Testing Models for Continental Growth and Melt-Rock Interaction from 186Os-187Os-Hf-Nd-Sr Isotopes in SW USA Mantle Xenoliths

测试美国西南部地幔包体中 186Os-187Os-Hf-Nd-Sr 同位素的大陆生长和熔岩相互作用模型

• 批准号: 1048583
• 负责人: Alan Brandon
• 金额: $ 33.81万
• 依托单位: University of Houston
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-02-15 至 2015-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1048583&HistoricalAwards=false
• 关键词: Testing; Models; Continental; Growth; Melt

Testing Models for Continental Growth and Melt-Rock Interaction from 186Os-187Os-Hf-Nd-Sr Isotopes in SW USA Mantle XenolithsIntellectual Merit. Peridotite and pyroxenite xenoliths provide a compositional record of processes that form and subsequently modify the subcontinental mantle lithosphere (SCLM). Large-scale melting in the convecting mantle results in lower density residual peridotite that may become stabilized as lithosphere. The melt removed from the peridotite that ultimately forms SCLM likely results in juvenile crust production. Hence, the formation of SCLM via partial melting in the convecting mantle is likely a primary mechanism to grow continents over Earth history. The 187Os/188Os isotopic compositions of peridotites can be used to constrain the timing of Re removal from partial melting, hence an approximate age for lithosphere generation. Tis approach will be applied to 4 xenolith suites from the Southwestern United States (SW USA): Dish Hill (California), Kilbourne Hole (New Mexico), San Carlos and Vulcan's Throne (Arizona). Together they span an 860 kilometer-wide region within two crustal provinces, the 2.0 to 2.3 Ga Mojavia (Dish Hill), and the 1.7 to 2.0 Ga Yavapai-Mazatzal (the other 3) provinces. Preliminary 187Os/188Os data for spinel-bearing peridiotites show positive correlations with melt depletion indicators such as Al2O3, consistent with a melt depletion control for the Os isotopic compositions. Applying the aluminachron age concept to these data gives 187Re-187Os model ages ranging from 2.0 to 2.3 Ga, broadly consistent with the ages of the overlying continental crustal provinces, and supporting models whereby continental growth is directly linked to stabilization of its underlying SCLM via partial melting in the convecting mantle. It also indicates that large off-craton continental regions may grow in rapid pulses through large-scale mantle melting events. An equally viable alternative is that the aluminachrons record melt-rock interaction subsequent to earlier partial melting, in which case the data only provide minimum model ages of partial melting and thus may be unrelated to the initial stages of juvenile crust production leading to continental growth. The detailed petrogenesis of these xenoliths must be known to accurately evaluate the timing and mechanisms of SCLM stabilization and to test the two contrasting conclusions drawn above. Al-augite bearing pyroxenites from these locales will also be studied to constrain compositional signatures that may be imparted to the peridotites via melt addition or melt-rock interaction. Platinum group element, lithophile trace element concentrations, and radiogenic (Sr, Nd, and Hf) isotopic compositions will be measured on clinopyroxene separates to comprehensively examine the petrogenesis of these rocks. The Lu-Hf system in particular is commonly resistant to metasomatism under many conditions, and will likely provide a robust indicator of melt depletion processes and the ages thereof in these peridotites. In addition, high-precision 186Os-187Os measurements of continental peridotite xenoliths will be measured to monitor Pt-Re-Os fractionation events, and to evaluate partial melting versus melt-rock interaction contributions to the Os budgets of each of these xenolith suites.Broader Impacts. The proposed work supports the NSF research program objectives by providing high-quality geochemical and isotope data of terrestrial materials, to address important questions about the nature of our planet. The PI and Co-PI are and have been committed to and have demonstrated the incorporation of undergraduate students in active research through coursework, mentorship and internship programs. The proposed work involves students, post-doctoral researchers, and faculty, and thus provide an educational platform spanning from very junior to senior levels of research experience in an interactive environment within the UH chronology laboratory, in strong support of NSF's educational goals for the training of future geochemists. Undergraduate students will be included in the research via various available internship, senior thesis, and work-study programs. Students from underrepresented populations (a large portion of enrolled students at UH) will have important access to state-of-the-art equipment and cutting-edge research in petrology and geochemistry.

