Heat capacity of hydrous trachybasalt from Mt Etna: comparison with CaAl2Si2O8 (An)-CaMgSi2O6 (Di) as basaltic proxy compositions
Heat capacity of hydrous trachybasalt from Mt Etna: comparison with CaAl2Si2O8 (An)-CaMgSi2O6 (Di) as basaltic proxy compositions
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DOI：
10.1007/s00410-015-1196-6
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发表时间：
2015-12-01
影响因子：
3.5
通讯作者：
Russell, J. K.
中科院分区：
文献类型：
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作者：
Giordano, D.;Nichols, A. R. L.;Russell, J. K.
使用差示扫描量热法从室温到玻璃化转变区域测量了六种不同水合（类似于 3.5 wt% H2O）含铁埃特纳火山粗基玄武岩玻璃和液体的比热容 (C-p)。将这些数据与无铁钙长石 (An)-透辉石 (Di) 系统中 13 种熔体组合物在相似的 H2O 含量范围内的热容测量值进行比较。这些数据大大扩展了已发表的含水熔体和玻璃的 C-p 测量值。埃特纳火山粗面玄武岩的结果表明，随着 H2O 含量的增加，玻璃转变开始时的玻璃热容（即 C-p(g)）和完全松弛液体（即 C-p(l)）均呈现非线性变化。类似地，“构型热容”（即 C-p(c) = C-p(l) - C-p(g)）随 H2O 含量呈非线性变化。研究的 An-Di 含水组合物显示出类似的趋势，C-p 值随熔体组成和 H2O 含量的函数而变化。结果表明，解聚玻璃和液体中的水合 C-p(g)、C-p(l) 和 C-p(c) 值与先前研究的更多聚合水合钠长石、淡色花岗岩、粗面质和声石质多组分组合物中观察到的值显着不同。相对于更解聚的组合物，聚合熔体具有较低的C-p(l)和C-p(c)以及较高的C-p(g)。玻璃和熔体中 C-p 值与聚合度之间的协变可以用 SM 水合和 NBO/T-水合来很好地描述。随着解聚作用的增加，C-p(c) 值急剧增加，直至 SMHydrous 接近 30-35 mol%（NBO/T-Hydrous 接近 0.5），然后稳定到几乎恒定的值。玻璃 (C-p(H2O)g) 和液体 (C-p(l) (H2O)) 的 H2O 偏摩尔热容似乎与成分无关，假设理想混合，我们得到的 C-p(l) (H2O) 值为 79 J mol(-1) K-1。然而，我们注意到，先前工作人员提出的一系列 C-p(H2O)l 值（即类似于 78-87 J mol(-1) K-1）将在实验不确定性范围内重现扩展数据。我们的分析表明，需要更多数据才能将成分依赖性（即非理想混合）归因于 C-p(l) (H2O)。
The specific heat capacity (C-p) of six variably hydrated (similar to 3.5 wt% H2O) iron-bearing Etna trachybasaltic glasses and liquids has been measured using differential scanning calorimetry from room temperature across the glass transition region. These data are compared to heat capacity measurements on thirteen melt compositions in the iron-free anorthite (An)-diopside (Di) system over a similar range of H2O contents. These data extend considerably the published C-p measurements for hydrous melts and glasses. The results for the Etna trachybasalts show nonlinear variations in, both, the heat capacity of the glass at the onset of the glass transition (i.e., C-p(g)) and the fully relaxed liquid (i.e., C-p(l)) with increasing H2O content. Similarly, the "configurational heat capacity" (i.e., C-p(c) = C-p(l) - C-p(g)) varies nonlinearly with H2O content. The An-Di hydrous compositions investigated show similar trends, with C-p values varying as a function of melt composition and H2O content. The results show that values in hydrous C-p(g), C-p(l) and C-p(c) in the depolymerized glasses and liquids are substantially different from those observed for more polymerized hydrous albitic, leucogranitic, trachytic and phonolitic multicomponent compositions previously investigated. Polymerized melts have lower C-p(l) and C-p(c) and higher C-p(g) with respect to more depolymerized compositions. The covariation between C-p values and the degree of polymerization in glasses and melts is well described in terms of SMhydrous and NBO/T-hydrous. Values of C-p(c) increase sharply with increasing depolymerization up to SMhydrous similar to 30-35 mol% (NBO/T-hydrous similar to 0.5) and then stabilize to an almost constant value. The partial molar heat capacity of H2O for both glasses (C-p(H2O)g) and liquids (C-p(l) (H2O)) appears to be independent of composition and, assuming ideal mixing, we obtain a value for C-p(l) (H2O) of 79 J mol(-1) K-1. However, we note that a range of values for C-p(H2O)l (i.e., similar to 78-87 J mol(-1) K-1) proposed by previous workers will reproduce the extended data to within experimental uncertainty. Our analysis suggests that more data are required in order to ascribe a compositional dependence (i.e., nonideal mixing) to C-p(l) (H2O).