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

美国西南部地幔捕虏体中186 Os-187 Os-Hf-Nd-Sr同位素对大陆生长和熔体-岩石相互作用的测试模型橄榄岩和辉石岩捕虏体提供了形成和随后修改陆下地幔岩石圈（SCLM）过程的成分记录。对流地幔的大规模熔融导致低密度残留橄榄岩，可能成为稳定的岩石圈。最终形成SCLM的橄榄岩中的熔体可能导致幼年地壳的产生。因此，通过对流地幔部分熔融形成SCLM可能是地球历史上大陆生长的主要机制。橄榄岩的~（187）Os/~（188）Os同位素组成可以用来确定部分熔融过程中Re的去除时间，从而得到岩石圈形成的大致年龄。该方法将应用于美国西南部（SW USA）的4个捕虏体套：Dish Hill（加州）、基尔本洞（新墨西哥州）、San卡洛斯和Vulcan's Throne（亚利桑那州）。它们共同跨越了两个地壳省内860亿光年宽的区域，即2.0至2.3 Ga的Mojavia（Dish Hill）和1.7至2.0 Ga的Yavapai-Mazatzal（其他3个）省。初步的187 Os/188 Os数据的尖晶石轴承橄榄岩显示正相关性与熔体亏损指标，如Al 2 O3，符合熔体亏损控制的Os同位素组成。应用铝年代概念，这些数据给出了187 Re-187 Os模型年龄范围从2.0到2.3 Ga，大致上与上覆大陆地壳省份的年龄一致，并支持模型，即大陆的增长直接与稳定其底层SCLM通过对流地幔的部分熔融。它还表明，大的克拉通外大陆地区可能通过大规模的地幔熔融事件快速脉冲增长。一个同样可行的替代方法是，铝年代记录熔体岩石相互作用后，早期的部分熔融，在这种情况下，数据只提供最低模型年龄的部分熔融，因此可能是无关的初始阶段的青年地壳生产导致大陆的增长。这些捕虏体的详细岩石成因必须知道，以准确地评估SCLM稳定的时间和机制，并测试上述两个对比的结论。还将研究这些地区的含铝辉石辉石岩，以约束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。铂族元素，亲石微量元素的浓度，和放射成因（Sr，Nd和Hf）同位素组成将测量单斜辉石分离，全面研究这些岩石的成因。特别是Lu-Hf系统在许多条件下通常抵抗交代作用，并且可能提供这些橄榄岩中熔体耗尽过程及其年龄的强有力指标。此外，将测量大陆橄榄岩捕虏体的高精度186 Os-187 Os测量值，以监测Pt-Re-Os分馏事件，并评估部分熔融与熔体-岩石相互作用对每个捕虏体套件的Os预算的贡献。拟议的工作通过提供陆地材料的高质量地球化学和同位素数据来支持NSF研究计划的目标，以解决有关地球性质的重要问题。PI和Co-PI一直致力于通过课程，导师和实习计划将本科生纳入积极的研究。拟议的工作涉及学生，博士后研究人员和教师，从而提供了一个教育平台，从非常初级到高级水平的研究经验，在UH年表实验室内的互动环境，在NSF的教育目标的强有力的支持，为未来的地球化学家的培训。本科生将通过各种可用的实习，高级论文和勤工俭学计划包括在研究中。来自代表性不足的人口（在UH注册的学生的很大一部分）的学生将有重要的访问国家的最先进的设备和岩石学和地球化学的前沿研究。

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.