Reliability of Os model ages in pervasively metasomatized continental mantle lithosphere: a case study of Sidamo spinel peridotite xenoliths (East African Rift, Ethiopia)
Reliability of Os model ages in pervasively metasomatized continental mantle lithosphere: a case study of Sidamo spinel peridotite xenoliths (East African Rift, Ethiopia)
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DOI：
10.1016/j.chemgeo.2004.04.008
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发表时间：
2004-08
期刊：
影响因子：
3.9
通讯作者：
L. Reisberg;J. Lorand;R. Bedini
中科院分区：
文献类型：
--
作者：
L. Reisberg;J. Lorand;R. Bedini
报道了埃塞俄比亚Sidamo地区一套尖晶石橄榄岩捕虏体的Re-Os结果，该捕虏体位于埃塞俄比亚主裂谷（MER）最南端附近。硫化物岩相学和铂族元素（PGE）特征，以及这些样品的主要和微量元素地球化学已在以前的研究[地球行星。Sci. Lett. 153（1997）67; Chem.Geol.196（2003）57]，并显示出反映了熔体渗透的影响，可能与引发裂谷的地幔柱的撞击有关。Os同位素比值大致与橄榄岩中熔体贫化指数相关，为古熔体提取事件后的长时间放射性向内生长提供了证据。然而，这种相关性已经受到干扰，可能在早期研究记录的熔体渗流过程中。考虑的Os数据与硫化物岩相学，铂族元素光谱，地球化学和纹理变化的岩石表明，机制的Os流动性改变的熔体向上迁移通过渗滤柱。在柱体底部附近的高孔隙度区域，现在由具有颗粒结构的橄榄岩代表，来自通过熔体的Os被单硫化物固溶体相清除。在这些岩石中的Os同位素比值的明显修改表明，大量的熔体，也许2至5克每克岩石，通过粒状相橄榄岩。在较浅的，低孔隙度的橄榄岩与变形的纹理为代表的区域，熔体是高度进化和挥发分丰富。这些挥发分可能作为载体的Os流动性明显的高187 Os/188 Os的比例的方辉橄榄岩样品。尽管最近的熔体渗透的影响，Os系统的主要特点是一般相关的187 Os/188 Os与熔体消耗指数。这表明，Os同位素仍然可以提供有用的年代学信息，即使在岩石圈已强烈扰动的地幔柱的冲击的地区。模型年龄从这个相关性，以及最贫化样品的再贫化年龄，表明熔体提取发生在2.4和2.8 Ga之间。这比上覆地壳的泛非年龄要古老得多[J.非洲地球科学。26（1998）207]，并可能表明该地区地壳和岩石圈地幔之间的主要脱钩，地幔代表了附近的坦桑尼亚岩根的延伸。然而，Sidamo橄榄岩的地球化学并不像典型的地幔[地球行星。Sci. Lett. 96（1989）15]。此外，最近的几项研究[地球行星。Sci. Lett. 177（2000）319;第九届年度V.M. Goldschmidt Conference，Abstract #7389，LPI Contribution No. 971，Lunar and Planetary Institute，Houston（CD-ROM），1999; Geochim. Cosmochim Acta 55（1991）1421; Science 281（1998）2011]已经表明，大量耗尽的物质可以在对流地幔中存活数亿年。因此，在一个给定的地区，将熔体抽取的时间与岩石圈稳定化的时间等同起来可能是不明智的。
Re–Os results are reported for a suite of spinel peridotite xenoliths from the Sidamo region of Ethiopia, near the southernmost tip of the Main Ethiopian Rift (MER). The sulfide petrography and platinum group element (PGE) characteristics, as well as the major and trace element geochemistry of these samples have been investigated in previous studies [Earth Planet. Sci. Lett. 153 (1997) 67; Chem. Geol. 196 (2003) 57], and shown to reflect the effects of melt percolation, probably related to the impingement of the mantle plume that initiated rifting. The Os isotopic ratios correlate roughly with indices of melt depletion in the peridotites, providing evidence for a long period of radiogenic ingrowth subsequent to an ancient melt extraction event. Nevertheless, this correlation has been perturbed, probably during the melt percolation process documented by the earlier studies. Consideration of the Os data in conjunction with the sulfide petrography, PGE spectra, and geochemical and textural variations of the rocks indicates that the mechanism of Os mobility changed as the melts migrated upwards through the percolation column. In the high porosity region near the bottom of the column, now represented by the peridotites with granular textures, Os from the passing melts was scavenged by monosulfide solid solution phases. The apparent modification of the Os isotopic ratios in these rocks suggests that large quantities of melt, perhaps 2 to 5 g per gram of rock, passed through the granular facies peridotites. In the shallower, low porosity region represented by peridotites with deformed textures, the melt was highly evolved and volatile rich. These volatiles may have served as the vector for Os mobility evident in the high187Os/188Os ratios of the harzburgitic samples. Despite the effects of recent melt percolation, the main feature of the Os systematics is the general correlation of187Os/188Os with indices of melt depletion. This demonstrates that Os isotopes may still provide useful chronological information, even in regions where the lithosphere has been strongly perturbed by the impingement of a mantle plume. Model ages obtained from this correlation, as well as the Re-depletion age of the most depleted sample, indicate that melt extraction occurred between 2.4 and 2.8 Ga. This is much older than the Pan-African age of the overlying crust [J. African Earth Sci. 26 (1998) 207], and could suggest a major decoupling between the crust and the lithospheric mantle in this region, with the mantle representing an extension of the nearby Tanzanian cratonic root. However, the geochemistry of the Sidamo peridotites does not resemble that of typical cratonic mantle [Earth Planet. Sci. Lett. 96 (1989) 15]. Furthermore, several recent studies [Earth Planet. Sci. Lett. 177 (2000) 319; Ninth Annual V.M. Goldschmidt Conference, Abstract #7389, LPI Contribution No. 971, Lunar and Planetary Institute, Houston (CD-ROM), 1999; Geochim. Cosmochim. Acta 55 (1991) 1421; Science 281 (1998) 2011] have shown that large masses of depleted material can survive in the convecting mantle for hundreds of millions of years. Thus it may be unwise to equate the time of melt extraction with the time of lithosphere stabilization in a given region.