Bazanova, L. I.

Bindemann, I. N.

Krasheninnikov, S. P.

Portnyagin, Maxim

English

OceanRep

10/14/121/2CONTROL ID: 1456301TITLE: Role of crystallizational differention in the origin of island-arc andesitic melts: evidence fromdata on melt inclusions and oxygen isotopesAUTHORS (FIRST NAME, LAST NAME): Stepan Petrovich Krasheninnikov1, MaximPortnyagin1, 2, Ilya N Bindeman3, Liliya Ivanovna Bazanova4INSTITUTIONS (ALL): 1. Vernadsky Institute of Geochemistry and Analytical Chemistry RAS,Moscow, Russian Federation. 2. Helmholtz-Zentrum für Ozeanforschung Kiel (GEOMAR), Kiel, Germany. 3. Department of Geological Sciences, University of Oregon, Eugene, OR, United States. 4. Institute of Volcanology and Seismology, Petropavlovsk-Kamchatsky, Russian Federation. ABSTRACT BODY: Several recent studies of melt inclusions in island-arc rocks revealed a strongbimodality of the melt compositions at the predominance of basic and silicic melts and the scarcity ofintermediate melts with SiO2=59-66 wt% (e.g. [1]). These observations were used to interpret theorigin of island-arc andesites by magma mingling, crustal assimilation and crystal accumulation ratherthan by fractional crystallization of basaltic magmas.In this work we addressed the question about the scarcity of andesitic melts in island-arc setting bysystematic study of bulk compositions, melt inclusions and oxygen isotopes in minerals fromAvachinskiy volcano in Kamchatka. We studied ~500 melt inclusions in 6 different mineral phases(Ol, Cpx, Opx, Pl, Amph, Mt), and concentrated on rapidly-quenched tephra samples from 40Holocene eruptions of andesites and basaltic andesites. The melt inclusions span a large range ofcompositions from basalts to rhyolites. In comparison with host bulk tephra samples, melt inclusionstend to have more silicic compositions (up to 10 wt% of SiO2), and this disparity tend to increasewith increasing SiO2 content in the host rocks. Both melt inclusion and host rock compositions formtrends along the line dividing low- and middle-K island-arc series, and variations of major elementsare continuous, without apparent bimodality, which is observed in data set from [1]. The MI statisticaldistribution is rather three-modal with maxima at ~56-58, ~66 and 74 wt% of SiO2. Much of themajor element variability in MI can be explained by fractional crystallization from parental basalticmelts using numerical modeling of crystallization path. Magnetite crystallization starts at ~58 wt% ofSiO2 and affects significantly on the evolutional path of melts. Abundant crystallization of magnetitelead to formation of more silica rich coexistent melts and change of crystallizing assemblage occurredat ~60 wt% of SiO2, when Opx replaced Ol, and Amph and Ap become stable. Paragenesis ofOPx, CPx, Amph, Pl, Mt, Ilm and Ap dominated the following evolution of melts toward stronglyacid compositions with 78-80 wt% SiO2. Individual Pl and Amph crystals are in magmatic isotopic equilibrium, have heavy δ18O valuesincreasing from 6.3 ‰ in basaltic andesites to 7.1 ‰ in andesites, suggesting that magmatic evolutionstarted from primary high-d18O basalt likely related to the abundant high-d18O sources describedfor Kamchatkan primitive magmas. The oxygen isotopic data support the conclusion that island-arcandesitic melts of Avachinsky Volcano generate predominantly due to the processes of fractionalcrystallization of high-d18O.The new data on composition of melt inclusions allowed us to reconstruct the entire spectrum ofparental melts for Avacha volcano. Melt inclusions in different minerals form coherent trends of majorelements, which can be well explained by fractional crystallization. Unlike some other island-arcvolcanoes, Avachinskiy melts do not display clear bimodality of SiO2 content. Melts of intermediatecompositions are relatively abundant and found in minerals from basaltic andesites. [1] Reuby &Blundy (2009) Nature, 461(7268), 1269-1273.10/14/122/2KEYWORDS: [1038] GEOCHEMISTRY / Mantle processes, [1041] GEOCHEMISTRY /Stable isotope geochemistry.(No Image Selected)(No Table Selected)Additional DetailsPreviously Presented Material:Contact DetailsCONTACT (NAME ONLY): Stepan KrasheninnikovCONTACT (E-MAIL ONLY): spkrasheninnikov@mail.ruTITLE OF TEAM:

Role of crystallizational differention in the origin of island-arc andesitic melts: evidence from data on melt inclusions and oxygen isotopes

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