The Effect of H2O and Pressure on Multiple Saturation and Liquid Lines of Descent in Basalt from the Shatsky Rise
The Effect of H2O and Pressure on Multiple Saturation and Liquid Lines of Descent in Basalt from the Shatsky Rise
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DOI：
10.1093/petrology/egw008
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发表时间：
2016-02
影响因子：
3.9
通讯作者：
Anika Husen;R. Almeev;F. Holtz
中科院分区：
文献类型：
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作者：
Anika Husen;R. Almeev;F. Holtz
为了模拟岩浆对热力学变量（如压力、温度、氧逸度和水活度）变化的反应，已经建立了许多模型。然而，大量的实验数据仍然缺乏关于少量水对拉斑玄武岩中橄榄石+斜长石+斜辉石共晶结晶的明显影响的信息。本文研究了压力（100、200、400和700 MPa）和少量水对代表沙斯基隆起海洋高原岩浆系统不同演化阶段的三种拉斑玄武岩（组成AH6、AH3和AH5， MgO含量分别为8.6、8.0和6.4 wt %）相关系和液态下降线的影响。设计了两种实验方法（干燥和低水）来保持结晶过程中不同的水活性，使用(1)石墨-铂双胶囊进行几乎无水的实验（熔体中H2O < 0.15 wt %）和(2)铁预饱和au20pd80胶囊获得熔体水含量在0.4至1.1 wt % H2O之间的低含量。在干燥条件下，低压（≤400 MPa）条件下，富镁的AH6和中间AH3玄武岩的结晶过程遵循典型的拉斑分异-橄榄石结晶-橄榄石+斜长石-橄榄石+斜长石+斜辉石的结晶顺序。在400 ~ 700 MPa压力范围内，两种玄武岩均接近多次饱和。在高压下（700兆帕）结晶顺序相反，从液相的斜辉石开始。在低水条件下，即使在100和200 MPa的低压下，AH6和AH3也非常接近多次饱和，并且在400 MPa时已经观察到反向结晶顺序（斜辉石、斜长石+斜辉石、橄榄石+斜长石+斜辉石）。与另外两种富含镁的玄武岩相比，在AH5玄武岩中，在所有研究压力下，在干燥和低水条件下，斜辉石都是液相相，然后是斜长石+斜辉石和橄榄石+斜长石+斜辉石的结晶。在我们的实验中，最引人注目的观察结果是，斜辉石的稳定性不仅随着压力的增加而增加，而且在存在少量H2O的情况下（与相同压力下的干燥对偶物相比）也会增加。少量的H2O增加了橄榄石+斜长石+斜辉石相组合中斜辉石的比例。我们的实验清楚地表明，在共晶熔体成分中加入0.4 wt %的H2O（例如在给定MgO下的CaO/ al2o3比）的效果与将压力从100 MPa增加到~ 300 MPa相似。这意味着少量的H2O会导致对共晶结晶压力的严重高估（高达300兆帕），并且在地球气压模型中需要考虑到H2O的含量。我们的新实验强调了低熔体水含量在稳定斜辉石中的作用，并为“辉石悖论”问题提供了一些新的见解。如果存在少量的水，使用“干”地球气压法获得的一些洋中脊玄武岩的表观地幔压力实际上可以代表下地壳的深度。我们对天然沙斯基隆起玄武岩的实验数据的应用表明，岩浆记录了部分结晶过程，主要发生在低压（100 MPa）下，对应于前扩张中心下~ 3 km的深度，尽管更原始的熔岩显示了证据…
Numerous models have been developed to simulate the reaction of magmas to changes of thermodynamic variables, such as pressure, temperature, oxygen fugacity, and water activity. However, the extensive experimental database still lacks information on the distinct effect of small amounts of H2O on olivine + plagioclase + clinopyroxene cotectic crystallization in tholeiitic basalt. We present an experimental study addressing the effects of pressure (at 100, 200, 400, and 700 MPa) and small amounts of H2O on phase relations and liquid lines of descent in three tholeiitic basalts representing different evolutionary stages of the Shatsky Rise oceanic plateau magmatic system (compositions AH6, AH3, and AH5 with 8·6, 8·0, and 6·4 wt % MgO, respectively). Two experimental approaches (dry and low H2O) are designed to maintain contrasting H2O activities during crystallization using (1) graphite–platinum double capsules to perform nearly anhydrous experiments (<0·15 wt % H2O in the melt) and (2) Fe pre-saturated Au20Pd80capsules to obtain low melt H2O contents ranging from 0·4 to 1·1 wt % H2O. Under dry conditions, at lower pressures (≤400 MPa), the crystallization in the MgO-rich AH6 and intermediate AH3 basalts follows the typical sequence of tholeiitic differentiation with olivine crystallization at the liquidus followed by olivine + plagioclase and olivine + plagioclase + clinopyroxene. Both basalts are close to multiple saturation at pressures between 400 and 700 MPa. At high pressure (700 MPa) the crystallization sequence is reversed, starting with clinopyroxene at the liquidus. Under low-H2O conditions, AH6 and AH3 are very close to multiple saturation, even at the low pressures of 100 and 200 MPa, and the reversed crystallization sequence (clinopyroxene, plagioclase + clinopyroxene, olivine + plagioclase + clinopyroxene) is observed already at 400 MPa. In contrast to the two more MgO-rich basalts, in the most evolved AH5 basalt, clinopyroxene is the liquidus phase at all investigated pressures and under both dry and low-H2O conditions, followed by crystallization of plagioclase + clinopyroxene and olivine + plagioclase + clinopyroxene. The most striking observation in our experiments is that the stability of clinopyroxene increases not only with pressure increase but also in the presence of small amounts of H2O (when compared with dry counterparts at similar pressures). Small amounts of H2O increase the proportion of clinopyroxene in the olivine + plagioclase + clinopyroxene phase assemblage. Our experiments clearly show that the effect of adding 0·4 wt % H2O to cotectic melt compositions (e.g. CaO/Al2O3ratio at a given MgO) is similar to that caused by an increase of pressure from 100 to ∼ 300 MPa. This implies that small amounts of H2O can lead to significant overestimation of cotectic crystallization pressures (by up to 300 MPa) and that H2O contents need to be taken into account in geobarometric models. Our new experiments emphasize the role of low melt H2O contents in stabilizing clinopyroxene and provide some new insights into the problem of the ‘pyroxene paradox’. The apparent mantle pressures obtained for some mid-ocean ridge basalts using ‘dry’ geobarometric approaches can actually represent depths within the lower crust, if small amounts of H2O are present. The application of our experimental data to natural Shatsky Rise basalts implies that the magmas record partial crystallization processes occurring mainly at low pressure (100 MPa), corresponding to depths of ∼3 km beneath the former spreading center, although the more primitive lavas show evidence …