Sequestration of Halogens in the Sub-continental Lithospheric Mantle: Implications for Global Element Cycling

卤素元素（氯、氟、碘和溴）在改变地球上流体和矿物的性质方面发挥着重要作用。例如，向流体中添加卤素可以导致矿物在不同的温度和压力下分解，以及增加流体运输金属的能力。卤素在表面储层中富集（例如，沉积孔隙流体，沉积物）与地幔相比。这些储层的俯冲将卤素带入地幔。然后，卤素可以通过弧火山返回地球表面。然而，质量平衡的限制表明，更多的卤素是俯冲到地幔比返回到地球表面通过弧火山作用。这些观察结果使大多数研究人员认为，过量的卤素不断被输送到地幔。这种不平衡对地球表层储层和地幔的长期化学演化有影响。然而，几个卤素水库仍然没有量化，可能是这种脱节的原因。这里提出卤素可能被隔离在次大陆岩石圈地幔中（即，大陆地壳底部以下的上地幔），因此在计算中没有考虑。通过定量分析陆下岩石圈地幔中卤素的浓度，可以对整个卤素收支进行限制。除了通过俯冲带提高对元素转移的理解的智力影响外，更广泛的影响还包括对研究生和本科生的培训以及对GeoFORCE的参与，一个暑期项目，吸引优秀学生从选择低表现的少数民族-这项工作将限制俯冲作用对次大陆岩石圈地幔卤素收支的影响，以及次大陆岩石圈地幔可能充当主要的卤素储存库，并通过将卤素与俯冲带循环“隔离”很长一段时间来控制总体卤素收支。这项工作还将提供有关俯冲过程如何修改返回对流地幔的物质中的卤素浓度和比例的见解。本体卤素浓度，以及含水的原位卤素浓度（例如，闪石、叶蛇纹石）和无水矿物（例如，辉石，橄榄石）的地幔捕虏体来自美国西部，已被俯冲法拉隆板地球化学修改将被测量。这些数据将与俯冲输入和熔体消耗的其他亲石示踪剂进行比较。受俯冲影响的捕虏体将与非俯冲改造地幔捕虏体和深海橄榄岩进行比较，后者将作为对流地幔中卤素含量的基准。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The halogen elements (chlorine, fluorine, iodine, and bromine) play a fundamental role in modifying the properties of fluids and minerals on Earth. For example, the addition of halogens to fluids can cause minerals to break down at different temperatures and pressures, as well as, increase the ability of the fluid to transport of metals. Halogens are enriched in surface reservoirs (e.g., sedimentary pore fluids, sediments) compared to the Earth's mantle. The subduction of these reservoirs brings halogens into the mantle. Halogens can then be returned to the Earth's surface at arc volcanoes. However, mass balance constraints suggest that more halogens are subducted to the mantle than are returned to the Earth's surface via arc volcanism. These observations lead most researchers to suggest that an excess of halogens are being continually delivered to the mantle. This imbalance as implications for the long-term chemical evolution of the Earth's surficial reservoirs and mantle. However, several halogen reservoirs remain unquantified and may account for this disconnect. Here it is proposed that halogens may be sequestered in the sub-continental lithospheric mantle (i.e., the upper mantle below the base of the continental crust) and thus not accounted for in calculations. By quantifying the concentrations of halogens in the sub-continental lithospheric mantle, the overall halogen budget can be constrained. In addition to the intellectual impacts of improved understanding of elemental transfer through subductions zones, the broader impacts involve training of graduate and undergraduate students and participation in GeoFORCE, a summer program that engages outstanding students from select low performing minority-serving high schools in STEM fields.This work will constrain the effects of subduction on the subcontinental lithospheric mantle halogen budget and the extent to which the subcontinental lithospheric mantle may act as a major halogen reservoir and control the overall halogen budget by "isolating" halogens from the subduction zone cycle for potentially long periods of time. This work will also provide insights as to how subduction processes modify the halogen concentrations and ratios in material returned to the convecting mantle. Bulk halogen concentrations, as well as, in situ halogen concentrations of hydrous (e.g., chlorite, amphibole, antigorite) and anhydrous minerals (e.g., pyroxene, olivine) of mantle xenoliths from the western United States that have been geochemically modified by the subducting Farallon slab will be measured. These data will be compared with other lithophile tracers of subduction input and melt depletion. The subduction-influenced xenoliths will be compared to non-subduction-modified mantle xenoliths and abyssal peridotites, which will serve as a baseline for halogen content in the convecting mantle.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.