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起着重要作用。例如，在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，并增加流体运输金属的能力。与地幔相比，卤素在地表储集层(如沉积孔隙流体、沉积物)中富含卤素。这些储集层的俯冲将卤素带入地幔。然后，卤素可以在弧形火山中返回地球表面。然而，物质平衡的限制表明，更多的卤素被俯冲到地幔，而不是通过弧形火山作用返回地球表面。这些观察结果导致大多数研究人员认为，过量的卤素正在不断地输送到地幔。这种不平衡对地球表面储集层和地幔的长期化学演化有影响。然而，几个卤素储集层仍未量化，可能是造成这种脱节的原因。这里提出，卤素可能被隔离在次大陆岩石圈地幔(即大陆地壳底部以下的上地幔)中，因此不在计算中考虑。通过量化次大陆岩石圈地幔中的卤素浓度，可以限制总的卤素收支。除了通过俯冲带更好地了解元素迁移的智力影响外，更广泛的影响还包括对研究生和本科生的培训，以及参与GeoFORCE，这是一个暑期项目，吸引表现不佳的少数族裔高中的优秀学生参与STEM领域。这项工作将限制俯冲对次大陆岩石圈地幔卤素预算的影响，以及次大陆岩石圈地幔可能充当主要卤素储藏库的程度，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预算。这项工作还将提供关于俯冲过程如何改变返回对流地幔的物质中的卤素浓度和比率的见解。将测量经俯冲法拉隆板块地球化学改造的美国西部地幔包体的散装卤素浓度以及含水矿物(如绿泥石、角闪石、斜长石)和无水矿物(如辉石、橄榄石)的原地卤素浓度。这些数据将与其他消减输入和熔体耗尽的亲石示踪剂进行比较。受俯冲影响的包体将与非俯冲修改的地幔包体和深海橄榄岩进行比较，这将作为对流地幔中卤素含量的基线。这一裁决反映了NSF的法定使命，并通过使用基金会的智力优势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.