表生钾锰矿物~(40)Ar/~(39)Ar年代学及其古气候意义

红土型风化壳和次生锰矿床形成于温暖和潮湿的古气候条件 ,其中含有丰富的表生钾锰矿物。因此 ,对表生钾锰矿物进行精确的40 Ar/ 3 9Ar年龄测定 ,不仅能查明大陆化学风化和矿床次生富集的时间和过程 ,而且可以为区域古气候的反演提供重要的年代学资料。透射电子显微镜、热重分析、离子交换实验和40 Ar/ 3 9Ar同位素分析表明 ,层状结构的黑锌锰矿、锂锰矿和钠水锰矿以及具有 1× 1隧道结构的软锰矿不适合于40 Ar/ 3 9Ar年龄测定 ;而隐钾锰矿、锰钡矿和锰铅矿因具有致密和稳定的 2× 2隧道结构及很强的保存K Ar体系的能力 ,是40 Ar/ 3 9Ar同位素定年的理想对象。硬锰矿和钙锰矿分别具有 2× 3和 3× 3隧道结构 ,由于隧道孔径过大 ,晶体结构的稳定性较差 ,其作为40Ar/ 3 9Ar测年的适用性有待于进一步证实。采用精细的激光阶段加热技术 ,可以有效克服表生钾锰矿物40 Ar/ 3 9Ar测年过程中3 9ArK 的反冲损失、多世代表生钾锰矿物的共生 ,以及表生钾锰矿物中原生矿物的污染和过量大气氩的存在等问题 ,并获得有意义的风化年龄。已有数据表明 ,表生钾锰矿物的形成主要集中在白垩纪末期、始新世末期—渐新世早期、中新世和上新世中期等 4个时期 ,可能记录了地史时期周期性的化学风化及气候的交替演变

40Ar/39Ar GEOCHRONOLOGY OF SUPERGENE K-BEARING MANGANESE OXIDES AND ITS PALEOCLIMATIC IMPLICATIONS

Laterites and supergene manganese ore deposits, which have been generally formed under warm and humid environments, are rich in potassium-bearing Mn-oxide minerals. Therefore, direct and precise ~(40)Ar/~(39)Ar dating on these K-Mn oxides will provide important constraints not only on the timing and process of continental weathering and supergene enrichments of metals, but also on the paleoclimatic evolution of regional significance. Integrated studies of transmission electron microprobe, thermogravimetric analysis, cation exchange experiments, and ~(40)Ar/~(39)Ar isotopic analysis suggest that chalcophanite, lithiophorite, birnessite, and pyrolusite, which are characterized by layered or 1×1 tunnel structure, are not suitable for ~(40)Ar/~(39)Ar dating. However, cryptomelane, hollandite, and coronadite, which have the 2×2 tunnel structure being very thermally stable and argon retentive, are ideal minerals for ~(40)Ar/~(39)Ar dating. The suitability of the 2×3 tunnel structured romanechite and the 3×3 tunnel structured todorokite to ~(40)Ar/~(39)Ar dating is still to be proven, as these minerals possess large tunnel size and are less thermally stable. Previous studies demonstrated that, using laser incremental heating ~(40)Ar/~(39)Ar method, the ~(39)Ar_K recoil loss, the overgrowth of multiple generation Mn-oxides, the hypogene contaminants in supergene phases, and the excess argon would not cause the problems in obtaining meaningful weathering ages. Available data indicate that there were four major stages for the formation of thick weathering crusts and supergene manganese ore deposits, including the latest Cretaceous, the late Eocene to the early Oligocene, the Miocene, and the middle Pliocene. This reflects possibly episodic weathering facilitated by alternating paleoclimates.