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Durham University

Department of Earth Sciences

Profile

Publication details for Professor Yaoling Niu

Niu, Yaoling (1997). Mantle Melting and Melt Extraction Processes beneath Ocean Ridges: Evidence from Abyssal Peridotites. Journal of Petrology 38(8): 1047-1074.

Author(s) from Durham

Abstract

This paper presents the results of the first quantitative petrological
modelling of abyssal peridotites. The mantle beneath a ridge may
be considered as two regions: (1) the melting region between the
solidus (Po) at which upwelling mantle begins to melt, and the
depth (Pf ) at which melting stops because of conductive cooling to
the surface; (2) the thermal boundary layer between Pf and the
base of crust. In the melting region, decompression near-fractional
melting is characterized by the reaction aCpx+bOpx+cSpl=
dOl + 1Melt, i.e. clinopyroxene, orthopyroxene and spinel melt
whereas olivine crystallizes as melting proceeds. In much of the
pressure range (PoZ25 kbar), orthopyroxene contributes more than
clinopyroxene to the melt (i.e. b>a) during decompression melting,
which is unexpected from isobaric melting experiments, but is
constrained by the incongruent melting of OpxÞOl + SiO2 with
decreasing pressure. The melting reaction also explains the so-
called local trend of mid-ocean ridge basalt (MORB) chemistry
characteristic of slow-spreading ridges. Melts produced over a wide
region and depth range in the mantle will ascend and migrate
laterally towards the axial zone of crustal accretion. These melts
cool and crystallize olivine as they pass through previously depleted beresidues
in the thermal boundary layer. This explains why abyssal
peridotites have excess olivine relative to simple melting residues.
The greater the ambient extent of mantle melting, the more melt is
produced in the mantle. Thus, greater extents of melting lead to
more olivine (up to 50% of the rock mass in abyssal peridotites)
crystallization at shallow levels. Additional important implications
are: (1) neither MORB melts nor the bulk igneous crust is
compositionally comparable with partial melts produced in peridotite
melting experiments because primary mantle melts crystallize olivine
back in the mantle; (2) diffusive porous flow is the primary mode
of melt migration even at very shallow levels because excess olivine
is observed on thin-section scales in abyssal peridotites; (3) low-
pressure melt equilibration during ascent is inevitable because the melting reaction preserved in residual peridotites requires continuous
solid–liquid equilibration, and because olivine crystallization in the
thermal boundary layer is the natural consequence of melt ascent
and cooling; (4) perfect fractional melting is unlikely because melt
porosity (a few percent?) in the melting mantle is required by
the melting reaction, whole-rock major element data and other
observations; (5) compositional variations of both MORB and
abyssal peridotites are consistent with varying extents (~10–22%)
of mantle melting beneath global ocean ridges.