Petrology and Mineral Chemistry of Crust and Mantle Fragments in an Archean Ophiolite from the North China Craton

关于太古代地幔中FeO含量的理论很难检验，因为熔化产生玄武质地壳的直接岩石样品从未被鉴定和描述过。地幔中FeO含量的不稳定性对理解地幔动力学有着深远的影响，因为Fe比所有其他造岩元素都重得多，即使是适度的铁富集也必须通过大幅度的温度升高来补偿，以保持中性浮力。因此，最近在华北地区发现的一个完整的蛇绿岩和构造地幔橄榄岩（Kusky等，二○ ○一年; Science 292，1142-1145）具有巨大的重要性，因为它为限制太古代地幔的FeO含量创造了前所未有的机会。 不幸的是，华北陆壳的地幔样品已经变质，原始太古代地球化学的改变程度目前尚不清楚。我们计划在一个为期一年的探索性试点项目中评估从这些样品中提取原始地幔FeO含量是否可行。我们将使用罗格斯大学的JEOL-8600超级探针和先进的微束升级，以获得变质和可能的残留火成岩矿物的化学数据。显微图像软件将用于获得每个薄片1-4百万个点的模式分析，这将允许对整个岩石主要元素地球化学进行质量平衡计算。罗格斯大学获得的矿物和全岩化学将与合作者A获得的全岩主要元素地球化学进行交叉检查。Polat在温莎大学（加拿大）使用XRFS。 这些数据将与中生代的深海橄榄岩和蛇绿岩中的地幔橄榄岩的成分进行比较。 任何成分上的差异都将根据以下可能的情况进行评估：1）不同的火成岩温压历史，2）变质蚀变效应，或3）地幔成分的长期变化。

HerzbergEAR-0228592Theories on the FeO content of the Archean mantle are difficult to test because direct rock samples that were melted to produce basaltic crust have never been identified and described. Uncertainties in mantle FeO content have profound consequences on understanding mantle dynamics because Fe is much heavier than all other rock-forming elements, and even a modest iron-enrichment must be compensated by large temperature increases in order to maintain neutral buoyancy. Therefore, the recent discovery of a complete ophiolite with tectonized mantle peridotite from the North China Craton (Kusky et al., 2001; Science 292, 1142-1145) is of monumental importance because it has created an unprecedented opportunity for constraining the FeO content of the Archean mantle. Unfortunately, the mantle samples from the North China Craton have been metamorphosed, and the extent to which the original Archean geochemistry has been modified is not presently known. We plan on evaluating whether it is feasible to extract from these samples the original mantle FeO content in a 1-year exploratory pilot project. We will use the JEOL-8600 Superprobe with Advanced Microbeam upgrade at Rutgers University to obtain chemistry data on metamorphic and possible relect igneous minerals. Microimage software will be used to obtain modal analyses of 1-4 million points per thin section, and this will permit a mass balance calculation to be made of whole rock major element geochemistry. Mineral and whole rock chemistry obtained at Rutgers University will be crosschecked with whole rock major element geochemistry obtained by collaborator A. Polat at Windsor University (Canada) using XRFS. These data will be compared with the composition of mantle peridotite from abyssal peridotites and ophiolites of Phanerozoic ages. Any differences in composition will be evaluated in terms of possible: 1) different igneous T-P histories, 2) metamorphric alteration effects, or 3) secular variations in mantle composition.