First noble gas results from fluid inclusions of the Late Miocene-Pleistocene Macedonian volcanics

2021 ◽  
Author(s):  
Kata Molnár ◽  
Marjan Temovski ◽  
László Palcsu
Keyword(s):  
Fluid Inclusions ◽  
Late Miocene ◽  
Lithospheric Mantle ◽  
Noble Gas ◽  
Mantle Xenoliths ◽  
Gas Composition ◽  
The European Union ◽  
Large Area ◽  
Development Fund ◽  
Wide Range

<p>Late Miocene to Pleistocene volcanism within the Vardar zone (N. Macedonia) covers a large area, has a wide range in composition and it is largely connected to the tectonic evolution of the South Balkan extensional system, the northern part of the Aegean extensional regime. A recent study indicated an increasing rate of mantle metasomatism towards the younger centers in the region [1]. During the last stage of activity, ultrapotassic (UK) centers that formed between ca. 3.2 and 1.5 Ma originated from the lithospheric mantle beneath the region [2]. Although there are no reported mantle xenoliths from these centers, the erupted mafic rocks contain abundant olivine as phenocrysts [3]. Noble gas isotopic characteristics of fluid inclusions in olivine can reveal important information about the origin of the fluid and the metasomatic state of the lithospheric mantle. We analyzed for the first time the noble gas composition of fluid inclusions of olivine phenocrysts from the Mlado Nagoričane volcanic center, the northernmost member of the UK centers with an eruption age of 1.8 ± 0.1 Ma [2]. The R/R<sub>A</sub> ratios give a range of 3.1-4.5 with <sup>4</sup>He/<sup>20</sup>Ne values of 11.7-14.6. These R/R<sub>A</sub> values are lower than the MORB and the averaged subcontinental lithospheric values, and considering the negligible amount of atmospheric contribution, imply a more metasomatized character for the underlying lithospheric mantle beneath the region. Mantle-derived noble gases were detected in a recent geochemical study on the thermal springs and gas exhalations in the region, with up to 20% of mantle contribution calculated based on their noble gas composition using the MORB R/R<sub>A</sub> value [4]. These new Mlado Nagoričane fluid inclusion noble gas values indicate that the mantle contribution in the recent gas emissions in the region could be higher than what was thought.</p><p>This research was supported by the European Union and the State of Hungary, financed by the European Regional and Development Fund in the project of GINOP-2.3.2-15-2016-00009 ‘ICER’ project</p><p>[1] Molnár et al. 2020 – EGU2020-13101.</p><p>[2] Yanev et al., 2008 – Mineralogy and Petrology, 94(1-2), 45-60.</p><p>[3] Yanev et al., 2008 – Geochemistry, Mineralogy and Petrology, Sofia, 46, 35-67.</p><p>[4] Temovski et al. 2020 – EGU2020-2763.</p>

scholarly journals Geochemistry of noble gas and CO2 in fluid inclusions from lithospheric mantle beneath Eifel and Siebengebirge (Germany)

2020 ◽  
Author(s):  
Andrea Luca Rizzo ◽  
Massimo Coltorti ◽  
Barbara Faccini ◽  
Federico Casetta ◽  
Theodoros Ntaflos ◽  
...  
Keyword(s):  
Fluid Inclusions ◽  
Lithospheric Mantle ◽  
Noble Gas ◽  
Mantle Xenoliths ◽  
Gas Content ◽  
Great Opportunity ◽  
Chem Geol ◽  
Wide Range ◽  
Eruptive Centers ◽  
West Eifel

