Whole rock and Cr-rich spinel geochemistry of ODP Legs 152 and 163 and DSDP Hole 38-338 samples

Coring during Ocean Drilling Program and Deep Sea Drilling Project Legs 163, 152, 104, 81, and 38 recovered sequences of altered basalt from North Atlantic seaward-dipping reflector sequences (SDRS) erupted during the initial rifting of Greenland from northern Europe and likely associated with excessive mantle temperatures caused by an impacting mantle plume head. Cr-rich spinel is found abundantly as inclusions and groundmass crystals within the olivine-rich lavas of Hole 917A (Leg 152) cored into the Southeast Greenland SDRS, but only rarely as inclusions within plagioclase in the lavas of the Vøring Plateau SDRS, and it is absent from other cored SDRS lavas from the Rockall Plateau and Southeast Greenland. Eruptive melt compositions determined from inferred, thermodynamically-defined, spinel-melt exchange equilibria indicate that the most primitive melts represented by Hole 917A basalts have Mg/(Mg + Fe2+) at least as high as 0.70 and approach near-primary mantle melt compositions. In contrast, Cr-rich spinels from Hole 338 (Leg 38) lavas on the Vøring Plateau SDRS give evidence for melt with Mg/(Mg + Fe2+) only as high as 0.64. This study underlines that primitive melts similar to those from Hole 917A comprise only a small fraction of the eruptive North Atlantic SDRS melts, and that most SDRS basalts were, in fact, too evolved to have precipitated Cr-rich spinel, with true melt Mg/(Mg + Fe2+) likely below 0.60. The evolved nature of the SDRS basalts implies large amounts of fractionation at the base of the crust or deep within it, consistent with seismic results that indicate an abnormally thick Layer 3 underlying the SDRS.本研究依托大洋钻探计划（Ocean Drilling Program, ODP）与深海钻探计划（Deep Sea Drilling Project, DSDP）第163、152、104、81及38航次的岩心，从北大西洋格陵兰与北欧初始裂谷期喷发的向海反射层序（seaward-dipping reflector sequences, SDRS）中回收了蚀变玄武岩序列。该层序的形成可能与撞击型地幔柱头部引发的异常地幔升温密切相关。在第152航次钻取的917A钻孔东南格陵兰SDRS富橄榄石熔岩中，富铬尖晶石作为包裹体与基质晶体大量产出；而沃林高原SDRS的熔岩中，该矿物仅以斜长石包裹体的形式零星出现；罗卡尔高原与东南格陵兰其他SDRS熔岩岩心则完全缺失富铬尖晶石。通过热力学定义的尖晶石-熔体交换平衡关系推断的喷发熔体成分显示，917A钻孔玄武岩所代表的最原始熔体，其Mg/(Mg + Fe²+)比值至少可达0.70，接近原生地幔熔体的成分特征。与之相比，沃林高原SDRS第38航次338钻孔熔岩中的富铬尖晶石则指示其对应熔体的Mg/(Mg + Fe²+)比值仅为0.64。本研究表明，类似917A钻孔所代表的原始熔体仅占北大西洋SDRS喷发熔体的极小部分；绝大多数SDRS玄武岩均经历了显著演化，以至于无法析出富铬尖晶石，其真实熔体的Mg/(Mg + Fe²+)比值大概率低于0.60。SDRS玄武岩的演化特征暗示其在地壳底部或地壳深部发生了大规模岩浆分异作用，这与地震学观测结果一致——该区域SDRS下方存在异常增厚的地壳三层（Layer 3）。