Department of Earth and Planetary Sciences, Washington University, St Louis, Missouri 63130, USA.
Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA.
Nature. 2015 Feb 19;518(7539):395-8. doi: 10.1038/nature14113. Epub 2015 Feb 2.
Processes of melt generation and transport beneath back-arc spreading centres are controlled by two endmember mechanisms: decompression melting similar to that at mid-ocean ridges and flux melting resembling that beneath arcs. The Lau Basin, with an abundance of spreading ridges at different distances from the subduction zone, provides an opportunity to distinguish the effects of these two different melting processes on magma production and crust formation. Here we present constraints on the three-dimensional distribution of partial melt inferred from seismic velocities obtained from Rayleigh wave tomography using land and ocean-bottom seismographs. Low seismic velocities beneath the Central Lau Spreading Centre and the northern Eastern Lau Spreading Centre extend deeper and westwards into the back-arc, suggesting that these spreading centres are fed by melting along upwelling zones from the west, and helping to explain geochemical differences with the Valu Fa Ridge to the south, which has no distinct deep low-seismic-velocity anomalies. A region of low S-wave velocity, interpreted as resulting from high melt content, is imaged in the mantle wedge beneath the Central Lau Spreading Centre and the northeastern Lau Basin, even where no active spreading centre currently exists. This low-seismic-velocity anomaly becomes weaker with distance southward along the Eastern Lau Spreading Centre and the Valu Fa Ridge, in contrast to the inferred increase in magmatic productivity. We propose that the anomaly variations result from changes in the efficiency of melt extraction, with the decrease in melt to the south correlating with increased fractional melting and higher water content in the magma. Water released from the slab may greatly reduce the melt viscosity or increase grain size, or both, thereby facilitating melt transport.
类似于大洋中脊的减压熔融和类似于弧下地幔柱的部分熔融。拉乌盆地区拥有大量处于俯冲带不同距离处的扩张脊，为区分这两种不同熔融过程对岩浆生成和地壳形成的影响提供了机会。在这里，我们根据利用陆地和海底地震仪获得的瑞利波层析成像得到的地震波速度，对推断出的部分熔融三维分布进行了限制。中央拉乌扩张中心和北东拉乌扩张中心之下的低地震速度向弧后延伸得更深、更向西，这表明这些扩张脊是由从西部上涌区的熔融所供给的，这有助于解释与南部的瓦卢法脊的地球化学差异，南部的瓦卢法脊没有明显的深部低地震速度异常。在中央拉乌扩张中心和东北拉乌盆地之下的地幔楔中，推断出存在一个高熔融含量导致的低速 S 波速度区，即使在没有活跃扩张中心的地方也是如此。该低速异常在沿着东拉乌扩张中心和瓦卢法脊向南的方向上向南逐渐减弱，而推断出的岩浆生产力却在增加。我们提出，异常变化是由于熔融提取效率的变化引起的，南部熔融的减少与向南增加的部分熔融和岩浆中更高的含水量有关。从板块释放的水可能会大大降低熔融的粘度或增加晶粒尺寸，或者两者兼而有之，从而促进熔融传输。
