科学家最近又有新的研究成果：来自加拿大北部古老岩石样品中的碳物质的年龄比之前所认为的“年轻”数百万年。
图： 来自波士顿大学的Papineau教授
该研究小组的成员包括来自美国波士顿大学（Boston College），华盛顿卡内基研究所（Carnegie Institution of Washington），美国航天局约翰逊航天中心（NASA's Johnson Space Center）和海军研究实验室（Naval Research Laboratory）多领域的科学家，新的研究证据显示，加拿大哈德逊湾地区（Canada's Hudson Bay）的碳颗粒并不是与岩石的形成年代一致，这中间有数百万年的差距。
样品来自于太古代（Archean）苏必利尔克拉通地区（Superior craton）的条带状铁建造（BIF）。为了更确切的认识岩石中所保存的碳物质信息，科学家们利用一系列的高科技检测手段。传统的做法会将采集的样品研碎磨成粉，然后对碳质进行分析研究。新的研究方法则利用显微镜和光谱学方法直接从岩石样品中分析研究石墨结晶的原始完整微结构。研究结果显示这些碳质相比其围岩的年龄要晚上很多。
波士顿大学科学家D. Papineau介绍说：“这些石墨结晶的特征与岩石的变质历史并不一致，表明石墨中碳的形成年代并没有岩石的年代古老。这给我们敲响了警钟，以前的研究结果需要我们重新审视了。”
Papineau还说道：“格陵兰距今约40亿年的样品被认为是最早生命出现的证据。但新的研究结果表明最早生命出现的时间要往后推数百万年，而且仍需更严谨深入的研究。科学家们现在要回去格陵兰重新确定这些碳物质是否与这些变质岩的年龄一样古老了！”
随着地球的演化，岩石和其它物质都会受到一系列高温高压的影响，古老的岩石绝大多数会遭受这样高变质作用影响。早期地球的碳物质主要来自于早期的微生物，它是早期生命存在的主要信息来源之一，是地球演化历史重要的里程碑。
Papineau最后补充道：“现在我们的建议是：在最古老的变质岩中，碳物质可能并不是本来就有的，在研究这些时代非常古老的岩石时，需要解决的两个根本问题是碳质是否来源于生物，以及其与岩石是否是同一时间形成保存至今的。”（生物谷Bioon.com）
生物谷推荐原文出处：
Geochimica et Cosmochimica Acta doi:10.1016/j.gca.2010.05.025
Ancient graphite in the Eoarchean quartz–pyroxene rocks from Akilia in southern West Greenland I: Petrographic and spectroscopic characterization
Dominic Papineau , Bradley T. De Gregorio, George D. Cody, Marc D. Fries, Stephen J. Mojzsis, Andrew Steele, Rhonda M. Stroudb and Marilyn L. Fogel
Because all known Eoarchean (>3.65 Ga) volcano-sedimentary terranes are locked in granitoid gneiss complexes that have experienced high degrees of metamorphism and deformation, the origin and mode of preservation of carbonaceous material in the oldest metasedimentary rocks remain a subject of vigorous debate. To determine the biogenicity of carbon in graphite in such rocks, carbonaceous material must be demonstrably indigenous and its composition should be consistent with thermally altered biogenic carbon as well as inconsistent with abiogenic carbon. Here we report the petrological and spectroscopic characteristics of carbonaceous material, typically associated with individual apatite grains, but also with various other minerals including calcite, in a >3.83 Ga granulite-facies ferruginous quartz-pyroxene unit (Qp rock) from the island of Akilia in southern West Greenland. In thin sections of the fine-grained parts of Akilia Qp rock sample G91-26, mapped apatites were found to be associated with graphite in about 20% of the occurrences. Raman spectra of this carbonaceous material had strong G-band and small D-band absorptions indicative of crystalline graphite. Three apatite-associated graphites were found to contain curled graphite structures, identified by an anomalously intense second-order D-band (or 2D-band) Raman mode. These structures are similar to graphite whiskers or cones documented to form at high temperatures. Raman spectra of apatite-associated graphite were consistent with formation at temperatures calculated to be between 635 and 830 °C, which are consistent with granulite-facies metamorphic conditions. Three graphite targets extracted by focused ion beam (FIB) methods contained thin graphite coatings on apatite grains rather than inclusions sensu stricto as inferred from transmitted light microscopy and Raman spectroscopy. TEM analyses of graphite in these FIB sections showed a (0 0 0 2) interplanar spacing between 3.41 and 3.64 ? for apatite-associated graphite, which is larger than the spacing of pure graphite (3.35 ?) and may be caused by the presence of non-carbon heteroatoms in interlayer sites. Samples analyzed by synchrotron-based scanning transmission X-ray microscopy (STXM) also confirmed the presence of crystalline graphite, but abundances of N and O heteroatoms were below detection limit for this method. Graphite in the Akilia Qp rock was also found to occur in complex polyphase mineral assemblages of hornblende ± calcite ± sulfides ± magnetite that point to high-temperature precipitation from carbon-bearing fluids. These complex mineral assemblages may represent another generation of graphitization that could have occurred during the amphibolite-facies metamorphic event at 2.7 Ga. Several observations point to graphitization from high-temperature fluid-deposition for some of the Akilia graphite and our results do not exclude a biogenic source of carbon in graphite associated with apatite, but ambiguities remain for the origin of this carbon.
