Allaz Julien

Last Name

Julien

First Name

Allaz

ORCID

0000-0003-3783-0502

Organisational unit

03958 - Bachmann, Olivier / Bachmann, Olivier

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Publications 1 - 10 of 23

Beryllium-silicon disorder and rare earth crystal chemistry in gadolinite from the White Cloud pegmatite, Colorado, USA
Julien, Allaz; Smyth, Joseph R.; Henry, Rhiana E.; et al. (2021)
The Canadian Mineralogist
Gadolinite, REE2FeBe2Si2O10, is a monoclinic orthosilicate member of the gadolinite supergroup of minerals and occurs in beryllium and rare earth element (REE) bearing granites, pegmatites, and some metamorphic rocks. Gadolinite from the White Cloud pegmatite, South Platte Pegmatite district, Colorado, USA, has been investigated and shows unusually variable REE compositions and distinct Be-Si disorder. Crystal structure and chemistry of two petrographically distinct gadolinite samples from this locality have been studied by electron microprobe chemical analysis, laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS), single-crystal X-ray diffraction (XRD), and micro-Raman spectroscopy. Within these samples, the gadolinite was found to range from gadolinite-(Y) to gadolinite-(Ce). Regions of nearly full occupancy of Fe at the M site, and partial substitution of Si for Be at the Q tetrahedral site, as well as substitution of Be for Si at the T site were observed, with up to 15% vacancy at the Fe site and up to 15% disorder between Be and Si at distinct tetrahedral sites elsewhere. The layered nature of the crystal structure allows for large variation of the radius of the cation at the A site which contains the REE. This study shows that Be may substitute for Si and that Be may be more abundant in geochemical systems than previously assumed.
Rare-earth crystal chemistry of thalénite-(Y) from different environments
Raschke, Markus B.; Anderson, Evan J.D.; Van Fosson, Jason; et al. (2018)
Mineralogical Magazine
Formation of (F+P+REE)-rich segregations by liquid immiscibility within aplite near Jamestown, CO (USA)
Julien, Allaz; Stern, Charles R.; Raschke, Markus B.; et al. (2019)
Evaluating Consensus in Experimental K-ratios from over 40 WDS and EDS Measurement Systems
Nachlas, William O.; Moy, Aurélien; Ritchie, Nicholas; et al. (2023)
Microscopy and Microanalysis
Water exsolution in the magma chamber favors effusive eruptions: Application of Cl-F partitioning behavior at the Nisyros-Yali volcanic area
Popa, Răzvan-Gabriel; Tollan, Peter M.E.; Bachmann, Olivier; et al. (2021)
Chemical Geology
Changes from effusive to explosive behavior are common during single eruptive events. However, in many cases these shifts are recorded between distinct eruptions, separated by periods of volcanic repose. In the quiescent periods, magma chamber processes are likely to play a major role in controlling the style of the next eruption, by modifying the properties of the magmas. Crystallinity, viscosity, dissolved water content and the water saturation state of the melt are among the properties that can easily change during periods of repose, and that can influence the behavior of the magmas during eruption. Here, we focus on the active Nisyros-Yali volcanic center (South Aegean Volcanic Arc) to investigate the water saturation state of the magma reservoir, before effusive and explosive events were triggered. We investigate the evolution of Cl in melt inclusions, and the fractionation of Cl from F in apatite included in other mineral phases. We present an analytical protocol for minimizing F diffusion during EPMA apatite measurements, and to correct for diffusion resulting from variable crystal orientation. The relatively high concentrations of Cl positively correlated with the degree of differentiation in melt inclusions, and the lack of a clear fractionation trend of Cl from F in apatite, indicate that explosive eruptions were generated by magmas that were water-undersaturated during magmatic storage. In effusive eruptions, both melt and apatite inclusions record Cl depletion at magmatic storage conditions. This suggests that effusive events were generated by magmas stored in areas of the reservoir with sufficient volumes of exsolved water to significantly extract Cl from the melt. This is in accordance with mineral-melt hygrometry estimates, which suggest dissolved water contents of ~4.5 wt% for magmas generating explosive events, and in excess of 5.5 wt% for magmas behaving effusively. The presence of exsolved volatiles in the magma reservoirs immediately prior to eruption favors outgassing in the conduit and effusive behavior despite the high water content. This is partly due to increasing the compressibility of the magmatic reservoir upon mafic recharge, and partly due to fostering the development of early gas permeability, at deep levels in the conduit.
