Phase Relations of a Depleted Peridotite Fluxed by a CO 2 ‐H 2 O Fluid—Implications for the Stability of Partial Melts Versus Volatile‐Bearing Mineral Phases in the Cratonic Mantle
Phase Relations of a Depleted Peridotite Fluxed by a CO 2 ‐H 2 O Fluid—Implications for the Stability of Partial Melts Versus Volatile‐Bearing Mineral Phases in the Cratonic Mantle
复制标题
CO 2 -H 2 O 流体熔融的贫化橄榄岩的相关系——对克拉通地幔中部分熔体与挥发性矿物相稳定性的影响
DOI：
10.1029/2019jb017653
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发表时间：
2019
期刊：
影响因子：
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通讯作者：
Dasgupta, Rajdeep
中科院分区：
文献类型：
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作者：
Saha, Sriparna;Dasgupta, Rajdeep
我们用混合CO2-H2O流体(体积分数为0.5wt.%CO2和0.94wt.%H2O)对一块含K的贫化橄榄岩(MG#92)进行相平衡实验，以了解贫化橄榄岩体系中部分熔体相对于富含挥发分矿物相的稳定性。实验是在850-1150°C和2-4 Gpa的条件下，使用活塞-圆柱体和多顶锤装置进行的。橄榄石、斜方辉石、单斜辉石和尖晶石/石榴石在所有实验条件下都存在。部分熔融的织构确认是在2 Gpa时低至1000℃、3 Gpa时低至950℃、4 Gpa时低至1000℃时进行的，标志着2 Gpa时900-1000°C、3 Gpa时850-950°C和3 Gpa时950-1000°C开始熔化。金云母和菱镁矿在2 GPa900-1000°C、3 GPa950-1000°C和4 GPa1000-1050°C条件下崩解。通过硅质熔体引入的CO_2-H_2O含量相同的贫化橄榄岩体系的实验结果表明，引入CO_2-H_2O作为流体可使金云母在2-4 GPa时的破裂温度降低150-200℃，并在较低温度下稳定部分熔体。因此，我们的研究表明，克拉通地幔中存在的挥发分相受主体成分控制，并受俯冲带克拉通形成过程中挥发分加成过程的影响。此外，挥发分通过熔体和水相引入，导致不同比例的无水相，如橄榄石和斜方辉石。因此，考虑交代作用对于评估贫化橄榄岩的主体成分如何被改变至关重要，这将导致含挥发分相的潜在稳定性，这是导致大陆下岩石圈中部不连续等地幔区域异常低剪切波速的原因。
We present phase‐equilibria experiments of a K‐bearing, depleted peridotite (Mg# 92) fluxed with a mixed CO2‐H2O fluid (0.5 wt.% CO2and 0.94 wt.% H2O in the bulk) to gain insight into the stability of volatile‐bearing partial melts versus volatile‐bearing mineral phases in a depleted peridotite system. Experiments were performed at 850–1150 °C and 2–4 GPa using a piston‐cylinder and a multianvil apparatus. Olivine, orthopyroxene, clinopyroxene, and spinel/garnet are present at all experimental conditions. Textural confirmation of partial melt is made at temperatures as low as 1000 °C at 2 GPa, 950 °C at 3 GPa, and 1000 °C at 4 GPa marking the onset of melting at 900–1000 °C at 2 GPa, 850–950 °C at 3 GPa, and 950–1000 °C at 3 GPa. Phlogopite and magnesite breakdown at 900–1000 °C at 2 GPa, 950–1000 °C at 3 GPa, and 1000–1050 °C at 4 GPa. Comparison with previously published experiments in depleted peridotite system with identical CO2‐H2O content introduced via a silicic melt show that introduction of CO2‐H2O as fluid lowers the temperature of phlogopite breakdown by 150–200 °C at 2–4 GPa and stabilizes partial melts at lower temperatures. Our study thus, shows that the volatile‐bearing phase present in the cratonic mantle is controlled by bulk composition and is affected by the process of volatile addition during craton formation in a subduction zone. In addition, volatile introduction via melt versus aqueous fluid, leads to different proportion of anhydrous phases such as olivine and orthopyroxene. Considering the agent of metasomatism is thus critical to evaluate how the bulk composition of depleted peridotite is modified, leading to potential stability of volatile‐bearing phases as the cause of anomalously low shear wave velocity in mantle domains such as mid lithospheric discontinuities beneath continents.