Effects of H2O–CO2 Fluids, Temperature, and Peridotite Fertility on Partial Melting in Mantle Wedges and Generation of Primary Arc Basalts
Effects of H2O–CO2 Fluids, Temperature, and Peridotite Fertility on Partial Melting in Mantle Wedges and Generation of Primary Arc Basalts
复制标题
H2O·CO2流体、温度和橄榄岩肥力对地幔楔部分熔融和原生弧玄武岩生成的影响
DOI：
10.1093/petrology/egad047
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发表时间：
2023
影响因子：
3.9
通讯作者：
Dasgupta, Rajdeep
中科院分区：
文献类型：
--
作者：
Lara, Michael;Dasgupta, Rajdeep
来自高p - t实验、热力学模型和自然观测的许多证据表明，板块衍生的含水流体（流经地幔楔）含有不同数量的溶解碳。然而，关于h2o - co2流体对地幔熔融的影响的限制，特别是在地幔楔型条件下，是有限的。本文对含3.5 wt.% H2O和XCO2[=摩尔CO2/ (CO2+ H2O)]为0.04 ~ 0.17的富橄榄岩和贫橄榄岩组成进行了活塞缸实验。实验在2-3 GPa和1350℃条件下进行，以评估温度、橄榄岩肥沃度和板状流体xco2对地幔楔块部分熔融的影响。所有的实验都得到橄榄石+正辉石+7 ~ 41 wt.%的部分熔体。我们的新数据，以及之前的低温数据表明，随着地幔楔块温度的升高，在h2o - co2流体的影响下，初级熔体中SiO2、FeO*和MgO的含量增加，CaO、Al2O3和碱的含量减少。在恒定体积H2O含量条件下，地幔楔体的熔融程度主要受橄榄岩肥力和板状流体xco2的控制。高xco_2贫化组分产生~ 7wt .%的熔体，而在相同p - t下，低xco_2富化组分产生~30 ~ 40wt .%的熔体。此外，橄榄岩的肥力和xco2对橄榄岩部分熔体成分有显著影响。当p - t - xco2恒定时，肥沃的橄榄岩生成的熔体富含CaO和al2o3，而较少含有SiO2、MgO + FeO和碱。与以往的实验研究相似，在恒定温度条件下，随着xco2的增加，熔体的sio2和CaO分别有系统地减少和增加。如果含碳流体（如含ch4）被还原，则无法观察到溶解碳的氧化形式对橄榄岩部分熔体组成的这种独特影响。考虑到xco2对熔体SiO2和CaO浓度的影响较大，以及弧岩浆的相对氧化性质，我们将实验熔体的SiO2/CaO与之前的橄榄岩+ H2O±co2研究中熔体的SiO2/CaO与原始弧玄武岩和超钙硅欠饱和弧熔体包裹体的SiO2/CaO进行了比较。通过比较，我们发现，在大多数预测的地幔楔的p - t富性条件下，即使校正了橄榄石分馏，XCO2≥0.11的大块成分部分熔体的SiO2/CaO含量也低于全球所有原始弧熔体，而XCO2= 0.04的大块成分部分熔体的SiO2/CaO含量重叠在自然数据定义的SiO2/CaO场的下端。这些结果表明，影响原生弧岩浆形成的板块流体的XCO2上限为0.04 < XCO2< 0.11，这一上限可能在全球范围内适用。最后，我们发现在超钙弧熔体中观察到的异常SiO2/CaO和CaO/ al2o3特征可以通过2-3 GPa含co2富水橄榄岩和贫水橄榄岩（0 < XCO2< 0.11）的部分熔融或P bbb30 GPa名义上不含co2的富水橄榄岩的部分熔融来重现。
Many lines of evidence from highP–Texperiments, thermodynamic models, and natural observations suggest that slab-derived aqueous fluids, which flux mantle wedges contain variable amounts of dissolved carbon.However, constraints on the effects of H2O–CO2fluids on mantle melting, particularly at mantle wedgeP–Tconditions, are limited. Here, we present new piston cylinder experiments on fertile and depleted peridotite compositions with 3.5 wt.% H2O and XCO2[= molar CO2/ (CO2+ H2O)] of 0.04–0.17. Experiments were performed at 2–3 GPa and 1350°C to assess how temperature, peridotite fertility, and XCO2of slab-derived fluid affects partial melting in mantle wedges. All experiments produce olivine + orthopyroxene +7 to 41 wt.% partial melt. Our new data, along with previous lower temperature data, show that as mantle wedge temperature increases, primary melts become richer in SiO2, FeO*, and MgO and poorer CaO, Al2O3,and alkalis when influenced by H2O–CO2fluids. At constantP–Tand bulk H2O content, the extent of melting in the mantle wedge is largely controlled by peridotite fertility and XCO2of slab-fluid. High XCO2depleted compositions generate ~7 wt.% melt, whereas, at identicalP–T, low XCO2fertile compositions generate ~30 to 40 wt.% melt. Additionally, peridotite fertility and XCO2have significant effects on peridotite partial melt compositions. At a constantP–T–XCO2,fertile peridotites generate melts richer in CaO and Al2O3and poorer in SiO2, MgO + FeO, and alkalis. Similar to previous experimental studies, at a constantP–Tfertility condition, as XCO2increases, SiO2and CaO of melts systematically decrease and increase, respectively. Such distinctive effects of oxidized form of dissolved carbon on peridotite partial melt compositions are not observed if the carbon-bearing fluid is reduced, such as CH4-bearing. Considering the large effect of XCO2on melt SiO2and CaO concentrations and the relatively oxidized nature of arc magmas, we compare the SiO2/CaO of our experimental melts and melts from previous peridotite + H2O ± CO2studies to the SiO2/CaO systematics of primitive arc basalts and ultra-calcic, silica-undersaturated arc melt inclusions. From this comparison, we demonstrate that across mostP–T–fertility conditions predicted for mantle wedges, partial melts from bulk compositions with XCO2≥ 0.11 have lower SiO2/CaO than all primitive arc melts found globally, even when correcting for olivine fractionation, whereas partial melts from bulk compositions with XCO2= 0.04 overlap the lower end of the SiO2/CaO field defined by natural data. These results suggest that the upper XCO2limit of slab-fluids influencing primary arc magma formation is 0.04 < XCO2< 0.11, and this upper limit is likely to apply globally. Lastly, we show that the anomalous SiO2/CaO and CaO/Al2O3signatures observed in ultra-calcic arc melt inclusions can be reproduced by partial melting of either CO2-bearing hydrous fertile and depleted peridotites with 0 < XCO2< 0.11 at 2–3 GPa, or from nominally CO2-free hydrous fertile peridotites at P > 3 GPa.