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首页> 外文期刊>Chemical geology >Platinum-group element geochemistry of the continental flood basalts in the central Emeisihan Large Igneous Province, SW China
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Platinum-group element geochemistry of the continental flood basalts in the central Emeisihan Large Igneous Province, SW China

机译：中国西南埃梅西汉中部大火成岩省大陆性玄武岩的铂族元素地球化学

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摘要

In the central zone of the Emeishan Large Igneous Province (ELIP), southwestern China, the Permian Emeishan continental flood basalts (ECFB) can be divided into high- and low-Ti basalts and alkaline basalts. At the southern margins of the central ELIP, the low- and high-Ti basalts comprise the lower and upper parts of the ECFB sequences, respectively; the alkaline basalts occur in the lower part of the ECFB sequence in a few locations. The high-Ti basalts are high in Pd (4-16 ppb) and Pt (7-18 ppb), but have large variations in It (0.04-0.88 ppb). The alkaline basalts have low TiO2 contents (1.8-2.15 wt%) and are depleted in PGE (<1 ppb Pt and Pd with exception of one sample). The low-Ti basalts may be platinum-group element (PGE) depleted or undepleted. The PGE-depleted low-Ti basalts and alkaline basalts are marked by <2 ppb Pd, and <3 ppb Pt, whereas the PGE-undepleted low-Ti basalts contain 5-18 ppb Pd and have large variations in Pt (3-19 ppb) and Ir (0.02-0.36 ppb). For the PGE-depleted low-Ti basalts and alkaline basalts, Pd and Pt decrease significantly with decreasing of Ir. Very high Cu/Pd ratios (>30,000) and very low Pt/Y ratios (230 to as low as 6) strongly suggest that the PGE-depletion in the low-Ti basalts and alkaline basalts is related to sulfide removal. The positive relationship between Pt and Pd depletion and increases in Zr/Nb and Th/Nb ratios indicate that crustal contamination played a role in driving the low-Ti basaltic magmas to S-saturation. In contrast, for the PGE-undepleted high- and low-Ti basalts, Pd decreases slightly with decreasing MgO and It, and Pd/Cr ratios increase with decreasing Pt/Y, indicating that fractionation occurred under S-under-saturated conditions. Some PGE-undepleted low-Ti basalts have very high Pd/Pt ratios (up to 6, much higher than that of the primitive mantle similar to 0.6) and extremely low It concentrations (<0.03 ppb), suggesting that the magmas experienced fractionation of olivine, chromite, pyroxene and PGE minerals rich in Os, It, Ru, and Pt. The remaining high-Ti basalts have moderate Pd/Pt ratios (<2.3) and very high Ir concentrations, up to 0.88 ppb, indicating that they possibly contain chromite and PGE minerals rich in Os, It, and Ru.
机译：在中国西南部的峨眉山大火成岩省（ELIP）的中部地区，二叠纪的峨眉山大陆性洪水玄武岩（ECFB）可分为高，低钛玄武岩和碱性玄武岩。在中央ELIP的南部边缘，低钛和高钛玄武岩分别构成ECFB序列的下部和上部。碱性玄武岩出现在ECFB序列的下部，有几个位置。高钛玄武岩的Pd（4-16 ppb）和Pt（7-18 ppb）较高，但It的变化较大（0.04-0.88 ppb）。碱性玄武岩的TiO2含量低（1.8-2.15 wt％），并且PGE贫乏（<1 ppb Pt和Pd，只有一个样品除外）。低钛玄武岩可以是耗尽的或未耗尽的铂族元素（PGE）。 PGE贫化的低Ti玄武岩和碱性玄武岩的特征是<2 ppb Pd和<3 ppb Pt，而PGE未缺失的低Ti玄武岩包含5-18 ppb Pd，并且Pt差异很大（3-19 ppb）和Ir（0.02-0.36 ppb）。对于贫PGE的低Ti玄武岩和碱性玄武岩，Pd和Pt随Ir的减少而显着降低。很高的Cu / Pd比（> 30,000）和非常低的Pt / Y比（230至低至6）强烈表明，低钛玄武岩和碱性玄武岩中的PGE损耗与硫化物的去除有关。 Pt和Pd耗尽与Zr / Nb和Th / Nb比增加之间呈正相关关系，表明地壳污染在将低Ti玄武岩浆驱动至S饱和中发挥了作用。相反，对于没有PGE的高和低Ti玄武岩，Pd随着MgO和It的减少而略有下降，Pd / Cr比随着Pt / Y的减少而增加，这表明在S-饱和条件下发生了分馏。一些未经PGE释放的低Ti玄武岩具有很高的Pd / Pt比（高达6，远高于原始地幔的0.6）和极低的It浓度（<0.03 ppb），表明岩浆经历了分馏。富含Os，It，Ru和Pt的橄榄石，铬铁矿，辉石和PGE矿物。其余的高钛玄武岩具有中等的Pd / Pt比（<2.3）和非常高的Ir浓度，最高可达0.88 ppb，表明它们可能含有富含Os，It和Ru的铬铁矿和PGE矿物。

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