Geochemistry of Apatite from rapid cooled samples@en

Apatite (U-Th-Sm)/He (AHe) thermochronology is increasingly used for reconstructing geodynamic processes of the upper crust and the surface. Results of AHe thermochronology, however, are often in conflict with apatite fission track (AFT) thermochronology, yielding an inverted age-relationship with AHe dates older than AFT dates of the same samples. This effect is mainly explained by radiation damage of apatite, either impeding He diffusion or causing non-thermal annealing of fission tracks. So far, systematic age inversions have only been described for old and slowly cooled terranes, whereas for young and rapidly cooled samples 'too old' AHe dates are usually explained by the presence of undetected U and/or Th-rich micro-inclusions. We report apatite (U-Th-Sm)/He results for rapidly cooled volcanogenic samples deposited in a deep ocean environment with a relatively simple post-depositional thermal history. Robust age constraints are provided independently through sample biostratigraphy. All studied apatites have low U contents (< 5 ppm on average). While AFT dates are largely in agreement with deposition ages, most AHe dates are too old. For leg 43, where deposition age of sampled sediment is 26.5-29.5 Ma, alpha-corrected average AHe dates are up to 45 Ma, indicating overestimations of AHe dates up to 50%. This is explained by He implantation from surrounding host U-Th rich sedimentary components and it is shown that AHe dates can be \"corrected\" by mechanically abrading the outer part of grains. We recommend that particularly for low U-Th-apatites the possibility of He implantation should be carefully checked before considering the degree to which the alpha-ejection correction should be applied.

磷灰石(U-Th-Sm)/He热年代学（Apatite (U-Th-Sm)/He thermochronology，简称AHe）正日益被用于重建上地壳与地表的地球动力学过程。然而，磷灰石裂变径迹热年代学（apatite fission track thermochronology，简称AFT）的结果常与AHe热年代学结果存在矛盾：同一试样的AHe测年结果往往老于AFT测年结果，呈现倒转的年龄对应关系。该效应主要归因于磷灰石的辐射损伤——此类损伤既会阻碍氦扩散，也会引发裂变径迹的非热退火。迄今为止，系统性年龄倒转现象仅在古老且冷却缓慢的地体中被报道；而对于年轻且冷却迅速的试样，其“过老”的AHe测年结果通常被归因于未被检出的富U和/或富Th微包裹体的存在。本研究报道了沉积于深海环境、沉积后热历史相对简单的快速冷却火山成因样品的磷灰石(U-Th-Sm)/He测年结果，并通过样品生物地层学独立提供了可靠的年龄约束。所有被研究的磷灰石U含量均较低（平均<5 ppm）。尽管AFT测年结果与沉积年龄基本吻合，但多数AHe测年结果却偏老。以第43航次为例，所采沉积物的沉积年龄为26.5~29.5 Ma，经α校正后的平均AHe测年结果可达45 Ma，表明AHe测年结果的高估幅度可达50%。该现象可由周围富U-Th沉积基质组分的氦植入作用解释，且研究表明，通过机械研磨颗粒外层可对AHe测年结果进行“校正”。我们建议，尤其针对低U-Th含量磷灰石，在确定α逸出校正的应用程度前，应仔细排查氦植入作用的可能性。