i. Introduction - Science Direct

Earth and Planetary Science Letters, 29 (1976) 7 -20 © I-lsevierScientific Publishing Company, Amsterdam

Printed in The Netherlands

SPINELS IN MID-ATLANTIC RIDGE BASALTS: CHEMISTRY AND OCCURRENCE ItARALI)UR SI(;URDSSON and J.-G. SCHILLING Graduate School of Oceanography, University of Rhode Island, Kingston, R.I. {USA)

Received June 24, 1975 Revised version received November 18. 1975

Three groups of spinels have been identified in dredged basalts from the Mid-AtlanticRidge in the Azores region (30 -40°N): (1) magnesiochromites with 0.4-0.5 Cr/(Cr + AI) are most common and characteristic of olivine tholciires of the region; (2) titaniferous magnesiochromites are found in an olivine basalt with alkali affinities, of local occurrence and evolved in relatively high fugacity of oxygen; ~3) chromian spinels with 0.23 ('r/(('r + AI) occur in unusual high-Al picrites of local occurrencc and possible high-pressure origin. Spincls are restricted in occurrence to the least fractionated' lavas, with t:eO*/FeO* + MgO ratio less than 0.575 and with Cr content greater than 350 ppm. A close relationship between AI content of liquidus spinel and AI content of magma has been observed for basaltic types. Iligh-Al spinels deviating from this relationship, such as those found in picritie lavas from the Mid-Atlantic Ridgc, may have crystallized at high pressure. The use of spincls as geobarometers in magmas of a restricted compositional range seems a promising prospect. There is no evidence of systematic variation in spinel chemistry or occurrence ahmg the Mid-Atlantic Ridge, such as could be related to different mantle sources of the basalts, plume versus non-plutne or binary mantle mixing.

i. Introduction Spinel mineral chemistry in igneous rocks is potentially an important tool in petrogenetic studies as pointed out by lrvine [1]. In vicw of the extensivc substitution ofCr, A1, Fe 3. and Ti in the octahedral (B) site and Mg and Fe 2÷ in tetrahedral (A) site, spinels in basaltic rocks may prove to be as informative about the earliest stages of magmatic crystallization as pyroxenes have been in the middle and late stages. Spinels are, however, very sensitive to subsolidus reequilibration and thus spinel chemical trends in holocrystalline rocks have to be interpreted with caution due to the possibility of reaction between early-formed spinel and residual melt. Reports on the presence and crystal chemistry of spinels in submarine basalts have been few so far [2 5]. However, spinels are characteristic accessory minerals in many Mid-Atlantic Ridge (MAR) basalts, occurring as tiny euhedral crystals enclosed in or attached to magnesian olivine phenocrysts or, more rarely, enclosed in plagioclase phenocrysts. Their

paragenesis indicates that spinels may be the earliest phase to crystallize from MAR basaltic melts: thus some of these spinels may have coexisted and perhaps equilibrated with upper mantle partial melt or a primitive basaltic liquid. We report here on the distribution and chemical composition of spinels in submarine basalts from the Mid-Atlantic Ridge between 40°N and 30°N (Fig. 1). Most of the basalts are from the R/V "Trident" dredged rock collection of the University of Rhode Island. The spinels reported occur as accessory crystals or inclusions in quenched glassy rims of variolitic zones of dredged pillow lavas. The effects of subsolidus reequilibration have thus been minimized, providing compositional data on primary spinels from basaltic melts erupting along the rift of the Mid-Athmtic Ridge.

2. Spinel distribution Dredged rocks from some 21 R/V "Trident" stations along the rift of the Mid-Atlantic Ridge between 40°N

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