The Holocene volcanism at El Hierro consists of basaltic monogenetic volcanic fields associated with o the three rift systems present in this island. In this work we report preliminary petrological and geochemical data of Holocene lava flows belonging to the WNW-striking rift. Sampling was focused in three zones: Orchilla, Verodal-Sabinosa, and Tanganasoga. Petrography of the studied lavas shows that they are homogeneous. All samples are porphyritic with macrocrysts of clinopyroxene and olivine immersed in a groundmass formed by microcrysts of plagioclase, Fe-Ti oxides and clinopyroxene. Clinopyroxenes are diopsides, olivines have forsterite contents ranging from 74 to 84 % and anorthite in plagioclase varies from 66 to 76% (labradorite). Whole-rock geochemical results evidence that all magmas are basic in composition, ranging from picrobasalts to phonotephrites. Major, trace elements and isotope suppor fractional crystallization as the main process of magma evolution. However, petrography and chemistry of clinopyroxene cores agree with a xenocrystic nature of some of them. We suggest that these clinopyroxene cores crystallized from a genetically related magma and subsequently were entrapped o cannibalized by the basic rising magmas

Arienzo, I.

Aulinas Juncà, Meritxell

Carmona, H.

Carracedo, J.C.

D'Antonio, M.

Dominguez, D.

Fernández Turiel, José Luis

Rodriguez-Gonzalez, A.

English

Carracedo, Juan C.

