Diamond formation episodes at the southern margin of the Kaapvaal Craton: Re-Os systematics of sulfide inclusions from the Jagersfontein Mine
Diamond formation episodes at the southern margin of the Kaapvaal Craton: Re-Os systematics of sulfide inclusions from the Jagersfontein Mine
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DOI：
10.1007/s00410-008-0350-9
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发表时间：
2009-04-01
影响因子：
3.5
通讯作者：
Harris, Jeff W.
中科院分区：
文献类型：
--
作者：
Aulbach, Sonja;Shirey, Steven B.;Harris, Jeff W.
对侵入Kaapva al克拉通南缘的90 Ma Jagersfontein金伯利岩中的硫化物包裹体进行了Re-Os同位素系统分析，以确定其寄主钻石的年龄和成因。后者具有-3.5~-9.8a欧氏度的增量C-13和与大陆岩石圈地幔深处的时间/温度历史相对应的氮聚集状态(从纯IAA型到51%的总氮作为B中心)。大多数硫化物是贫镍的(17个包裹体中15个为[Ni+Co]/Fe=0.05-0.25)，具有高的铜/[Fe+Ni+Co]比值(0.02-0.36)和元素Re-Os比值在0.5-46之间(14个包裹体中有12个)，这是榴辉岩-辉石岩地幔来源的典型特征。Re-Os同位素系统学指示了两代钻石：(1)在1.7Ga年龄阵列上的钻石，初始Os-187/Os-188(Os-187/Os-188(I))为0.46+/-A0.07；(2)在1.1Ga阵列上的钻石，其Os-187/Os-188(I)为0.30+/-A0.11。两代钻石的放射性成因初始Os同位素组成表明，参与了具有高时间积分Re-Os的成分，可能是通过古俯冲洋壳的再动员和橄榄岩的杂化。聚集态氮含量较低的钻石中含有较高的Os和Ni含量但明显较低的Os-187/Os-188的单一硫化物可能代表了晚一代橄榄岩钻石的一部分。来自卡瓦尔克拉通的Jagersfontein和其他金伯利岩中的钻石中橄榄岩硫化物包裹体的稀少，与具有橄榄岩硅酸盐包裹体的钻石的总体相对丰度形成对比。这可能与Kaapvaal克拉通根形成过程中的极度枯竭和硫消耗有关，结果是在橄榄岩中，只有在后来重新引入硫的过程中才能形成包含硫化物的钻石。
Sulfide inclusions in diamonds from the 90-Ma Jagersfontein kimberlite, intruded into the southern margin of the Kaapvaal Craton, were analyzed for their Re-Os isotope systematics to constrain the ages and petrogenesis of their host diamonds. The latter have delta C-13 ranging between -3.5 and -9.8aEuro degrees and nitrogen aggregation states (from pure Type IaA up to 51% total N as B centers) corresponding to time/temperature history deep within the subcontinental lithospheric mantle. Most sulfides are Ni-poor ([Ni + Co]/Fe = 0.05-0.25 for 15 of 17 inclusions), have elevated Cu/[Fe + Ni + Co] ratios (0.02-0.36) and elemental Re-Os ratios between 0.5 and 46 (12 of 14 inclusions) typical of eclogitic to more pyroxenitic mantle sources. Re-Os isotope systematics indicate two generations of diamonds: (1) those on a 1.7 Ga age array with initial Os-187/Os-188 (Os-187/Os-188(i)) of 0.46 +/- A 0.07 and (2) those on a 1.1 Ga array with Os-187/Os-188(i) of 0.30 +/- A 0.11. The radiogenic initial Os isotopic composition for both generations of diamond suggests that components with high time-integrated Re-Os are involved, potentially by remobilization of ancient subducted oceanic crust and hybridization of peridotite. A single sulfide with higher Os and Ni content but significantly lower Os-187/Os-188 hosted in a diamond with less aggregated N may represent part of a late generation of peridotitic diamonds. The paucity of peridotitic sulfide inclusions in diamonds from Jagersfontein and other kimberlites from the Kaapvaal craton contrasts with an overall high relative abundance of diamonds with peridotitic silicate inclusions. This may relate to extreme depletion and sulfur exhaustion during formation of the Kaapvaal cratonic root, with the consequence that in peridotites, sulfide-included diamonds could only form during later re-introduction of sulfur.