Open‐system melting in the upper mantle: Constraints from the Hayachine‐Miyamori ophiolite, northeastern Japan

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Abstract
The Hayachine and Miyamori hydrous ultramafic complexes, northeastern Japan, exhibit not only diverse lithological variations from very primordial spinel lherzolites (olivine forsterite mol % = 89 (Fo89), spinel 100Cr/(Cr+Al) (Cr #) =10) to refractory harzburgites (Fo93, spinel Cr #=70) but also show diverse trace element characteristics in amphiboles and clinopyroxenes correlated with the major element behavior. Some of the most primitive lherzolites are from the Hayachine complex and contain a clinopyroxene with strongly light rare earth element (REE) depleted abundances. The slightly refractory aluminous spinel peridotites of the Miyamori complex forming kilometric blocks in chromian spinel peridotites, exhibit flat to weakly V‐shaped REE patterns with lower heavy REE (HREE) abundances. The chromite‐bearing peridotites of the Miyamori complex exhibit light REE (LREE) enriched patterns with further lower HREE abundances than the aluminous spinel peridotites. The most refractory clinopyroxene‐free harzburgite (Fo93) contains amphibole with the highest LREE/HREE ratio among the Miyamori‐Hayachine peridotites. The observed covariance between major and trace elements together with geological and petrological data for the complex is reproduced with an open‐system melting model, in which continuous supply of LREE‐enriched melt, melting, and melt segregation are assumed to be coupled. Nonmodal melting with specified amounts of trapped melt is also taken into consideration. The result shows that REE and trace element data for less refractory peridotites can be reproduced fairly well by the model. Accounting for the REE patterns of more refractory peridotites, however, proved less tractable, indicative of the importance of a melting and melt separation process involving melt migration and reactions during the decompression of the mantle material.