<p>The study of fluid inclusions (FI) composition (He, Ne, Ar, CO<sub>2</sub>) integrated with the petrography and mineral chemistry of mantle xenoliths representative of the Sub Continental Lithospheric Mantle (SCLM) is a unique opportunity for constraining its geochemical features and evaluating the processes and the evolution that modified its original composition. An additional benefit of this type of studies is the possibility of better constraining the composition of fluids rising through the crust and used for volcanic or seismic monitoring.  </p><p>In this respect, the volcanic areas of Eifel and Siebengebirge in Germany represent a great opportunity to test this scientific approach for three main reasons. First, these volcanic centers developed in the core of the Central European Volcanic Province where it is debated whether the continental rift was triggered by a plume (Ritter, 2007 and references therein). Second, Eifel and Siebengebirge formed in Quaternary (0.5-0.01 Ma) and Tertiary (30-6 Ma), respectively, thus spanning a wide range of age. Third, Eifel is characterized by the presence of CO<sub>2</sub>-dominated gas emissions and weak earthquakes that testify that local magmatic activity is nowadays dormant, but not ended (e.g., Bräuer et al., 2013). It is thus important to better constrain the noble gas signature expected in surface gases in case of magmatic unrest.</p><p>This work focuses on the petrological and geochemical study of mantle xenoliths sampled in the West Eifel and Siebengebirge volcanic areas (Germany) and aims at enlarging the knowledge of the local SCLM. Gautheron et al. (2005) carried out the first characterization of noble gases in FI of crystals analyzed by crushing technique (as in our study) but limited to olivines and to West Eifel eruptive centers. Here, we integrate that study by analyzing olivines, orthopyroxenes and clinopyroxenes from a new suite of samples and by including two eruptive centers from Siebengebirge volcanic field (Siebengebirge and Eulenberg quarries).</p><p>Xenoliths from the Siebengebirge localities are characterized by the highest Mg# for olivine, clinopyroxene and Cr# for spinel, together with the lowest Al<sub>2</sub>O<sub>3</sub> contents for both pyroxenes, suggesting  that the mantle beneath Siebengebirge experienced high degree of melt extraction (up to 30%) while metasomatic/refertilization events were more efficient in the mantle beneath West Eifel.</p><p>In terms of CO<sub>2</sub> and noble gas concentration, clinopyroxene and most of the orthopyroxene show the highest gas content, while olivine are gas-poor. The <sup>3</sup>He/<sup>4</sup>He varies between 5.5 and 6.9 Ra. These values are comparable to previous measurements in West Eifel, mostly within the range proposed for European SCLM (6.3±0.4 Ra), and slightly below that of MORB (Mid-Ocean Ridge Basalts; 8±1Ra). The Ne and Ar isotope ratios fall along a binary mixing trend between air and MORB-like mantle. He/Ar* in FI and Mg# and Al<sub>2</sub>O<sub>3</sub> content in minerals confirm that the mantle beneath Siebengebirge experienced the highest degree of melting, while the metasomatic/refertilization events largely affected the Eifel area.</p><p>References</p><p>Bräuer, K., et al. 2013. Chem. Geol. 356, 193–208.</p><p>Gautheron, C., et al. 2005. Chem. Geol. 217, 97–112.</p><p>Ritter, J.R.R., 2007. In: Ritter, J.R.R., Christensen, U.R. (Eds.), Mantle Plumes: A Multidisciplinary Approach. Springer-Verlag, Berlin Heidelberg, pp. 379–404.</p>

Noble gas composition of subcontinental lithospheric mantle: An extensively degassed reservoir beneath Southern Patagonia

2016 ◽  
Vol 450 ◽  
pp. 263-273 ◽  
Author(s):  
Tiago Jalowitzki ◽  
Hirochika Sumino ◽  
Rommulo V. Conceição ◽  
Yuji Orihashi ◽  
Keisuke Nagao ◽  
...  
Keyword(s):  
Lithospheric Mantle ◽  
Noble Gas ◽  
Subcontinental Lithospheric Mantle ◽  
Gas Composition ◽  
Southern Patagonia

Eclogite and garnet pyroxenite xenoliths from kimberlite pipes of north Siberian craton - evidences of subduction processes and cumulate origin