U-Th-Pb(total) dating of REE-phosphate by electron microprobe: Review and progress
Julien, Allaz; Jercinovic, Michael J.; Williams, Michael L. (2020)
IOP Conference Series: Materials Science and Engineering
Monazite (Mnz) and xenotime (Xnt) are rare earth element (REE) phosphate minerals that occur in a large variety of rocks. They are known to contain very limited amounts of common Pb, and thus, a date can be calculated by simply measuring the total lead- and actinide-contents. Such a measurement can be done at high spatial resolution with an electron probe microanalyser (EPMA), together with a complete mineral analysis. The U-Th-Pbtotal dating method was developed almost 30 years ago, to provide an in-situ age for several (U, Th)-bearing minerals such as uraninite, Mnz, and Xnt. This technique permits identification of events that are not recorded by other minerals, allows dates to be related to structural or geochemical features, and ultimately reinforces the age interpretation. The principal challenge is the accurate measurement of Pb, which is commonly present at the trace level (< 1,000 ppm). This paper reviews the most recent advances in the U-Th-Pbtotal dating technique of Mnz and Xnt, notably in terms of a) accurate background acquisition using the multipoint background technique, and b) improved peak interference correction for trace and REE elements. This paper also discusses a new method of quantitative element mapping, which helps with the identification of compositional domains in Mnz (or Xnt). Finally, a date cannot be interpreted as an age without the proper context, and several key factors in the interpretation are discussed and illustrated with several case studies.
Control of effusive and explosive eruptions of Ciomadul volcano: constraints by apatite composition
Hajdu, Krisztina; Lukács, Réka; Popa, Razvan-Gabriel; et al. (2024)
EGUsphere
The Ciomadul Volcanic Complex, Eastern Carpathians, Romania is the youngest volcano in the Carpathian-Pannonian region, eastern-central Europe, where volcanic activity occurred between 160 ka and 30 ka. It is a typical long-dormant volcano, where active stages were divided by several 10’s ka quiescence. Geophysical studies indicate that it is still underlain by a potentially active magma storage (PAMS volcano). Two main stages of volcanism are distinguished that was separated by ca. 40 ka dormancy: the first one (160–95 ka) was characterized by lava dome extrusions, whereas the second one (56–30 ka) was mostly explosive. Magma composition, however, remained homogeneous, i.e., high-K dacitic, which contains plagioclase, amphibole, biotite as well as accessory apatite, titanite and zircon. In this study, we focus on the variation of apatite composition, particularly the changes in the volatile content and its effects on the type of eruptions. We analyzed the chemical composition of apatite microphenocrysts and inclusions enclosed by amphibole and biotite phenocrysts by microprobe, with special attention to the volatile contents (Cl, F, OH). Samples represent different eruption ages, and both effusive and explosive eruption types. In the xCl/xOH vs. xF/xOH diagram, a breakpoint in the compositional variation of apatite indicates the change of water-saturation state in the magma reservoir, due to the different behavior of Cl and F in water-saturated and unsaturated magmas. Fluorine remains in the melt during water-saturation state, and follows the same trend as in water-undersaturated conditions. In contrast Cl shows similar incompatible behavior in water-undersaurated state, but in water-(over)saturated conditions it enters to the gas phase, so its content in the melt (and in crystal lattice of apatite) is buffered or decreased. We also used MgO content of apatite to follow the behavior of the halogens during the magma differentiation, where high amount of MgO represents the less evolved magma. We interpret our results that effusive eruptions occurred when the magma reached water-(over)saturated state (constant Cl content). In that case, the eruption triggering recharge event was not able to return the magma to water-undersaturated condition. On the other hand, the explosive events were characterized by magmas became in water-undersaturated state. In the case of the three oldest explosive eruptions the less evolved recharge material was able to return the already saturated magma to water-undersaturated state before the eruptions. The or one of the youngest explosive eruption of the Ciomadul (Bixad) was also explosive, but in this case water-saturated state was not detectable by the apatite record. Similarly to earlier studies of the Ciomadul, our results on apatite composition also indicate the role of mafic magma recharge in the petrogenesis of the rocks and shows that it could have turned the system to water-undersaturated state that eventually led to explosive eruptions. This study belongs to the K135179 NKFIH-OTKA research project.
