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Experimental study of a low-alkali tholeiite at 1–5 kbar: optimal condition for the crystallization of high-An plagioclase in hydrous arc tholeiite.
Авторы: Takagi D., Sato H., Nakagawa M.
2005 г.
Contrib.Miner.Petrol.
We conducted melting experiments on a lowalkali 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 1,000–1,300oC 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/ SiO2 ratios) on plagioclase composition and to reveal the effect of H2O on An (=100·Ca/(Ca+Na)) content and KCaNa D (=(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/SiO2 ratios. Our experimental results show that, at each experimental pressure, An content of the nearliquidus plagioclase and the KD Ca-Na almost linearly increases as H2O content in melt increases. Each of the An content and the KCaNa D variations in a low-alkali tholeiitic system (CaO/Na2O4.0–4.5, Al2O3/SiO2 0.27–0.33) can be described by one equation using temperature, pressure, and melt H2O content as parameters. An content and KCaNa D of 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 lowalkali 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.
Файл: 701.pdf
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