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Late Tertiary Petrified Wood from Nevada, USA: Evidence of Multiple Silicification Pathways

机译：美国内华达州的第三纪晚期木化木：多重硅化途径的证据

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Late Tertiary fossil woods from the state of Nevada provide an opportunity for observing the mineralization sequences that cause buried wood to become permineralized. Oligocene and Miocene caldera basins contain abundant petrified wood that ranges in composition from incipient silicification to complete permineralization. Examination of specimens from 21 localities reveals that the petrifaction sequence can follow multiple pathways. Fossil wood specimens from a single stratum may have different mineralization; silicification may vary even within a single specimen. Despite these variations, several trends are evident. Features in Nevada specimens suggest that two fundamental processes are involved: early mineralization of cell walls, and later silica deposition in lumina, vessels, and rot pockets from groundwater that permeated these open spaces. The process of open-space filling may be analogous to the genesis of geodes and veins, where multiple episodes of hydrothermal precipitation may produce opal, chalcedony, and quartz as deposits within a single cavity. Silica polymorphs may coexist as primary precipitates, or they may originate from solid-state transformation of a single parent material. Relic lepisphere textures observed in some chalcedony wood specimens are evidence of opal→chalcedony transition. In Nevada, specimens that contain crystalline quartz, this mineral appears to have been formed by direct precipitation in open spaces, not from recrystallization of chalcedony. Opal-A has seldom been reported in fossil wood, but this amorphous material is fairly common in Nevada specimens.
机译：内华达州的晚第三纪化石木材为观察导致埋藏的木材矿化的矿化序列提供了机会。渐新世和中新世破火山口盆地含有丰富的石化木材，其成分从初期硅化到完全矿化。对21个地区的标本进行检查后发现，石化序列可以遵循多种途径。来自单个地层的化石木材标本可能具有不同的矿化作用;硅化甚至可能在单个样品中也不同。尽管有这些变化，但还是有一些趋势是显而易见的。内华达州标本的特征表明，这涉及两个基本过程：细胞壁的早期矿化作用，以及随后二氧化硅在管腔，容器中的渗透，以及渗透到这些开放空间中的地下水的腐烂物的沉积。开放空间填充的过程可能类似于大地脉和脉脉的成因，在这里，多次热液降水可能会在单个腔中生成蛋白石，玉髓和石英作为沉积物。二氧化硅多晶型物可以作为主要沉淀物共存，或者它们可以源自单一母体材料的固态转变。在一些玉髓的木材标本中观察到的遗迹的叶球质地是蛋白石→玉髓过渡的证据。在内华达州，样品中含有晶体石英，这种矿物似乎是在露天空间直接沉淀形成的，而不是玉髓的重结晶。很少在化石木材中报道蛋白石-A，但是这种无定形材料在内华达州的标本中相当普遍。

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