Oxygen and U-Th isotopes and the timescales of hydrothermal exchange and melting in granitoid wall rocks at Mount Mazama, Crater Lake, Oregon
Oxygen and U-Th isotopes and the timescales of hydrothermal exchange and melting in granitoid wall rocks at Mount Mazama, Crater Lake, Oregon
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俄勒冈州火山口湖马扎马山花岗岩围岩中的氧和 U-Th 同位素以及热液交换和熔化的时间尺度
DOI：
10.1016/j.gca.2017.04.043
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发表时间：
2017
影响因子：
5
通讯作者：
Johnson, Clark M.
中科院分区：
文献类型：
--
作者：
Ankney, Meagan E.;Bacon, Charles R.;Valley, John W.;Beard, Brian L.;Johnson, Clark M.
本文报道了7.7ka火山口形成过程中喷出的部分熔融（0- 50vol%熔融）、低δ 18 O的更新世花岗岩类块体的新的全岩U-Th和原位氧同位素组成。马扎马（火山口湖，俄勒冈州）。块被解释为代表的气候岩浆房，在喷发之前，经历了变量的交换与流星热液流体和随后的部分熔融的围岩。U-Th和氧同位素的结果使我们能够检查热液循环和部分熔融的时间尺度，并提供了一个“外”的角度上的建设，以火山爆发的高潮。马扎马在单个石英（n= 126）和斜长石（n= 91）晶体的核心和边缘以及横跨10个石英晶体的横断面中测量的氧同位素组成，证明了石英的环带（Δ 18 O Core-Rim ≤ 0.1-5.5‰），但斜长石显示出均匀性（Δ 18 O Core-Rim ≤ ±0.8‰）。我们建议，石英中的氧同位素环带记录热液交换，然后由高温交换，以响应部分熔融所造成的玄武岩安山质补给岩浆注入到更深的部分室。石英中氧扩散的模拟结果表明，石英中的热液交换发生在1000- 63000年的时间内。模型还表明，花岗岩类的熔融开始至少发生在Mazama火山爆发前10-200年，这一推断与其他人先前报道的磁铁矿均一化和熔体中Zr扩散的结果基本一致。并与以前在Mt.马扎马花岗岩类中的铀过剩可能是由于热液循环富集，因为它们的δ 18 O值较低。铀过量最高（≥5.8%）的样品也具有最大的18 O同位素亏损（平均δ 18 Oplag = −4.0‰）。花岗岩类可能是火山岩中铀过剩的同化物和来源。马扎马两个花岗岩类具有Th过量和低δ 18 O值，解释为记录在热液蚀变过程中的U淋溶。基于花岗岩类和火山岩的U过剩阵列的U-Th等时线表明，热液循环开始前40-75 kyrs的高潮喷发，可能标志着一个持久的上地壳岩浆房的开始。U-Th年龄与根据石英氧同位素分带推断的热液蚀变最大时间尺度一致。
We report new whole rock U-Th andin-situoxygen isotope compositions for partially melted (0–50 vol% melt), low-δ18O Pleistocene granitoid blocks ejected during the ∼7.7 ka caldera-forming eruption of Mt. Mazama (Crater Lake, Oregon). The blocks are interpreted to represent wall rocks of the climactic magma chamber that, prior to eruption, experienced variable amounts of exchange with meteoric hydrothermal fluids and subsequent partial melting. U-Th and oxygen isotope results allow us to examine the timescales of hydrothermal circulation and partial melting, and provide an “outside in” perspective on the buildup to the climactic eruption of Mt. Mazama. Oxygen isotope compositions measured in the cores and rims of individual quartz (n= 126) and plagioclase (n= 91) crystals, and for transects across ten quartz crystals, document zonation in quartz (Δ18OCore-Rim≤ 0.1–5.5‰), but show homogeneity in plagioclase (Δ18OCore-Rim≤ ±0.8‰). We propose that oxygen isotope zonation in quartz records hydrothermal exchange followed by high-temperature exchange in response to partial melting caused by injection of basaltic to andesitic recharge magma into the deeper portions of the chamber. Results of modeling of oxygen diffusion in quartz indicates that hydrothermal exchange in quartz occurred over a period of ∼1000–63,000 years. Models also suggest that the onset of melting of the granitoids occurred a minimum of ∼10–200 years prior to the Mazama climactic eruption, an inference which is broadly consistent with results for magnetite homogenization and for Zr diffusion in melt previously reported by others.Uranium-thorium isotope compositions of most granitoid blocks are in238U excess, and are in agreement with a238U enriched array previously measured for volcanic rocks at Mt. Mazama. Uranium excess in the granitoids is likely due to enrichment via hydrothermal circulation, given their low δ18O values. The sample with the highest U excess (≥5.8%) also has the most18O isotope depletion (average δ18Oplag= −4.0‰). The granitoids are a probable assimilant and source of U excess in volcanic rocks from Mt. Mazama. Two granitoids have Th excess and low δ18O values, interpreted to record leaching of U during hydrothermal alteration. A U-Th isochron based on the U excess array of the granitoids and volcanic rocks indicates that hydrothermal circulation initiated ∼40–75 kyrs before the climactic eruption, potentially marking the initiation of a persistent upper-crustal magma chamber. The U-Th ages are consistent with the maximum timescales inferred for hydrothermal alteration based on oxygen isotope zoning in quartz.
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