Potassium-argon/argon-40-argon-39 geochronology of Cenozoic alkali basalts from the South China Sea

Based on the isotopic chronologic results of Cenozoic alkali basalts from the South China Sea, the characteristics of volcanic activi- ty of the South China Sea after spreading were studied. The potassium - argon ages of eight alkali basalt samples from the South China Sea, and the argon - argon ages of two samples among them are reported. Apparent ages of the whole rock are 3.80 to 7. 91 Ma with an average value of 5.43 Ma （potassium- argon, whole rock）, and there is little difference among samples at the same location, e. g. , 4. 76 - 5.78 Ma for location S（M-12. The argon - argon ages for the two samples are 6.06 and 4. 71 Ma, which lie within the age scope of potassium - argon method. The dating results indicate that rock-forming age is from late Miocene to Pliocene, which is consistent with erupting event for alkali basalts from adjacent regions of the South China Sea. Volcanic activities occur after the cessation of spreading of the South China Sea, which are controlled by lithospheric fault and the spreading center formed during the spreading period of the South China Sea. These dating results, combined with geochemical characteristics of these basalts, the published chronological data for the South China Sea and its adjacent regions, and the updated geophysical data near Hainan Island, suggest that after the cessation of spreading of the South China Sea, there occur widely distributing magmatic activities which primarily is alkali basalt, and the volcanic activity continues to Quaternary. The activity may be relative to Hainan mantle plume originated from core/mantle boundary.

Evidence for Large Planetary Climate Altering Thermonuclear Explosions on Mars in the Past

Mars data presents a collection of startling and seemly contradictory isotopic data: a glaring excess of the two radiogenic isotopes 129Xe/132Xe @ 2.5 and 40Ar/36Ar @ 3000 enabled identification of MM (Mars Meteorites) because they are so different than any other major Solar System reservoir. Mars appears to have lost an original atmosphere of pressure 1 bar or greater, yet the ratio 14N/15N indicates only a loss of a few millibar by Solar Wind Erosion. The LPARE (Large Planet Altering R-process Event) hypothesis attempts to explain these major isotopic puzzles at Mars by postulating that two massive, anomalous thermonuclear explosions, rich in R-process physics, occurred over the surface of Northern Mars in the past, approximately 500 million years ago, and that these explosions created the 129Xe/132Xe excess, and the accompanying intense neutron bombardment of Mars atmosphere and regolith created the 40Ar/36Ar excess off of potassium in the surface rocks. The collateral massive and non-mass fractionating atmospheric loss, and the intense neutron bombardment of 14N in the atmosphere primarily created the 14N/15N ratio we presently observe, with some mass fractionating erosion of the residual atmosphere. This LPARE hypothesis is found to explain other isotopic features of Mars atmosphere and surface. 80Kr and 82Kr are hyperabundant in the Mars atmosphere and in the youngest MMs indicating intense irradiation of Mars surface with neutrons. Although there is presently no plausible explanation for the nuclear events, the hypothesis can be tested through related nuclear products such as Pu-244.

晚中新世南海珍贝-黄岩海山岩浆活动及其演化:岩石地球化学和年代学证据

南海海盆15°N附近呈东西向展布的珍贝-黄岩海山被认为是32~17 Ma前南海海盆的残留扩张中心。对采自黄岩海山的两个火山岩样品(9DG,9DG-2)进行了岩石学、地球化学和年代学研究。两个样品的SiO2含量分别为60.3%和63.6%,Al2O3含量分别为17.56%和17.55%,TiO2含量分别为0.48%和0.31%,碱度率分别为3.88和3.62。根据岩石学和岩石化学分类,样品属碱性系列的粗面岩。对稀土元素和微量元素分析表明岩石具有洋岛玄武岩(OIB)型配分型式,轻重稀土总量比(∑c(LREE)/∑c(HREE))和球粒陨石标准化镧镱比((La/Yb)N)分别高达17.22和27.23,并具有铕负异常和锶、磷、钛亏损的特点。样品9DG的锶-钕-铅同位素分析结果为锶-87的含量与锶-86的含量之比值为0.704 183,钕-143的含量与钕-144的含量之比值为0.512 827,铅-206的含量与铅-207的含量之比值为18.686 68,铅-207的含量与铅-204的含量之值为15.679 62,铅-208的含量与铅-204的含量之比值为39.002 61,表明初始岩浆来自软流圈地幔,具有与珍贝海山玄武岩相似的同位素组成。经钾-氩法测年,粗面岩的年龄为(7.77±0.49)Ma,略晚于珍贝海山玄武岩的年龄[(9.1±1.29)^(10.0±1.80)Ma],属于南海扩张期后晚中新世火山活动的产物。对比珍贝海山玄武岩的地球化学和同位素特征,认为两者有相同的岩浆源区,但是它们经历了不同程度的结晶分异过程,在晚中新世期间珍贝-黄岩海山可能有地幔柱活动。

