Origin of geochemical heterogeneity in mid-ocean ridge upper mantle, ODP Leg 209

Ocean Drilling Program (ODP) Leg 209预计沿15°20′n （Capo Verde）断裂带南北的Mid Atlantic Ridge （MAR）钻探6 - 7个100米规模的地幔橄榄岩岩心。全球范围内的疏浚已经证明了热点和扩张速度对地幔橄榄岩化学的影响。远离这些热点的地幔异常的重要性才刚刚开始被充分认识到。然而，熔体岩石反应与熔体运输的程度仍然存在激烈的争论。多种多样的模型，部分是由第209号研究小组的共同负责人彼得·克莱门的工作激发的，从与孤立（通道）熔体运输有关的部分熔融到全球多孔流动，后者意味着所有的橄榄岩都受到熔体-岩石反应的影响。在岩心中可以很好地确定橄榄岩宿主中辉石岩、泥质岩、剪切带和模态相变化的丰度变化，它们是指示熔融-岩石反应变化程度的主要候选物。我们建议选择两个合适的岩心剖面（长度尺度为20-30 m），显示岩性和应变的变化，并要求对其进行详细采样。通过获得这些样品的完整地球化学数据集，我们期望更好地约束熔融岩反应和多孔流动的程度和控制反应，并约束在一段原位海洋地幔中控制典型“噪声”或背景变化的过程。重要的是，建议的调查规模弥补了目前海洋橄榄岩调查的空白，与任何蛇绿岩剖面相比，所选地点的构造情况是众所周知的。

Ocean Drilling Program (ODP) Leg 209 expects drilling six to seven, 100 m scale cores of mantle peridotite along the Mid Atlantic Ridge (MAR) north and south of the 15°20'N (Capo Verde) Fracture Zone. Global scale dredging has demonstrated the influence of hot spots and spreading rate on mantle peridotite chemistry. The importance of mantle anomalies away from such hot spots is just beginning to be fully realized. The extent of melt rock reaction as associated with melt transport remains, however, fiercely debated. Diverse models, in part stimulated by the work of Leg 209 co-chief Peter Kelemen, range from fractional melting linked with isolated (channeled) melt transport to global porous flow, the latter implying that all peridotites have been affected by melt-rock reaction. Variations in the abundance of pyroxenites, dunites, shear zones and modal phase variation in peridotite hosts can be well determined in drill core and are the prime candidates to indicate variable extents of melt-rock reaction. We propose to select two suitable sections of core (length scale 20-30 m) showing variations in lithology and strain and request their detailed sampling. By obtaining a complete geochemical data set on these samples, we expect to better constrain the extent of, and the reactions governing, melt rock reaction and porous flow and to constrain what processes control the typical "noise" or background variation in a section of in-situ oceanic mantle. Importantly, the suggested scale of investigation bridges a current gap in the investigations of oceanic peridotite and the tectonic situation of the chosen site is, in contrast to any ophiolitic section, well known.