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.

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智力价值。橄榄岩和辉石岩捕虏体提供了形成并随后改变次大陆地幔岩石圈（SCLM）过程的成分记录。对流地幔中的大规模熔化导致密度较低的残余橄榄岩，这些橄榄岩可能会稳定为岩石圈。从橄榄岩中去除的熔体最终形成 SCLM，可能会导致新生地壳的产生。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 187Os/188Os 同位素组成可用于限制 Re 从部分熔融中去除的时间，从而确定岩石圈生成的大致年龄。这种方法将应用于美国西南部 (SW USA) 的 4 个捕虏体套件：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）。它们总共横跨两个地壳省份内 860 公里宽的区域，即 2.0 至 2.3 Ga 莫哈维亚（Dish Hill）省份和 1.7 至 2.0 Ga Yavapai-Mazatzal（其他 3 个）省份。含尖晶石橄榄岩的初步 187Os/188Os 数据显示与 Al2O3 等熔体损耗指标呈正相关，这与 Os 同位素成分的熔体损耗控制一致。将 aluminachron 年龄概念应用于这些数据，得出 187Re-187Os 模型年龄范围为 2.0 至 2.3 Ga，与上覆大陆地壳省的年龄大致一致，并支持大陆生长通过对流地幔部分熔融与其底层 SCLM 稳定直接相关的模型。它还表明，大片克拉通外大陆区域可能会通过大规模地幔融化事件而快速增长。一个同样可行的替代方案是，aluminachrons 记录了早期部分熔融之后的熔体-岩石相互作用，在这种情况下，数据仅提供部分熔融的最小模型年龄，因此可能与导致大陆生长的新生地壳产生的初始阶段无关。必须了解这些捕虏体的详细岩石成因，才能准确评估 SCLM 稳定的时间和机制，并测试上面得出的两个对比结论。还将研究来自这些地点的含铝辉石辉石岩，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。将在单斜辉石分离物上测量铂族元素、亲石微量元素浓度和放射性（Sr、Nd 和 Hf）同位素组成，以全面检查这些岩石的岩石成因。特别是 Lu-Hf 系统在许多条件下通常能抵抗交代作用，并且可能为这些橄榄岩中的熔体消耗过程及其年龄提供可靠的指标。此外，将对大陆橄榄岩捕虏体进行高精度 186Os-187Os 测量，以监测 Pt-Re-Os 分馏事件，并评估部分熔融与熔岩相互作用对每个捕虏体套件 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陆地材料地球化学和同位素数据来支持美国国家科学基金会的研究计划目标，以解决有关地球性质的重要问题。 PI 和 Co-PI 一直致力于并证明了通过课程作业、指导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，从而在夏威夷大学年代学实验室的交互式环境中提供了一个涵盖从初级到高级研究经验的教育平台，有力支持 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参与研究。来自弱势群体的学生（夏威夷大学注册学生的很大一部分）将有机会获得最先进的设备以及岩石学和地球化学领域的前沿研究。