Time scales of magmatic processes
Time scales of magmatic processes
复制标题
岩浆过程的时间尺度
DOI：
10.1016/s0012-821x(03)00634-4
复制
发表时间：
2004
影响因子：
5.3
通讯作者：
G. Zellmer
中科院分区：
文献类型：
--
作者：
C. Hawkesworth;R. George;S. Turner;G. Zellmer
改进的分析技术的发展促进了短寿命同位素在岩浆过程研究中的应用，并使人们对其他一些计时器重新产生了兴趣。两种方法已被用来确定岩浆过程的时间尺度。同位素测年法提供矿物相生长的绝对年龄。这通常涉及矿物分离的分析，因此难以建立单个颗粒的结构关系。唯一的例外是锆石，它可以在现场进行分析。第二种方法是使用相对测时法的基础上，主要的，微量元素和同位素分布的晶体，可能已被修改的扩散。这些产生了晶体在特定温度下持续多久的信息，但没有表明这是何时发生的。年龄是在单个晶体上获得的，因此可以在同一整块岩石的不同晶体上确定年龄分布。在火成岩中，晶体和液体的年龄（以基质为代表）可能不同，在许多情况下，已经表明，即使是较大的，因此通常较老的晶体也是在对整个岩石成分负责的分步结晶之后形成的。其中一个含义是，岩浆分异的过程，负责整个岩石成分可能不一定是推断出的组成记录较大的晶体。因此，采用不同的方法来研究结晶历史和岩浆岩套的分异。结晶速率为每秒10 - 10 - 11厘米，而分化成高硅质岩浆可能需要2× 105年。火山爆发时晶体的年龄可以追溯到105- 106年，更老的年龄往往是在更进化的岩石类型中，而在个别火山中心产生高硅岩浆可能需要105年。看来，这种演化的岩浆的产生是热控制的，对于分步结晶和地壳熔体的产生，分步结晶的速率可以，例如，与火山的功率输出。如果结晶作用是岩浆脱气或减压的反应，那么结晶作用将很快，而且可能没有足够的时间来进行分步结晶。
The development of improved analytical techniques has facilitated the application of short-lived isotopes to the study of magmatic processes, and resulted in a renewed interest in a number of other chronometers. Two approaches have been used to determine the time scales of magmatic processes. Isotopic dating provides absolute ages for the growth of mineral phases. This usually involves analyses of mineral separates such that the textural relations of the individual grains are difficult to establish. An exception is zircon, which can be analysed in situ. The second approach is to use relative chronometry based on major, trace element and isotope profiles in crystals that may have been modified by diffusion. These yield information on how long crystals were at a particular temperature, without indicating when this occurred. The ages are obtained on individual crystals, and so age distributions can be determined on different crystals from the same whole rock. The ages of crystals and the liquid, as represented by the groundmass, in an igneous rock can be different, and in a number of cases it has been shown that even the larger, and therefore typically older crystals formed after the fractional crystallisation responsible for the whole rock composition. One implication is that the processes of magma differentiation responsible for whole rock compositions may not necessarily be inferred from the compositional record of the larger crystals. Different approaches are therefore used to investigate the crystallisation history and the differentiation of magmatic suites. Crystallisation rates are ∼10−10–10−11cm/s, whereas differentiation to high-silica magmas may take up to 2×105years. The ages of crystals at the time of eruption can range back to 105–106years, the older ages tend to be in the more evolved rock types, and it can take 105years for high-silica magmas to be generated at individual volcanic centres. It appears that the generation of such evolved magmas is thermally controlled, for both fractional crystallisation and the generation of crustal melts, and the rates of fractional crystallisation can, for example, be linked to volcanic power outputs. If crystallisation is in response to magma degassing or decompression, it will be fast and there may be too little time for fractional crystallisation to take place.