Electron Microprobe Analysis of Minor and Trace Elements in Beam Sensitive Materials: How Far Can We Go?
Julien, Allaz; Popa, Răzvan-Gabriel; Reusser, Eric; et al. (2019)
Microscopy and Microanalysis
Magmatic to hydrothermal conditions in the transition from the A-type Pikes Peak granite (Colorado) to its related pegmatite
Fonseca Teixeira, Ludmila Maria; Troch, Juliana; Julien, Allaz; et al. (2022)
Frontiers in Earth Science
Fluid exsolution in magmas is a process that, in many silicic upper crustal reservoirs, starts at relatively low crystallinities (near liquidus), and precedes the precipitation of many ore bodies, including pegmatites. As any magmatic system approaches its solidus, the amount of the exsolved fluid phase increases and becomes progressively dominant over melt, allowing local over pressurization and the generation of pegmatitic pods/dykes. Such pegmatitic bodies show several features that point to both magmatic and hydrothermal environments, linking those realms and providing a unique opportunity to document and understand the magmatic-hydrothermal transition within silicic magmatic systems. We studied the 1.1 Ga classic A-type Pikes Peak granite (Colorado, United States) and one of its many internally-hosted pegmatites, the Wellington Lake pegmatite, to investigate the changes that occur within a granitic system as it crosses its theoretical water- saturated solidus and continues crystallizing beyond it. Textural and geochemical analyses of quartz, plagioclase, and K-feldspar minerals, as well as fluid inclusion studies, demonstrate this magmatic to hydrothermal transition in the granite and the pegmatite. Different thermometers (Ti-in-quartz, 2-feldspars, fluid inclusions) document the temperature evolution of the granitic system, from >850°C for the hottest magmatic minerals to <400°C for the pegmatite core. The magmatic- hydrothermal transition is recorded by plagioclase and quartz rims that yield temperatures well below the traditionally inferred haplogranite solidus. In the pegmatite, the magmatic-hydrothermal transition is observed between the graphic granite wall zone, which shows homogeneous quartz geochemical signatures at near-solidus conditions (700–670°C), and the intermediate zone, which crystallized at much colder temperatures (470–420°C). Although a significant process, our calculations suggest that subsolidus precipitation from exsolved, solute-rich magmatic fluids represent less than 20% of the total volume of the granite.
A New Albite Microanalytical Reference Material from Piz Beverin for Na, Al and Si Determination, and the Potential for New K-Feldspar Reference Materials
Julien, Allaz; Guillong, Marcel; Tavazzani, Lorenzo; et al. (2023)
Geostandards and Geoanalytical Research
Determination of alkali elements is important to Earth scientists, yet suitable and reliable microanalytical reference materials are lacking. This paper proposes a new albite reference material and evaluates the potential for future K-feldspar reference materials. The proposed Piz Beverin albite reference material from Switzerland yields a homogeneous composition at the centimetre- to micrometre-scale for Si, Al and Na with < 2000 mu g g(-1) total trace elements (mostly heterogeneously distributed Ca, K and Sr). EPMA and LA-ICP-MS measurements confirm a composition of 99.5(2)% albite component, which is supported further by bulk XRF measurements. A round robin evaluation involving nine independent EPMA laboratories confirms its composition and homogeneity for Si, Al and Na. In addition, a set of five distinct clear K-feldspar samples was evaluated as possible reference materials. The first two crystals of adular and orthoclase yield unacceptable inhomogeneities with > 2% relative local variations of Na, K and Ba contents. The three other investigated sets of K-feldspar crystals are yellow sanidine crystals from Itrongay (Madagascar). Despite distinct compositions, EPMA confirms they are each homogeneous at the centimetre to micrometre scale for Si, Al and K and have no apparent inclusions; further investigation to find larger amounts of these materials is therefore justified.

Publications 1 - 10 of 23

Allaz Julien

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