Collaborative Research: Tracking nitrogen in mélange matrix from fore-arc to sub-arc depths with implications for deep nitrogen cycling: A combined field and experimental approach

由于氮在地球大气中的丰度和对生命的重要性，氮(N)循环与生物圈、大气圈和水圈的关系已经得到了很好的研究。在人们知之甚少的固体地球氮循环中，在地球历史的大部分时间里，板块构造调节了地表和深层地球储层之间的氮通量，影响了千年时间尺度上地球氮的总体分布。因此，在俯冲过程中输送到地球深处，然后在火山活动和脱气过程中返回到地表的氮的质量平衡是至关重要的，然而效率估计是高度变化的。目前的估计表明，45-74%的俯冲氮不会通过弧火山作用返回地表。这意味着氮以不同的数量被隔离在地球深处，这可能反映了俯冲板块组成或俯冲条件等因素。深部、特征不明显的含氮储层候选者包括中下大陆地壳、次大陆地幔、弧前至弧次地幔或更深的地幔（即更深的上地幔、过渡带或下地幔）。这突出了对适当样品进行氮测量的必要性，以及对这些储层中氮行为的彻底限制。为俯冲带弧前至弧次区域内氮的分布和加工提供了新的关键约束条件。首先，将对富沉积物和富蛇纹岩基质岩石和矿物进行一些首次N组成测量，以表征弧前加工过程中N的分布。其次，将进行相平衡实验，以评估关键含氮矿物的稳定性，并测量N熔体/流体-矿物分配系数（作为几个因素的函数）（压力、温度、氧易度、氯含量和部分熔体成分），以跟踪亚弧深度板坯脱水和部分熔化过程中的N行为。拟议研究的数据以及先前研究的数据将用于量化从弧前到弧下加工带的氮量，其中含有矿物，以及它如何随主要岩性（沉积物或蛇纹岩）而变化。在亚弧过程中，有多少氮从板块中释放出来，以及在不同热状态的俯冲带中有多少氮被隔离在亚弧地幔中，然后将其量化。这些将构成对氮行为的新限制，可以应用于贯穿地球历史的俯冲机制。因此，它也将用于解决跨耦合固体地球-大气系统的N的反馈和演变。该提案支持来自亚利桑那大学（UA）和南加州大学（USC）的两名早期职业女性pi，两名研究生和两名以上本科生。为了加强合作和扩大参与，pi将提供一个关于深度挥发性循环的联合虚拟研究生研讨会，包括UA和USC的学生。UA的团队将在UA的Alfie Norville Gem & Mineral museum开发一个关于高压-高温地球科学研究（包括实验室设备和研究应用）的博物馆展览。作为地球大气中最丰富的成分和生命的基本成分，氮(N)在当今大气、海洋、地壳和生物圈（统称为地表储层）中的行为已经得到了比较充分的研究。然而，在整个地球历史中，地球表面储层的N组成可能不会保持不变，这可能对早期地球气候和生命进化有影响。氮通过板块构造，特别是俯冲带，在地球表层储层和深部内部进行交换。在俯冲带，地壳中的氮（以及来自大气、海洋和生物圈的成分）被拉入地幔或地球内部。一部分来自地幔的氮通过火山脱气逃逸回大气和海洋。表面和内部之间的氮交换没有得到很好的约束，本研究旨在填补这一知识缺口的关键组成部分。随着压力和温度的升高，将测量典型俯冲带岩石的N组成，以确定N在何处被赋存。将在地幔条件下进行实验室实验，以了解地壳进入地幔并融化后N的行为。目标是最终利用这些结果来估计地球历史上地幔和大气的N组成是如何变化的。该提案支持来自亚利桑那大学（UA）和南加州大学（USC）的两名早期职业女性pi，两名研究生和两名以上本科生。为加强合作和扩大参与，两所院校的学生将会就拟议的主题举办联合虚拟研究生研讨会。UA的团队将在UA的Alfie Norville宝石和矿物博物馆展出连接地球表面和内部的板块构造，这将是教育公众了解最新研究的绝佳媒介。该奖项反映了美国国家科学基金会的法定使命，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 nitrogen (N)-cycle as it relates to the bio-, atmo-, and hydro-spheres have been well studied due to N’s abundance in Earth’s atmosphere and importance for life. In the less understood solid Earth N cycle, plate tectonics has regulated N fluxes between surface and deep Earth reservoirs over much of Earth’s history, affecting the bulk Earth N distribution over millennial timescales. Thus, the mass balance of nitrogen (N) delivered to the deep Earth during subduction and then returned to the surface during volcanism and degassing is critically important, yet efficiency estimates are highly variable. Current estimates suggest that 45-74% of subducted N does not return to the surface through arc volcanism. This implies that N is being sequestered in the deep Earth in variable amounts, which could reflect factors such as subducting plate composition or subduction conditions. Candidates for deep, poorly characterized N reservoirs include mid-lower continental crust, subcontinental mantle, fore-arc to sub-arc mantle, or deeper mantle (i.e., deeper upper mantle, transition zone or lower mantle). This highlights a need for N measurements on appropriate samples as well as thorough constraints on N behavior in these reservoirs. New key constraints on nitrogen (N) distribution and processing within the fore-arc to sub-arc regions of subduction zones will be provided. First, some of the first N composition measurements of sediment-rich and serpentinite-rich mélange matrix rocks and minerals to characterize the distribution of N during fore-arc processing will be made. Second, phase equilibria experiments to assess the stability of key N hosting minerals and measure N melt/fluid-mineral partition coefficients on mélange-matrix materials as a function of several factors (pressure, temperature, oxygen fugacity, chlorine