Melting Processes in Infant Subduction Zones: HFSE fractionation in boninites

一个板块如何开始俯冲到另一个板块之下是板块构造学的一个基本问题。不幸的是，没有我们可以研究的俯冲开始的活跃例子。因此，主要的限制来自于对新生俯冲带保存下来的岩浆产物的研究。这类熔岩通常出现在俯冲带的弧前，即与主弧有关的火山链和海沟之间的区域。它们也可以在蛇绿岩中找到，蛇绿岩是被冲到大陆上或以其他方式带到表面的海洋地壳的碎片。在新生弧中产生的岩浆通常具有独特的主元素和微量元素组成，称为玻安岩。玻安岩的主要元素组成记录了它们产生的地幔的压力和温度。通过观察这些地幔P-T条件的时间演化，人们可以限制早期弧下的地幔如何流动。这反过来又提供了宝贵的线索，为什么俯冲开始摆在首位。玻安岩所记录的P-T条件虽然复杂，但却是迄今为止对早期弧幔流最直接的记录，是理解俯冲作用开始的关键。许多研究已经限制了玻安岩的熔融条件，使用高压实验来模拟熔融条件。估计值在很宽的P-T条件范围内变化，主要取决于为实验选择的特定玻安岩组合物。造成这种混淆的原因之一可能是玻安岩熔体与岩石圈的相互作用，这可能记录在玻安岩的微量元素组成中。玻安岩除了主量元素组成奇特外，还具有独特的微量元素组成。一个显著的特征是Hf、Ta和Ti元素经常亏损，而Zr和Hf元素则富集。通常这些元素的行为是一致的（全部贫化或全部富集），因此通常被归为高场强元素（HFSE）。已经提出了一些过程来解释HFSE分馏，但最新的数据表明，它起源于地幔玻安岩熔体和地幔矿物之间的分配。不同的假设有不同的影响熔化过程和估计的P-T条件。然而，目前的分区数据不足以在各种假设之间做出决定。拟议的研究将测量HFSE之间的分配熔体和地幔矿物使用高压实验。研究结果将被用来检验现有的假设，并将导致更有信心和更准确的估计地幔条件在早期弧。由于其独特的化学性质，玻安岩相对容易识别，即使是在已经严重变质的古代岩石中。一些已知的最古老的岩浆（3.7 Ga）是玻安岩。由于玻安岩只在俯冲带中形成，因此它们是少数几种仅从岩石的地球化学特征就能给出明确构造背景的样品之一。因此，玻安岩代表了一个相当独特的机会，从现在追溯到地球最早的历史和解决问题，如长期冷却和板块构造的启动板块构造（俯冲）的一个元素。因此，对现代地球中玻安岩形成的更好理解将为地球最早的历史提供关键线索。

How subduction of one plate under another begins is a fundamental problem in plate tectonics. Unfortunately, there are no active examples of subduction initiation that we can study. So the primary constraints come from looking at the preserved magmatic products of infant subduction zones. Such lavas are often found in the fore-arc of subduction zones, the area between the main arc-related volcanic chain and the trench. They can also be found in ophiolites, slivers of oceanic crust that have been thrust onto continents or otherwise brought to the surface. The magmas produced in infant arcs often have a distinctive major and trace element composition and are called boninites. The major element composition of boninites records the pressures and temperatures of the mantle from which they were produced. By looking at the temporal evolution of these mantle P-T conditions, one can constrain how the mantle beneath the early arc was flowing. This in turn provides invaluable clues to why subduction started in the first place. While complicated, the P-T conditions recorded by boninites are by far the most direct record of early-arc mantle flow, and are key to understanding the initiation of subduction. A number of studies have constrained the melting conditions of boninites using high-pressure experiments to mimic melting conditions. The estimates vary over a wide-range of P-T conditions, largely depending upon the particular boninite composition chosen for the experiments. One source of the confusion may come from interaction of boninite melts with the lithosphere, which may be recorded in the trace element composition of boninites. In addition to their odd major element composition, boninites also have unique trace element compositions. One notable feature is that the elements Hf, Ta and Ti are often depleted, while Zr and Hf are enriched. Usually these elements behave coherently (all depleted or all enriched) and so are usually grouped together as the high field-strength elements (HFSE). A number of processes have been proposed to explain the HFSE fractionation but the most current data suggest it originates in mantle by partitioning between boninite melts and mantle minerals. The different hypotheses have distinct implications for the melting process and for estimated P-T conditions. However, current partitioning data is insufficient to decide between the various hypotheses. The proposed research will measure the partitioning of HFSE between melt and mantle minerals using high-pressure experiments. The results will be used to test the existing hypothesis and will lead to more confident and accurate estimates of mantle conditions in early arcs. Because of their distinctive chemistry, boninites are relatively easy to identify, even in ancient terranes that have been heavily metamorphosed. Some of the oldest magmas known (3.7 Ga) are boninites. As boninites are only formed in subduction zones, they are one of a very few types of sample that can give an unambiguous tectonic setting from just the geochemistry of the rocks. Thus boninites represent a fairly unique chance to trace one element of plate tectonics (subduction) from the present, back to the Earth's earliest history and address issues such as secular cooling and the initiation of plate tectonics. Thus, an improved understanding of boninite formation in the modern Earth will provide key clues to the Earth's earliest history.