An experimental study of the effects of melt composition on plagioclase-melt equilibria at 5 and 10 kbar: implications for the origin of magmatic high-An plagioclase

An experimental investigation of plagioclase crystallization in broadly basaltic/andesitic melts of variable Ca# (Ca/(Ca+Na)*100) and Al# (Al/(Al+Si)*100) values and H₂O contents has been carried out at high pressures (5 and 10 kbar) in a solid media piston-cylinder apparatus. The H₂O contents of glasses coexisting with liquidus or near-liquidus plagioclases in each experiment were determined via an FTIR spectroscopic technique. This study has shown that melt Ca# and Al#, H₂O content and crystallization pressure all control the composition of liquidus plagioclase. Increasing melt Ca# and Al# increase An content of plagioclase, whereas the effect of increasing pressure is the opposite. However, the importance of the role played by each of these factors during crystallization of natural magmas varies. Melt Ca# has the strongest control on plagioclase An content, but melt Al# also exerts a significant control. H₂O content can notably increase the An content of plagioclase, up to 10 mol % for H₂O-undersaturated melts, and 20 mol % for H₂O-saturated melts. Exceptionally calcic plagioclases (up to An₁₀₀) in some primitive subduction-related boninitic and related rocks cannot be attributed to the presence of the demonstrated amounts of H₂O (up to 3 wt %). Rather, they must be due to the involvement of extremely refractory (CaO/Na₂O>18) magmas in the petrogenesis of these rocks. Despite the refractory nature of some primitive MORB glasses, none are in equilibrium with the most calcic plagioclase (An₉₄) found in MORB. These plagioclases were likely produced from more refractory melts with CaO/Na₂O = 12–15, or from melts with exceptionally high Al₂O₃(>18%). Magmas of appropriate compositions to crystallize these most calcic plagioclases are sometimes found as melt inclusions in near liquidus phenocrysts from these rocks, but are not known among wholerock or glass compositions. The fact that such melts are not erupted as discrete magma batches indicates that they are effectively mixed and homogenized with volumetrically dominant, less refractory magmas. The high H₂O contents (∼ 6 wt%) in some high-Al basaltic arc magmas may be responsible for the existence of plagioclases up to An₉₅ in arc lavas. However, an alternative possibility is that petrogenesis involving melts with abnormally high CaO/Na₂O values (> 8) may account for the presence of highly anorthitic plagioclases in these rocks. Received: 31 August 1993 / Accepted: 20 May 1994     

西北冰洋冰筏碎屑指示的MIS 5以来冰盖和洋流的演化历史

通过西北冰洋楚科奇边缘地区4个柱状沉积物组成的岩芯断面,分析MIS 5以来粗颗粒（>250 μm）冰筏碎屑（IRD）的岩矿组成,确定西北冰洋不同时期IRD的来源和搬运模式,进而重建MIS 5以来欧亚冰盖和北美冰盖的消长以及北冰洋海盆中表层洋流的演化历史。研究结果显示MIS 5.5和MIS 5.1期间,波弗特环流的影响范围大于MIS 1期,而在MIS 5.4~5.2期间,波弗特环流的影响范围小于现代;MIS 4期间,岩芯中发现了大量来源于欧亚冰盖或东西伯利亚海冰盖的冰川浊积物,冰盖可能扩张到了楚科奇边缘地区南部;在同一时期,在楚科奇边缘地区南部的岩芯中IRD几乎全部为铁锰结核,可能受到了欧亚冰盖或东西伯利亚海冰盖的影响,但铁锰结核与欧亚冰盖的关系仍需进一步研究;MIS 3期间,波弗特环流的影响范围和强度略弱于现代;在末次冰盛期,研究区受到以劳伦冰盖为主要来源的冰川浊积物输入;在末次冰消期,研究区发生了一次来源于北美劳伦冰盖的冰融水排泄事件;在MIS 1期,西北冰洋主要受到波弗特环流的控制,将来源于加拿大北极富含碎屑碳酸岩的IRD带入北冰洋海盆。