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186Os-187Os-Hf-Nd-Sr同位素对大陆生长和熔体-岩石相互作用的检验模型橄榄岩和辉石岩包体提供了形成和随后修改次大陆地幔岩石圈（SCLM）过程的成分记录。对流地幔的大规模熔融导致密度较低的残余橄榄岩可能成为稳定的岩石圈。从橄榄岩中移除的熔体最终形成了SCLM，可能导致了幼年地壳的形成。因此，在地球历史上，通过对流地幔的部分熔融形成陆地板块可能是大陆形成的主要机制。橄榄岩的187Os/188Os同位素组成可用于限制部分熔融去除Re的时间，从而确定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套捕虏岩：Dish Hill（加利福尼亚州）、Kilbourne Hole（新墨西哥州）、San Carlos和Vulcan’s Throne（亚利桑那州）。它们横跨两个地壳省，2.0至2.3 Ga Mojavia（碟山）和1.7至2.0 Ga Yavapai-Mazatzal（其他3个）省，共860公里宽的区域。含尖晶石橄榄岩的187Os/188Os初步数据与Al2O3等熔体耗尽指标呈正相关，与Os同位素组成的熔体耗尽控制相一致。将铝年代概念应用于这些数据，得出187Re-187Os模型年龄范围为2.0 - 2.3 Ga，与上覆大陆地壳省份的年龄大致一致，并支持大陆生长与通过对流地幔的部分熔融使其下伏SCLM稳定直接相关的模型。它还表明，大型离克拉通大陆区域可能通过大规模的地幔熔融事件以快速脉冲的方式生长。另一种同样可行的选择是，铝年表记录了早期部分熔融之后的熔融岩相互作用，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幼年地壳形成的初始阶段无关。必须了解这些捕虏体的详细岩石成因，才能准确地评估SCLM稳定的时间和机制，并检验上述两个相反的结论。还将研究这些地区含铝辉石岩的辉石岩，以限制可能通过熔体添加或熔体-岩石相互作用传递给橄榄岩的成分特征。测定斜辉石分离物的铂族元素、亲石微量元素浓度和放射性同位素（Sr、Nd、Hf）组成，全面考察这些岩石的成因。特别是在许多条件下，Lu-Hf系统通常具有抗交代作用，并且可能为这些橄榄岩中的熔体耗尽过程及其年龄提供强有力的指标。此外，将测量大陆橄榄岩捕虏体的186Os-187Os的高精度测量，以监测Pt-Re-Os分馏事件，并评估部分熔融与熔融岩相互作用对每个捕虏体套件的Os预算的贡献。更广泛的影响。拟议的工作通过提供高质量的地球化学和陆地物质同位素数据来支持NSF研究计划的目标，以解决有关我们星球本质的重要问题。PI和Co-PI一直致力于通过课程作业、指导和实习项目将本科生纳入积极的研究中，并且已经证明了这一点。这项工作涉及学生、博士后研究人员和教职员工，因此在UH年表实验室的互动环境中提供了一个涵盖初级到高级研究经验的教育平台，有力地支持了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、毕业论文和勤工俭学项目参与研究。来自代表性不足群体的学生（在休斯敦大学注册的学生中占很大一部分）将有机会获得最先进的设备和岩石学和地球化学方面的前沿研究。

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.