A geochronological 40Ar/39Ar and 87Rb/87Sr study of K-Mn oxides from the weathering sequence of Azul, Brazil
A geochronological 40Ar/39Ar and 87Rb/87Sr study of K-Mn oxides from the weathering sequence of Azul, Brazil
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DOI：
10.1016/0016-7037(96)00080-4
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发表时间：
1996-06
影响因子：
5
通讯作者：
G. Ruffet;C. Innocent;A. Michard;G. Féraud;A. Beauvais;D. Nahon;B. Hamelin
中科院分区：
文献类型：
--
作者：
G. Ruffet;C. Innocent;A. Michard;G. Féraud;A. Beauvais;D. Nahon;B. Hamelin
锰锰矿族矿物的 KMn 氧化物，如钾锰矿 (K1–2(Mn3+, Mn4+)8O16nH2O) 经常在风化剖面中自生沉淀。结构 K 的存在使得这些矿物可以通过 KAr 和 40Ar/39Ar 方法进行测年，从而可以研究风化过程中氧化锋的进展。 Vasconcelos 等人最近对 Carajàs 地区（巴西亚马逊）Azul 锰矿床的锰矿进行 40Ar/39Ar 研究。 （1994）定义了三个年龄簇（65-69、51-56 和 40-43 Ma），并提出它们对应于矿床中已发现的三代锰氧化物的幕式沉淀（Beauvais 等，1987）。我们对来自同一锰矿床的新样品进行了激光探针 40Ar/39Ar 和 87Rb/87Sr 研究。我们的 40Ar/39Ar 数据证实锰矿是适合 40Ar/39Ar 测年的矿物，尽管在某些情况下我们清楚地确定了 39Ar 反冲效应的存在。尽管相应的年龄谱通常受到强烈干扰，但我们的结果也证实了最早的钾锰矿的产生是在上白垩世-古新世时代。我们从第二代隐黑岩的矿脉和结核中获得了良好的平台年龄。其中一些结果允许定义 Vasconcelos 等人未观察到的 46.7-48.1 Ma 的严格约束年龄簇。 （1994）。岩相学研究证实，本研究中分析的样品均不含与第三代钾锰矿相关的材料。我们认为这些新结果支持了 KMn 氧化物或多或少连续结晶的想法，主要受局部因素的限制，而不是 Vasconcelos 等人提出的模型。 (1994)，这表明每一代钾锰矿都对应于与全球气候变化相关的不同风化事件。87Sr/86Sr 数据显示出很大的变化，明显继承自 Mn 原矿的 2.1 Ga 母岩。 Rb/Sr 结果表明，在钾锰矿结晶过程中发生的分馏最少，但最新一代除外，其 Sr 已耗尽。这排除了使用 Rb/Sr 放射计时器对红土中二次锰氧化物进行测年的可能性。
KMn oxides of hollandite group minerals such as cryptomelane (K1–2(Mn3+, Mn4+)8O16nH2O) are often precipitated authigenically in weathering profiles. The presence of structural K allows these minerals to be dated by the KAr and40Ar/39Ar methods, making it possible to study the progression of oxidation fronts during weathering processes. Within the context of a recent40Ar/39Ar study of cryptomelane from the Azul Mn deposit in the Carajàs region (Amazônia, Brazil), Vasconcelos et al. (1994) defined three age clusters (65–69, 51–56, and 40–43 Ma) and proposed that they correspond to the episodic precipitation of the three generations of Mn oxide that have been identified in the deposit (Beauvais et al., 1987). We performed a laser probe40Ar/39Ar and87Rb/87Sr study on new samples from the same Mn deposit. Our40Ar/39Ar data confirm that cryptomelane is a suitable mineral for40Ar/39Ar dating, although in some cases we clearly identify the existence of39Ar recoil effects. Although the corresponding age spectra are generally strongly disturbed, our results also confirm that the earliest cryptomelane generation is of Upper Cretaceous-Paleocene age. We obtained good plateau ages from veins and concretions of the second cryptomelane generation. Some of these results allow definition of a well-constrained age cluster at 46.7–48.1 Ma not observed by Vasconcelos et al. (1994). A petrographic study confirms that none of the samples analyzed in the present study contained material associated with the third generation of cryptomelane. We propose that these new results support the idea of a more or less continuous crystallization of KMn oxides, mainly constrained by local factors, rather than the model advanced by Vasconcelos et al. (1994), which suggests that each cryptomelane generation corresponds to distinct weathering events related to global climatic changes.87Sr/86Sr data show large variations, clearly inherited from the 2.1 Ga parent rock of the Mn protore. The Rb/Sr results demonstrate that minimum fractionation occurs during cryptomelane crystallization, except for the latest generation, which is depleted in Sr. This precludes use of the Rb/Sr radiochronometer for dating secondary Mn oxides in laterites.