Serpentinized peridotites from selected sections from Holes 920B and 920D, drilled in the Mid-Atlantic Ridge near the Kane Transform (MARK) area, include harzburgites and minor olivine-rich lherzolites. They are characterized by porphyroclastic textures with large (up to 1.2 cm), mostly ellipsoidal orthopyroxenes. The original modal composition (70%-85% olivine, 15%-22% orthopyroxene, 0%-8% clinopyroxene, and 0.5%-1% spinel) is close to that of refractory mantle peridotites in ophiolites and in other Mid-Atlantic Ridge regions.
The primary mineral constituents are present in a number of morphological varieties, which show evidence of high-temperature deformation and moderate recrystallization. Olivine (forsterite [Fo] = 90.1-90.3 mol%) is present as kink-banded or undulatory grains up to 4 mm in size, and as a mosaic of equidimensional smaller grains, particularly in the vicinity of orthopyroxene porphyroclasts. Aside from porphyroclasts, orthopyroxene (wollastonite [Wo] = 1.2-6.6, enstatite [En] = 84.0-89.4, ferrosilite [Fs] = 8.9-10.1 mol%) is present as small, undulatory, matrix grains, as small, anhedral, kink-banded grains in a mosaic arrangement, and as small, polygonal, undeformed grains. The porphyroclasts are heterogeneous, with more Ca, A1, Cr, and Ni in the cores than in the rims. Small, undulatory, orthopyroxenes are chemically comparable to the rims of the porphyroclasts. The orthopyroxene in mosaic arrangement is lower in Ca, Ti, Ni, and Cr than the porphyroclasts. We explain this by a combined process of porphyroclast disintegration and exsolution of clinopyroxene.
Clinopyroxenes display morphological varieties similar to orthopyroxene, but have a comparatively wider compositional range (Wo = 40.7-50.1, En = 46.1-53.8, and Fs = 3.4-5.5 mol%). Porphyroclastic grains display similar core-rim variations as orthopyroxenes. Undeformed interstitial clinopyroxene, believed to have formed from exsolved clinopyroxene in orthopyroxene, is chemically indistinguishable from other textural varieties. Evidence for magmatic impregnation is absent. Spinels are chromium bearing, but belong to the alumina-rich group (Cr# = 0.241–0.344). They are believed to have preserved their original composition during high-temperature deformation, or they may have formed as symplectites by reaction of A1-rich orthopyroxene with olivine.
The bulk chemical composition of the serpentinized peridotites was modified by alteration. Heavy rare-earth elements together with Sc, Al, Si, Ti, and Cr are the only elements that do not indicate mobility during alteration. All other determined elements, including light rare-earth elements, were more or less mobile. Positive europium anomalies in some samples may be due to the reduction of Eu3+ to significantly more mobile Eu2+ under conditions of low oxygen fugacity.
The platinum group elements also reveal some mobility. Enrichment of Pd in secondary sulfides is indicated. Elevated Os contents occur in samples with more chromium-bearing spinel.
Date of initial receipt: 8 August 1995
Date of acceptance: 26 February 1996
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