Amphibole stability in primitive arc magmas: effects of temperature, H2O content, and oxygen fugacity
Amphibole stability in primitive arc magmas: effects of temperature, H2O content, and oxygen fugacity
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DOI：
10.1007/s00410-012-0740-x
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发表时间：
2012-04
影响因子：
3.5
通讯作者：
M. Krawczynski;T. Grove;H. Behrens
中科院分区：
文献类型：
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作者：
M. Krawczynski;T. Grove;H. Behrens
测定了本区原始镁质安山岩(57wt%SiO_2，9wt%MgO)的饱和水相关系。压力范围200-800兆帕，温度范围915-1,070摄氏度，氧逸度从镍-氧化镍(NNO)缓冲液到NNO(NNO+3)以上三个对数单位不等。原始玄武岩安山岩(52wt%SiO_2，10.5wt%MgO)的相图也来自该山。沙斯塔地区(Grove et al.在Contrrib Miner Petroleum 145：515-533；2003)中补充了500兆帕的额外实验数据。这些成分的水合相关系允许比较溶解的H2O对结晶序列的显著影响。液相线矿物相稳定性和出现温度随压力和H2O含量的变化而敏感地变化，这些信息被用于校准原始弧形岩浆的岩浆气压计-湿度计。水饱和实验表明，角闪石的稳定性强烈依赖于水的分压。在原始安山岩成分中橄榄石和角闪石共存的狭窄稳定区在500 Mpa以上，至少持续到800 Mpa，在975-1,025°C之间。随着H2O压力()的增加，液相线和角闪石外观之间的温差减小，导致第一个角闪石化学成分的变化结晶。提出了角闪石初现气压计-湿度计的经验定标方法，该气压计使用角闪石中的镁元素和：P_{\Text{H}_{{\rM_O}}({\rm Mpa})=\Left[{\rM_mG_#}}{52.7}}-0.014*\\上增量{\rM_nno}\右]^{15.12}该气压计给出了原始弧状玄武岩和安山岩中首次出现角闪石所记录的最小值。我们将这个晴雨表应用于从山上喷发的安山岩和英安岩混合熔岩中的角闪石前结晶。加利福尼亚州沙斯塔，平流层。高H2O压力(500-900 Mpa)和高喷发前岩浆H2O含量(10-14wt%H2O)是保存在沙斯塔熔岩中的岩浆混合原始端元的标志。我们还使用这些新的实验数据来探索和评估Larocque和Canil(2010)的经验角闪石气压计。
The water-saturated phase relations have been determined for a primitive magnesian andesite (57 wt% SiO2, 9 wt% MgO) from the Mt. Shasta, CA region over the pressure range 200–800 MPa, temperature range of 915–1,070 °C, and oxygen fugacities varying from the nickel–nickel oxide (NNO) buffer to three log units above NNO (NNO+3). The phase diagram of a primitive basaltic andesite (52 wt% SiO2, 10.5 wt% MgO) also from the Mt. Shasta region (Grove et al. in Contrib Miner Petrol 145:515–533; 2003) has been supplemented with additional experimental data at 500 MPa. Hydrous phase relations for these compositions allow a comparison of the dramatic effects of dissolved H2O on the crystallization sequence. Liquidus mineral phase stability and appearance temperatures vary sensitively in response to variation in pressure and H2O content, and this information is used to calibrate magmatic barometers-hygrometers for primitive arc magmas. H2O-saturated experiments on both compositions reveal the strong dependence of amphibole stability on the partial pressure of H2O. A narrow stability field is identified where olivine and amphibole are coexisting phases in the primitive andesite composition above 500 MPa and at least until 800 MPa, between 975–1,025 °C. With increasing H2O pressure (), the temperature difference between the liquidus and amphibole appearance decreases, causing a change in chemical composition of the first amphibole to crystallize. An empirical calibration is proposed for an amphibole first appearance barometer-hygrometer that uses Mg# of the amphibole and:$$ P_{\text{H}_{2}{\rm O}}({\rm MPa})=\left[{\frac{{\rm Mg\#}}{52.7}}-0.014 * \Updelta {\rm NNO}\right]^{15.12} $$This barometer gives a minimumrecorded by the first appearance of amphibole in primitive arc basaltic andesite and andesite. We apply this barometer to amphibole antecrysts erupted in mixed andesite and dacite lavas from the Mt. Shasta, CA stratocone. Both high H2O pressures (500–900 MPa) and high pre-eruptive magmatic H2O contents (10–14 wt% H2O) are indicated for the primitive end members of magma mixing that are preserved in the Shasta lavas. We also use these new experimental data to explore and evaluate the empirical hornblende barometer of Larocque and Canil (2010).