The fall and rise of metamorphic zircon
The fall and rise of metamorphic zircon
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DOI：
10.2138/am-2015-5064
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发表时间：
2015-04-01
影响因子：
3.1
通讯作者：
Markley, Christopher
中科院分区：
文献类型：
--
作者：
Kohn, Matthew J.;Corrie, Stacey L.;Markley, Christopher
锆石地质年代学和地球化学对于理解变质过程越来越重要，特别是在极端条件下，但对锆石溶解和再生长的驱动因素知之甚少。在这里，我们模拟锆质量平衡，以确定P-T地区的锆石应该溶解或生长。根据已发表的数据和新的测量结果对主要变质矿物的锆含量进行了评估，并沿沿着有代表性的P-T轨迹对含水MORB和变质岩成分的矿物学发展和锆石丰度建立了模型。不包括锆石，矿物金红石，石榴石，和角闪石强烈影响锆质量平衡的变基性岩，占全岩锆预算的40%。单斜辉石和石榴石比斜长石含有更多的Zr，因此斜长石在角闪岩到榴辉岩相转变处的破裂，应该会导致锆石轻微溶解，而不是生长。超高压锆石的生长被预测在一个有限的区域，大多数锆石生长在低得多的压力。在变岩中，锆石在固态组合中的模态丰度预计只会发生微小的变化。然而，部分熔融驱动大量锆石溶解，而熔融结晶再生长锆石。从质量平衡的角度来看，锆石生长不能归因于一个先验的斜长角闪岩榴辉岩过渡，超高压变质，或部分熔融。相反，锆石应该主要在后期折返和冷却过程中生长，特别是在从金红石到钛铁矿的氧化物转变和熔融结晶过程中。正如预测的那样，大多数锆石从高压/超高压榴辉岩的西部片麻岩区和巴布亚新几内亚基本上postdate榴辉岩的形成和最大压力。
Zircon geochronology and geochemistry are increasingly important for understanding metamorphic processes, particularly at extreme conditions, but drivers of zircon dissolution and regrowth are poorly understood. Here, we model Zr mass balance to identify P-T regions where zircon should dissolve or grow. Zirconium contents of major metamorphic minerals were assessed from published data and new measurements, and models were constructed of mineralogical development and zircon abundance for hydrous MORB and metapelitic compositions along representative P-T paths. Excluding zircon, the minerals rutile, garnet, and hornblende strongly influence Zr mass balance in metabasites, accounting for as much as 40% of the whole-rock Zr budget. Clinopyroxene and garnet contain more Zr than plagioclase, so breakdown of plagioclase at the amphibolite to eclogite facies transition, should cause zircon to dissolve slightly, rather than grow. Growth of UHP zircon is predicted over a restricted region, and most zircon grows subsequently at much lower pressure. In metapelites, zircon is predicted to undergo only minor changes to modal abundance in solid state assemblages. Partial melting, however, drives massive zircon dissolution, whereas melt crystallization regrows zircon. From a mass-balance perspective, zircon growth cannot be attributed a priori to the prograde amphibolite-eclogite transition, to UHP metamorphism, or to partial melting. Instead, zircon should grow mainly during late-stage exhumation and cooling, particularly during oxide transitions from rutile to ilmenite and melt crystallization. As predicted, most zircons from HP/UHP eclogites of the Western Gneiss Region and Papua New Guinea substantially postdate eclogite formation and maximum pressures.