A noble gas study of cubic diamonds from Zaire: constraints on their mantle source
A noble gas study of cubic diamonds from Zaire: constraints on their mantle source
复制标题
对扎伊尔立方钻石的稀有气体研究：对其地幔来源的限制
DOI：
10.1016/s0016-7037(98)00139-2
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发表时间：
1998
影响因子：
5
通讯作者：
J. Matsuda
中科院分区：
文献类型：
--
作者：
N. Wada;J. Matsuda
众所周知，立方钻石含有亚微米包裹体中的流体，这些包裹体被认为来自大陆地壳下的地幔。来自扎伊尔的16颗立方钻石的稀有气体元素和同位素组成与MORB的钻石明显不同。金刚石中的3he /4He比值为（5 - 12）× 10−6，低于典型的MORB值1 × 10−5。20ne /22Ne比率（≤11）和21ne /22Ne比率（≤0.06）表现出粗略的线性相关，斜率低于MORB定义的斜率。造成较浅斜坡的过量ne可能是由源地幔或钻石中被困流体中的核反应产生的。相对于MORB，立方钻石中过量的21ne的核源起源与它们较低的3he /4He比率一致。还观测到129xe和131 - 136xe相对于大气Xe比值高达10%的正相关过剩。它们分别归因于灭绝129i的衰变和灭绝244pu和/或现存238u的自发裂变。立方钻石的氙同位素数据与136xe /130Xe vs.129Xe/130Xe三同位素图中为MORB定义的相关线相同。发现了非常高的40ar /36Ar比率，高达35000，与MORB源地幔的期望值相似。根据3he /4He和40ar /36Ar比值，可以对立方钻石的起源进行一些限制。观察到的比例可以解释，如果立方钻石结晶的流体是从以前富含不相容元素的地幔源分离出来的。这与先前提出的含流体钻石起源模型是一致的。立方钻石中的38ar /36Ar比值略高于大气中的38ar /36Ar比值。这种高38ar /36Ar比值可能代表了保存在地幔中的原始同位素比值。如果核原38Ar的生成速率比先前报道的值高10倍左右，并且流体包裹体的年龄相当老（> ~ 109yr），则所观察到的过量38Ar可能是由于流体中35cl (α,p)38Ar的核反应。
Cubic diamonds are known to contain fluid in submicrometer inclusions which are thought to originate from the mantle beneath the continental crust. Noble gas elemental and isotopic compositions of sixteen cubic diamonds from Zaire are noticeably different from those in MORB. The3He/4He ratios observed in the diamonds are (5–12) × 10−6and lower than the typical MORB value of 1 × 10−5. The20Ne/22Ne ratios (≤11) and the21Ne/22Ne ratios (≤0.06) show a rough linear correlation and have a slope that is lower than that defined for MORB. The excess21Ne responsible for the shallower slope may be produced by nuclear reactions either in the source mantle or in the fluid trapped in the diamonds. A nucleogenic origin for excess21Ne in the cubic diamonds relative to MORB is consistent with their lower3He/4He ratios. Positively correlated excesses of129Xe and131–136Xe of up to 10% relative to the atmospheric Xe ratios are also observed. They are attributed to the decay of extinct129I and to spontaneous fission of extinct244Pu and/or extant238U, respectively. Xenon isotopic data for the cubic diamonds lie on the same correlation line as that defined for MORB in the three isotope diagram of136Xe/130Xe vs.129Xe/130Xe. Very high40Ar/36Ar ratios of up to 35000 are found and are similar to the expected value for the MORB source mantle. From the3He/4He and40Ar/36Ar ratios, some constraints can be made on the origin of cubic diamonds. The observed ratios can be explained if the cubic diamonds crystallized in a fluid which separated from a mantle source previously enriched in incompatible elements. This is consistent with the previously suggested model for the origin of fluid- bearing diamonds. The38Ar/36Ar ratios in the cubic diamonds are slightly but significantly higher than the atmospheric ratio. This high38Ar/36Ar ratio may represent the primordial isotopic ratio preserved in the mantle. Alternatively, the observed excess38Ar may be attributed to the nuclear reaction35Cl(α,p)38Ar in the fluid phase if the production rate of nucleogenic38Ar is about ten times higher than the previously reported value and the age of the fluid inclusions is quite old (>109yr).