Mantle melting and melt extraction processes beneath ocean ridges: Evidence from abyssal peridotites

Niu Y. (1997) Mantle melting and melt extraction processes beneath ocean ridges: Evidence from abyssal peridotites. Journal of Petrology, 38 8: 1047-1074. doi:10.1093/petroj/38.8.1047

Author Niu Y.
Title Mantle melting and melt extraction processes beneath ocean ridges: Evidence from abyssal peridotites
Journal name Journal of Petrology   Check publisher's open access policy
ISSN 0022-3530
Publication date 1997-01-01
Sub-type Article (original research)
DOI 10.1093/petroj/38.8.1047
Open Access Status Not yet assessed
Volume 38
Issue 8
Start page 1047
End page 1074
Total pages 332
Publisher Oxford University Press
Language eng
Subject 1906 Geochemistry and Petrology
1908 Geophysics
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 (P) at which upwelling mantle begins to melt, and the depth (P) at which melting stops because of conductive cooling to the surface; (2) the thermal boundary layer between P 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 (P < 25 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 Opx⇒*Ol + SiO 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 residues in the thermal boundary layer. This explains why abyssal paidotites 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 we: (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 pendotites 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 abyssalperidotites are consistent with varying extents (∼10-22%) of mantle melting beneath global ocean ridges.
Keyword Abyssal peridotites
Mantle melting
Melt extraction
Mid-ocean ridges
Ridge dynamics
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Unknown

Document type: Journal Article
Sub-type: Article (original research)
Collection: Scopus Import - Archived
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Citation counts: TR Web of Science Citation Count  Cited 294 times in Thomson Reuters Web of Science Article | Citations
Scopus Citation Count Cited 307 times in Scopus Article | Citations
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