The quench control of water estimates in convergent margin magmas
The quench control of water estimates in convergent margin magmas
复制标题
DOI：
10.2138/am-2019-6735
复制
发表时间：
2017-12
影响因子：
3.1
通讯作者：
Maxim Gavrilenko;M. Krawczynski;P. Ruprecht;Wenlu Li;J. Catalano
中科院分区：
文献类型：
--
作者：
Maxim Gavrilenko;M. Krawczynski;P. Ruprecht;Wenlu Li;J. Catalano
本文研究了含水镁铁质熔体的可淬性。我们通过水热实验表明，在与天然样品相关的冷却速率下，将镁铁质含水熔体淬火成均匀玻璃的能力具有熔体中溶解的H2O不超过9 ± 1重量%的限制。我们在1-1.5 GPa下对镁铁质起始组合物进行了超液相线实验，实验范围从H2O欠饱和到H2O饱和条件（从~1到~21重量%）。在溶解H_2O并平衡后，将含水镁铁熔体实验淬灭。在玻璃化转变温度下实现了20至90 K/s的淬火速率，并且允许一些实验从热收缩中恢复，而其他实验在淬火期间保持在等压条件下。我们发现，淬火的含水熔体的均匀玻璃在淬火速率可比的自然条件下是可能的，在水含量高达6wt%。含有6- 9wt%的H2O的熔体被部分淬火成玻璃，并且总是含有显著分数的淬火晶体和玻璃变/失透产物。水含量大于9wt%的实验在淬火后没有光学透明的玻璃，并导致蚀变/失透产物（矿物和无定形材料）的细粒混合物。我们的9 ± 1wt%的极限与天然玻璃质熔体包裹体中溶解的H2O含量的最大值（8.5wt%H2O）非常一致。使用岩石学和地球化学建模估计喷发前溶解H2O含量的其他技术已被用于论证某些弧岩浆的含水量为16重量% H2O。因此，我们的研究结果提出了一个问题，即所观察到的玻璃熔体夹杂物的记录是否有一个上限，这部分是由淬火过程控制。这可能会导致低估的最大量的H2O回收弧时，玻璃熔体包裹体的结果主要用于估计水通量从地幔。
Abstract Here we present a study on the quenchability of hydrous mafic melts. We show via hydrothermal experiments that the ability to quench a mafic hydrous melt to a homogeneous glass at cooling rates relevant to natural samples has a limit of no more than 9 ± 1 wt% of dissolved H2O in the melt. We performed supra-liquidus experiments on a mafic starting composition at 1–1.5 GPa spanning H2O-undersaturated to H2O-saturated conditions (from ~1 to ~21 wt%). After dissolving H2O and equilibrating, the hydrous mafic melt experiments were quenched. Quenching rates of 20 to 90 K/s at the glass transition temperature were achieved, and some experiments were allowed to decompress from thermal contraction while others were held at an isobaric condition during quench. We found that quenching of a hydrous melt to a homogeneous glass at quench rates comparable to natural conditions is possible at water contents up to 6 wt%. Melts containing 6–9 wt% of H2O are partially quenched to a glass, and always contain significant fractions of quench crystals and glass alteration/devitrification products. Experiments with water contents greater than 9 wt% have no optically clear glass after quench and result in fine-grained mixtures of alteration/devitrification products (minerals and amorphous materials). Our limit of 9 ± 1 wt% agrees well with the maximum of dissolved H2O contents found in natural glassy melt inclusions (8.5 wt% H2O). Other techniques for estimating pre-eruptive dissolved H2O content using petrologic and geochemical modeling have been used to argue that some arc magmas are as hydrous as 16 wt% H2O. Thus, our results raise the question of whether the observed record of glassy melt inclusions has an upper limit that is partially controlled by the quenching process. This potentially leads to underestimating the maximum amount of H2O recycled at arcs when results from glassy melt inclusions are predominantly used to estimate water fluxes from the mantle.