用钾-氩法求地质事件真实年龄的方法探讨

钾-氩法所测出的地质事件的表面年龄一般小于地质事件发生的真实年龄,其中很多没有地质意义。本文利用随机过程理论,在对地质体内^(40)Ar的逸散作用进行分析的基础上,推导出如下年龄计算公式: 在一定条件下,利用这些公式,可以据钾-氩法测试资料求出地质体形成时代t+t^(0)、热扰动结束时代t^(0)及热事件所持续的时期t'。据所导出的公式,对冀东迁西群地层的年龄进行讨论,求得2500Ma左右结束的那期热事件所持续的时期t'为4.76亿年,迁西群形成时代为3000—3300Ma。检验表明,公式有效。

浅析成藏年代学的若干定量分析方法

油气藏成藏期的确定对于油气勘探有着重要的指导意义。传统意义上的成藏期分析方法仅能定性分析油气藏的成藏时间。讨论了油气成藏期的若干定量分析方法,包括油藏地球化学、流体包裹体测年法、钾——氩测年法、裂变径迹法,以及这些方法在应用过程中存在的问题。

胶东区域成矿断裂带蚀变年龄研究及其矿床学意义

胶东金矿是中生代构造岩浆活化区岩浆期后中温热液蚀变脉状矿床。前人曾认为该类金矿属于壳源深融花岗岩-热液石英脉-蚀变岩型金矿。近期研究表明,胶东金矿围岩蚀变年龄的测定,对于矿床成因的研究有重要意义。为解决胶东区域断裂热液蚀变年龄问题,对胶东半岛主要断裂矿化蚀变带开展如下相关研究:(1)选择胶东地区各蚀变岩带普遍发育的黄铁绢英岩化蚀变岩带和强钾长石化蚀变岩带作为取样目标,进行区域对比;(2)该两类岩石中的蚀变矿物受到脆性断裂影响较小,能够得到蚀变阶段的稳定化学成分;(3)该两类岩石中的蚀变矿物发育有与蚀变同期的韧性变形,能够揭示蚀变阶段的构造应力应变状态。对样品中的钾长石和斜长石颗粒进行K-Ar法同位素年龄测试,获得蚀变岩同位素年龄数据。测试结果显示,胶东区域构造热液蚀变作用时期为(148.3±4.3)^(52.03±0.70)Ma,即早白垩世燕山晚期;同位素年龄自西向东逐渐变小,证明胶东西部地区的热液蚀变作用早于东部地区。通过分析已有的构造、热液成分和成矿物理化学参数等资料,结合本研究的最新成果,总结出区域成矿时代与成矿规律:胶东区域断裂蚀变系统中的构造附加静水压力、流体温度压力条件和成矿元素的分布与变化控制了热液蚀变成矿规律。上述地质特征显示,胶东金矿床是复成构造热液作用下的一种新型矿床类型。

塔中地区天然气氦、氩同位素地球化学特征

氦同位素组成对大构造背景下的构造演化特征具有指示性,而对同一盆地内部较小区块的构造演化差异特征指示性不明显。塔中地区天然气的3He/4He组成具有典型壳源天然气特征,和塔里木盆地整体大地构造背景一致。与中国其他油气区寒武系-奥陶系天然气氩同位素组成相比,塔中地区天然气氩同位素40Ar/36Ar组成有一定特殊性,原因是该区中、下寒武统和中、上奥陶统烃源岩中氩的母体元素钾的丰度较低。利用元素衰变原理估算塔中地区天然气40Ar的母体元素钾的百分含量表明,塔中地区中、下寒武统和中、上奥陶统烃源岩中的泥质和泥质条带对该区天然气具有重要贡献。

如何实现油气成藏期的精确定年

随着油气田"精细勘探"的进一步发展,油气藏的精确定年日益成为一种必要。目前可实现油气藏精确定年的方法多依靠放射性同位素体系,如钾—氩、氩—氩和铼—锇等。按照所测定样品的不同,又分为成藏矿物定年、油气包裹体定年和储层有机质定年等。但在实际勘探工作中,受地质定年理论或实验方法学的影响,单一定年技术所提供的年代数据的准确度和精密度仍有所欠缺,在对地质事件的解释上也容易产生一些疑议。在如何实现油气成藏期精确定年这项命题上,不仅需要不断地提高和改进实验分析技术,还需要完善定年方法的基础地质理论,以实现对数据结果的合理解释。油气成藏年代学分析不能仅仅停留在单一的方法技术上,应从各油气藏的特点出发,寻找与油气成藏直接相关的产物,采用多技术手段,获得较全面的年代学信息。