Petrogenesis of Gabbro from Ultraslow-Spreading Gakkel Ridge



Journal Title

Journal ISSN

Volume Title



Petrologic evolution under mid-ocean ridges (MORs) has long been a major means to study the complex structure of the oceanic crust. Gakkel Ridge, the Arctic mid-ocean ridge, is especially interesting with its unique “ultraslow” spreading rate. Geochemical studies of Gakkel Ridge basalts and peridotites have shown the primitive magma to be created by a low degree of partial melting and a heterogeneous parental mantle. However, the extremely thin seismic “Layer 3”, correlated generally with gabbro, and its properties are not yet well studied. We studied the geochemistry of gabbros recovered from the AMORE program (Arctic Mid-Ocean Ridge Expedition, 2001), using Electron Probe Microanalyzer (EPMA) and LA-ICP-MS. A total of 25 thin sections from 6 stations were analyzed. The results of the clinopyoxene (cpx) Mg# (Mg/(Mg+Fe)) versus anorthite content (An% = (Ca/Ca+Na)) of the studied gabbros show a relatively low trend in An% compared to global gabbros. This implies the degree of partial melting is among the lowest compares to the global MORs. In addition, MELTS models of isobaric fractional crystallization of average Gakkel primitive basalt indicates that the SLD (solid line of descent) of (Mg# cpx vs An% plag) matches with our results quite well. Such models indicate that the SMZ (Sparsely Magmatic Zone) and the WVZ (Western Volcanic Zone) are generated with about 7.5% and 11% of partial melting respectively. Using these solid line of descent models, the major element trends can be used to predict (or infer) the thickness of the oceanic crust. The crustal thickness calculated by the Gakkel gabbros is about 2 – 4 km, in accordance with predictions made with the basalt and the seismic data that infers a cold mantle. This low-pressure melting phenomenon under Gakkel Ridge can also be determined by the lowest SLD of MgO and FeO in cpx compared to global data. Trace elements in clinopyroxene generally also show the result of crystallization of primary magmas. There seems to be little influence of the degree of partial melting. Clinopyroxene however shows us a substantial and consistent core and rim fractionation. This could be explained by melt-rock reaction or in-situ crystallization.



Gakkel Ridge, Gabbro