Diposit Digital de la Universitat de Barcelona

GEOGACETA, 65, 201935Copyright© 2019 Sociedad Geológica de España / www.geogaceta.comRecepción: 1 de julio de 2018Aceptación: 23 de octubre de 2018Aceptación: 23 de noviembre de 2018Geogaceta, 65 (2019), 35-38ISSN (versión impresa): 0213-683XISSN (Internet): 2173-6545The Holocene volcanism at El Hierro: insights from petrologyand geochemistryEl volcanismo holoceno en El Hierro: petrología y geoquímicaMeritxell Aulinas1, Diego Domínguez1, Alejandro Rodríguez-González2, Héctor Carmona1, José Luis Fernández-Turiel3, Francisco JoséPérez-Torrado2, Juan Carlos Carracedo2, Ilenia Arienzo4 and Massimo D’Antonio51 Departamento de Mineralogía, Petrología y Geología Aplicada. Facultad de Ciencias de la Tierra. Universidad de Barcelona. C/ Martí Franquès s/n 08028, Barcelona.meritxellaulinas@ub.edu; ddominguezcarretero@gmail.com; hector.carmona87@gmail.com  2 Instituto de Estudios Ambientales y Recursos Naturales (i–UNAT), Universidad de Las Palmas de Gran Canaria (ULPGC), 35017 Las Palmas de Gran Canaria, Spainalejandro.rodriguezgonzalez@ulpgc.es; franciscojose.perez@ulpgc.es 3 Instituto de Ciencias de la Tierra Jaume Almera ICTJA-CSIC. C/ Solé i Sabaris s/n. 08028 Barcelona. jlfernandez@ictja.csic.es 4 Osservatorio Vesuviano (INGV). Via Diocleziano 328, 80124 Nápoles (Italia). ilenia.arienzo@ingv.it  5 Dipartimento di Scienze della Terra, dell'Ambiente e delle Risorse. Largo San Marcellino, 10 - 80138 Nápoles (Italia) masdanto@unina.itABSTRACTThe Holocene volcanism at El Hierro consists of basaltic monogeneticvolcanic fields associated with o the three rift systems present in this island. Inthis work we report preliminary petrological and geochemical data of Holocenelava flows belonging to the WNW-striking rift. Sampling was focused in threezones: Orchilla, Verodal-Sabinosa, and Tanganasoga. Petrography of the studiedlavas shows that they are homogeneous. All samples are porphyritic withmacrocrysts of clinopyroxene and olivine immersed in a groundmass formedby microcrysts of plagioclase, Fe-Ti oxides and clinopyroxene. Clinopyroxenesare diopsides, olivines have forsterite contents ranging from 74 to 84 % andanorthite in plagioclase varies from 66 to 76% (labradorite). Whole-rockgeochemical results evidence that all magmas are basic in composition, rangingfrom picrobasalts to phonotephrites. Major, trace elements and isotope supportfractional crystallization as the main process of magma evolution. However,petrography and chemistry of clinopyroxene cores agree with a xenocrysticnature of some of them. We suggest that these clinopyroxene cores crystallizedfrom a genetically related magma and subsequently were entrapped orcannibalized by the basic rising magmas.Key-words: El Hierro, Holocene, volcanism, petrology, geochemistry.RESUMENEl volcanismo holoceno en El Hierro está formado por campos volcánicos mono-genéticos asociados a las tres estructuras de tipo rift que modelan la isla. En esetrabajo se presentan los resultados petrológicos y geoquímicos preliminares de laslavas holocenas del rift de dirección ONO. El muestreo se concentró en tres zonas:Orchilla, Verodal-Sabinosa y Tanganasoga. La petrografía de las lavas estudiadasmuestra que son bastante homogéneas. Todas las lavas son porfídicas con macro-cristales de clinopiroxeno y olivino inmersos en una matriz formada por microcristalesde plagioclasa, óxidos de Fe-Ti y clinopiroxeno. Los clinopiroxenos son diópsidos, losolivinos presentan contenidos en forsterita que varían entre 74 y 84% y el contenidoen anortita en las plagioclasas varía entre 66 y 76% (labradorita). Los resultados geo-químicos de roca total evidencian que todos los magmas son básicos, concomposiciones que varían entre picrobasaltos y fonotefritas. Los elementos mayori-tarios, trazas e isótopos sugieren que la cristalización fraccionada es el principalproceso de evolución magmática. Aun así, la petrografía y química de los núcleos delos clinopiroxenos sugieren que algunos de ellos son xenocristales. Se sugiere queestos núcleos cristalizaron a partir de un magma genéticamente relacionado y queposteriormente fueron atrapados o canibalizados por el magma básico en ascenso.Palabras clave: El Hierro, Holoceno, volcanismo, petrología, geoquímica.IntroductionEl Hierro (1.12 Ma) is, together with LaPalma, the youngest island of the