Heterogeneity of oceanic peridotite from the Western Canyon Wall at MARK: results from Site 920

Abstract

Drilling on the western canyon wall of the Mid-Atlantic Ridge south of the Kane Fracture Zone (23 °N at MARK) penetrated 200 m into serpentinized spinel peridotite exposed in the footwall of a low-angle, east-dipping, normal fault system. A total penetration of 320 m was achieved in two holes (Holes 920B and 920D), from which recovery averaged approximately 48%. This degree of recovery is unique in sampling of oceanic peridotite. These holes thus represent an opportunity to examine not only the mineralogical and bulk composition of in situ oceanic peridotite, but also the degree and character of any heterogeneity in those parameters. Mineral compositions in peridotite are remarkably uniform, considering the large apparent modal heterogeneity. The peridotites were classified during shipboard studies as predominantly harzburgite and orthopyroxene-rich harzburgite (orthopyroxene ranging up to 35 modal%) with volumetrically minor dunite and lherzolite. Mg numbers in olivine and orthopyroxene average 90.7, consistent with the peridotites being moderately depleted in terms of the previously identified, regional variation in North Atlantic oceanic peridotite mineralogy. Chrome spinel is correspondingly Al2O3-rich (Cr/[Cr + Al] ~ 28), and orthopyroxene contains moderate A12O3 and Cr2O3 contents. Trace-element analyses of whole-rock samples using inductively coupled plasma mass spectrometry indicate that background peridotites have very low abundances of high-field-strength elements, with Y <l ppm and Zr averaging about 60 ppb. Nb and Ta abundances are less than ~30 ppb, and typically are <IO ppb. Chondrite-normalized rare-earth element (REE) patterns show strong depletions in light rare-earth elements (LREE <103 times chondrite). REE abundances in the most depleted compositions are consistent with the harzburgite forming the residue to 15% to 20% fractional melting, approaching clinopyroxene exhaustion, of a fertile lherzolite. Erratic enrichment in LREE that is not reflected in changes in mineral major-element chemistry is consistent with selective enrichment of the peridotites by a fluid rich in LREE and, to a lesser extent, Zr. Selective enrichment is suggested to have accompanied the infiltration of melt through the peridotite, although textural or mineral chemistry evidence for melt infiltration is not preserved.