研究表明地幔熔融程度与洋壳厚度呈正相关性。然而，慢速扩张大西洋中脊15°N区却表现出相反趋势。该区脊轴裂谷两侧大面积出露地幔橄榄岩，加上较高的剩余重力异常，表明洋壳很薄，而地球化学特征却指示地幔经历了强烈的部分熔融。地幔强烈熔融下为何洋壳很薄？针对这个问题，前人提出过若干解释，但尚缺乏足够地球化学数据支撑。为此，本课题拟从洋中脊堆晶岩入手，堆晶岩单矿物微区具有比全岩更广泛的同位素组成，可以反映更丰富的源区信息，为解答这一问题提供条件。本课题拟对从该区采集到的辉长岩和伟晶辉石岩开展地球化学研究，分析其全岩、单矿物微区的Sr-Nd-Pb-Hf同位素和元素地球化学组成，探究其源区物质组成和岩浆演化过程，寻找堆晶岩和橄榄岩以及洋中脊玄武岩之间存在的成因关联，为解决大西洋中脊15°N区“地幔强烈熔融而洋壳减薄”这一问题提供更全面的地球化学制约。本研究将有助于进一步认识慢速扩张洋中脊的深部岩浆过程。

It is known from many studies that high degrees of mantle melting will generate a thick oceanic crust. However, the case at the slow spreading Mid-Atlantic Ridge 15°N region shows an opposite scenario. The large exposure of mantle peridotites on both sides of its axial valley and the associated residual gravity highs suggest obvious crustal thinning along the substantial portion of the segment. But, geochemistry of these peridotites show indicators of extreme mantle depletion. So there exists an apparent contradiction that the high degrees of mantle melting are associated with crustal thinning. To address this contradiction, many workers have proposed different explanations, but all of them lack concrete geochemical supports. Cumulates minerals can record wider ranges of isotopes than the whole-rock analyses, thus could reveal a more complete information about the source mantle. Therefore, this research will choose cumulates including gabbros and pegmatitic pyroxenites from the MAR 15°N region to analyze their elemental and isotopic geochemistry on both bulk rock samples and micro-milled mineral crystals. Based on these geochemistry, we will discuss the magma evolution trajectory during the cumulates petrogenesis process and trace the compositions of their source mantle, and find the geochemical linkages between peridotites, cumulates and mid-ocean ridge basalts, to provide more geochemical constraints on the mechanism of “strong mantle depletion but crust thinning”. This work will lead a further understanding of the magmatic process beneath the slow spreading mid-ocean ridges.