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

近十年来，人们利用橄榄石斑晶的成分分带和扩散模型来估计岩浆的停留时间和岩浆上升的持续时间。基本假设是，新注入的岩浆与储层的混合引发了原有橄榄石晶体与新到达的岩浆之间的扩散交换，这种岩浆混合最终引发了火山喷发。如果混合深度是已知的，这就转化为岩浆上升到地表的速率。我们将把这种方法应用于堪察加半岛一系列不同的弧玄武岩熔岩，以更好地限制岩浆上升的速度，以及居住在世界上最活跃的俯冲带之一的岩浆，该俯冲带也由丰富的异常基性岩浆主导。我们对堪察加含橄榄玄武岩的初步研究表明，岩浆房的混合、晶体生长和居住可能更为复杂。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Mg-Fe， Ca， Ni和其他微量元素）的分区分布曲线，找到共同的模型解决方案，并评估作为Mg/Fe在橄榄石和温度中的函数的可变扩散系数的作用。从早期与我们的俄罗斯同事（Tatiana Churikova和Boris Gordeychik）的合作中，我们已经收集了大量的样本，其中大部分样本之前已经进行了地球化学分析（参见我们的出版物）。我们将特别选择涵盖不同火山环境的样本，包括从大容量层状火山的中央喷口喷发的玄武岩，从层状火山周围地区的小体积单成因火山渣锥以及广泛的高原玄武岩产点（例如Klyuchevskoy， Tolbachik, Shiveluch, Kharchinsky, Zarechny, Ploskie Sopki， Sredny以及来自Kostakan地区的火山渣锥，Bakening火山附近的Mount Peschanaya）。通过大量含橄榄石的样品，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机制。对于代表这些不同岩浆体系的样品，我们期望不同的橄榄石分带历史，这将限制它们形成和向地表运输过程的年代学。

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.