2021 ◽  
Author(s):  
Tatiana Kalashnikova ◽  
Lidia Solov'eva ◽  
Sergey Kostrovitsky ◽  
Konstantin Sinitsyn ◽  
Elvira Yudintseva
Keyword(s):  
Mineral Composition ◽  
Siberian Craton ◽  
Lithospheric Mantle ◽  
Isotope Composition ◽  
Bulk Composition ◽  
Mantle Xenoliths ◽  
Rock Composition ◽  
Geochemical Properties ◽  
Eu Anomaly ◽  
Wide Range

<p>The lithospheric mantle structure and evolution is one of the fundamental problems of the Earth's history. Eclogites and clinopyroxenite xenoliths are characterized by a similar two-mineral composition (garnet and clinopyroxene), but differ in mineralogical and petrographic features (Gonzaga et al., 2010). Questions of their origin and relationship with peridotites remain controversial. There are several classifications of eclogites based on various attributes: structural and textural features (Mercier & Nicolas, 1975; MacGregor & Carter, 1970), chemical composition of garnet (Coleman, 1965), clinopyroxene (Taylor & Neal, 1989), as well as the whole rock composition (Aulbach et al., 2016 and other), the given classifications may not coincide. The geochemical properties of eclogite xenoliths from kimberlite pipes suggest two main points of view for genesis: implication of subduction processes or cumulates of high-pressure melting in lithosphere mantle (Condie, 1993; Jacob et al., 1994). The "classical" cratonic eclogites represent an ancient oceanic crust subsequently subducted and altered possible further metasomatic processes. These rocks are characterized by significant variations in the composition of minerals, a relatively high content of Al<sub>2</sub>O<sub>3</sub> (14-20 wt%) and a low MgO content (10-15 wt%), depletion of elements of the LREE and an Eu anomaly (Gonzaga et al., 2010). In addition, eclogites have a wide range of oxygen isotopic composition in garnet δ<sup>18</sup>O 4.51 - 8.69 (much higher than mantle values ​​5.3 ± 0.3) (9). Garnet pyroxenites are characterized by a more magnesian garnet - pyrope and bulk composition (MgO - 15-20 wt.%). The oxygen isotope composition of Grt from clinopyroxenites is close to that of the mantle - δ<sup>18</sup>O 5.2 - 5.8. It is assumed that these rocks are a consequence of the polybaric partial melting at high temperatures and pressures (Gonzaga et al., 2010). The mantle xenoliths from upper-Jurassic Obnajennaya kimberlite pipe (Kuoika field, Yakutia) were studied. Eclogites and clinopyroxenites occupy about 10-15% population among xenoliths. Garnet in the eclogites differs from that in the clinopyroxenites by a higher content of CaO and FeO (Prp<sub>55-62 </sub>Alm<sub>22-30</sub>Grs<sub>8-18 </sub>in clinopyroxenites and Prp<sub>40-45</sub>Alm<sub>13-29</sub>Grs<sub>15-30 </sub>in eclogites). Clinopyroxenes are distinguished by reduced magnesia content (Mg# 91-84), as well as low calcium content (16-18 wt.%). The high contents of jadeite components in the clinopyroxene (NaAl[Si<sub>2</sub>O<sub>6</sub>] - 25-32%) classify this group of rocks as eclogites. The high δ<sup>18</sup>O varies in eclogite Cpx (more than 6.0), positive Eu anomaly is assumed that the formation of the protolith of the xenolith group occurred as melts in the subduction zone during accretion of the Birekte block to the Siberian craton (Rosen, 2003). However, the presence of garnet clinopyroxenites with narrow variations in mineral composition and relatively low δ<sup>18</sup>O suggests melting processes in the lithospheric mantle and the formation of megacrystalline pyroxene cumulates.</p><p>The research was supported by Russian Science Foundation grant №20-77-00074.</p>

First assessment of the noble gas and CO2 isotopic composition of fluid inclusions hosted in mantle xenoliths from El Hierro (Canary Islands) 

