The clumped-isotope geochemistry of exhumed marbles from Naxos, Greece
The clumped-isotope geochemistry of exhumed marbles from Naxos, Greece
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DOI：
10.1016/j.epsl.2017.04.026
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发表时间：
2017-07
影响因子：
5.3
通讯作者：
U. Ryb;M. Lloyd;D. Stolper;J. Eiler
中科院分区：
文献类型：
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作者：
U. Ryb;M. Lloyd;D. Stolper;J. Eiler
折返作用和伴随的退变质作用通过变形、矿物-矿物反应、水-岩石反应以及扩散控制的矿物内和矿物间原子活动性改变了变质岩的成分和结构。在这里，我们证明了这些过程被记录在大理岩的块状和单同位素(δ13C和δ18O)成分中，可以用来约束退变历史。在希腊纳克索斯变质核杂岩边缘至中心的断面上采集了27个方解石和白云石大理石，分析了它们的碳酸盐单同位素和块状同位素组成。全岩样品的Δ-47值大部分与变质杂岩的折返控制冷却一致。然而，这些数据也揭示了水-岩相互作用、变形驱动的重结晶和与热液蚀变相关的热震可能对Δ-47值的总体分布产生重大影响。我们分析了受变形和流体-岩石反应影响的特定碳酸盐组构，以研究这些过程如何登记在碳酸盐块状同位素系统中。从方解石大理岩中钻取的磁区的Δ-47值呈双峰分布。低Δ47值对应的表观温度为260C，在静态织物中很常见；高Δ47值对应的表观温度为200°C，在动态再结晶织物中很常见。我们认为，低的Δ47值反映了冷却过程中扩散控制的同位素重新排序，而高的Δ47值反映了动态再结晶驱动的同位素重新排序。通过控制加热实验，进一步研究了动态再结晶改变Δ-47值的机理。结果表明，静态和动态组构中的实验室反应速率没有显著差异，这与矿物-外在机制一致，即沿晶面滑移与方解石晶格中原子尺度的同位素重新排序有关。这些实验表明了一种内在机制(变形矿物中增强的同位素重排速度)。我们认为，在方解石扩散控制阻挡温度以下形成的动态再结晶组构的Δ_(47)值限制了变形温度。我们发现，纳索斯大理岩静态组构的Δ-47温度比在缓慢冷却的变质岩中通常观察到的温度高∼60-80℃，并建议冷却速度为∼105℃-−1。来自核心附近的白云岩大理岩也有类似的热历史，其视温度保持在比典型阻塞温度(∼300℃)高出200℃的水平。这一发现可以用一次驱动短暂热脉冲的热液事件来解释，并在当地重新设置Δ47值。核杂岩区域的快速冷却与已发表的冷却年龄汇编和新的磷灰石U-Th/He年龄相一致，将热事件与∼12 Ma的花岗闪长岩体侵位联系在一起。
Exhumation and accompanying retrograde metamorphism alter the compositions and textures of metamorphic rocks through deformation, mineral–mineral reactions, water–rock reactions, and diffusion-controlled intra-and inter-mineral atomic mobility. Here, we demonstrate that these processes are recorded in the clumped-and single-isotope (δ 13 C and δ 18 O) compositions of marbles, which can be used to constrain retrograde metamorphic histories. We collected 27 calcite and dolomite marbles along a transect from the rim to the center of the metamorphic core-complex of Naxos (Greece), and analyzed their carbonate single-and clumped-isotope compositions. The majority of Δ 47 values of whole-rock samples are consistent with exhumation-controlled cooling of the metamorphic complex. However, the data also reveal that water–rock interaction, deformation driven recrystallization and thermal shock associated with hydrothermal alteration may considerably impact the overall distribution of Δ 47 values. We analyzed specific carbonate fabrics influenced by deformation and fluid–rock reaction to study how these processes register in the carbonate clumped-isotope system. Δ 47 values of domains drilled from a calcite marble show a bimodal distribution. Low Δ 47 values correspond to an apparent temperature of 260° C and are common in static fabrics; high Δ 47 values correspond to an apparent temperature of 200° C and are common in dynamically recrystallized fabrics. We suggest that the low Δ 47 values reflect diffusion-controlled isotopic reordering during cooling, whereas high Δ 47 values reflect isotopic reordering driven by dynamic recrystallization. We further studied the mechanism by which dynamic recrystallization may alter Δ 47 values by controlled heating experiments. Results show no significant difference between laboratory reactions rates in the static and dynamic fabrics, consistent with a mineral-extrinsic mechanism, in which slip along crystal planes was associated with atomic-scale isotopic reordering in the calcite lattice. An intrinsic mechanism (enhanced isotopic reordering rate in deformed minerals) is contraindicated by these experiments. We suggest that Δ 47 values of dynamically recrystallized fabrics that form below the diffusion-controlled blocking-temperature for calcite constrain the temperature of deformation. We find that Δ 47-based temperatures of static fabrics from Naxos marbles are∼ 60–80° C higher than commonly observed in slowly cooled metamorphic rocks, and would suggest cooling rates of∼ 10 5° C Myr− 1. A similar thermal history is inferred for dolomite marbles from the core vicinity, which preserve apparent temperatures up to 200° C higher than a typical blocking temperature (∼ 300° C). This finding could be explained by a hydrothermal event driving a brief thermal pulse and locally resetting Δ 47 values. Rapid cooling of the core-complex region is consistent with a compilation of published cooling ages and a new apatite U–Th/He age, associating the thermal event with the emplacement of a granodiorite pluton at∼ 12 Ma.