Evidence for primitive magma storage and eruption following prolonged equilibration in thickened crust
Evidence for primitive magma storage and eruption following prolonged equilibration in thickened crust
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加厚地壳长期平衡后原始岩浆储存和喷发的证据
DOI：
10.1007/s00445-020-01406-3
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发表时间：
2020
影响因子：
3.5
通讯作者：
Amigo, Alvaro
中科院分区：
文献类型：
--
作者：
Winslow, Heather;Ruprecht, Philipp;Stelten, Mark;Amigo, Alvaro
在大陆弧中，原始喷发产物在地表的暴露通常是岩浆通过地壳快速转移的结果。因此，最初的原始岩浆经历了最小的地壳居住，从而对更进化的产品微不足道的分化。这种原始岩浆通过增厚地壳的快速转移通常是从较小的单成因火山锥中记录下来的。Manantial Pelado（南纬35.5°）是南安第斯火山带（SVZ）中的一个长期存在的层锥，覆盖在厚的大陆地壳（45-50公里）上，几乎只产生镁铁质物质。由于Manantial Pelado被广泛的火山活动所包围，对其作为平流层的镁铁质暴露的研究可用于进一步了解长寿火山系统的岩浆起源。我们的研究使用从Manantial Pelado收集的熔岩的纹理，地球化学和地质年代学数据来表征其岩浆成因，评估原始性质，并解释SVZ内的地壳过程。对火山的地质描述揭示了玄武岩安山岩的大多数单调的喷发历史，现在可以通过冰川雕刻的山谷进入。新的~（40）Ar/~（39）Ar定年结果表明，该火山锥的形成期约为220 ~ 190 ka。在~ 30 ka，小体积的活动和不同的岩相学的更多的中间岩浆，反映了火山的性质的变化。全岩和矿物尺度数据的结合表明，Manantial Pelado的玄武安山岩是SVZ增厚地壳中最原始的岩浆之一。这种原始签名的证据包括橄榄石中的纹理和环带图案，橄榄石芯中铬尖晶石的存在，以及橄榄石芯中的Fo和Ni含量升高。这些数据与元素扩散模型相结合，为这些熔岩的原始签名提供了证据。具有均匀核心组成的中间Fo橄榄石（Fo 80 -84）表明，玄武安山岩在喷发前（约25-6000年）在准封闭系统环境中长期储存在地壳中。在这些中间Fo橄榄石的扩散平衡掩盖了岩浆的原始性质。这些结果表明，镁铁质岩浆可以有一个长期的存储历史在地壳中，并没有显着改变其原始的散装组成之前，到达表面。我们认为，这些都是重要的过程，在了解岩浆起源的长寿系统和存在的成分均匀的橄榄石在中间Fo含量可能代表神秘的证据与原始岩浆经历了长期的地壳储存的补给。
In continental arcs, the exposure of primitive eruptive products at the surface is typically a result of rapid magmatic transfer through the crust. As a result, the initially primitive magma experiences minimal crustal residence and thus insignificant differentiation towards more evolved products. This rapid transfer of primitive magma through thickened crust is commonly recorded from smaller, monogenetic cinder cones. Manantial Pelado (35.5° S) is a long-lived stratocone in the Southern Andean Volcanic Zone (SVZ) overlying thick continental crust (45–50 km) that produces almost exclusively mafic material. As Manantial Pelado is surrounded by extensive silicic volcanism, the study of its mafic exposure as a stratocone can be used to further understand magmatic origins of long-lived volcanic systems. Our study uses textural, geochemical, and geochronological data from lavas collected from Manantial Pelado to characterize its magmatic petrogenesis, assess the primitive nature, and explain processes in the crust within the SVZ. A geologic description of the volcano reveals a mostly monotonous eruptive history of basaltic andesites that are now accessible through glacially carved valleys. New40Ar/39Ar dating constrains most of the volcano’s cone constructing phase to last from ~ 220 to 190 ka. At ~ 30 ka, small-volume activity and different petrography of more intermediate magmas were present reflecting a change in the volcano’s character. A combination of the whole-rock and mineral-scale data reveals that basaltic andesites at Manantial Pelado are among the most primitive magmas in the thickened crust of the SVZ. Evidence for this primitive signature consists of textural and zonation patterns in olivine, the presence of Cr-spinel in olivine cores, and elevated Fo and Ni content within olivine cores. This data combined with elemental diffusion modeling provides evidence for a primitive signature for these lavas. Intermediate Fo olivines with uniform core compositions (Fo80–84) suggest that basaltic andesites reside in the crust in quasi-closed system environments for extended storage prior to eruption (~ 25–6000 years). Diffusive equilibration in those intermediate Fo olivines masks the primitive nature of the magmas. These results suggest that mafic magmas can have a protracted storage history in the crust that does not significantly alter their primitive bulk composition before reaching the surface. We argue that these are important processes in understanding the magmatic origin of long-lived systems and the presence of compositionally homogenous olivines at intermediate Fo content may represent cryptic evidence for recharge with primitive magmas that experienced prolonged crustal storage.
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