Possible effects of grain-boundary REE on the REE distribution in felsic melts derived by partial melting
Possible effects of grain-boundary REE on the REE distribution in felsic melts derived by partial melting
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晶界稀土元素对部分熔融长英质熔体中稀土元素分布的可能影响
DOI：
10.2343/geochemj.24.57
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发表时间：
1990
影响因子：
0.8
通讯作者：
Koshi Yamamoto
中科院分区：
文献类型：
--
作者：
Kazuhiro Suzuki;M. Adachi;Koshi Yamamoto
用同位素稀释法和电子探针方法测定了日本中部白垩纪Naegi花岗岩全岩及其伴生副矿物中稀土元素的丰度。每千克花岗岩中含有副矿物：萤石853.8 mg、磷灰石215.2 mg、钛铁矿166.5 mg、锆石145.5 mg、独居石78.1m g、纤锌矿10.5m g、褐柱石9.5m g、金红石3.6m g、铀矿1.7m g、针铁矿1.4m g。花岗岩中的稀土主要赋存于副矿物中，但在轻稀土端，副矿物对全岩稀土丰度的贡献变得不那么重要，其稀土总量仅占全岩La、Ce、Nd、Sm和Gd-Yb的53±6%、59±6%、62±8%、66±10%和74±11%。岩石中难以解释的稀土元素储存在组成矿物的晶界上。花岗岩晶界的球粒陨石归一化稀土配分模式(全岩减去配位体之和)为轻稀土富集型，由两个以Gd为转折点的线段组成。由于认为晶界在部分熔融时首先熔融，早期形成的熔体将保留晶界的稀土元素特征。有些花岗岩类的稀土配分模式由两个转折点位于Gd附近的线段组成，被认为是由地壳岩石部分熔融形成的，它们可能来自具有与Naegi花岗岩类似的晶界稀土分布的源岩。
Abundances of rare earth elements (REEs) in the whole rock and coexisting accessory minerals from a sample of the Cretaceous Naegi granite from central Japan, were determined with isotope dilution and electron microprobe methods. The granite contains the following amounts of accessory minerals: 853.8 mg fluorite, 215.2 mg apatite, 166.5 mg ilmenite, 145.5 mg zircon, 78.1 mg monazite, 10.5 mg fergusonite, 9.5 mg allanite, 3.6 mg rutile, 1.7 mg columbite and 1.4 mg thorite per 1 kg. REEs in the granite reside mainly in the accessory minerals, but the contribution of the accessories to the whole-rock REE abundance becomes less important toward the light REE end; the sum of REEs in the accessories accounts for only 53±6% of the whole-rock La, 59±6% Ce, 62±8% Nd, 66±10% Sm and 74±11% Gd-Yb. The unaccountable REEs in the rock are stored at the grain-boundaries of constituent minerals. The chondrite-normalized REE pattern for the grain-boundary (whole-rock minus sum of accessories) of the granite is light REE-enriched and consists of two linear segments with turning point at Gd. Since grain-boundaries are considered to melt first at partial melting, the early-formed melt will retain the REE characteristics of grain-boundaries. Some granitoids, which possess REE patterns consisting of two linear segments with turning point at around Gd and are considered to be formed by partial melting of crustal rocks, may have originated from source rocks with a grain-boundary REE distribution like that of the Naegi granite.