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

Abstract

and these appear to be derived from the lower crust (0.5-0.8 GPa). The suppressed stability of plagioclase and early saturation of amphibole in these cumulates are indicative of high-pressure crystallisation from primitive hydrous melts (≥ 3 wt% H 2 O). Clinopyroxene in these cumulates has Al 2 O 3 contents up to 9 wt% due to the absence of crystallising plagioclase, and is subsequently consumed in a peritectic reaction to form primitive, Al-rich amphibole (Mg# > 73, 12-15 wt% Al 2 O 3 ). The composition of these peritectic amphiboles is distinct from trace element-enriched interstitial amphibole in shallower cumulates. Phenocryst compositions and assemblages in both suites differ markedly from the cumulates. Phenocrysts, therefore, reflect shallow crystallisation and do not record magma differentiation in the deep arc crust.

Evidence for a Moist to Wet Source Transition Throughout the Oman‐UAE Ophiolite, and Implications for the Geodynamic History

“…Although the petrography and trace element data indicate amphibole fractionation in Phase 2 magmas, extensive amphibole cumulates are not found in the ophiolite. Arc volcanic suites typically obtain their geochemical amphibole signature through the reaction of melts with earlier‐formed cumulate mushes to form amphibole in the lower crust; a process that drives the generation of intermediate and felsic magmas (Davidson et al, , ; Klaver et al, , ; Smith, ). These crystal‐poor felsic melts ascend to shallower levels where amphibole might not be stable as phenocryst phase, thus giving rise to the concept of cryptic amphibole fractionation (Davidson et al, ).…”

“…Generating the tonalites through partial melting of an amphibole‐bearing source (slab, sediments, crust) is clearly inconsistent with the isotopic evidence. The presence of subhedral, possible relic, clinopyroxene indicates that amphibole formed in response to a reaction between a hydrous melt and clinopyroxene bearing cumulates (Best, ; Coltorti et al, ; Debari et al, ; Francis, ; Klaver et al, ; Neal, ; Smith, ). Moreover, the observed increase in LREE with decreasing MgO indicates fractional crystallization of a mafic magma, producing tonalitic compositions (Figure a; Brophy, , ; Brophy & Pu, ).…”

Fiat Lux: Functional Analysis of Three Saxo-Norman Pottery Lamps from Berkeley, Gloucestershire

“…The style of volcanism changed gradually from effusive and mildly explosive to larger Plinian eruptions and the emplacement of viscous lava flows and domes. Based on petrographic features, Klaver et al (2017) divided the eruptive products into two distinct suites: a phenocryst-poor andesite (LPA) and a phenocryst-rich rhyodacite (HPRD) suite. The LPA suite comprises (basaltic-)andesites and rare dacites with low crystal contents (<10 vol.% on average in the andesites).…”

“…In addition, the HPRD suite hosts a wide variety of plutonic xenoliths. Several of these cumulate fragments record a high-pressure crystallisation trend of hydrous primitive melts, ranging from (hornblende-)wehrlite to plagioclase-hornblendites (Klaver et al, 2017). Hornblende forms at the expense of clinopyroxene in these cumulates and the role of this reaction in generating the (rhyo)dacites is explored in this study.…”

“…A representative suite of 39 whole rock samples spanning the entire subaerial volcanic history of Nisyros was analysed for major element, trace element and Sr-Nd-Hf-Pb isotope composition. Major element data for 20 of these samples were reported together with mineral major-and trace element compositions in Klaver et al (2017), whereas trace element and Nd-Pb isotope data for 11 samples have been previously published in Klaver et al (2016). Here we provide the complete geochemical dataset for our Nisyros samples, supplemented with two samples of the KPT.…”