U-PB SYSTEMATICS OF PHOSPHATES FROM EQUILIBRATED ORDINARY CHONDRITES
U-PB SYSTEMATICS OF PHOSPHATES FROM EQUILIBRATED ORDINARY CHONDRITES
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DOI：
10.1016/0012-821x(94)90038-8
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发表时间：
1994-01-01
影响因子：
5.3
通讯作者：
ALLEGRE, CJ
中科院分区：
文献类型：
--
作者：
GOPEL, C;MANHES, G;ALLEGRE, CJ
从平衡的普通陨石和同一陨石的小块碎片中分离出15个磷酸盐，测定了U-Pb系统学。这些磷酸盐分离物中有13个的U-238/Pb-204比值很高，这导致了放射成因的铅，其Pb-206/Pb-204比值范围从250到3500。这些磷酸盐的Pb/Pb模式年龄范围为4.563 ~ 4.502Ga，分析精度为10（6）y，U-Pb体系具有明显的一致性。所观察到的时间间隔为60 × 10（6）y，反映了平衡辉长岩的热处理过程，与以前从Rb/Sr、K/Ar和Pu年代学中得到的结果一致。7种镁铁质岩石中磷酸盐的Pb/Pb年龄与变质程度呈负相关。这是迄今为止第一次观察到的长寿命计时器与变质作用强度之间的明确关系，变质程度反映了变质作用的强度。假设Pb/Pb年龄指示了磷酸盐中准确的U-Pb闭合时间，Pb/Pb年代学与层状H球粒陨石母体模型是相容的。然而，这种解释的U/Pb系统并不是唯一的，它假定了一个缓慢冷却的平衡材料在高温下，在明显的冲突与岩石学观察。除了与Pu系统学相当一致的H_2O_3外，Pb/Pb年代学与已发表的放射性年代学数据的比较没有揭示简单的相关性。在目前关于榴辉岩热历史的争论中，来自每个同位素系统的计时信息被解释为它的热闭合时间。然而，这一基本假设可能并不适用于所有的射电年代学，必须在射电年代学数据被应用于4.56-4.4 Ga的原行星历史之前进行评估。
U-Pb systematics were determined from fifteen phosphate separates from equilibrated ordinary chondrites and from small bulk fragments of the same meteorites. The high U-238/Pb-204 ratios of thirteen of these phosphate separates lead to extremely radiogenic Pb whose Pb-206/Pb-204 ratios range from 250 up to 3500. The Pb/Pb model ages for these phosphates range from 4.563 to 4.502 Ga, with an analytical precision of 10(6) y and the U-Pb system is apparently concordant. The time interval observed, 60 x 10(6) y, reflects the thermal processing of the equilibrated chondrites and is consistent with that previously derived from the Rb/Sr, K/Ar and Pu chronologies. The Pb/Pb ages of the phosphates from the seven H chondrites show a negative correlation versus their metamorphic grade. This is the first clear relationship ever observed between a long-lived chronometer and the intensity of metamorphism as reflected by metamorphic grade.Assuming that the Pb/Pb age indicates the accurate U-Pb closure time in phosphates, the Pb/Pb chronology is compatible with the model of a layered H chondrite parent body. However, this interpretation of the U/Pb systematics is not unique; it postulates a slow cooling of the equilibrated materials at high temperature, in apparent conflict with petrological observations. Except for the H chondrites, which agree rather well with Pu systematics, comparison of the Pb/Pb chronology with published radiochronometric data does not reveal simple correlations. In the present debate concerning the thermal history of chondrites, the chronometric information derived from each isotope system is interpreted as the time of its thermal closure. However, this basic assumption may not be correct for all the radiochronologies and must be evaluated before the radiochronometric data can be applied as compelling time constraints for the period of 4.56-4.4 Ga of proto-planetary history.