Mineral chemistry of ultramafic cumulates from the North Arm Mountain Massif of the Bay of Islands ophiolite: Evidence for high‐pressure crystal fractionation of oceanic basalts

Abstract

1973].

Petrological Systematics of Mid-Ocean Ridge Basalts: Constraints on Melt Generation Beneath Ocean Ridges

“…In addition, some aspects of the residual variability may result from more complex fractionation processes. These may include (a) magma evolution in periodically replenished magma chambers [O 'Hara, 1977], where a magma chamber can be modeled simply as a holding tank with inputs and outputs; (b) in situ fractionation [Langmuir, 1989], where solidification takes place along the margins of a magma chamber in a temperature gradient that extends from solidus to liquidus; and (c) highpressure fractionation, which has been suggested to be an important factor in MORB evolution [e.g., Bender et al, Elthon et al, 1982]. To evaluate the importance of these fractionation processes in contributing to the diversity of MORB compositions, it is necessary to quantitatively examine their chemical effects.…”

Comparison of major- and trace-element geochemistry of abyssal peridotites and mafic plutonic rocks with basalts from the MARK region of the Mid-Atlantic Ridge

“…Preferential dissolution of clinopyroxene eventually will produce a clinopyroxene-saturated melt. Melts that are always near saturation in clinopyroxene may crystallize clinopyroxene during their ascent initially in the absence of Plagioclase, and possibly aided by moderate pressure cooling and fractionation (e.g., Elthon et al, 1982Elthon et al, , 1984aCasey et al, 1983;Tormey et al, 1987;Elthon, 1987). This may explain the late wehrlite and clinopyroxenite veins that intrude the residual mantle as it cools and becomes part of the mechanical lithosphere.…”

“…Higher pressure (>IO kbar) fractionation of these melts could lead to the early crystallization of olivine, olivine + clinopyroxene, olivine + clinopyroxene + orthopyroxene, and localized refertilization at deeper levels. The clinopyroxenes in these veins often have high Mg#s, outside the range predicted at lower pressures (Elthon et al, 1982;Grove et al, 1992). The significance of high Mg# (92-82) clinopyroxene in these pyroxenite veins may indeed be that they indicate moderately high pressures of crystallization (Elthon et al, 1982).…”

“…The clinopyroxenes in these veins often have high Mg#s, outside the range predicted at lower pressures (Elthon et al, 1982;Grove et al, 1992). The significance of high Mg# (92-82) clinopyroxene in these pyroxenite veins may indeed be that they indicate moderately high pressures of crystallization (Elthon et al, 1982). However, a perplexing feature of these veins is the lack of significant ductile deformation of the pyroxenites, if one is to accept the highpressure origin.…”

“…9) and the mineral chemistry variations across the veins within Holes 920B and 920D, clear evidence of a crystallization orders of olivine, then olivine + clinopyroxene^ orthopyroxene) or alternatively of olivine, then olivine + clinopyroxene(± orthopyroxene), then olivine + clinopyroxene + Plagioclase ± Fe-Ti oxides ± sulfides exist (also see plates for Holes 920B and 920D; Shipboard Scientific Party, 1995). Olivine + clinopyroxene or clinopyroxene + orthopyroxene coprecipitation in the absence of Plagioclase is not predicted when MORB melts crystallize at pressures less than -10 kbar (e.g., Elthon et al, 1982;Grove et al, 1992). Composite veins that show Plagioclase at the center probably crystallized at lower pressures than 10 kbar, but the fact that clinopyroxene precedes Plagioclase in the crystallization sequence documented would not be typical of MORB crystallization.…”