Petrology of ultramafic rocks at DSDP Holes 82-556, 82-558, and 82-560

Strongly serpentinized peridotites occur at shallow crustal levels at DSDP Sites 556 (35-Ma-old crust), 558 (35-Maold crust), and 560 (12-Ma-old crust) in the North Atlantic. Primary peridotite silicate minerals are preserved only at Sites 556 and 558, but all three sites have primary spinel preserved. The serpentinites at all three sites were probably spinel harzburgite tectonites, with little or no clinopyroxene.
For orthopyroxene from Site 556, Mg/(Mg+Fe) ratios average 0.904, and Al2O3 contents are about 2.0-3.3 wt.%. Compositions of pyroxene-derived bastites from Sites 558 and 560 suggest that the original pyroxenes were probably low-alumina orthopyroxene. Spinels from all three sites have low Fe3+ and TiO2. Spinels from Sites 556 and 560 have high Cr/(Cr+Al) (0.45-0.53). Those from Site 558 have Cr/(Cr+Al) values that are among the highest measured for any plagioclase-free oceanic peridotites (0.55-0.62).
Whole-rock chemistry, although affected by serpentinization, suggests that the peridotites had relatively low CaO, TiO2, Al2O3, and Na2O. Mineral and whole-rock chemistry and modal abundances suggest that Leg 82 peridotites are residua from partial melting, and that the extent of partial melting may have been greater in this region than farther south in the North Atlantic, or that the mantle was originally more depleted here.

北大西洋的深海钻探计划（Deep Sea Drilling Project, DSDP）556号站位（对应35 Ma年龄的地壳）、558号站位（对应35 Ma年龄的地壳）及560号站位（对应12 Ma年龄的地壳）均在地壳浅部环境产出强蛇纹石化橄榄岩。仅556与558号站位保留有原始橄榄岩硅酸盐矿物，但三个站位均保存有原始尖晶石。三处站位的蛇纹岩原岩大概率为尖晶石方辉橄榄岩构造岩，几乎不含或完全不含单斜辉石。
对于556号站位的斜方辉石，其Mg/(Mg+Fe)比值平均为0.904，三氧化二铝（Al₂O₃）含量约为2.0~3.3 wt.%。取自558与560号站位的由辉石蚀变而来的纤蛇纹石（bastite）的成分特征表明，原始辉石大概率为低铝斜方辉石。三处站位的尖晶石均具有较低的Fe³⁺与TiO₂含量。556与560号站位的尖晶石具有较高的Cr/(Cr+Al)比值（0.45~0.53）；而558号站位的尖晶石Cr/(Cr+Al)比值（0.55~0.62）为全球无斜长石大洋橄榄岩中已测得的最高值之一。
尽管全岩地球化学特征受蛇纹石化作用影响，但仍表明该类橄榄岩具有相对较低的CaO、TiO₂、Al₂O₃及Na₂O含量。矿物与全岩地球化学特征及矿物百分含量均表明，深海钻探计划第82航次采集的橄榄岩为部分熔融作用的残余体，且该区域的部分熔融程度可能高于北大西洋更南部区域，抑或是该区域原始地幔本身更为亏损。