Carbonate-fluxed Melting of MORB-like Pyroxenite at 2•9 GPa and Genesis of HIMU Ocean Island Basalts
Carbonate-fluxed Melting of MORB-like Pyroxenite at 2•9 GPa and Genesis of HIMU Ocean Island Basalts
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DOI：
10.1093/petrology/egq049
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发表时间：
2010-10-01
影响因子：
3.9
通讯作者：
Dasgupta, Rajdeep
中科院分区：
文献类型：
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作者：
Gerbode, Christine;Dasgupta, Rajdeep
我们提出了部分熔融实验碳酸盐添加，大洋中脊玄武岩（MORB）的辉石岩组合物（G2 C; 5重量% CO2）。在2中心点9 GPa和1000-1500 ℃下进行实验，并将所得部分熔体成分与碱性洋岛玄武岩（OIB）的成分进行比较。固相线估计在1000和1050摄氏度之间，液相线在1450和1475摄氏度之间。近固相线熔体为碳酸盐质（<5wt%SiO_2，< 1中心点3wt%TiO_2，< 0中心点5wt%Al_2O_3，31wt%< CaO <25wt%）。在1245-1275 ℃，随着金红石的消失，发现碳酸盐玄武岩熔体与碳酸盐熔体、cpx和石榴石共存。硅酸盐熔体是碱性玄武岩，在无挥发物的基础上具有类似于44- 47wt%的SiO2，并且熔体在碳酸盐和硅酸盐熔体完全混合的温度（即，在1345-1375 ℃）下变得最贫二氧化硅和富CO2。在我们的研究中，碳酸化硅酸盐熔融的开始类似于60-70摄氏度，比在类似压力下无碳酸盐的MORB-辉石岩的固相线低。G2 C衍生的碳酸盐硅酸盐部分熔体类似于一般的霞石玄武质洋岛玄武岩，特别是那些来自HIMU地幔端员。主要和次要元素特征的关键相似性包括低SiO2和高TiO 2、FeO*、CaO和Na 2 O。G2 C部分熔体与天然碱性OIB的主要区别是前者的高Al 2 O3、低CaO/Al 2 O3和低MgO。我们假设，如果碳酸化的MORB状辉石岩在较高的压力下产生部分熔融，并且如果考虑混合橄榄岩-碳酸化辉石岩来源，这种差异可能会得到解决。海洋岛屿下辉石岩体碳酸盐硅酸盐熔融的地球动力学考虑表明，具有HIMU特征的富含挥发物的碱性OIB可能产生于俯冲碳酸盐地壳的深度范围内，熔融的起始深度为180-200 km，潜在温度接近1500 ℃。然而，而不是碳酸盐洋壳的直接减压，在与MORB-辉石岩平衡的碳酸盐硅酸盐熔体的生成中所涉及的关键过程可能涉及熔体-岩石反应和熔体-熔体混合。
We present partial melting experiments on a carbonate-added, mid-ocean ridge basalt (MORB)-like pyroxenite composition (G2C; 5 wt % CO2). Experiments were conducted at 2 center dot 9 GPa and 1000-1500 degrees C and the resulting partial melt compositions were compared with those of alkalic ocean island basalts (OIBs). The solidus is estimated between 1000 and 1050 degrees C and the liquidus is between 1450 and 1475 degrees C. The subsolidus assemblage is cpx + garnet + rutile + calcio-dolomitic solid solution, and the near-solidus melt is carbonatitic (< 5 wt % SiO2, < 1 center dot 3 wt % TiO2, < 0 center dot 5 wt % Al2O3, 31 wt % < CaO < 25 wt %). At 1245-1275 degrees C, with the disappearance of rutile, a carbonated basaltic melt is found to coexist with carbonatitic melt, cpx, and garnet. The silicate melts are alkalic basalts with SiO2 of similar to 44-47 wt % on a volatile-free basis, and the melt becomes most silica-poor and CO2-rich at the temperature of complete mixing of carbonate and silicate melt (i.e. at 1345-1375 degrees C). The onset of carbonated silicate melting in our study is similar to 60-70 degrees C cooler than the solidus of the carbonate-free MORB-pyroxenite at a similar pressure. G2C-derived carbonated silicate partial melts are similar to nephelinitic to basanitic ocean island basalts in general, and those derived from the HIMU mantle end-member in particular. The key similarities in major and minor element signatures include low SiO2 and high TiO2, FeO*, CaO, and Na2O. The main discrepancies between G2C partial melts and natural alkalic OIBs are higher Al2O3, lower CaO/Al2O3, and lower MgO of the former. We hypothesize that such discrepancies might be resolved if carbonated MORB-like pyroxenite produces partial melts at somewhat higher pressures and if a hybrid peridotite-carbonated pyroxenite source is considered. Geodynamic consideration of carbonated silicate melting of pyroxenite bodies beneath ocean islands suggests that volatile-enriched alkalic OIBs with the HIMU signature are probably generated from subducted, carbonated crust over a depth range, with the onset of melting as deep as 180-200 km, for a potential temperature of similar to 1500 degrees C. However, rather than a direct decompression of carbonated ocean crust, the key processes involved in the generation of a carbonated silicate melt in equilibrium with MORB-pyroxenite may involve melt-rock reaction and melt-melt mixing.