Mg-Fe and trace element zonation in olivine phenocrysts of Kamchatka basalts : Implications for magma ascent and residence times

在过去的十年里，橄榄石斑晶的成分分带和扩散模型被用于估算岩浆停留时间和岩浆上升的持续时间。基本的假设是，与新注入的岩浆混合进入储集层，会触发常驻橄榄石晶体和新到达的岩浆之间的扩散交换，这种岩浆混合最终会触发喷发。如果混合深度已知，这将转化为岩浆上升到地表的速率。我们将把这种方法应用于来自堪察加的一系列不同的弧状玄武岩熔岩，以更好地限制岩浆上升的速度和岩浆驻留在世界上最活跃的俯冲带之一，那里也是由异常丰富的镁铁质岩浆主导的。我们对来自堪察加的含橄榄石玄武岩的初步研究表明，混合、晶体生长和在岩浆室中的停留可能更加复杂。因此，我们将分析和模拟一系列具有不同扩散系数的元素(例如，镁-铁、钙、镍和其他微量元素)的分带轮廓，寻找共同的模型解决方案，并评估不同扩散系数作为橄榄石和温度中镁/铁的函数所起的作用。我们已经从早期与我们的俄罗斯同事(Tatiana Churikova和Boris Gordeychik)的合作中获得了大量样本，其中大多数样本以前已经进行了地球化学分析(见我们的出版物)。我们将专门从大型层状火山中央喷口喷发的玄武岩、层锥周围地区的小体积单生煤渣锥以及广泛的高原玄武岩产状(例如来自科斯塔坎地区的Klyuchevskoy、Tolbachk、Shiveluch、Kharchinsky、Zarechny、Ploskie Sopki、Sredny和煤渣锥，贝克宁火山附近的Peschanaya火山)中选择涵盖不同火山环境的样本。通过大量的橄榄石样本选择，我们涵盖了堪察加半岛岩浆演化、上升、混合和喷发的主要模式和制度。对于代表这些不同岩浆制度的样品，我们预计会有不同的橄榄石分带历史，这将限制其形成和运移到地表的过程的年代学。

Compositional zonation in olivine phenocrysts and diffusion modelling have been used in the last ten years to estimate magma residence times and the duration of magma ascent. The fundamental assumption is that mixing with newly injected magma into a reservoir triggers diffusional exchange between resident olivine crystals and newly arrived magma and that this magma mixing eventually triggers eruption. If depth of mixing is known, this translates to ascent rates of magmas to the surface.We will apply this approach to a series of different arc basalt lavas from Kamchatka to better constrain the rates of magma ascent and magma resident in what is one of the most active subduction zones in the world that is also dominated by an abundance of unusually mafic magmas. In our preliminary studies from olivine-bearing basalts from Kamchatka have shown that mixing, crystal growth and residence in magma chambers may be more complex. Therefore we will analyse and model zonation profiles for a range of elements with different diffusivities (e.g. Mg-Fe, Ca, Ni and other trace elements), find common model solutions and assess the role of variable diffusivities as a function of Mg/Fe in the olivines and temperature. A large sample collection already is available to us from earlier collaboration with our Russian colleagues (Tatiana Churikova and Boris Gordeychik) and most of these samples have previously been geochemically analysed (see our publications). We will specifically select samples that cover different volcanological settings from basalts erupted from the central vent of large-volume stratovolcanoes, small-volume monogenetic cinder cones from areas surrounding the stratocones as well as from extensive plateau-basalt occurrences (e.g. Klyuchevskoy, Tolbachik, Shiveluch, Kharchinsky, Zarechny, Ploskie Sopki, Sredny and cinder cones from the Kostakan area, Mount Peschanaya near Bakening volcano). With this large selection of olivine-bearing samples we cover the principal modes and regimes of magma evolution, ascent, mixing and eruption in Kamchatka. For samples representing these distinct magmatic regimes we expect different olivine zoning histories, which will constrain the chronology of processes of their formation and transport to the surface.