2021 ◽  
Author(s):  
Andres Sandoval Velasquez ◽  
Andrea Luca Rizzo ◽  
Alessandro Aiuppa ◽  
Maria Luce Frezzotti ◽  
Samantha Remigi ◽  
...  
Keyword(s):  
Isotopic Composition ◽  
Oceanic Crust ◽  
Canary Islands ◽  
Fluid Inclusions ◽  
Upper Mantle ◽  
Noble Gas ◽  
Mantle Metasomatism ◽  
Mantle Xenoliths ◽  
Local Source ◽  
El Hierro

<p>Studying the isotopic composition of fluids trapped in mantle xenoliths opens avenues to understanding the origin and cycling of volatiles in the Earth’s upper mantle. Here, we present the first isotopic results for noble gases and CO<sub>2</sub> in fluid inclusions (FI) trapped in mantle xenoliths from El Hierro the youngest island of the Canarian archipelago. Our results are based on 6 mantle xenolith samples (3 Spinel-lherzolites and 3 Spinel-harzburgites) collected from the El Julan cliff valley (Oglialoro et al., 2017), from which we hand-picked crystals of Ol, Opx, and Cpx. Isotopic determinations were performed at the INGV (Sezione di Palermo) noble gas and stable isotopes laboratories, following the preparation methods and analytical procedures described in Rizzo et al. (2018 and references therein).</p><p>The Ne-Ar isotopic compositions reveal the presence of an atmospheric component in the FI. Most of the samples exhibit <sup>4</sup>He/<sup>20</sup>Ne ratios > 60, <sup>20</sup>Ne/<sup>22</sup>Ne ratios between 9.84 and 10.49, <sup>21</sup>Ne/<sup>22</sup>Ne ratios from 0.0295 to 0.0330, and <sup>40</sup>Ar/<sup>36</sup>Ar > 800, suggesting mixing between MORB-like mantle fluids and an air-derived component. We argue this latter may (at least in part) derive from upper mantle recycling of atmospheric fluids via paleo-subduction event(s). Excluding samples possibly affected by diffusive fractionation processes, the average Rc/Ra ratio (<sup>3</sup>He/<sup>4</sup>He ratio corrected for atmospheric contamination) measured in El Hierro xenoliths is ~7.45 + 0.26 Ra, within the MORB range (8 + 1 Ra; Graham, 2002). The He homogeneous signature of these xenoliths agrees well with the <sup>3</sup>He/<sup>4</sup>He compositions previously reported in lava phenocrysts and cumulates (Day and Hilton, 2011) and is slightly below the maximum ratios measured in groundwater samples during the 2012 volcanic unrest (~8.2 Ra; Padron et al., 2013). All these pieces of evidence argue against a primordial source involved in the local lithospheric mantle. Putting these data in the context of previous literature results for FI and surface gases in the Canary Islands (La Palma, La Gomera, Tenerife, Gran Canaria, and Lanzarote), we identify an eastward <sup>3</sup>He/<sup>4</sup>He decreasing trend that parallels a corresponding increase of the oceanic crust thickness. In addition to the mantle heterogeneity, we propose that part of the <sup>3</sup>He/<sup>4</sup>He east-to-west variation along the archipelago is caused by the variable thickness of the oceanic crust (and hence, different interactions with <sup>4</sup>He-rich crustal fluids during emplacement).</p><p>The FI δ<sup>13</sup>C(CO<sub>2</sub>) isotopic composition ranges from -2.38 to -1.23‰ in pyroxenes and -0.2 to +2.0‰ in olivine. These unusually positive δ<sup>13</sup>C compositions support the existence of a recycled crustal carbon component in the local source mantle, likely pointing to mantle metasomatism (Oglialoro et al., 2017) from fluids carrying carbon from subducted sediments and/or altered oceanic crust (AOC).</p>

The westernmost Late Miocene-Pliocene volcanic activity in the Vardar Zone (North Macedonia) – geochronology, petrology and geochemistry of Pakoševo, Debrište and Šumovit Greben volcanic centers

