Trace-element composition of hydrothermal zircon and the alteration of Hadean zircon from the Jack Hills, Australia
Trace-element composition of hydrothermal zircon and the alteration of Hadean zircon from the Jack Hills, Australia
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DOI：
10.1016/j.gca.2004.07.006
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发表时间：
2005-02-01
影响因子：
5
通讯作者：
Hoskin, PWO
中科院分区：
文献类型：
--
作者：
Hoskin, PWO
热液锆石可用于测定流体渗透事件和水/岩石相互作用。在澳大利亚东部的Boggy Plain带状岩体（BPZP）中，热液锆石与热液白钨矿共生。在早期蚀变细晶岩和二长花岗岩中发现了辉绿石、钍石和金红石。热液锆石与岩浆锆石在同一岩石中的结构不同。在岩浆核上以20-50 μ m厚的暗褐色半透明地幔形式出现，较少以单个晶体形式出现。在背散射电子和阴极发光图像中，热液地幔内部无纹理，而岩浆锆石则呈振荡环带。热液锆石的年龄与岩浆锆石的年龄难以区分，表明在结晶的最后阶段，从岩浆锆石演化而来的流体的沉淀。尽管无法区分的U-Pb同位素组成，微量元素组成的热液和岩浆锆石是不同的。相对于同一岩石中的岩浆锆石，热液锆石的所有测得的微量元素都是富集的。包括V、Ti、Nb、Hf、Sc、Mn、U、Y、Th和稀土元素（REE）。球粒陨石标准化REE丰度形成两个不同的模式组：类型1（岩浆）模式从La到Lu急剧增加，并有Cc和Eu异常，这是陆壳岩石类型中未蚀变岩浆锆石的典型模式; 2型（热液）模式一般具有较高的稀土元素丰度，较平坦的锂-稀土配分模式[（Sm/La）（N）= 1.5-4.4，岩浆锆石为22-110]和较小的Ce异常（Ce/Ce* = 1.8-3.5，岩浆锆石为32-49）。2型模式也被描述为水热蚀变锆石从加贝尔Hamradom花岗岩，埃及，和花岗岩脉从Pasta片麻岩复杂。加拿大。杰克山的冥古宙（类似于4.5 -4.0 Ga）锆石。西澳大利亚。具有可变的标准化REE模式。特别是，最古老的一块地球锆石晶体W74/2-36（在4.4 Ga）-包含类型-1和类型-2的模式在50微米的规模，一个现象尚未报告的未蚀变的岩浆锆石。在BPZP和文献中约束样品中记录的岩浆和热液锆石成分的背景下，晶体W74/2-36和其他Jack Hills Hadean（JHH）锆石中的2型模式被解释为热液蚀变岩浆成分。一个改变的场景。受到同位素和微量元素数据的限制。以及α衰变事件计算。部分JHH锆石可能在4.27 Ga时发生了阶段性的溶解/再沉淀，其中包括流体/锆石阳离子和氧同位素交换。JHH锆石中的2型成分被解释为与轻稀土、高δ（1 S）O（相当于6 -10%o或更高）流体的局部交换。因此，一个复杂的解释涉及“永久”液态水海洋。太古代早期的大规模水/岩相互作用和板块构造是不必要的，因为锆石的结构和成分可以简单地解释为部分变质锆石和少量短暂流体之间的交换。版权所有（C）2005 Elsevier Ltd.
Hydrothermal zircon can be used to date fluid-infiltration events and water/rock interaction. At the Boggy Plain zoned pluton (BPZP), eastern Australia, hydrothermal zircon occurs with hydrothermal scheelite. molybdenite, thorite and rutile in incipiently altered aplite and monzogranite. The hydrothermal zircon is texturally distinct from magmatic zircon in the same rocks. occurring as murky-brown translucent 20-50 mum-thick mantles on magmatic cores and less commonly as individual crystals. The hydrothermal mantles are internally textureless in back-scatter electron and cathodoluminescence images whereas magmatic zircon is oscillatory zoned. The age of the hydrothermal zircon is indistinguishable from magmatic zircon, indicating precipitation from a fluid evolved from the ma.arna during the final stages of cnstallizaLion. Despite indistinguishable U-Pb isotopic compositions, the trace-element compositions of the hydrothermal and magmatic zircon are distinct. Hydrothermal zircon is enriched in all measured trace-elements relative to magmatic zircon in the same rock. including V, Ti, Nb, Hf, Sc, Mn, U, Y, Th and the rare-earth elements (REE). Chondrite-normalized REE abundances form two distinct pattern groupings: type-1 (magmatic) patterns increase steeply from La to Lu and have Cc and Eu anomalies-these are patterns typical for unaltered magmatic zircon in continental crust rock types; type-2 (hydrothermal) patterns generally have higher abundances of the REE, flatter liqhi-REE patterns [(Sm/La)(N) = 1.5-4.4 vs. 22-110 for magmatic zircon] and smaller Ce anomalies (Ce/Ce* = 1.8-3.5 vs. 32-49 for magmatic zircon). Type-2 patterns have also been described for hydrothermally-altered zircon from the Gabel Hamradom granite, Egypt, and a granitic dyke from the Acasta Gneiss Complex. Canada.Hadean (similar to4.5-4.0 Ga) zircon from the Jack Hills. Western Australia. have variable normalized REE patterns. In particular, the oldest piece of Earth-zircon crystal W74/2-36 (dated at 4.4 Ga)-contains both type-1 and type-2 patterns on a 50 mum scale, a phenomenon not yet reported for unaltered magmatic zircon. In the context of documented magmatic and hydrothermal zircon compositions from constrained samples from the BPZP and the literature, the type-2 patterns in crystal W74/2-36 and other Jack Hills Hadean (JHH) zircon are interpreted as hydrothermally-altered magmatic compositions. An alteration Scenario. constrained by isotope and trace-element data. as well as a-decay event calculations. involving fluid/zircon cation and oxygen isotope exchange within partially and minor dissolution/reprecipitation, may have occurred episodically for some JHH zircon and at similar to4.27 Ga for zircon W74/2-36. Type-2 compositions in JHH zircon are interpreted to represent localized exchange with a light-REE-bearing, high delta(1S)O (similar to6-10%o or higher) fluid. Thus, a complex explanation involving "permanent" liquid water oceans. large-scale water/rock interaction and plate tectonics in the very early Archean is not necessary as the zircon textures and compositions are simply explained by exchange between partially metamict zircon and a low volume ephemeral fluid. Copyright (C) 2005 Elsevier Ltd.