Experimental study of a low-alkali tholeiite at 1–5 kbar: optimal condition for the crystallization of high-An plagioclase in hydrous arc tholeiite
Experimental study of a low-alkali tholeiite at 1–5 kbar: optimal condition for the crystallization of high-An plagioclase in hydrous arc tholeiite
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DOI：
10.1007/s00410-005-0666-7
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发表时间：
2005-05
影响因子：
3.5
通讯作者：
D. Takagi;Hiroaki Sato;M. Nakagawa
中科院分区：
文献类型：
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作者：
D. Takagi;Hiroaki Sato;M. Nakagawa
通过对低碱拉斑玄武岩(SiO_2~52wt%，MgO~6.5wt%，CaO/Na2O~4.4，Al_2O_3/SiO_2~0.33)在H2O不饱和和H2O饱和条件下的熔融实验，研究了H2O对斜长石与熔体之间Ca-Na分配的影响。实验分别在1000-1300℃和1-5kbar的温度和压力范围内进行，水含量为0-12wt%。氧化还原条件为NNO(镍-氧化镍)缓冲液以上0-2个对数单位。绘制了低碱拉斑玄武岩在Kbar为1、2、5℃时的温度-体积H2O图，并测定了近液相线斜长石和共存熔体的成分。为了排除熔体组成(CaO/Na2O和Al_2O_3/SiO_2比值)对斜长石成分的影响，揭示H_2O对AN(=100×Ca/(Ca+Na))含量和(=(Ca/Na)pl/(Ca/Na)熔体)的影响，我们重点研究了从CaO/Na_2O和Al_2O_3/SiO_2比值基本不变的熔体中析出的近液相线斜长石的组成。我们的实验结果表明，在每个实验压力下，随着熔体中H2O含量的增加，近液相线斜长石和KDCA-NaA的含量几乎呈线性增加。低碱拉斑玄武岩体系(CaO/Na2O~4.0-4.5，Al_2O_3/SiO_2~0.27-0.33)中AN含量及其变化均可用一个以温度、压力和熔体H2O含量为参数的方程来描述。随着熔体H2O的增加和压力的降低，液相线斜长石的含量和含量增加，说明2-3kbar的近H2O饱和条件对最富An的斜长石(>；An88)的结晶是最佳的。我们认为，这种2-3kbar的压力条件对富水低碱拉斑玄武岩中富An斜长石的成因起着重要的作用。在超过4kbar的压力下，富钙单斜辉石的结晶降低了液体的CaO/Na2O比，从而阻止了含水拉斑玄武岩中高AN斜长石的结晶。
We conducted melting experiments on a low-alkali tholeiite (SiO2~52 wt%, MgO ~6.5 wt%, CaO/Na2O~4.4, Al2O3/SiO2~0.33) under both H2O-undersaturated and H2O-saturated conditions to investigate the effect of H2O on the Ca–Na partitioning between plagioclase and melt. Experiments were performed in the temperature and pressure ranges of 1,000–1,300°C and 1–5 kbar, respectively, with varying H2O contents of 0–12wt%. Redox condition was 0–2 log unit above NNO (nickel–nickel oxide) buffer. Temperature-bulk H2O diagrams for the low-alkali tholeiite are constructed at 1, 2, and 5 kbar, and compositions of near-liquidus plagioclase and coexisting melt are determined. To exclude the effect of melt composition (CaO/Na2O and Al2O3/SiO2ratios) on plagioclase composition and to reveal the effect of H2O on An (=100×Ca/(Ca+Na)) content and(=(Ca/Na)pl/(Ca/Na)melt), we focused on the composition of near-liquidus plagioclases which crystallized from melts with nearly constant CaO/Na2O and Al2O3/SiO2ratios. Our experimental results show that, at each experimental pressure, An content of the near-liquidus plagioclase and the KDCa-Naalmost linearly increases as H2O content in melt increases. Each of the An content and thevariations in a low-alkali tholeiitic system (CaO/Na2O~4.0–4.5, Al2O3/SiO2~0.27–0.33) can be described by one equation using temperature, pressure, and melt H2O content as parameters. An content andof liquidus plagioclase increases with increasing melt H2O and with decreasing pressure, elucidating that nearly H2O-saturated conditions of 2–3 kbar is optimal for the crystallization of the most An-rich plagioclase (>An88). We suggest this pressure condition of 2–3 kbar, corresponding to depth of 7–11 km, plays an important role for the origin of An-rich plagioclase in H2O-rich low-alkali tholeiite. At pressures more than ca. 4 kbar, crystallization of liquidus Ca-rich clinopyroxene decreases the CaO/Na2O ratio of liquid, thus prohibiting the crystallization of high-An plagioclase from hydrous tholeiite.