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

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.

Continuous (Dynamic) Melting Theory Revisited

“…It has not yet been specified whether the matter added is solid or liquid; Ozawa & Shimizu (1995) referred to fluid. The choice is significant, and at least three options are open.…”

“…Sobolev & Shimizu (1993) applied continuous melting theory to the origin of "super-depleted" (SL) melt (glass) trapped in mantle materials. Ozawa & Shimizu (1995) invoked an opensystem continuous melting regime to explain features of the Hayachine and Miyamori ultramafic complexes of Japan. Albarède (1995) discussed the formalism of continuous melting, but without considering the nonmodal theory.…”

“…The added mass at some degree of evolution (Fig. 9b) is denoted by W aa , and the analysis is simplified if the rate of addition is assumed to be constant (see Ozawa & Shimizu 1995) and equal to ␤ per unit of melting F, so that…”

Consistent patterns of rare earth element distribution in accessory minerals from rocks of mafic-ultramafic complexes

“…Among the first the estimates of D (apatite / melt) values were obtained by the example of samples from the natural system -basanites [106], and later on -based on the results of experimental studies of basaltic melts [109]. These observations showed that all the REE have values of D (apatite / melt) > 1.…”

Petrography and Geochemistry of the mantle xenoliths: Implications for lithospheric mantle beneath the Japan arcs

“…The small letter n denotes the chondrite normalized value. Arrows are melt extraction trends estimated from abyssal peridotite (Johnson et al, 1990;Hellebrand et al, 2001 (Johnson et al, 1990;Ozawa and Shimizu, 1995;Xu et al, 2000) HREE REE pattern Ti, Zr HFSE (Vasseli et al, 1997), (c) for the peridotite xenoliths from Eifel, Germany (Witt Eickschen et al, 1993), (d) for the peridotite xenoliths from South Patagonia (Gorring and Kay, 2000), (e) for the peridotite xenoliths from Samoa islands (Hauri and Hauri, 1994).…”