Partial melting of subducting oceanic crust
Partial melting of subducting oceanic crust
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DOI：
10.1016/0012-821x(94)90042-6
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发表时间：
1994
影响因子：
5.3
通讯作者：
S. Peacock;T. Rushmer;A. Thompson
中科院分区：
文献类型：
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作者：
S. Peacock;T. Rushmer;A. Thompson
俯冲洋壳发生部分熔融的条件可以通过结合玄武岩成分的部分熔融模型和俯冲带的二维热模型来确定。对于含H_2O的孔隙度≤ 1%，在湿玄武质固相线处产生的部分熔体的量被限制为<5vol%。在较高的温度下（1.5 GPa，1000°C），大量的部分熔融，高达50体积%，通过角闪石的分解和结构结合的H2O的释放形成。在大多数俯冲带中，俯冲洋壳只有在岩石能够维持接近或高于其熔融温度的高剪切应力（> 100 MPa）时才会发生大量部分熔融。在没有高剪切应力的情况下，大洋地壳的大量熔融只会发生在非常年轻（< 5 Ma）的大洋岩石圈俯冲过程中。有人提出来自洋中脊的水合玄武岩（斜长角闪岩）的部分熔融[例如，1-3]作为负责某些现代高铝安山质英安质火山岩（埃达克岩）的生成。其中三个火山岩组（圣海伦斯山、智利南部和巴拿马）位于火山弧中，在那里，25 Ma以下的洋壳正以1-3厘米/年的速度俯冲，计算出的热状况比更典型的俯冲带环境高出几百度。然而，海洋岩石圈目前没有被俯冲下巴哈和新几内亚，最近埃达克岩也存在，这表明一些埃达克岩岩浆可能形成的水不饱和部分熔融的下板基性下地壳或以前俯冲的海洋地壳。需要对代表大洋地壳的成分进行进一步的实验工作，以更准确地确定可能的埃达克岩源区的深度。
The conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two-dimensional thermal models of subduction zones. For porosities of ∼ 1% containing H2O the amount of partial melt generated at the wet basaltic solidus is limited to < 5 vol%. At higher temperatures (∼ 1000°C at 1.5 GPa) large amounts of partial melt, up to 50 vol%, form by the breakdown of amphibole and the release of structurally bound H2O. In most subduction zones, substantial partial melting of subducting oceanic crust will only occur if high shear stresses ( > ∼ 100 MPa) can be maintained by rocks close to, or above, their melting temperatures. In the absence of high shear stresses, substantial melting of the oceanic crust will only occur during subduction of very young ( < 5 Ma) oceanic lithosphere.Partial melting of hydrated basalt (amphibolites) derived from the mid-ocean ridge has been proposed [e.g., 1–3] as being responsible for the generation of certain recent high-Al andesitic to dacitic volcanic rocks (adakites). Three of these volcanic suites (Mount St. Helens, southern Chile, and Panama) occur in volcanic arcs where oceanic crust < 25 Ma is being subducted at rates of 1–3 cm/yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments. However, oceanic lithosphere is not currently being subducted beneath Baja and New Guinea, where recent adakites are also present, suggesting that some adakite magmas may form by water-undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust. Further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately.