Bull. Geol. Soc. Amer. : Shipek C.G., 1962. Photographic survey of sea floor on southwest slope of Eniwetok Atoll. 73 (7): 805–812

Abstract

Basalts and basaltic rocks are the most abundant igneous rocks on the earth and their petrologic and geochemical studies have formed our knowledge base on the thermal structure and composition of the mantle with which we have developed workable models on the chemical differentiation of the earth. All this would not have been possible without innovative and painstaking experimental petrology on mantle peridotite melting, basaltic magma generation and evolution largely done in the period of 1960s -1980s. However, the ~30 year lively debate on the nature of “primary magma” among experimental petrologists and the petrology community during this time had inadvertently shelved the development of consensus models on mantle melting in the context of plate tectonics. Continued experimental petrology in parallel with worldwide sampling and study of mid-ocean ridge basalts (MORB) brought about new insights, culminating with a model in 1980s that mantle potential temperature (TMP) variation controls the extent and pressure of mantle melting and basalt compositions. The tenet of this model is that hotter rising mantle begins to melt deeper and thus has greater decompression depth interval to melt more with the melt having the petrological signature of higher extent and pressure of melting than cooler mantle. This model has gained wide acceptance in MORB studies and has also been invoked in the study of intra-plate basalts in ocean basins and in continental settings. Basalt generation above subduction zones, on the other hand, has been generally accepted as resulting from slab-dehydration induced mantle wedge melting since early 1980s, but recent studies also advocate mantle temperature variation as the primary control on the extent of mantle wedge melting. All these views with laudable merits have formed a paradigm on mantle melting and basaltic magmatism. In this paper, I review the historical developments towards this paradigm and demonstrate in simple clarity that it is the lithosphere thickness, not TMP, that controls the extent of mantle melting, depth of melt extraction and basalt compositions, i.e., the lid effect. The lithospheric lid caps the rising melting mantle, thus limiting the extent of decompression melting and equilibrium pressure/depth of melt extraction, which is well registered in the compositions of MORB, intra-plate ocean island basalts (OIB), volcanic arc basalts above subduction zones (VAB) and basalts in continental interiors (CIB). Hence, lithosphere thickness is the governing variable that controls mantle melt compositions in all tectonic settings on earth. Major element compositions (e.g., Si-Mg-Fe) of erupted basalts have no memory of initial depth of melting because of effective and efficient melt-solid (e.g., olivine [Mg,Fe]2SiO4) equilibration in the rising melting mantle. Therefore, basalt-olivine based thermobarometry, albeit useful, supplies no information on TMP. It is also the lithosphere thickness that controls whether “mantle plumes” can surface or not and the large igneous provinces (LIPs) serve as effective manifestations for thin or thinned lithosphere at the time of emplacement. This new understanding based on global observations, well-understood experimental petrology and rigorous analysis is fundamental and requires a major change to the current paradigm.