美国西南部地幔Xenolits 186Os-187Os-Hf-Ndsr同位素大陆生长和熔岩相互作用测试模型橄榄岩和辉石岩包体提供了形成并随后改变次大陆地幔岩石圈(SCLM)的过程的成分记录。对流地幔中的大规模熔融导致较低密度的残余橄榄岩，可能成为岩石圈。从橄榄岩中移出的熔体最终形成SCLM，可能会导致年轻地壳的产生。因此，在对流地幔中通过部分熔融形成的SCLM很可能是地球历史上大陆生长的主要机制。橄榄岩的187Os/188Os同位素组成可以用来限制部分熔融中Re的去除时间，从而获得岩石圈形成的大致年龄。TIS方法将应用于美国西南部的4个捕虏体套件：迪什希尔(加利福尼亚州)、基尔伯恩洞(新墨西哥州)、圣卡洛斯和火神宝座(亚利桑那州)。它们总共横跨两个地壳省份，即2.0至2.3Ga Mojavia(Dish Hill)和1.7至2.0ga Yavapai-Mazatzal(其他3个)省内860公里宽的地区。含尖晶石的橄榄岩的187Os/188Os初步数据显示，与熔体亏损指标如Al_2O_3呈正相关，这与Os同位素组成的熔体亏损控制相一致。对这些数据应用Al-Achron年龄概念，给出了从2.0到2.3Ga的187Re-187Os模型年龄，与上覆大陆壳区的年龄基本一致，并支持以下模型：大陆增长通过对流地幔的部分熔融与其下伏的SCLm的稳定直接相关。它还表明，大型克拉通外大陆区可能通过大规模的地幔熔融事件以快速脉冲的方式增长。一种同样可行的替代方案是，铝陨石记录了早先部分熔融之后的熔体-岩石相互作用，在这种情况下，数据只提供了部分熔融的最小模型年龄，因此可能与导致大陆生长的年轻地壳产生的初始阶段无关。必须知道这些包体的详细岩石成因，才能准确地评价SCLM稳定化的时间和机制，并检验上面得出的两个相反的结论。来自这些地点的含铝辉石辉石岩也将被研究，以限制可能通过熔体添加或熔体-岩石相互作用赋予橄榄岩的成分特征。为了全面研究这些岩石的成因，将在单斜辉石分离物上测定铂族元素、亲岩石元素浓度和放射性(锶、钕、氢)同位素组成。特别是，Lu-HF系统在许多条件下通常抵抗交代作用，并可能提供这些橄榄岩中熔体耗竭过程及其年龄的可靠指示。此外，将测量大陆橄榄岩包体的高精度186Os-187Os测量，以监测铂-Re-Os分馏事件，并评估部分熔融和熔岩相互作用对每个包体的Os预算的贡献。这项拟议的工作通过提供高质量的地球材料的地球化学和同位素数据来支持NSF的研究计划目标，以解决有关我们星球性质的重要问题。PI和Co-PI一直致力于并已经通过课程作业、指导和实习计划将本科生纳入到积极的研究中。这项拟议的工作涉及学生、博士后研究人员和教职员工，因此提供了一个教育平台，在密歇根大学年代学实验室内的互动环境中提供从初级到高级的研究经验，大力支持NSF培养未来地球化学家的教育目标。本科生将通过各种可用的实习、高级论文和勤工俭学项目被纳入研究范围。来自人数不足的学生(UH在校学生的很大一部分)将有重要的机会获得最先进的设备和岩石学和地球化学方面的尖端研究。