An experimental study of bromine behaviour in water-saturated silicic melts
An experimental study of bromine behaviour in water-saturated silicic melts
复制标题
DOI：
10.1016/s0016-7037(02)01339-x
复制
发表时间：
2003-05-01
影响因子：
5
通讯作者：
Métrich, N
中科院分区：
文献类型：
--
作者：
Bureau, H;Métrich, N
为了评估熔体成分对岩浆中溴浓度的影响，我们研究了具有可变 Na+K/Al 和 Si/Al 摩尔比的水饱和、无铁硅熔体（钠长石、单长花岗岩、流纹岩和磷钨矿）的溴化物溶解度。实验在快速淬火冷封高压釜中进行，压力（1、1.5 和 2 kbar）和温度（900、1000 和 1080 摄氏度）范围内，运行时间为 5 至 7 天。通过 PIXE（质子诱导 X 射线发射）对岩浆矿物中的一系列天然火山玻璃和熔体包裹体以及合成玻璃进行了分析。钠长石的 Br 浓度范围为 5360 至 7850 ppm，单方花岗岩的 Br 浓度范围为 2800 至 3900 ppm，流纹岩的 Br 浓度范围为 4300 至 5900 ppm，而镁铁矿的 Br 浓度范围为 9745 至 11,250 ppm。 Br 浓度与 H2O 饱和硅熔体中的压力呈负相关，并随 (Na+K)/Al 摩尔比变化，在比率接近 1 时具有最小值。对于所有这些熔体组合物，Br 的行为与氯相似。溴化物溶解度在钠长石熔体和流纹岩熔体中相似，这意味着两种成分的 D-f/m 几乎相同，并且适用于天然流纹岩，正如我们之前的研究（Bureau 等人，2000 年）所建议的那样。这意味着火山溴对大气的贡献可能很大。在天然黑曜石样品和石英、橄榄石和白榴石中的 MI 中，Br 浓度变化范围为 < 3 至 28 ppm，其中在泛紫晶熔体中浓度最高。我们将天然熔体的低溴浓度归因于地幔中这种卤素的初始丰度较低。然而，由于 Br 在水溶出之前表现为不相容元素，因此我们的结果表明，岩浆在喷发和水脱气之前可能含有比通常在火山岩中测量的几 ppm 多得多的溶解 Br。岩浆结晶过程中的 Br 行为是通过其分配到富含 H2O 的流体相中来控制的。此外，它在硅酸盐熔体中潜在的高溶解度使其成为海水和富含溴材料的岩浆污染的非常敏感的化学示踪剂。这表明对俯冲带样品中溴行为的研究可能有助于更好地了解地球储层之间的挥发物循环。版权所有 (C) 2003 爱思唯尔科学有限公司
To assess the effect of the melt composition on bromine concentrations in magmas, we have investigated bromide solubility for water-saturated, iron-free silicic melts with variable Na+K/Al and Si/Al molar ratios (albite, haplogranite, rhyolite, and pantellerite). The experiments were performed in rapid quench cold-seal autoclaves over a range of pressure (1, 1.5, and 2 kbar) and temperature (900, 1000, and 1080 degreesC) with run durations from 5 to 7 days. A series of natural volcanic glasses and melt inclusions hosted in magmatic minerals were analysed together with the synthetic glasses by PIXE (proton-induced X-ray emission). The Br concentrations range from 5360 to 7850 ppm for albite, from 2800 to 3900 ppm for haplogranite, from 4300 to 5900 ppm for rhyolite, and from 9745 to 11,250 ppm for pantellerite. Br concentrations are negatively correlated with pressure in H2O-saturated silicic melts and vary with (Na+K)/Al molar ratio with a minimum value at the ratio close to unity. Br behaves similarly to chlorine for all of these melt compositions. The bromide solubility is similar in albitic and rhyolitic melts, which implies that D-f/m is nearly the same for both compositions and is applicable for natural rhyolites as suggested in our previous study (Bureau et al., 2000). This means that the volcanic Br contribution to the atmosphere may be significant. In natural obsidian samples and MI hosted in quartz, olivine, and leucite, the Br concentration varies from < 3 to 28 ppm, with the highest concentrations in pantelleritic melts. We attribute the low Br concentrations of natural melts to a low initial abundance of this halogen in the Earth mantle. However, because Br behaves as an incompatible element before water exsolution, our results imply that magmas could contain much more dissolved Br before eruption and water degassing than the few ppm usually measured in volcanic rocks. Br behaviour during magma crystallisation is controlled by its partitioning into the H2O-rich fluid phase when this occurs. In addition, its potential high solubility in silicate melts makes it a very sensitive chemical tracer of magma contamination by seawater and Br-rich material. This infers that the investigation of Br behaviour in subduction-zone samples may help for a better understanding of volatiles cycling between the Earth reservoirs. Copyright (C) 2003 Elsevier Science Ltd.