2020 ◽  
Author(s):  
Kata Molnár ◽  
Stéphane Dibacto ◽  
Pierre Lahitte ◽  
Marjan Temovski ◽  
Samuele Agostini ◽  
...  
Keyword(s):  
Late Miocene ◽  
Isotopic Ratio ◽  
Crustal Contamination ◽  
Sedimentary Basins ◽  
Geochemical Data ◽  
Large Area ◽  
Quaternary Volcanism ◽  
Horizon 2020 ◽  
Vardar Zone ◽  
Wide Range

<p>Late Miocene to Pleistocene volcanism within the Vardar zone (North Macedonia) covers a large area, has a wide range in composition and it is largely connected to the tectonic evolution of the South Balkan extensional system, the northern part of the Aegean extensional regime. The scattered potassic to ultrapotassic volcanism developed south from the Scutari-Peć fault zone since 6.57 Ma [1]. The focus of this study is on three volcanic centers located on deep structures or thrust faults along the western part of the Vardar zone, for which there is none to very little geochronological and geochemical data available. Pakoševo and Debrište localities are represented as small remnants of lava flows cropping out at the southern edge of Skopje basin and at the western edge of Tikveš basin, respectively. Šumovit Greben center is considered as part of the Kožuf-Kozjak/Voras massif (6.5-1.8 Ma [1]), and it is located on its westernmost side, at the southern edge of Mariovo basin, which is largely comprised of volcanoclastic sediments. Here we present new eruption ages applying the unspiked Cassignol-Gillot K-Ar technique on groundmass, petrological and geochemical data, supplemented with Sr and Nd isotopes to complement and better understand the Neogene-Quaternary volcanism in the region. Obtaining the eruption ages of these volcanic centers could also help to better constrain the evolution of the sedimentary basins. All of the three centers belong to the shoshonitic series based on their elevated K-content. The oldest center amongst these three localities, as well as other Late Miocene centers within the region, is the trachyandesitic Debrište, which formed at ca. 8.1 Ma, and exhibits the highest Nd isotopic ratios (0.512441-0.512535). The trachybasaltic Pakoševo center formed at ca. 3.8 Ma and, based on its Nd isotopic ratio (0.512260), represents the strongest sign of crustal contamination. The rhyolitic Šumovit Greben center is a composite volcanic structure formed at ca. 3.0-2.7 Ma. Its youngest eruption unit has a slightly larger Nd isotopic ratio (0.512382), representing a less evolved magma at the end of its activity.</p><p>This research was funded by the GINOP-2.3.2-15-2016-00009 ‘ICER’ project, the French-Hungarian Cooperation Program TÉT-FR-2018-00018 and the HORIZON 2020 grant N 676564.</p><p>References:</p><p>[1] Yanev et al., 2008 – Mineralogy and Petrology, 94(1-2), 45-60.</p>

Transmission electron microscopy investigations of (hydrous) chain silicates from the lithospheric mantle beneath the Carpathian Pannonian Region

2020 ◽  
Author(s):  
Zsófia Pálos ◽  
Péter Pekker ◽  
Mihály Pósfai ◽  
Thomas Pieter Lange ◽  
Nóra Liptai ◽  
...  
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Chemical Composition ◽  
Fluid Inclusions ◽  
Lithospheric Mantle ◽  
Ion Beam ◽  
Mantle Xenoliths ◽  
Formation Mechanisms ◽  
Alkaline Basalt ◽  
Transmission Electron

