权益分类	功能权益	普通用户	{{item.name}}会员
{{category.name}}	{{benefitItem.name}}

A study of origin and history of magma by trace element partitioning and U-Io dating method.

用微量元素分配和 U-Io 测年方法研究岩浆的起源和历史。

基本信息

批准号：
01470042
负责人：
NAGASAWA Hiroshi
金额：
$ 3.33万
依托单位：
Gakushuin University
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (B)
财政年份：
1989
资助国家：
日本
起止时间：
1989 至 1990
项目状态：
已结题

项目摘要

We have found in the lst-year research, that zircon is very difficult to synthesize from silicate melts. Thus we used Li molybdate flux to synthesize zircon containing trace elements at the level of natural rocks. Good agreement in solia-liquia partition coefficients of trace elements was found between the synthesized zircon and natural zircon, when latter values were calculated by assuming the concentrations of trace elements in glass are equal to those in the liquid magma.For the purpose of studying the diffusional change of distributions of elements in zircon crystals, synthesized zircon (ZrSiO_4) attached to synthesized hafnon (HfSiO_4) and double layered Zircon-Hafnon crystals synthesized by epitaxial growth technique were heated to 1600ﾟC for diffusion annealing. It is found that the distribution of Zr and Hf in zircon do not change significantly within 10^5 years. Taking diffusivities of U and Th are equal to those of zr and Hf in zircon, U and Th isotopes in zircon which had been crystallized in the early stage of formation of the magma did not move significantly durina-residence of zircon in magma at temperatures 800-900ﾟC for the order of 10^4 years.The U-Io age of pumtice of Nevado del Ruiz volcano erupted in 1985 in Columbia South America determined for zircon-glass pair exceeded 10^4 years. The reason for this unreasonably old age was examined by partition coefficients and major mineral compositions etc. We concluded that the zircon crystals in the pumice are mixture of phenocryst and xenocryst zircons.

我们在一年的研究中发现，从硅酸盐熔体中合成锆石非常困难。因此，我们用锂离子熔剂合成了天然岩石水平上的微量元素锆石。在假定微量元素在玻璃中的浓度与在液态岩浆中的浓度相等的情况下，合成锆石与天然锆石的固液分配系数具有很好的一致性。将合成锆英石（ZrSiO_4）附着在合成铪英石（HfSiO_4）上，并将外延生长的锆英石-铪英石双层晶体加热到1600 ℃进行扩散退火。结果表明，锆石中Zr和Hf的分布在10^5年内变化不大。假设U、Th与Zr、Hf在锆石中的扩散系数相等，在岩浆形成早期结晶的锆石中，U、Th同位素在800-900 ℃的温度下，在岩浆中停留了10^4年左右，没有发生明显的迁移。1985年南美洲哥伦比亚火山喷发的火山灰的锆石-玻璃对测得的木卫一年龄超过10^4年。从分配系数、主要矿物成分等方面分析了其年龄偏老的原因，认为浮岩中的锆石晶体是斑晶和捕虏晶锆石的混合体。