INTERPRETATION OF DISCORDANT AR-40-AR39 AGE-SPECTRA OF MESOZOIC THOLEIITES FROM ANTARCTICA
INTERPRETATION OF DISCORDANT AR-40-AR39 AGE-SPECTRA OF MESOZOIC THOLEIITES FROM ANTARCTICA
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DOI：
10.1016/0016-7037(77)90184-3
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发表时间：
1977-01-01
影响因子：
5
通讯作者：
ELLIOT, DH
中科院分区：
文献类型：
--
作者：
FLECK, RJ;SUTTER, JF;ELLIOT, DH
横贯北极山脉中部的费拉群拉斑玄武岩的常规K-Ar年龄表明，在其露头的大部分地区，这一单位的放射性40Ar损失很大。Ar损失与玄武岩中非玻璃化基质的数量成反比，玄武岩自挤压以来没有受到过热或构造干扰。这些拉斑玄武岩的40Ar/19Ar年龄谱总体上是不一致的，表明放射性成因40Ar相对于39Ar的分布明显不均匀，但与热扰动样品的释放模式明显不同。Ar再分配的量与反玻璃化的量直接相关，并反映在年龄谱的渐进式修改中。提出了一个与常规K-Ar系统学的扰动以及40Ar/39Ar年龄谱相一致的原玻璃基质脱玻璃化的~(40)Ar再分配模型。具有大量再分布但损失很小的放射性成因的Ar的样品产生了年龄谱，其视年龄从低温阶梯递减到高温阶梯，类似于一些月球玄武岩、角砾岩和土壤的报道。尽管Turner和Cadogan(1974)提出的39Ar反冲效应在某些情况下可能是一个因素，但所有年龄谱的修改都归因于在渐进去玻璃化过程中40Ar的重新分配。玄武岩中的大部分钾位都与失透作用有关，可能形成不具地质意义的40Ar/39Ar年龄平台。
Conventional K-Ar ages of tholeiitic basalts of the Ferrar Group in the central Transantarctic Mountains indicate significant loss of radiogenic40Ar from this unit over much of its outcrop area. Argon loss varies inversely with amount of devitrified matrix in the basalts, which have not been thermally or tectonically disturbed since extrusion.40Ar/19Ar age-spectra of these tholeiites are generally discordant and indicate significant inhomogeneity in the distribution of radiogenic40Ar with respect to39Ar, but are distinctly different from release patterns of thermally disturbed samples. Amounts of argon redistribution vary directly with amounts of devitrification and are reflected in progressive modification of the age spectra. A model of redistribution of radiogenic40Ar by devitrification of originally glassy matrix is suggested that is consistent with disturbance of the conventional K-Ar systematics as well as the40Ar/39Ar age-spectra. Samples with substantial redistribution but minor loss of radiogenic argon yield age spectra whose apparent ages decrease from low-temperature to high-temperature steps, similar to those reported for some lunar basalts, breccias, and soils. Modification of all the age spectra is attributed to redistribution of radiogenic40Ar during progressive devitrification, although39Ar-recoil effects suggested byTurnerandCadogan(1974) may be a factor in some cases. Where devitrification involves most potassium sites within the basalt,40Ar/39Ar age-plateaux may be formed that have no geologic significance.