<p>Transmission electron microscopy (TEM) is a powerful, yet scarcely used technique when it comes to investigating mantle minerals and fluid inclusions. It is capable to collect structural information of the studied mineral, its precise chemical composition, and makes nanofeatures visible, such as dislocations and nano-inclusions.</p><p>In this study TEM and STEM (scanning transmission electron microscopy) measurements were carried out on a set of ortho- and clinopyroxene samples from central and marginal localities of Carpathian Pannonian region (CPR), where Plio-Pleistocene alkaline basalt volcanism sampled the lithospheric mantle retrieving lithospheric mantle xenoliths. Objective of the study was to constrain the presence and formation mechanisms of sub-microscopic occurrence of pargasitic amphibole.</p><p>The detailed investigation of pargasite in the upper mantle is rather timely, because its presence may be the major cause for the rheologic contrast experienced between the lithosphere and the asthenosphere [1], [2]. The nominally anhydrous minerals’ (NAMs, as ortho- and clinopyroxene) structural hydroxyl [3] content or volatiles in fluid inclusions could lead to formation of pargasite [4]. In addition, pargasite could form interstitially during metasomatic intereactions.</p><p>Our observations so far suggest that hydrous silicate formation as sub-solidus exsolution in the central CPR may not have taken place. Ordering of the Ca forming Ca-rich and Ca-poor domains in an orthopyroxene grain was identified. Precursors of H<sup>+</sup> diffusion were also recorded, such as dislocations and nanosized fluid inclusions. Diffusion of H<sup>+</sup> could be active in the lattice scale through the disclinations along subgrain boundaries [3], [5] or dislocations in the host mineral along the boundary of nanoscale fluid inclusions [6], [7]. Clinopyroxene-amphibole phase boundary has been prepared by focused ion beam (FIB) milling technique from the marginal area of CPR. The chemical composition of the amphibole lamella provides evidence that the H<sub>2</sub>O content of the nearby fluid inclusion migrated into the host clinopyroxene producing an amphibole lamella growing along the ‘c’ crystallographic axis [4].</p><p>Observations of the boundary of clinopyroxene and amphibole confirm that the amphibole octahedral layers penetrate the clinopyroxene structure. The precise nanoscale measurements (STEM mapping) of chemical composition of both the host and the lamellae can lead to profound implications on the original composition of the studied fluid inclusions.</p><p>[1] Green, D. H., Hibberson, W. O., Kovács, I. J., & Rosenthal, A. (2010). <em>Nature</em>, 467(7314), 448–451.</p><p>[2] Kovács, I. J., Lenkey, L., Green, D. H., Fancsik, T., Falus, G., Kiss, J., Orosz, L., Angyal, J., Vikor, Zs. (2017). <em>Acta Geodaetica et Geophysica</em>, 52, 183–204.</p><p>[3] Liptai, N., Kovács, I.J., Lange, T.P., Pálos, Zs., Berkesi, M., Szabó, Cs., Wesztergom, V. (2019). <em>Goldschmidt Abstracts</em>, 2019 1981.</p><p>[4] Lange, T.P., Liptai, N., Patkó, L., Berkesi, M., Kesjár, D., Szabó, Cs., Kovács, I. J. (2019). 25th European Current Research on Fluid Inclusions (ECROFI) , <em>Abstract Series</em>, 68.</p><p>[5] Demouchy, S., & Bolfan-Casanova, N. (2016). <em>Lithos</em>, 240–243, 402–425.</p><p>[6] Bakker, R. J., & Jansen, J. B. H. (1994). <em>Contributions to Mineralogy and Petrology</em>, 116, 7–20.</p><p>[7] Viti, C., & Frezzotti, M. L. (2000). <em>American Mineralogist</em>, 85(10), 1390–1396.</p>

scholarly journals SPECTORS Structure and Goals

Proceedings ◽  
2020 ◽  
Vol 30 (1) ◽  
pp. 55
Author(s):  
Rolf Becker ◽  
Birgit Mosler
Keyword(s):  
Remote Sensing ◽  
Economic Development ◽  
Business Models ◽  
Conservation Agriculture ◽  
Development Project ◽  
Border Region ◽  
Direct Access ◽  
The European Union ◽  
Development Fund ◽  
Wide Range

