Chemical Differentiation by Mineralogical Buffering in Crustal Hot Zones
Chemical Differentiation by Mineralogical Buffering in Crustal Hot Zones
复制标题
地壳热区矿物学缓冲的化学分异
DOI：
10.1093/petrology/egac054
复制
发表时间：
2022
影响因子：
3.9
通讯作者：
Blundy J
中科院分区：
文献类型：
--
作者：
Blundy J
会聚边缘岩浆的化学多样性是地壳热区火成岩分异的产物，垂直延伸的区域以分散在富含晶体的糊状物中的低体积（<20%）移动的熔体相为特征。浮力、膨胀的熔体和周围的泥浆之间的化学反应导致了当地矿物组合的化学缓冲。在这种组合具有低的热力学变化（例如，六个矿物相加上熔体和H2O-CO2流体），所得到的多重饱和熔体将显示有限的化学变化。许多火山弧的深成捕虏体，以及挖出的弧地壳部分，证明了普遍存在的低方差，广泛辉长岩，矿物组合。在这里，我使用多饱和度的概念来探索地壳热区的分解反应熔体流动的化学后果，使用的数据从各种不同的起始材料上发表的实验研究。我表明，常见的，低方差角闪石辉长岩组合单斜辉石-角闪石-斜方辉石-磁铁矿-斜长石-钛铁矿（CHOMPI）共存于流体饱和熔体在宽范围的压力（1-10 kb）温度（800-1050 °C）和流体组成（摩尔分数H2O，XH 2 O，为1.0至0.3）。CHOMPI稳定性场受以下因素的限制：高压下石榴石的出现，低温下含水的单花岗岩花岗岩液相线，以及高温低压下角闪石的分解。CHOMPI熔体覆盖宽的组成范围（54-74重量% SiO2; 4.4-0.1重量% MgO），其可以根据五个独立变量参数化：压力、温度、fO 2、流体中的摩尔CO2/H2O和熔体K2 O含量。CHOMPI饱和熔体的成分多样性和广泛的稳定性领域使它们在岩石记录中非常常见。熔体组成参数化可以反演以恢复CHOMPI饱和熔体的压力（±1.3 kb）、温度（±16 °C）和流体摩尔CO2/H2O（±0.43）。如果一个天然的岩浆成分可以被证明是躺在或接近CHOMPI饱和面，然后在何种条件下，熔体是最后与这种矿物组合的平衡，可以建立。我应用这种方法的岩浆源温压和湿度最近的喷发从15瀑布弧火山中心。计算出的压力范围为1.3至5.8 kb（5-21 km深度），沿弧变化显著。温度与压力相关，并与矿物测温法对喷发温度的独立估计相匹配，但有两次喷发除外，在喷发前的岩浆储存期间发生了显着（≤10°C）的冷却。FluidXH 2 O的范围为0.47-0.92，与压力呈负相关。热区熔融化学的矿物学缓冲作用被认为是火山弧化学分异的重要机制。矿物学缓冲可以在弧壳地球物理调查中观察到的低熔体分数下操作，为在熔体丰富的系统中最有效地操作的同化-分步结晶和液体下降线的传统概念提供了一种替代方案。
Chemical diversity in convergent margin magmas is a product of igneous differentiation in crustal hot zones, vertically extensive regions characterised by a low-volume (<20%) mobile melt phase dispersed in a crystal-rich mush. Chemical reaction between buoyant, percolating melts and the surrounding mush leads to chemical buffering by the local mineral assemblage. Where this assemblage has low thermodynamic variance (e.g. six mineral phases plus melt and H2O-CO2fluid) the resultant multiply saturated melts will show limited chemical variability. Plutonic xenoliths from many volcanic arcs, as well as exhumed arc crustal sections, testify to the ubiquity of low-variance, broadly gabbroic, mineral assemblages. Here I use the concept of multiple saturation to explore the chemical consequences of percolative reactive melt flow in crustal hot zones using data from published experimental studies on a wide variety of different starting materials. I show that the common, low-variance hornblende gabbronorite assemblage clinopyroxene-hornblende-orthopyroxene-magnetite-plagioclase-ilmenite (CHOMPI) coexists with fluid-saturated melt over a wide range of pressure (1–10 kb) temperature (800–1050 °C) and fluid composition (molar fraction H2O,XH2O, of 1.0 to 0.3). The CHOMPI stability field is bounded by the following: the appearance of garnet at high pressure, the hydrous haplogranite granite liquidus at low temperature, and amphibole breakdown at high temperature and low pressure. CHOMPI melts cover a wide compositional range (54–74 wt% SiO2; 4.4–0.1 wt% MgO) that can be parameterised in terms of five independent variables: pressure, temperature,fO2, molar CO2/H2O in the fluid and melt K2O content. The compositional diversity and broad stability field of CHOMPI-saturated melts make them extremely common in the rock record. Melt composition parameterisations can be inverted to recover pressure (±1.3 kb), temperature (±16 °C) and fluid molar CO2/H2O (±0.43) of CHOMPI-saturated melts. If a natural magma composition can be shown to lie on or close to the CHOMPI saturation surface then the conditions under which that melt was last in equilibrium with this mineral assemblage can be established. I apply this method of magma source thermobarometry and hygrometry to the most recent eruptions from 15 Cascades arc volcanic centres. Calculated pressures range from 1.3 to 5.8 kb (5–21 km depth) with significant along-arc variation. Temperatures correlate with pressure and match independent estimates of eruption temperatures from mineral thermometry with the exception of two eruptions where significant (≤10°C) cooling occurred during pre-eruptive magma storage. FluidXH2O is in the range 0.47–0.92 and inversely correlates with pressure. Mineralogical buffering of melt chemistry in hot zones is proposed as an important mechanism of chemical differentiation in volcanic arcs. Mineralogical buffering can operate at the low-melt fractions observed in geophysical surveys of arc crust, providing an alternative to traditional concepts of assimilation-fractional crystallisation and liquid lines of descent that operate most effectively in melt-rich systems.