CanarianArchipelago. Both islands are in the shieldstage of their volcanic growth, which im-plies a high volcanic activity during the Ho-locene period. The submarine eruption ofOctober 2011 at El Hierro awakened the in-terest of the scientific community for the un-derstanding of El Hierro volcanism. Conse-quently, numerous scientific works relatedto this eruption were published in a shortperiod of time (e.g., geophysics: Gonzaleset al., 2013; Roberts et al., 2016; petrologyand geochemistry: Troll et al., 2012, Martíet al., 2013; geomorphology and bathy-metry: Rivera et al., 2013; monitoring andcrisis management: Carracedo et al., 2015;Sobradelo et al., 2015). By contrast, recentonshore studies are restricted to the worksof Longpré et al. (2011), Pedrazzi et al.(2014) and Becerril et al. (2015, 2016a, b).Nowadays, there is no full understanding ofthe magma system below the island (detai-led petrological and geochemical studiesare lacking), how and when Holocene erup-tions have taken place (radiocarbon agesare scarce), and how future eruption will(for hazard assessment).  In this work wereport preliminary petrological and geo-chemical data of Holocene lavas erupted inthe WNW rift necessary for a detailed vol-canic hazard evaluation.Geological settingEl Hierro is the smallest and western-most island in the Canarian Archipelago. Ithas a surface of circa 280 km2 and a maxi-mum elevation of 1501 m above sea level(Pico Malpaso). This island represents theemerged part of a volcanic edifice that restson an ocean floor located at 3500-4000 mdepth. The morphology and structure of theisland is determined by the presence of aregular three rift system and several giantlandslides that give the island its presentgeometry, like a “Mercedes star” (e.g., Ca-rracedo et al., 2001). The subaerial develop-ment of El Hierro includes three volcaniccycles: (1) Tiñor edifice (1.12-0.88 Ma), (2)El Golfo-Las Playas edifice (545-176 ka), and(3) Rift volcanism (158 ka-present) (Fig. 1).The Tiñor edifice represents the first stageof subaerial growth of El Hierro. Its volcanicproducts, which crop out in the NE sector ofthe island, consist of picritic to tephriticlavas (Guillou et al., 1996). El Golfo volcanicedifice is mainly located in the NW sectorand shows a variable chemical compositionranging from nephelintes to trachytes. Mag-mas belonging to the rift stage are picrites,basanites and tephrites. The three volcaniccycles are separated by giant gravitationalcollapses being the youngest ones (1) LasPlayas I and II (ca. 545-176 ka and 176-145ka, respectively), (2) El Julan (>158 ka) andEl Golfo (ca. 130 ka- 39 ka, 13 ka?) (e.g.,Guillou et al., 1996, Longpré et al., 2011).It seems there is a clear connection bet-ween these catastrophic events and the riftstructures. Moreover, the giant landslideshave also been related to changes in themagmatic system of El Hierro. Manconi etal. (2009) evidenced that the El Golfo gra-vitational collapse affected the magmaticplumbing system at that time, resulting inthe eruption of less evolved magmas thanpreviously.The petrological and geochemicalknowledge about the magmatic feedingsystem beneath the island suggests the pre-sence of small reservoirs located at 14-30km depth (Stroncik et al., 2009). Thesedepths are consistent with those determi-ned for the submarine eruption of October2011 (10-25 km, Gonzales et al., 2013). The subaerial Holocene volcanismThe Holocene volcanism in El Hierroconsists of basaltic monogenetic volcanicfields related to the three rift systems pre-sent in this island. The eruptive mecha-nisms are typically strombolian with minorphreatomagmatic pulsations. The most re-cent eruptions, which form coastal plat-forms, are younger than the Last GlacialMaximum (ca. 18 ka, Carracedo et al.,2001). The Tanganasoga volcano (6.74 ka,Pellicer, 1977) is the most voluminous andrepresentative Holocene volcano. Theavailable radiocarbon ages indicate thatMontaña Chamuscada is the youngestsubaerial eruption in El Hierro (2.5 ka, Ca-rracedo et al., 2001).Sample location and methodologyThe analyzed samples correspond tolavas from the WNW rift structure. Lavaswere collected in three distinguished zoneswhere Holocene volcanic products are ex-posed (Fig. 1): (1) Orchilla, (2) Verodal-Sa-binosa and (3) Tanganasoga.The geochemical study includes whole-rock major and trace element analyses