The very depleted nature of certain primary mid-ocean ridge basalts |
| |
Authors: | D. Elthon J.F. Casey |
| |
Affiliation: | Department of Geosciences, University of Houston, Houston, Texas 77004, USA |
| |
Abstract: | Many mid-ocean ridge basalts (MORBs) might be ultimately derived from primary magmas that are very depleted in Na2O and TiO2. These very depleted primary magmas have 0.60 to 1.50 wt.% Na2O and 0.10 to 0.50 wt.% TiO2 compared to MORBs, which typically have > 1.90% Na2O and >0.60% TiO2. Evidence for these depleted primary magma compositions is obtained from megacrysts in MORBs, from glass inclusions within these megacrysts, and from the highly calcic plagioclases (An91–96) and depleted clinopyroxenes (Na2O mostly between 0.10 and 0.35) in certain abyssal peridotites.Cumulate ultramanfi and gabbroic rocks from the North Arm Mountain Massif of the Bay of Islands ophiolite complex show a progressive increase in the Na2O and TiO2 abundances in clinopyroxene crystals with stratigraphic height in the ophiolite. The use of mineral-liquid distribution coefficients and cumulate mineral compositions indicate that the liquids from which these minerals crystallized had 0.10 to 0.20 wt.% TiO2 and 0.60 to 0.80 wt.% Na2O for the lowermost cumulate ultramafic rocks, with TiO2 and Na2O abundances of liquids increasing progressively to normal MORB abundances during crystallization of higher-level gabbroic cumulates. These data clearly demonstrate that primary basalts that are very depleted in Na2O and TiO2 can differentiate to form residual magmas that are indistinguishable from MORBs. |
| |
Keywords: | |
本文献已被 ScienceDirect 等数据库收录! |
|