Intragrain oxygen isotope zoning in titanite by SIMS: Cooling rates and fluid infiltration along the Carthage‐Colton Mylonite Zone, Adirondack Mountains, NY, USA
Intragrain oxygen isotope zoning in titanite by SIMS: Cooling rates and fluid infiltration along the Carthage‐Colton Mylonite Zone, Adirondack Mountains, NY, USA
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DOI：
10.1111/jmg.12059
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发表时间：
2014-01
影响因子：
3.4
通讯作者：
C. Bonamici;R. Kozdon;T. Ushikubo;J. Valley
中科院分区：
文献类型：
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作者：
C. Bonamici;R. Kozdon;T. Ushikubo;J. Valley
由于含氧矿物的普遍存在以及矿物18 O/16 O比值对温度和流体成分的依赖性，氧同位素是分区研究的一个有吸引力的目标。在这项研究中，微妙的晶粒内氧同位素分带在10 μm尺度的二次离子质谱解析。δ 18 O分带的模式取决于单个晶粒的显微结构背景，并反映了多个过程，包括扩散氧交换，部分再结晶，晶粒尺寸减小和晶粒生长。利用结构关系所提供的时间框架，这些过程可以与格伦维尔大地震期间的具体事件联系起来。从迦太基-科尔顿糜棱岩带（CCMZ）内的两个露头中采集了钛铁矿样品，CCMZ是一个长期存在的剪切带，最终在格伦维尔造山带的奥塔湾阶段（1090-1020 Ma）期间从阿迪朗达克低地下方对阿迪朗达克高地进行了折返。钛铁矿赋存于Diana变正长岩杂岩中，该杂岩保留了三个连续发育的组构：早期NW向原糜棱岩组构（S1）被近垂直的超糜棱岩剪切带（S2）横切，后者被NNW向糜棱岩组构（S3）局部重新定向。结构上的早期钛铁矿（前S2）显示出扩散主导的δ 18 O分带，记录了从奥塔旺峰（麻粒岩相条件）开始的冷却。快速晶界程序中的数值扩散模型与观测到的δ 18 O剖面在50 ± 20 °C Ma−1的冷却速率下拟合良好，这比先前从区域热年代学中推断的阿迪朗达克高地1-5 °C Ma−1的冷却速率要快得多。高的冷却速率与渥太华造山带在c.时重力塌陷期间沿CCMZ沿着的快速剪切和折返相一致。1050 Ma.结构较晚的钛铁矿（syn-S2）具有更高的总δ 18 O，并显示出从扩散主导向重结晶主导的δ 18 O分区的过渡，表明δ 18 O升高的流体沿着S2剪切带渗透，并在氧气阻断温度以下继续剪切（研究现场粒度约≤500 °C）。结构上最新的钛铁矿（后S3）具有生长主导的δ 18 O分带，与CCMZ的Harrisville段沿着剪切停止后的变斑晶颗粒生长一致。
Oxygen isotopes are an attractive target for zoning studies because of the ubiquity of oxygen‐bearing minerals and the dependence of mineral 18O/16O ratios on temperature and fluid composition. In this study, subtle intragrain oxygen isotope zoning in titanite is resolved at the 10‐μm scale by secondary ion mass spectrometry. The patterns of δ18O zoning differ depending on microstructural context of individual grains and reflect multiple processes, including diffusive oxygen exchange, partial recrystallization, grain‐size reduction, and grain growth. Using the chronological framework provided by structural relations, these processes can be related to specific events during the Grenville orogeny. Titanite was sampled from two outcrops within the Carthage‐Colton Mylonite Zone (CCMZ), a long‐lived shear zone that ultimately accommodated exhumation of the Adirondack Highlands from beneath the Adirondack Lowlands during the Ottawan phase (1090–1020 Ma) of the Grenville orogeny. Titanite is hosted in the Diana metasyenite complex, which preserves three sequentially developed fabrics: an early NW‐dipping protomylonitic fabric (S1) is crosscut by near‐vertical ultramylonitic shear zones (S2), which are locally reoriented by a NNW‐dipping mylonitic fabric (S3). Texturally early titanite (pre‐S2) shows diffusion‐dominated δ18O zoning that records cooling from peak Ottawan, granulite‐facies conditions. Numerical diffusion models in the program Fast Grain Boundary yield good fits to observed δ18O profiles for cooling rates of 50 ± 20 °C Ma−1, which are considerably faster than the 1–5 °C Ma−1 cooling rates previously inferred for the Adirondack Highlands from regional thermochronology. High cooling rates are consistent with an episode of rapid shearing and exhumation along the CCMZ during gravitational collapse of the Ottawan orogen at c. 1050 Ma. Texturally later titanite (syn‐S2) has higher overall δ18O and shows a transition from diffusion‐dominated to recrystallization‐dominated δ18O zoning, indicating infiltration of elevated‐δ18O fluids along S2 shear zones and continued shearing below the blocking temperature for oxygen (~≤500 °C for grain sizes at the study site). The texturally latest titanite (post‐S3) has growth‐dominated δ18O zoning, consistent with porphyroblastic grain growth following cessation of shearing along the Harrisville segment of the CCMZ.