Mantle flow, volatiles, slab-surface temperatures and melting dynamics in the north Tonga arc-Lau back-arc basin
Mantle flow, volatiles, slab-surface temperatures and melting dynamics in the north Tonga arc-Lau back-arc basin
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DOI：
10.1029/2012jb009526
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发表时间：
2012-11-21
影响因子：
3.9
通讯作者：
Handley, Heather
中科院分区：
文献类型：
--
作者：
Caulfield, John;Turner, Simon;Handley, Heather
Fonualei扩张中心提供了一个极好的机会，以评估地球化学变化与深度增加的板在劳弧后盆地。我们目前的H2O和CO2的浓度和Sr，Nd，Pb，Hf和U-Th-Ra同位素数据选定的玻璃，以及新的Hf同位素数据玻安岩和海山的汤加弧北部。Pb和Hf同位素数据表明，地幔流动方向为西南方向，板片北方端的撕裂可能不会向东延伸到玻安岩位置。沿着Fonualei扩展中心，关键的地球化学参数变化平稳，增加距离弧前和增加板片表面温度。后者的范围可以从720到866摄氏度，基于降低的H2O/Ce比率。与实验数据相一致，地球化学趋势的解释，以反映湿泥质岩熔体或超临界流体和含水流体来自板的量和成分的变化。除了一个例外，所有的熔岩都保留了过量的U-238和Ra-226。我们认为，从Fonualei扩展中心和Valu Fa山脊熔岩占主导地位的流体熔融，而从东部和中部刘扩展中心，板表面温度超过类似850-900摄氏度，主要是通过减压。马努斯和东斯科舍弧后盆地也有类似的观察结果，可能反映了俯冲玄武岩地壳中硬硅钙石等关键阶段的到期。
The Fonualei Spreading Center affords an excellent opportunity to evaluate geochemical changes with increasing depth to the slab in the Lau back-arc basin. We present H2O and CO2 concentrations and Sr, Nd, Pb, Hf and U-Th-Ra isotope data for selected glasses as well as new Hf isotope data from boninites and seamounts to the north of the Tonga arc. The Pb and Hf isotope data are used to show that mantle flow is oriented to the southwest and that the tear in the northern end of the slab may not extend east as far as the boninite locality. Along the Fonualei Spreading Center, key geochemical parameters change smoothly with increasing distance from the arc front and increasing slab surface temperatures. The latter may range from 720 to 866 degrees C, based on decreasing H2O/Ce ratios. Consistent with experimental data, the geochemical trends are interpreted to reflect changes in the amount and composition of wet pelite melts or super-critical fluids and aqueous fluids derived from the slab. With one exception, all of the lavas preserve both U-238 excesses and Ra-226 excesses. We suggest that lavas from the Fonualei Spreading Center and Valu Fa Ridge are dominated by fluid-fluxed melting whereas those from the East and Central Lau Spreading Centers, where slab surface temperatures exceed similar to 850-900 degrees C, are largely derived through decompression. A similar observation is found for the Manus and East Scotia back-arc basins and may reflect the expiry of a key phase such as lawsonite in the subducted basaltic crust.