Metamorphic rocks seek meaningful cooling rate: Interpreting 40Ar/39Ar ages in an exhumed ultra-high pressure terrane
Metamorphic rocks seek meaningful cooling rate: Interpreting 40Ar/39Ar ages in an exhumed ultra-high pressure terrane
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变质岩寻求有意义的冷却速率：解释挖掘出的超高压地体中的 40Ar/39Ar 年龄
DOI：
10.1016/j.lithos.2012.08.011
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发表时间：
2012
期刊：
影响因子：
3.5
通讯作者：
Warren C
中科院分区：
文献类型：
--
作者：
Warren C
变质白云母的40Ar/39Ar定年通常用于说明变质地体冷却和/或挖掘的时间。理论上，只要定年矿物的初始结晶温度高于其闭合温度，得到的40ar /39Ar年龄在样品内部或样品之间不应该有显著变化，而应该仅因晶粒尺寸而变化（根据估计的冷却速率而变化~3Ma）。挪威西部片麻岩杂岩（WGC）北峡湾地区基性榴辉岩体及其寄主片麻岩的红外激光单粒融合和紫外激光斑点白云母和/或云母数据表明，同一基性榴辉岩样中白色云母内部和之间的年龄差异高达284Ma。同一泥质片麻岩样品的年龄差异可达28Ma。总的来说，40ar /39Ar云母岩的年龄范围在~750 ~ 440Ma之间，比之前报道的限制北峡湾WGC峰变质作用的405 ~ 400ma U-Pb锆石年龄要早得多。长英质主片麻岩样品在峰后、与挖掘相关的角闪岩相叠印过程中重结晶，产生单粒融合和激光斑点40ar /39Ar白云母，时间跨度约420至385Ma，涵盖了之前对整个变质旋回时间的估计。这些数据表明，在所有样品中，Ar在cm -亚mm尺度上均呈非均匀分布，表明在变质旋回过程中Ar输运受限，去除效率低下。开放体系（零晶界Ar浓度）行为是通常应用的闭合温度公式中的固有假设，但除非可以证明这一点，否则将40ar /39Ar年龄与温度联系起来不再是微不足道的。紫外激光烧蚀提供的高空间和时间精度意味着颗粒内Ar浓度分布可用于评估开放与封闭系统的行为。这里提供的数据表明，在变质系统中，即使在理论上压力、温度和时间条件允许在变质循环中有效扩散的系统，也不能假设开放系统的行为。高空间分辨率40ar /39Ar数据以及与开放系统行为数值模型的可靠比较为揭示变质发掘时间尺度的复杂性提供了最大的未来潜力。
40Ar/39Ar dating of metamorphic white mica is commonly used to elucidate the timing of cooling and/or exhumation of metamorphic terranes. In theory, so long as the dated mineral originally crystallised above its closure temperature, the yielded40Ar/39Ar ages should not vary significantly within or between samples, and should vary only due to grain size (~3Ma variation depending on the estimated cooling rate). Infra-red laser single grain fusion and ultra-violet laser spot muscovite and/or phengite data from mafic eclogite bodies and their host gneisses in the Nordfjord region of the Western Gneiss Complex (WGC), Norway, show that the ages yielded both within and between white micas from the same mafic eclogite sample vary by up to 284Ma. Ages yielded within the same pelitic gneiss sample vary by up to 28Ma. Overall the40Ar/39Ar phengite dates from mafic eclogites range from ~750 to 440Ma and are significantly older than the previously reported 405–400Ma U–Pb zircon age constraining peak metamorphism in the Nordfjord WGC. The felsic host gneiss samples, which recrystalised during a post-peak, exhumation-related, amphibolite-facies overprint, yield single grain fusion and laser spot40Ar/39Ar white mica dates spanning ~420 to 385Ma, encompassing previous estimates of the timing of the entire metamorphic cycle. These data suggest that Ar is heterogeneously distributed both within grains and between grains on the cm to sub-mm scale in all samples, and imply restricted Ar transport and inefficient removal during the metamorphic cycle. Open system (zero grain boundary Ar concentration) behaviour is an inherent assumption in the commonly applied closure temperature formulation, but unless this can be demonstrated, linking40Ar/39Ar ages to temperature is no longer trivial. The high spatial and chronological precision afforded by UV laser ablation mean that intra-grain Ar concentration distributions may be used to assess open vs. closed system behaviour. The data presented here suggest that open system behaviour cannot be assumed in metamorphic systems, even those in which the pressure, temperature and time conditions in theory allow for efficient diffusion during the metamorphic cycle. High spatial resolution40Ar/39Ar data and robust comparison with numerical models of open system behaviour provide the greatest future potential for unravelling the complexities of metamorphic exhumation timescales.
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