Collaborative Research: Growth of oceanic lower crust: an integrated high-precision geochronologic and trace-element approach

尽管海洋岩石圈覆盖了地球表面的60%以上，但由于缺乏对海底喷发的火山岩下面的侵入岩的地质年代学限制，海洋中脊形成岩石圈的模型一直受到阻碍。锆石是一种难熔矿物，存在于海壳中的辉长岩和其他侵入岩中，最近在锆石U-Pb定年方面取得了进展，现在可以测量侵入岩的年代。本研究采用二次离子质谱（SIMS）和热电离质谱（TIMS）相结合的方法来实现这一目标。对大洋钻探项目中西南印度洋脊735B孔的侵入辉长岩样品进行分析。SIMS是一种分辨率高但不确定度大的技术，可以测量微米尺度的地球化学差异，将用于确定锆石中分带的微量元素组成。锆石的高精度同位素稀释TIMS定年将用于确定样品的岩浆演化。测量年龄的不确定性预计在12000年或更短。这种精确度将提供前所未有的洞察力，让我们了解海洋地壳在缓慢扩张的山脊上生长需要多长时间。更广泛的影响包括博士后培训和指导。它还促进机构间合作，并将支持两名pi，其中一名来自EPSCoR州的科学领域性别代表性不足的pi。在剑桥MA地区的高中将开展外展活动，并为他们制作地质年代学的教育材料。该项目的结果将存档在nsf资助的公开访问数据库中。

Although oceanic lithosphere covers more than 60% of the earth's surface, models for its formation at mid-ocean ridges have been hampered by a lack of geochronological constraints on intrusive rocks that underlie volcanic rocks erupted on the seafloor. Recent advances in the U-Pb dating of zircons, refractory minerals that are found in gabbors and other intrusive rocks in ocean crust, now allow dates of intrusions to be measured. This research pursues this objective using a combined secondary ion mass spectrometry (SIMS) and thermal ionization mass spectrometry (TIMS) approach. Samples of intrusive gabbros from Hole 735B on the Southwest Indian Ridge from the Ocean Drilling Program will be analyzed. SIMS, which is a high resolution but large uncertainty technique that can measure geochemical differences on a micron scale, will be used to determine the trace element composition of zoning in zircons. High precision isotope dilution TIMS dating of the zircons will be used to determine the magmatic evolution of the samples. Uncertainties in the ages measured are expected to be 12,000 years or less. This precision will provide unprecedented insight into how long it takes for ocean crust to grow at slow spreading ridges. Broader impacts of the work include postdoctoral training and mentoring. It also fosters inter-institutional collaboration and will support two PIs, one of whom is from a gender under-represented in the sciences, from an EPSCoR state. Outreach to high schools in the Cambridge MA area will be carried out and educational materials for them will be created on geochronology. Results of the project will be archived in NSF-funded publicly accessible databases.