content, and partial melt composition) to track N behavior during dehydration and partial melting in the slab at sub-arc depths will be performed. Data from the proposed study along with those from previous studies will be used to quantify the amount of N that is delivered from the fore-arc to the sub-arc processing zone, in which minerals it is hosted, and how it varies by dominant lithology (sediment or serpentinite). How much N is released from the slab during sub-arc processing versus how much is sequestered in the sub-arc mantle in subduction zones of different thermal states will then be quantified. These will constitute novel constraints on N behavior that can be applied to subduction regimes throughout Earth’s history. Hence, it will also be used to address the feedback and evolution of N across the coupled solid Earth-atmosphere systems. This proposal supports two early career female PIs, two graduate students, and two+ undergraduates from University of Arizona (UA) and University of Southern California (USC). To enhance collaboration and broaden participation, the PIs will offer a joint virtual graduate seminar on deep volatile cycling including students at UA and USC. The team at UA will develop a museum display at UA’s Alfie Norville Gem & Mineral Museum on high pressure-high temperature geoscience research (including laboratory equipment and research applications).As the most abundant constituent of the Earth’s atmosphere and as an essential ingredient of life, the behavior of nitrogen (N) in the present-day atmosphere, oceans, crust and biosphere (collectively known as the surficial reservoirs) have been relatively well-studied. However, the N composition of the Earth’s surficial reservoirs may not have remained the same throughout Earth’s history and this may have implications for early Earth climate and evolution of life. Nitrogen is exchanged between the Earth’s surficial reservoirs and the deep interior via plate tectonics, especially subduction zones. In subduction zones, N in the Earth’s crust (along with components from the atmosphere, ocean and biosphere) is pulled into the mantle or the interior of the Earth. Some proportion of the N from the mantle escapes back into the atmosphere and ocean by volcanic degassing. This N exchange between the surface and interior is not well-constrained and this proposed study aims to fulfill a key component of this knowledge gap. The N composition of typical subduction zone rocks will be measured to determine where N is hosted as pressure and temperature increase. Laboratory experiments at conditions in the Earth’s mantle will be performed to understand the behavior of N once the crust enters the mantle and melts. The objective is to eventually use these results to estimate how the N composition of the Earth’s mantle and atmosphere have changed through Earth’s history. This proposal supports two early career female PIs, two graduate students, and two+ undergraduates from University of Arizona (UA) and University of Southern California (USC). To enhance collaboration and broaden participation, the PIs will offer a joint virtual graduate seminar on the proposed theme including students from both institutions. The team at UA will develop a museum display at UA’s Alfie Norville Gem & Mineral Museum on plate tectonics connecting the surface and interior of the Earth, which would be an excellent medium to educate the public on state-of-the-art research.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.