火山岩U-Th矿物等时线年龄的新模型及其意义  

New model for the U-Th isochron systematics on volcanic rocks and its significance.

作　　者：隋建立[1] 樊祺诚[2] 刘嘉麒[1] R S J Sparks 

机构地区：[1]中国科学院地质与地球物理研究所,北京100029 [2]中国地震局地质研究所,北京100029 [3]Department of Earth Sciences,University of Bristol,Bristol BS8 1RJ,UK

出　　处：《岩石学报》2006年第6期1511-1516,共6页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(No.40372044);国家自然科学基金重大国际合作研究项目(中朝两侧长白山火山作用及其与东北亚板块体系的关系)资助。

摘　　要：U-Th矿物等时线法定年适用于小于350ka范围的年轻火山岩。但是,火山岩矿物U-Th同位素比值不仅反映了火山的喷发年龄,而且受到岩浆房过程和结晶分异过程时间尺度的影响,在一些研究中,出现了等时线年龄(表面年龄)比实际火山喷发年龄偏老的问题,这种现象为研究岩浆房滞留时间和矿物结晶分异时间尺度问题提供了很好的条件。长期以来,国际上不乏此类研究,但迄今为止,还没有准确、实用的数值模型来描述结晶分异过程对矿物等时线年龄的影响,更无从谈起合理的计算方程。本文以一个简化的晶体生长模型(岩浆房U-Th同位素初始比值保持不变;不考虑可能的熔蚀和结晶间断;矿物的结晶速率可以简化表示,例如dV=kt·dt)为例,考察了在晶体生长(结晶分异)条件影响下的U-Th同位素衰变关系: ((^(230)Th)/(^(232)Th))=((^(230)Th)/(^(232)Th))_0·1/(λT_m)·(e^(-λ(T_1-T_m))-e^(-λT_1))+((^(238)U)/(^(232)Th))·1/(λT_m)·(e^(-λT_1)-e^(-λ(T_1-T_m)))]这仍然是一个线性方程,斜率(m)表达了火山岩喷发年龄(T_2)和矿物结晶分异时间(T_m,岩浆房滞留时问)尺度的耦合关系,而不只是喷发年龄,用方程表示为m=1/(λT_m)·(e^(-λ(T_2+T_m))-e^(-λT_2))新的模型表明,等时线出现的一般条件是不同种类的矿物具有相同的结晶历史(包括火山岩喷发年龄和矿物结晶分异的时间尺度),而不是单一的喷发年龄,在大多数年轻火山岩喷发年龄相对表较明确的条件下,我们可以应用该模型估算很多火山的岩浆房滞留时间尺度。把新的模型和方程应用于天池火山的U-Th同位素年代学研究,根据Dunlap(1996)分析测试的数据,天池火山千年大喷发岩浆房滞留时间为100ka左右。这个时间与Dunlap(1996)所估计的时间(60～100ka)的上限一致;值得注意的是,在我们的新模型中,我们认为岩浆房滞留时间和矿物结晶分异时间尺度可能被低估,实际时间应当大于�Mineral U-Th isochron systematic is a popular method in young volcanism （ 〈 350ka） dating, based on U-series disequilibrium. In some cases, the appearance ages of the U-Th mineral isochron were much older than they should be, which was contribute to the prolonged history of fractional crystallization and hence the prolonged magma residence time. Since there is still no numerical model for interpretation and calculation of the values of the prolonged history of fractional crystallization based on the U-Th mineral isochron, here we provide a new model interpreting the U-Th isotopic variations in a prolonged history of fractional crystallization, and then calculate the value of the crystallization history. This model reveals that U-Th isotopic values can be described bv a linear equation：（230Th/^232Th）=（^230Th/^232Th）0·1/λTm·（e^-λ（T1-Tm）-e^-λT1）＋（^238U/^232Th）·1/λTm·（e^-λT1-e^-λ（T1-Tm）]This equation provides an alternative interpretation of the U-Th isochron, which including not only the eruption age but also the crystallization time. Based on the above equation, we can calculate the prolonged history of fractional crystallization and prolonged magma residence time of young volcanic eruptions. Use the above model on the Millennium eruption of the Tianchi Volcano, we may conclude that magma residence time was up to lOOka, based on the U-Th isochron data of Dunlap. Although this value is among the largest values of magma residence times and time scales of fractional crystallization, it is stiff a minimum estimation of model, because in the new model the erosion of the crystal cannot be calculated and will lead to underestimation of the real time. Anyway, this model starts a new method of numerical modeling on U-Th isochronology and on time scale of magma processes.

关 键 词：U-Th矿物等时线 结晶分异时间 数学模型 

分 类 号：P597.3[天文地球—地球化学]