores are about 5 ‰ lower than rims.  Taken together, the Li abundance and isotopic data make a compelling case for the plagioclase attempting to react to a sudden change in Li abundance in the surrounding melt.  Diffusion modelling of these gradients indicates that this sudden Li drop in the melt occurred over timescales of tens of minutes prior to quenching.  The volatile behaviour of Li implied by this result finds support in Li concentrations measured in quartz-hosted melt inclusions that reach 400 ppm while groundmass glass Li contents are much lower (36-55 ppm).  While equilibrium fractionation of stable isotopes is minimised at high temperatures, the large-magnitude, rapid loss of lithium from the melt phase may allow kinetic isotopic fractionation to occur, as recorded in the plagioclase crystals.  With glass / groundmass both volumetrically dominant and the main repository of Li in virtually all volcanic deposits, further consideration of how syn-eruptive processes may affect the bulk Li identity of a sample is warranted.        </p><p> </p>

Late Pleistocene rhyolitic explosive volcanism at Los Azufres Volcanic Field, central Mexico

2012 ◽  
pp. 45-82 ◽  
Author(s):  
José Luis Arce ◽  
José Luis Macías ◽  
Elizabeth Rangel ◽  
Paul Layer ◽  
Víctor Hugo Garduño-Monroy ◽  
...  
Keyword(s):  
Late Pleistocene ◽  
Explosive Volcanism ◽  
Volcanic Field ◽  
Central Mexico

The chlorine-isotopic composition of lunar KREEP from magnesian-suite troctolite 76535

2020 ◽  
Vol 105 (8) ◽  
pp. 1270-1274
Author(s):  
Francis M. McCubbin ◽  
Jessica J. Barnes
Keyword(s):  
Isotopic Composition ◽  
Melt Inclusions ◽  
Melt Inclusion ◽  
Isotopic Fractionation ◽  
Magma Ocean ◽  
Geochemical Composition ◽  
Isotopic Compositions ◽  
Stable Isotopic ◽  
Lunar Samples

Abstract We conducted in situ Cl isotopic measurements of apatite within intercumulus regions and within a holocrystalline olivine-hosted melt inclusion in magnesian-suite troctolite 76535 from Apollo 17. These data were collected to place constraints on the Cl-isotopic composition of the last liquid to crystallize from the lunar magma ocean (i.e., urKREEP, named after its enrichments in incompatible lithophile trace elements like potassium, rare earth elements, and phosphorus). The apatite in the olivine-hosted melt inclusion and within the intercumulus regions of the sample yielded Cl-isotopic compositions of 28.3 ± 0.9‰ (2σ) and 30.3 ± 1.1‰ (2σ), respectively. The concordance of these values from both textural regimes we analyzed indicates that the Cl-isotopic composition of apatites in 76535 likely represents the Cl-isotopic composition of the KREEP-rich magnesian-suite magmas. Based on the age of 76535, these results imply that the KREEP reservoir attained a Cl-isotopic composition of 28–30‰ by at least 4.31 Ga, consistent with the onset of Cl-isotopic fractionation at the time of lunar magma ocean crystallization or shortly thereafter. Moreover, lunar samples that yield Cl-isotopic compositions higher than the value for KREEP are likely affected by secondary processes such as impacts and/or magmatic degassing. The presence of KREEP-rich olivine-hosted melt inclusions within one of the most pristine and ancient KREEP-rich rocks from the Moon provides a new opportunity to characterize the geochemistry of KREEP. In particular, a broader analysis of stable isotopic compositions of highly and moderately volatile elements could provide an unprecedented advancement in our characterization of the geochemical composition of the KREEP reservoir and of volatile-depletion processes during magma ocean crystallization, more broadly.

scholarly journals Hydrogen Isotopic Variations in the Shergottites

Geosciences ◽  
2020 ◽  
Vol 10 (4) ◽  
pp. 148 ◽  
Author(s):  
Shuai Wang ◽  
Sen Hu
Keyword(s):  
Melt Inclusions ◽  
Crustal Rock ◽  
Evidence Based ◽  
Mantle Rock ◽  
Future Studies ◽  
Groundmass Glass ◽  
Impact Melt ◽  
Melt Glass ◽  
Isotopic Variations

Hydrogen isotopes in the shergottite Martian meteorites are among the most varied in Mars laboratory samples. By collating results of previous studies on major hydroxyl, deuterium, and H2O bearing phases, we provide a compendium of recent measurements in order to elucidate crustal-rock versus mantle-rock processes on Mars. We summarize recent works on volatile and δD measurements in a range of shergottite phases: from melt inclusions, apatite, merrillite, maskelynite, impact melt glass, groundmass glass, and nominal anhydrous minerals. We interpret these observations using an evidence-based approach, considering two particular scenarios: (1) water-rock crustal interactions versus (2) magmatic-based processes. We consider the implications of these measurements and the scope they have for future studies, paying particular attention to future works on H, S, and Cl isotopes in situ, shedding light on the nature of volatiles in the hydrosphere and lithosphere of Mars.

