McCarthy Anders, Müntener Othmar
1Institute of Earth Sciences, University of Lausanne, 1004 Lausanne, Switzerland.
2Present Address: School of Earth Sciences, University of Bristol, Bristol, BS8 1RJ UK.
Contrib Mineral Petrol. 2019;174(8):69. doi: 10.1007/s00410-019-1603-5. Epub 2019 Aug 1.
Western Tethyan peridotites exposed in the European Alps show limited amounts of partial melting and mostly fertile compositions. Here we investigate the Civrari Ophiolite (northwestern Italy), which is composed of depleted spinel-harzburgites and serpentinites associated with MOR-type gabbros and basalts. The ultramafic rocks are unique amongst western Tethyan peridotites, showing homogeneous residual compositions after ~ 15% near-fractional melting, lack of pervasive melt percolation and mineral compositions that indicate high-temperature equilibration ≥ 1200 °C. Clinopyroxene chemistry records some of the lowest abundances of NaO, Ce, and Zr/Hf amongst abyssal peridotites worldwide, suggesting that most abyssal peridotites have been affected by variable degrees of melt retention upon melting or cryptic melt percolation. Locally, cryptic MORB-like melt migration in Civrari peridotites produced orthopyroxene + plagioclase intergrowth around reacted clinopyroxene. These clinopyroxene preserve micron-scale chemical zoning indicating rapid cooling after melt crystallization. Nd/Nd isotopic data indicate that Civrari mantle rocks, gabbros, and basalts are not in isotopic equilibrium. Civrari spinel-peridotites represent a highly radiogenic endmember amongst Western Tethys depleted spinel-peridotites, which together form a partial melting errochron of 273 Ma ± 24 Ma. Ancient near-fractional melting and cryptic melt-rock reaction cause variations in radiogenic εNd and εHf, leading to isotopic heterogeneity of Western Tethys mantle rocks. Such inherited signatures in mantle rocks are most likely to be preserved along (ultra-)slow-spreading systems and ocean-continent transition zones.
出露于欧洲阿尔卑斯山的西特提斯橄榄岩显示出有限的部分熔融程度，且大多具有富集的成分。在此，我们研究了位于意大利西北部的奇夫拉里蛇绿岩，它由亏损的尖晶石二辉橄榄岩和蛇纹岩组成，并与洋中脊型辉长岩和玄武岩伴生。这些超镁铁岩在西特提斯橄榄岩中独具特色，在经历了约15%的近分离熔融后呈现出均匀的残余成分，缺乏普遍的熔体渗透现象，且矿物成分表明其经历了≥1200℃的高温平衡。单斜辉石的化学组成记录了全球深海橄榄岩中一些最低的NaO、Ce以及Zr/Hf丰度，这表明大多数深海橄榄岩在熔融过程中受到了不同程度的熔体保留或隐蔽熔体渗透的影响。在局部地区，奇夫拉里橄榄岩中类似洋中脊玄武岩的隐蔽熔体迁移在反应后的单斜辉石周围产生了斜方辉石+斜长石的交生体。这些单斜辉石保留了微米级的化学分带，表明熔体结晶后迅速冷却。Nd/Nd同位素数据表明，奇夫拉里地幔岩石、辉长岩和玄武岩并非处于同位素平衡状态。奇夫拉里尖晶石橄榄岩代表了西特提斯亏损尖晶石橄榄岩中一个高度放射性成因的端元，它们共同构成了一个273Ma±24Ma的部分熔融等时线。古老的近分离熔融和隐蔽的熔体-岩石反应导致了放射性成因的εNd和εHf的变化，从而造成了西特提斯地幔岩石的同位素不均一性。地幔岩石中的这种继承特征最有可能在（超）慢速扩张系统和洋陆过渡带得以保留。