and Srand Nd isotope ratios of selected elementcomposition of the main mineral samples, andmajor element composition the main mineralphases. Whole-rock major elements were me-asured by XRF at the Centres Científics i Tec-nològics de la Universitat de Barcelona(CCiTUB) whereas trace elements were deter-mined by HR-ICP-MS at the LabGEOTOP fromthe Institute of Earth Sciences Jaume Almera(ICTJA-CSIC). Sr and Nd isotope ratios of re-presentative samples were analysed at the Os-servatorio Vesuviano (OV-INGV). Mineralchemistry was done by EMPA at the CCiTUB.Petrography and mineralchemistryAll the studied lavas are porphyritic withmacrocrysts of olivine and clinopyroxene(and minor plagioclase in some samples)immersed in an microlitic groundmass. Thepresence of vesicles is also observed in allsamples. Locally, macrocrysts show glome-roporphyric textures. Groundmass is charac-terized by the presence of plagioclasemicrolites as well as clinopyroxene mi-crocrysts. Fe-Ti oxides are accessory mineralsmainly occurring as microcrysts in thegroundmass. GEOGACETA, 65, 2019 M. Aulinas, D. Domínguez, A. Rodríguez-González, H. Carmona, J.L. Fernández-Turiel, F.J. Pérez-Torrado, J.C. Carracedo, I Arienzo and M. D’Antonio36 Petrology and Geochemistry / Petrología y GeoquímicaFig. 1.- Simplified geological map of El Hierro. The studied areas are indicated (1 Orchilla, 2 Verodal-Sabinosa and 3 Tanganasoga). See color figure in the web.Fig.1.- Mapa geológico simplificado de El Hierro. Se señalan las áreas estudiadas (1 Orchilla, 2 Vero-dal-Sabinosa y 3 Tanganasoga). Ver figura en color en la web.Most of the clinopyroxene macrocrystsshow a complex growth history. They oftendisplay normal and oscillatory zoning. It isalso frequent the presence of cores with dis-equilibrium textures such as spongy cellu-lar/sieve texture (Fig. 2A). In addition,several clinopyroxenes present glome-rocrysts (Fig. 2B).The chemical analyses of clinopyroxenesindicate that they are all diopsides withWo46.7-49.3, En37.5-42.4, Fs9.5-15.8 (Fig. 2C).  Nor-mal zoning is reflected by cores with highercontents of MgO and CaO and lower con-tents of FeOT and TiO2 compared to rims.Forsterite (Fo) in olivine ranges from 74 to84% and NiO abundance from 0.07 to0.31%. Plagioclase composition is labrado-rite with anorthite (An) contents varyingfrom 56 to 66% (Fig. 2D). Finally, opaqueminerals are classified as ilmenites.Whole-rock geochemistryHolocene lava flows in El Hierro rangein composition from picrobasalts to phono-tephrites and follow an alkaline trend in theTotal Alkali Silica diagram of Le Bas et al.(1986) (Fig. 3A). They belong to the alkalinesodic series and are silica-undersaturatedrocks (nepheline normative).Overall, covariation diagrams (not re-presented) show a negative correlation ofSiO2, Al2O3, K2O, Na2O and positive correla-tion of FeOT and CaO with MgO, TiO2 versusMgO show a segmented trend, with a po-sitive correlation between 4 and 7 wt%MgO, followed by a negative correlation.Most trace elements show negative corre-lation except for Ni, Cr and Sc. REE contentsdepict a typical OIB pattern with enrichmentin LREE and depletion in HREE and withoutsignificant Eu anomalies. In addition, thebell-shaped trend in the multi-element dia-gram, together with a positive Nb anomalyis typical of OIB type (Fig. 3B). The Sr isotope ratios measured on se-lected Holocene lavas range from 0.702993to 0.703006, and 143Nd/144Nd varies from0.512936 to 0.512941 (Table I). These re-sults are in accordance with those reportedby Day et al. (2010) for subaerial lavas of ElHierro. Nd isotope signatures are also verysimilar to those reported for the submarineeruption of 2011-2012 in la Restinga area(e.g., Sigmarsson et al., 2013).Discussion and conclusionsThe studied lava flows are basic in com-position, with SiO2 ranging from 40 to 50wt%. They belong to the sodic alkaline seriesand are silica undersaturated.  Major and traceelements and Sr and Nd isotopes of the stu-died samples are consistent with an OIB origin.Moreover, all these results are in accordancewith those reported in previous works (e.g.,Stroncik et al., 2009; Day et al., 2010).Several geochemical trends such as po-sitive correlation of FeOT , CaO, Ni or Cr, andnegative correlation of Al2O3, K2O or mostGEOGACETA, 65, 201937Petrology and Geochemistry / Petrología y GeoquímicaThe Holocene volcanism at El Hierro: insights from petrology and geochemistryFig. 