SPECTORS—“Sensor products for enterprises creating technological opportunities in airborne remote sensing”—is a project on civil applications with drones lasting from Sept. 2016 to July 2020. The international cooperation project with more than 30 partners from small and medium-sized enterprises (SMEs), public sector as well as research institutions is aiming at economic development in the Dutch-German border region. It is financed by INTERREG-V-A Germany-Netherlands, a strong economic development instrument being supported by the “European Regional Development Fund (ERDF)”. The partners of SPECTORS are mainly located in the Euregio Rhine-Waal. In order to achieve the politically motivated economic development goals of the European Union, the entire project is completely tailored to support SMEs in product innovation and development. This is achieved through interdisciplinary and cross-disciplinary cooperation between Dutch and German partners. The project partners Oost NL and RheWaTech are advising the SMEs on developing appropriate business models for their intended innovations. The Business Model Canvas together with business and technology readiness level measures are tools to plan and reflect business. The project internal consultants meet regularly with collaborating SMEs and their potential customers to force the business development process. The continuous involvement of business consultants being part of the consortium leads to an improved target orientation in the research & development project. SPECTORS covers a wide range of civil drone applications, such as environmental and nature conservation, agriculture, surveying, hyperspectral remote sensing, surveillance, cloud computing and artificial intelligence. The extensive cooperation over the last years has resulted in a cross-border competence network, which already provides many companies and users in the region with uncomplicated and direct access to the diverse applications of drones in the civil sector.

scholarly journals Lithosphere tearing along STEP faults and synkinematic formation of lherzolite and wehrlite in the shallow subcontinental mantle

Solid Earth ◽  
2019 ◽  
Vol 10 (4) ◽  
pp. 1099-1121 ◽  
Author(s):  
Károly Hidas ◽  
Carlos J. Garrido ◽  
Guillermo Booth-Rea ◽  
Claudio Marchesi ◽  
Jean-Louis Bodinier ◽  
...  
Keyword(s):  
Grain Size ◽  
Lithospheric Mantle ◽  
Volcanic Field ◽  
Mantle Xenoliths ◽  
Coarse Grained ◽  
Fault System ◽  
Mantle Flow ◽  
Second Phase ◽  
Fine Grained ◽  
Wide Range

Abstract. Subduction-transform edge propagator (STEP) faults are the locus of continual lithospheric tearing at slab edges, resulting in sharp changes in the lithospheric and crustal thickness and triggering lateral and/or near-vertical mantle flow. However, the mechanisms at the lithospheric mantle scale are still poorly understood. Here, we present the microstructural study of olivine-rich lherzolite, harzburgite and wehrlite mantle xenoliths from the Oran volcanic field (Tell Atlas, northwest Algeria). This alkali volcanic field occurs along a major STEP fault responsible for the Miocene westward slab retreat in the westernmost Mediterranean. Mantle xenoliths provide a unique opportunity to investigate the microstructures in the mantle section of a STEP fault system. The microstructures of mantle xenoliths show a variable grain size ranging from coarse granular to fine-grained equigranular textures uncorrelated with lithology. The major element composition of the mantle peridotites provides temperature estimates in a wide range (790–1165 ∘C) but in general, the coarse-grained and fine-grained peridotites suggest deeper and shallower provenance depth, respectively. Olivine grain size in the fine-grained peridotites depends on the size and volume fraction of the pyroxene grains, which is consistent with pinning of olivine grain growth by pyroxenes as second-phase particles. In the coarse-grained peridotites, well-developed olivine crystal-preferred orientation (CPO) is characterized by orthorhombic and [100]-fiber symmetries, and orthopyroxene has a coherent CPO with that of olivine, suggesting their coeval deformation by dislocation creep at high temperature. In the fine-grained microstructures, along with the weakening of the fabric strength, olivine CPO symmetry exhibits a shift towards [010] fiber and the [010] and [001] axes of orthopyroxene are generally distributed subparallel to those of olivine. These data are consistent with deformation of olivine in the presence of low amounts of melts and the precipitation of orthopyroxenes from a melt phase. The bulk CPO of clinopyroxene mimics that of orthopyroxene via a topotaxial relationship of the two pyroxenes. This observation points to a melt-related origin of most clinopyroxenes in the Oran mantle xenoliths. The textural and geochemical record of the peridotites are consistent with interaction of a refractory harzburgite protolith with a high-Mg no. melt at depth (resulting in the formation of coarse-grained clinopyroxene-rich lherzolite and wehrlite) and with a low-Mg no. evolved melt in the shallow subcontinental lithospheric mantle (forming fine-grained harzburgite). We propose that pervasive melt–peridotite reaction – promoted by lateral and/or near-vertical mantle flow associated with lithospheric tearing – resulted in the synkinematic crystallization of secondary lherzolite and wehrlite and had a key effect on grain size reduction during the operation of the Tell–Rif STEP fault. Melt–rock reaction and secondary formation of lherzolite and wehrlite may be widespread in other STEP fault systems worldwide.

