The comparative behavior of apatite‐zircon U‐Pb systems in Apollo 14 breccias: Implications for the thermal history of the Fra Mauro Formation
The comparative behavior of apatite‐zircon U‐Pb systems in Apollo 14 breccias: Implications for the thermal history of the Fra Mauro Formation
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DOI：
10.1111/j.1945-5100.2009.tb01202.x
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发表时间：
2009-11
影响因子：
2.2
通讯作者：
A. Nemchin;R. Pidgeon;D. Healy;M. Grange;M. Whitehouse;J. Vaughan
中科院分区：
文献类型：
--
作者：
A. Nemchin;R. Pidgeon;D. Healy;M. Grange;M. Whitehouse;J. Vaughan
摘要-我们报道了从阿波罗14号着陆点采集的四个角砾岩样品中的锆石和磷灰石的二次离子质谱分析。在其中两个角砾岩中，锆石和磷灰石颗粒作为共生矿物出现在岩屑碎屑中，而在另外两个角砾岩的基质中，则作为无关的矿物碎屑出现。SIMS U-Pb分析表明，锆石颗粒的年龄为4023±24 Ma~4342±5 Ma，而磷灰石颗粒的年龄为3926±3 Ma。同生磷灰石和锆石的年龄差异表明，磷灰石U-铅系统在3926±3 Ma已经完全重置，而锆石的U-Pb系统此时并没有受到明显的干扰。在Apollo 14上，磷灰石U-Pb年龄比其他方法测得的年龄稍早，突出表明需要协调用于U-Pb、Ar-Ar和Rb-Sr系统的衰变常数。我们将磷灰石时代解释为毛罗岩组的形成时间。如果将这一地层解释为边缘喷出物是正确的，那么磷灰石也决定了边缘撞击的时间。磷灰石和锆石的铅损失行为的对比限制了阿波罗14号角砾岩的温度历史，我们根据实验确定的铅扩散常数和对Fra Mauro组出土样品的原始深度的近似估计，角砾岩经历了大约1300-1100°C的初始温度，但在最初的5-10年内冷却。
Abstract— We report secondary ion mass spectrometry (SIMS) U‐Pb analyses of zircon and apatite from four breccia samples from the Apollo 14 landing site. The zircon and apatite grains occur as cogenetic minerals in lithic clasts in two of the breccias and as unrelated mineral clasts in the matrices of the other two. SIMS U‐Pb analyses show that the ages of zircon grains range from 4023 ± 24 Ma to 4342 ± 5 Ma, whereas all apatite grains define an isochron corresponding to an age of 3926 ± 3 Ma. The disparity in the ages of cogenetic apatite and zircon demonstrates that the apatite U‐Pb systems have been completely reset at 3926 ± 3 Ma, whereas the U‐Pb system of zircon has not been noticeably disturbed at this time. The apatite U‐Pb age is slightly older than the ages determined by other methods on Apollo 14 materials highlighting need to reconcile decay constants used for the U‐Pb, Ar‐Ar and Rb‐Sr systems. We interpret the apatite age as a time of formation of the Fra Mauro Formation. If the interpretation of this Formation as an Imbrium ejecta is correct, apatite also determines the timing of Imbrium impact. The contrast in the Pb loss behavior of apatite and zircon places constraints on the temperature history of the Apollo 14 breccias and we estimate, from the experimentally determined Pb diffusion constants and an approximation of the original depth of the excavated samples in the Fra Mauro Formation, that the breccias experienced an initial temperature of about 1300–1100 °C, but cooled within the first five to ten years.