2.- A and B) Clinopyroxene crystals show-ing complex zoning (normal zoning plus oscil-latory zoning), cores with spongy texture, andglomerocrysts. C and D) Chemical classificationof clinpyroxenes and plagioclases. See colorfigure in the web.Fig. 2.- A y B) Cristales de clinopiroxenosmostrando una zonación compleja (zonaciónnormal y oscilatoria), núcleos con textura encolador, y glomerocristales.  C y D) Clasificaciónquímica de los clinopiroxenos y plagioclasas.Ver figura en color en la web.Fig. 3.- Whole-rock composition of analyzed samples are represented in the TAS diagram.Multielement diagram normalized to Primitive Mantle (Sun, 1980). See color figure in the web.Fig.3.- La composición de las muestras analizadas se representan en el diagrama TAS. Diagramamultielemenal normalizado al Manto Primitivo (Sun, 1980). Ver figura en color en la web.       Sample                   87Sr/86Sr             143Nd/144Nd HIR-25                  0.702993              0.512939 HIR-38                  0.702998              0.512936 HIR-39                  0.703005              0.512941 HIR-40                  0.703006              0.512940Table I.- Sr and Nd isotope ratios of the ana-lyzed lavas. Tabla I.- Isótopos de Sr and Nd de las lavasanalizadas.GEOGACETA, 65, 2019 M. Aulinas, D. Domínguez, A. Rodríguez-González, H. Carmona, J.L. Fernández-Turiel, F.J. Pérez-Torrado, J.C. Carracedo, I Arienzo and M. D’Antonio38 Petrology and Geochemistry / Petrología y Geoquímicaof trace elements with MgO, show goodgeochemical coherence. All these tenden-cies are compatible with a chemical evolu-tion of magmas by fractional crystallizationof the observed mineral phases. Normal zo-ning in some clinopyroxene crystals, withcores enriched in MgO and depleted inFeOT , also supports this hypothesis.However, clinopyroxene cores withspongy cellular/sieve textures due to re-sorption also indicate the involvement ofother magmatic processes, such as magmamixing/mingling or assimilation. Resorption inclinopyroxene cores is an indication of openmagmatic plumbing system. Thus, these corescould represent antecrysts that did not crysta-llize directly from the host magma in whichare contained, but from a previous event ge-netically related to the magmatic system (Je-rram and Martin, 2008). They may correspondto recycled crystals through different magmareplenishment events or to stored crystals incrystal accumulations from the magma. These antecrysts are wrapped by finegrowth bands showing an oscillatory zoningthat can be combined with sector zoning. Thesekinds of zoning are usually related to kineticeffects. Crystal growth rates depend on meltsupersaturation and undercooling at the crystalmelt interface but also on the crystal orienta-tion (sector zoning). It occurs because thegrowth rate is too fast relative to the rate ofdiffusion of chemical components in the melt. AcknowledgementsThis work has been funded by theAgencia Canaria de Investigación, Innova-ción y Sociedad de la Información (Sol-SubC200801000047). It was carried out inthe framework of the Research Consolida-ted Groups GEOVOL (Canary Islands Go-vernment, ULPGC) and GEOPAM (Genera-litat de Catalunya, 2017 SGR 1494).Authors would like to thank the reviewersGuillem Gisbert and Gumer Galán.ReferencesBecerril, L., Galindo, I., Martí, J. andGudmundsson, A. (2015). Tectonophysics647, 33-47.Becerril, L., Galve, J.P., Morales, J.M., Romero,C., Sánchez, N., Martí, J. and Galindo, I.(2016a). Journal of Maps 12, 43-52.Becerril, L., Ubide, T., Masafumi, S., Martí, J.,Galindo, I., Galé, C., Morales, J.M., Yepes,J. and Lago, M. (2016b). Journal of AfricanEarth Sciences 113, 88-94.Carracedo, J.C., Badiola, E.R., Guillou, H., deLa Nuez, J. and Pérez Torrado, F.J. (2001).Estudios Geológicos 57, 171-295.Carracedo, J.C., Troll, V.R., Zaczek, K.,Rodríguez-González, A., Soler, V. andDeegan, F.M. (2015). Earth ScienceReviews 150, 168-200.Day, J.M.D., Pearson, D.G., Macphearson, C.G.,Lowry, D. and Carracedo, J.C. 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The Holocene volcanism at El Hierro: insights from petrology and geochemistry

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The Holocene volcanism at El Hierro: insights from petrology and geochemistry

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