Preservation of biological information in thermal spring deposits: developing a strategy for the search for fossil life on Mars

Current interpretations of the early history of Mars suggest many similarities with the early Earth and therefore raise the possibility that the Archean and Proterozoic history of life on Earth could have a counterpart on Mars. Terrestrial experience suggests that, with techniques that can be employed remotely, ancient springs, including thermal springs, could well yield important information. By delivering water and various dissolved species to the sunlit surface of Mars, springs very likely created an environment suitable for life, which could have been difficult, if not impossible, to attain elsewhere. The chemical and temperature gradients associated with thermal springs sort organisms into sharply delineated, distinctive and different communities, and so diverse organisms are concentrated into relatively small areas in a predictable and informative fashion. A wide range of metabolic strategies are concentrated into small areas, thus furnishing a useful and representative sampling of the existing biota. Mineral-charged springwaters frequently deposit chemical precipitates of silica and/or carbonate which incorporate microorganisms and preserve them as fossils. The juxtaposition of stream valley headwaters with volcanoes and impact craters on Mars strongly implies that subsurface heating of groundwater created thermal springs. On Earth, thermal springs create distinctive geomorphic features and chemical signatures which can be detected by remote sensing. Spring deposits can be quite different chemically from adjacent rocks. Individual springs can be hundreds of meters wide, and complexes of springs occupy areas up to several kilometers wide. Benthic microbial mats and the resultant stromatolites occupy a large fraction of the available area. The relatively high densities of fossils and microbial mat fabrics within these deposits make them highly prospective in any search for morphological evidence of life, and there are examples of microbial fossils in spring deposits as old as 300 Myr.     

西班牙LaCodosera地区钻孔中金品位与矿脉厚度间的分形关系

西班牙西部钻孔中石英脉厚度和金品位的分析表明二者大体服从幂次律分布，暗含着一种分形关系。脉厚（ｔ）对累积频率（Ｎ）的双对数困反映出与金、品位有关的系统变化。在方程Ｎ＝ｃｔ￣（－Ｄ）中的幂指数Ｄ，在高品位区间内较低，表明较厚的脉中扩张作用较强，从而为含金流体的运移提供了通过。因此，脉体厚度分布的鉴别为区别脉状金矿床中矿化与非矿化构造提供了一种方法。     

浅成低温热液贵金属体系中金与氧化硅的胶体搬运(内华达州斯利泊矿床获得的证据)

内华达州斯利泊(Sleeper)矿床富矿脉型金矿石,系在小于500米的深度范围内沉淀形成,并具有细粒石英胶状条带与含有大量银金矿的蛋白石质氧化硅胶状条带交替的特殊结构.这种结构迹象表明石英条带是从原始的胶状非晶质状态重结晶而来,而富金条带系由氧化硅与金颗粒凝胶组成的非晶质析出物原始沉积而成.金与氧化硅首先在热液体系的较深部位以胶体形式析出,然后由成矿溶液的水力作用向上机械搬运,由于溶液的冷却或沸腾,并在复合阳离子Al~(3+)的水化物的催化下产生胶粒凝结。浅成低温热液体系中金以胶粒形式搬运表明,至少在某些条件下胶体中的金含量比真溶液中要高.欲解释斯利泊矿床富矿石的成因,需存在一种富金的成矿溶液;而胶体金的局部搬运可能对浸染状低温热液金矿床的形成起了一定的作用.     

麻粒岩相混合岩的出熔

麻粒岩相地体中的混合岩一般由发育于较细粒的叶理化基质中的一些粗粒的无水残留体细脉或网脉(活化体)所构成,我们认为这些特征是不一致的脱水熔融反应的结果(Waters,1988)。活化体并不代表分离熔融的成分,而是代表不同比例的(i)熔体(ii)不一致熔融的固体产物以及(iii)反应中没有被消耗掉的剩余相。在变质泥质岩和黑云母片麻岩中,近固相线的熔融成分为花岗质的,其水的含量可以做为T的函数估算出来,熔融反应可以达到平衡,因此生成相的相对体积也可预测。我们将计算的产物与实际观察的南非Nawagudand麻粒岩相混合岩的相分配和数量进行比较,在角闪岩与麻粒岩过渡带(约750℃)大约活化体中的典型成份表明有10     

太古宙和元古宙时期的地壳演化—从地壳地震学获得的证据

通过分析整个世界太古宇和元古宇地区的地震波速模型，发现太古宇地区的厚度为３５ｋｍ左右（碰撞带边缘除外），而元古宇地壳的厚度则显然要大得多，大致为４５ｋｍ，并且在其基底有一较厚的高速层（＞７ｋｍ／ｓ）．我们认为有两种模式可以解释这些差异。第一种模型把这种差异归因于上地幔成分的改变。太古宇地幔中的温度较高，因而导致了科马提熔岩的喷发，结果使得岩石圈过分贫化而不能产生大量的玄武质熔体。元古宇地是在富地幔