Inverse differentiation pathway by multiple mafic magma refilling in the last magmatic activity of Nisyros Volcano, Greece

Abstract

Based on detailed field, petrographic, chemical, and isotopic data, this paper shows that the youngest magmas of the active Nisyros volcano (South Aegean Arc, Greece) are an example of transition from rhyolitic to less evolved magmas by multiple refilling with mafic melts, triggering complex magma interaction processes. The final magmatic activity of Nisyros was characterized by subPlinian caldera-forming eruption (40 ka), emplacing the Upper Pumice (UP) rhyolitic deposits, followed by the extrusion of rhyodacitic post-caldera domes (about 31-10 ka). The latter are rich in magmatic enclaves with textural and compositional (basaltic-andesite to andesite) characteristics that reveal they are quenched portions of mafic magmas included in a cooler more evolved melt. Dome-lavas have different chemical, isotopic, and mineralogical characteristics from the enclaves. The latter have lower 87 Sr/ 86 Sr and higher 143 Nd/ 144 Nd values than dome-lavas. Silica contents and 87 Sr/ 86 Sr values decrease with time among dome-lavas and enclaves. Micro-scale mingling processes caused by enclave crumbling and by widespread mineral exchanges increase from the oldest to the youngest domes, together with enclave content. We demonstrate that the dome-lavas are multi-component magmas formed by progressive mingling/mixing processes between a rhyolitic component (post-UP) and the enclave-forming mafic magmas refilling the felsic reservoir (from 15 wt.% to 40 wt.% of mafic component with time). We recognize that only the more evolved enclave magmas contribute to this process, in which recycling of cumulate plagioclase crystals is also involved. The post-UP end-member derives by fractional crystallization from the magmas leftover after the previous UP eruptions. The enclave magma differentiation develops mainly by fractional crystallization associated with multiple mixing with mafic melts changing their composition with time. A time-related picture of the relationships between dome-lavas and relative enclaves is proposed, suggesting a delay between a mafic magma input and the relative dome outpouring. We also infer that the magma viscosity reduction by re-heating allows dome extrusion without explosive activity.

A mineral and cumulate perspective to magma differentiation at Nisyros volcano, Aegean arc

“…Major element data for the studied samples overlap with previously reported data for Nisyros (Di Paola 1974;Wyers and Barton 1989;Seymour and Vlassopoulos 1992;Francalanci et al 1995;Buettner et al 2005;Vanderkluysen et al 2005a;Zellmer and Turner 2007;Braschi et al 2012;Spandler et al 2012). Nisyros lavas range from medium-K basaltic andesites to high-K rhyodacites (Fig.…”

“…High crystal contents, a coarse microlite groundmass and plagioclase-dominated mineralogy are diagnostic features. Moreover, the HPRD lavas include abundant mafic enclaves with a texture of acicular amphibole and plagioclase that attests to quench crystallisation upon injection in the cooler (rhyo)dacitic host (Bacon 1986;Vanderkluysen et al 2005a;Braschi et al 2014) and hence supports mafic recharge as a likely eruption trigger (Bachmann et al 2012;Braschi et al 2012Braschi et al , 2014. Magma mingling and disaggregation of the enclaves leads to crystal transfer from the enclaves to the (rhyo)dacitic host and vice versa and can thus account for the complex zoning patterns recorded by HPRD crystals as well as similar, often bimodal plagioclase distributions in the enclaves and hosts (Humphreys et al 2009;Braschi et al 2014;Bergantz et al 2015).…”

“…2e). Of the studied units, the largest enclaves and highest abundance are found in the PFI domes (see Braschi et al 2012 for a detailed description). Crystals in the enclaves that appear to predate the quench crystallisation are referred to as phenocrysts.…”

“…Following these paroxysmal events, the style of volcanism on Nisyros underwent a gradual change from effusive and mildly explosive activity to the emplacement of (rhyo)dacitic domes and lava flows and larger Plinian eruptions. These porphyritic (rhyo)dacites are characterised by the conspicuous presence of mafic enclaves with a large range in phenocryst compositions and disequilibrium textures, which are indicative of magma mingling (Braschi et al 2012(Braschi et al , 2014Zouzias and St Seymour 2014). Lava flow 7 (LF7), underlying the Lies tuff cone, is the oldest studied enclave-bearing porphyritic dacite, followed by the Emborio domes (EMB) and the calderaforming Lower and Upper Pumice eruptions (LP and UP, respectively; Limburg and Varekamp 1991; Longchamp et al 2011).…”

“…A single plagioclase crystal is present in the otherwise plagioclase-free sample; f Type 3 amphibole-plagioclase cumulate with amphibole replacing clinopyroxene 95 Page 8 of 23 and hence they can be classified as primitive andesites (following the definition of Kelemen et al 2003). Mafic enclaves hosted in the HPRD suite largely overlap in composition with LPA basaltic andesites, although the former generally have lower Mg# (50-60) and SiO 2 contents, and Al 2 O 3 contents up to 20.5 wt% are found in enclaves in the Profitis Ilias domes (PFI; Braschi et al 2012). The lack of significant trace element and isotopic variations between the most primitive LPA and HPRD samples suggests that there is no pronounced difference in primary melts feeding the two suites (Klaver et al 2016).…”