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首页> 外文期刊>Contributions to Mineralogy and Petrology >Low-Ca boninite formation by second-stage melting of spinel harzburgite residues at mature subduction zones: new evidence from veined mantle xenoliths from the West Bismarck Arc
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Low-Ca boninite formation by second-stage melting of spinel harzburgite residues at mature subduction zones: new evidence from veined mantle xenoliths from the West Bismarck Arc

机译：低Ca Boninite在成熟的俯冲区中二级熔化的二阶熔化区：来自西俾斯麦弧的脉络Xenoliths的新证据

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摘要

We report the abundances of major and lithophile trace elements and volatiles (H2O, Cl and S) in orthopyroxenite veins cutting mantle-derived, spinel harzburgite xenoliths from the active Ritter volcano in the West Bismarck Arc (Papua New Guinea). The veins preserve sulfide-bearing glass coexisting with crystals. The glass formed by the quench of residual liquids left after crystallisation of abundant orthopyroxene, variable amounts of clinopyroxene, and minor olivine and spinel from silicate melts intruding spinel harzburgite. The glass compositions range from low-Ti (0.2wt% TiO2), intermediate to magnesian (2-5wt% MgO) and high-Mg# (0.45-0.6) andesite to dacite, with Mg#=Mg/(Mg+Fe-t) where Fe-t indicates all Fe treated as Fe2+. The glass is depleted in moderately incompatible lithophile trace elements and contains 1.2 to 2.5wt% H2O, 400-1800ppm Cl and 100-400ppm S. Thermo-barometric calculations indicate that the original vein-forming melts intruded the shallow sub-arc mantle lithosphere between 0.5 and 1 GPa, where they partially crystallised (50-60%) to form residual liquids during cooling from similar to 1200 degrees C down to similar to 650 degrees C. Our data and calculations show that West Bimarck orthopyroxenite veins are of high-temperature magmatic origin and bear no relation to the carrier magmas of the xenoliths. Instead, petrological modelling demonstrates that the veins formed from the intrusion of primitive low-Ca boninite (LCB) melts in the sub-arc mantle lithosphere. These melts were originally produced by low degrees of melting (5%) of spinel harzburgite at 1360 degrees C and = 1.5 GPa. Some key features of the parental LCB vein-forming melts from West Bismarck (e.g. lithophile trace element signatures and moderate volatile abundances and oxygen fugacity) suggest that, contrary to the widely held belief, their formation does not require persistent mantle wedge fluxing by slab-derived components. Instead, we propose that adiabatic decompression melting of spinel harzburgite residues may also form LCB in the sub-arc mantle. West Bismarck veins are very similar to those previously described in mantle xenoliths from the Kamchatka Arc; active boninite magmatism in these mature arcs shows that the generation of these melts is not as restricted as previously thought. In fact, the growing evidence for the prevalence of boninite in sub-arc mantle peridotites is consistent with the presence of boninite-like geochemical components in many island arcs.
机译：我们报告了在西俾斯麦弧（巴布亚新几内亚Papua New Guinea）的活性RITTER火山中切割甲状腺静脉静脉静脉静脉静脉静脉静脉静脉静脉静脉（H2O，Cl和S）的大量和鳞片痕量元素和挥发物（H2O，Cl和S）。静脉保护含硫化物玻璃与晶体共存。通过淬火末端脱落后的残留液体淬火形成的玻璃，从硅酸盐熔化的硅酸盐熔化的尖晶石麦芽酸盐的脱尾酸酯，可变量的冠状藻酸盐和小橄榄石和尖晶石后留下。玻璃组合物的范围是低Ti（0.2wt％TiO 2），中间体至氧化镁（2-5wt％MgO）和高Mg＃（0.45-0.6）和达氏菌，用Mg＃= Mg /（Mg + Fe- t）Fe-T表示所有Fe被视为Fe2 +。该玻璃在适度不相容的型鳞状物质中耗尽，含有1.2至2.5wt％H 2 O，400-1800ppm Cl和100-400ppm S.热压计算表明，原始静脉形成熔体侵入浅层岩石岩石圈之间0.5和1GPa，它们部分地结晶（50-60％），在冷却中形成残留液体，从类似于1200摄氏度，与650摄氏度相似。我们的数据和计算表明，西部Bimarck畸形静脉是高温的岩石原点和与Xenoliths的载体岩浆无关。相反，岩浆物学建模表明，由原始低Ca Boninite（LCB）的侵入中形成的静脉在亚弧地幔岩石圈中熔化。这些熔体最初由1360℃下的尖晶石哈尔茨伯根的低熔融（5％）产生，并且含量为1.5gPa。来自西俾斯麦的父母LCB静脉形成熔体的一些关键特征（例如绞线痕量元素签名和中等挥发性丰富和氧气不足）表明，与广泛的信仰相反，它们的形成不需要通过平板持久的地幔楔子通量派生组件。相反，我们提出了尖晶石麦芽矿石残留物的绝热减压熔化也可以在亚弧地幔中形成LCB。西俾斯麦静脉与先前在堪察加弧的岩石Xenoliths中描述的静脉非常相似;这些成熟弧中的活跃的Boninite Magmatism表明，这些熔体的产生并不像以前认为那样限制。事实上，亚弧形扁平岩石中Boninite患病率越来越多的证据与许多岛弧中的Boninite的地球化学成分的存在一致。

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