Silicate melt inclusions in ignimbrites, Bükkalja Volcanic Field, Northern Hungary - texture and geochemistry

2002 ◽  
Vol 45 (4) ◽  
pp. 341-358 ◽  
Author(s):  
Réka Lukács ◽  
György Czuppon ◽  
Szabolcs Harangi ◽  
Csaba Szabó ◽  
Theodor Ntaflos ◽  
...  
Keyword(s):  
Melt Inclusions ◽  
Volcanic Field ◽  
Silicate Melt

scholarly journals The characteristic of acid melts which formed tephra of pleistocene-holocene eruptions of Ichinsky Volcano, Kamchatka (Results of melt inclusions study)

2019 ◽  
Vol 64 (3) ◽  
pp. 237-262
Author(s):  
M. L. Tolstykh ◽  
M. M. Pevzner ◽  
V. B. Naumov ◽  
A. D. Babansky
Keyword(s):  
Trace Elements ◽  
Melt Inclusions ◽  
Magma Mixing ◽  
Major And Trace Elements ◽  
Magmatic System ◽  
Acid Melts

This paper presents the results of a study of melt inclusions in plagioclase, amphibole and pyroxene from Ichinsky volcano’s tephras of different age. Two types of melts have been identified, distinguished by different concentrations of potassium (K2O). Major and trace elements’ composition of these melts indicates that magma mixing was the dominating process in the Ichinsky magmatic system.

Explosive volcanism on Santorini, Greece

1989 ◽  
Vol 126 (2) ◽  
pp. 95-126 ◽  
Author(s):  
T. H. Druitt ◽  
R. A. Mellors ◽  
D. M. Pyle ◽  
R. S. J. Sparks
Keyword(s):  
Explosive Volcanism ◽  
Volcanic Field ◽  
Pyroclastic Flows ◽  
Coarse Grained ◽  
Magma Ascent ◽  
Explosive Eruptions ◽  
Caldera Collapse ◽  
Caldera Wall ◽  
Magma Chambers ◽  
Facies Associations

AbstrctSantorini volcanic field has had 12 major (1–10 km3 or more of magma), and numerous minor, explosive eruptions over the last ~ 200 ka. Deposits from these eruptions (Thera Pyroclastic Formation) are well exposed in caldera-wall successions up to 200 m thick. Each of the major eruptions began with a pumice-fall phase, and most culminated with emplacement of pyroclastic flows. Pyroclastic flows of at least six eruptions deposited proximal lag deposits exposed widely in the caldera wall. The lag deposits include coarse-grained lithic breccias (andesitic to rhyodacitic eruptions) and spatter agglomerates (andesitic eruptions only). Facies associations between lithic breccia, spatter agglomerate, and ignimbrite from the same eruption can be very complex. For some eruptions, lag deposits provide the only evidence for pyroclastic flows, because most of the ignimbrite is buried on the lower flanks of Santorini or under the sea. At least eight eruptions tapped compositionally heterogeneous magma chambers, producing deposits with a range of zoning patterns and compositional gaps. Three eruptions display a silicic–silicic + mafic–silicic zoning not previously reported. Four eruptions vented large volumes of dacitic or rhyodacitic pumice, and may account for 90% or more of all silicic magma discharged from Santorini. The Thera Pyroclastic Formation and coeval lavas record two major mafic-to-silicic cycles of Santorini volcanism. Each cycle commenced with explosive eruptions of andesite or dacite, accompanied by construction of composite shields and stratocones, and culminated in a pair of major dacitic or rhyodacitic eruptions. Sequences of scoria and ash deposits occur between most of the twelve major members and record repeated stratocone or shield construction following a large explosive eruption.Volcanism at Santorini has focussed on a deep NE–SW basement fracture, which has acted as a pathway for magma ascent. At least four major explosive eruptions began at a vent complex on this fracture. Composite volcanoes constructed north of the fracture were dissected by at least three caldera-collapse events associated with the pyroclastic eruptions. Southern Santorini consists of pryoclastic ejecta draped over a pre-volcanic island and a ridge of early- to mid-Pleistocene volcanics. The southern half of the present-day caldera basin is a long-lived, essentially non-volcanic, depression, defined by topographic highs to the south and east, but deepened by subsidence associated with the main northern caldera complex, and is probably not a separate caldera.

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