N-type MORB

Figurt 4.24 Trace clement concentrations normalized to the composition of chondridc meteorites and plotted from left to right in order of increasing compadbility in a Small fracnon melt of the mantle. The normalizing values are those of Thompson (1982) — Table 4.7, column 6. (a) Upper and lower continental crust from Weaver and Tarney (1984) — data in Table 4.8 (b) Average N-type MORB from Saunders and Tamey (1984) and OIB from Sun (1980) — data in Table 4.8.

Within-plate alkali basalts plot in field A within-plate riioleiites plot in fields All and C. E-type MORB plots in field B whilst N-type MORB plots in field D. Volcanic-arc basalts also plot in fields C and D. The several areas of overlap mean that only within-plate alkali basalts and E-type MORB can be identified without ambiguity,... 

Figure SJl The La/lO-Y/lS-Nb/8 discrimination diagram for basalts (after Cabanis and LecoUe, 1989). The plotting coordinates are shown around the margin of the diagram the broken lines are drawn normal to the sides of the triangular diagram. Field 1 contains volcanic-arc basalts, held 2 continental basalts and held 3 oceanic basalts. The subdivisions of the fields arc as follows I A, calc-alkali basalts 1C, volcanic-arc rholeiites IB is an area of overlap between lA and 1C 2A, continental basalts 2B, back-arc basin basalts (although this is less well defined) 3A, alkali basalts from intercontinental rift 3B, 3C, E-type MORB (3B enriched, 3C weakly enriched), 3D, N-type MORB,...

Honma et al. (1991) have shown that the Okinawa Trough basalts have significantly high K, Rb and Sr contents and D/H, 0/ 0 and Sr/ Sr ratios than N-Morb have and these are due to generation of magma from normal-type mantle peridotite modified by component from the subducted slab and crustal contamination. 

Rampone E., Hofmann A. W., Piccardo G. B., Vannucci R., Bottazzi P., and Ottolini L. (1996) Trace element and isotope geochemistry of depleted peridotites from an N-MORB type ophiolite (Internal Liguride N. Italy). Contrib. Mineral. Petrol. 123, 61-76. 

Table 2 Representative analyses of MORBs. Kolbeinsey Ridge, northern EPR, and mid-Cayman Rise encompass much of the global range in extents and pressures of melting. Also shown are examples of an Indian Ocean MORE and composite analyses of a normal (N-MORB), a transitional (T-type) MORE, and an enriched (E-type) MORE from the MAR. 

North Fiji Basin is an active, mature marginal basin with an 800 km-long spreading-ridge that produces normal Mid Ocean Ridge basalts (N-MORB). The White Lady vent field, named after an almost pure anhydrite chimney that vents low-Cl fluids at 290°C, occurs on the ridge crest in the north-central part of the basin. Immediately north of the White Lady site is a 2-km-wide fossil vent field with chimneys up to 15 m that are typical MOR types composed of anhydrite, chalcopyrite, and pyrrhotite. 

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