The evolution of mid-ocean ridge magma chambers and the growth of slow-spreading oceanic crust

海洋地壳几乎覆盖了地球表面的三分之二。它是由上升流地幔中形成的岩浆凝固在洋中脊产生的。这些岩浆被储存在脊轴以下的岩浆室中，在那里它们结晶形成海底地壳。岩浆输入和结晶所提供的热量驱动海底热液系统，通过岩石圈-水圈交换控制海洋化学，并为化学合成生态系统提供能量。海底地壳的恢复剖面提供了许多关于地壳增生机制的信息，但由于缺乏精确的测年工具，时间这一关键因素在很大程度上仍然没有得到限制。因此，吸积过程的时间演化仍然是个谜。然而，这不仅对了解大部分地壳的形成，而且对了解热液循环的控制都是至关重要的。我们最近的工作表明，U-Pb锆石定年技术现在具有足够的精度，可以让我们解决岩浆室的时间演化以及与地壳冷却的关系（Lissenberg等人，Science, 2009），表明锆石生长可能发生在一个显著的时间跨度（>100 ka）在单个洋中脊岩体。因此，我们现在有了一个工具来重建时间，温度和岩浆化学之间的关系。本提案旨在进一步发展该工具，并通过将其应用于从Vema岩石圈剖面（北纬11度，大西洋中脊）恢复的下海洋地壳样本，大大扩展其科学应用。结合高精度锆石测年和微量元素分析，我们将重建沿这一山脊段的岩浆房活跃了多长时间，它们是如何随着时间的推移而进化的，以及它们冷却的速度有多快。这将为中洋脊岩浆系统的演化提供一个前所未有的视角。样品的年龄随离扩张脊的距离的变化规律将约束岩浆进入地壳的位置。这可以检验我们的假设，即缓慢扩张的海洋地壳以两种根本不同的模式形成，一种是对称扩张和浅层熔融输送（从Vema推断），另一种是不对称扩张、脱离断裂和深部岩浆侵位。

Oceanic crust covers nearly two thirds of the Earth's surface. It is generated at mid-ocean ridges by the solidification of magmas formed in the upwelling mantle. These magmas are stored in magma chambers below the ridge axis, where they crystallise to yield the lower oceanic crust. The heat provided by magma input and crystallisation drives seafloor hydrothermal systems, which control ocean chemistry through lithosphere-hydrosphere exchange, and provide energy for chemosynthetic ecosystems. Recovered sections of lower oceanic crust have provided much information on crustal accretion mechanisms, but the key element of time has remained largely unconstrained due to the absence of precise dating tools. As a result, the temporal evolution of the accretion process has remained enigmatic. This is critical, however, not only for understanding formation of a large part of the Earth's crust, but also for understanding the controls on hydrothermal circulation. Our recent work has demonstrated that U-Pb zircon dating techniques are now of sufficient precision to allow us to resolve the temporal evolution of magma chambers and relationships with crustal cooling (Lissenberg et al., Science, 2009), showing that zircon growth may occur over a significant time span (>100 ka) within individual mid-ocean ridge plutons. As a result, we now have a tool to reconstruct the relationships between time, temperature and magma chemistry. This proposal seeks to further develop this tool and greatly expand its scientific application by applying it to samples from the lower oceanic crust recovered from the Vema Lithospheric Section (11 degrees N, Mid-Atlantic Ridge). Combining high-precision zircon dating with trace element analyses, we will reconstruct how long magma chambers along this ridge segment were active, how they evolved over time and how quickly they cooled. This will provide an unprecedented view of the evolution of mid-ocean ridge magmatic systems over time. The pattern of the age variation of the samples with distance from the spreading ridge will constrain where magma was delivered to the crust. This allows a test of our hypothesis that slow-spreading oceanic crust forms in two fundamentally different modes, one dominated by symmetric spreading and melt delivery at shallow levels (inferred for Vema), and the other by asymmetric spreading, detachment faulting and deep magma emplacement.