scholarly journals Lithosphere tearing along STEP faults and synkinematic formation of lherzolite and wehrlite in the shallow subcontinental mantle

2019 ◽  
Author(s):  
Károly Hidas ◽  
Carlos J. Garrido ◽  
Guillermo Booth-Rea ◽  
Claudio Marchesi ◽  
Jean-Louis Bodinier ◽  
...  
Keyword(s):  
Grain Size ◽  
Lithospheric Mantle ◽  
Volcanic Field ◽  
Mantle Xenoliths ◽  
Coarse Grained ◽  
Fault System ◽  
Mantle Flow ◽  
Second Phase ◽  
Fine Grained ◽  
Wide Range

Abstract. Subduction-Transform Edge Propagator (STEP) faults are the locus of continual lithospheric tearing at slab edges, resulting in sharp changes in the lithospheric and crustal thickness and triggering lateral and/or near-vertical mantle flow. However, the mechanisms at the lithospheric mantle scale are still poorly understood. Here, we present the microstructural study of olivine-rich lherzolite, harzburgite and wehrlite mantle xenoliths from the Oran volcanic field (Tell Atlas, NW Algeria). This alkali volcanic field occurs along a major STEP fault responsible for the Miocene westward slab retreat in the westernmost Mediterranean. Mantle xenoliths provide a unique opportunity to investigate the microstructures in the mantle section of a STEP fault system. The microstructures of mantle xenoliths show a variable grain size ranging from coarse granular to fine-grained equigranular textures uncorrelated with modal variations. The major element composition of the mantle peridotites provides temperature estimates in a wide range (790–1165 °C) but in general, the coarse-grained and fine-grained peridotites suggest deeper and shallower provenance depth, respectively. Olivine grain size in the fine-grained peridotites depends on the size and volume fraction of the pyroxene grains, which is consistent with pinning of olivine grain growth by pyroxenes as second phase particles. In the coarse-grained peridotites, well-developed olivine crystal preferred orientation (CPO) is characterized by orthorhombic and [100]-fiber symmetries, and orthopyroxene has a coherent CPO with that of olivine, suggesting their coeval deformation by dislocation creep at high-temperature. In the fine-grained microstructures, along with the weakening of the fabric strength, olivine CPO symmetry exhibits a shift towards [010]-fiber and the [010]- and [001]-axes of orthopyroxene are generally distributed subparallel to those of olivine. These data are consistent with deformation of olivine in the presence of low amounts of melts and the precipitation of orthopyroxenes from a melt phase. The bulk CPO of clinopyroxene mimics that of orthopyroxene via a topotaxial relationship of the two pyroxenes. This observation points to a melt-related origin of most clinopyroxenes in the Oran mantle xenoliths. The textural and geochemical record of the peridotites are consistent with interaction of a refractory harzburgite protolith with a high-Mg# melt at depth (resulting in the formation of coarse-grained clinopyroxene-rich lherzolite and wehrlite), and with a low-Mg# evolved melt in the shallow subcontinental lithospheric mantle (forming fine-grained harzburgite). We propose that pervasive melt-peridotite reaction – promoted by lateral and/or near-vertical mantle flow associated with lithospheric tearing – resulted in the synkinematic crystallization of secondary lherzolite and wehrlite and played a key effect on grain size reduction during the operation of the Rif-Tell STEP fault. Melt-rock reaction and secondary formation of lherzolite and wehrlite may be widespread in other STEP fault systems worldwide.

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