• journal_title：Geology
• Contributor：Bénédicte Abily ; Georges Ceuleneer
• Publisher：Geological Society of America
• Date：2013-01-01
• Format：text/html
• Language：en
• Identifier：10.1130/G33351.1
• journal_abbrev：Geology
• issn：0091-7613
• volume：41
• issue：1
• firstpage：67
• section：Articles

The contact between mantle peridotite and gabbro from the lower oceanic crust is usually underlined by a horizon of dunite. The origin of this dunitic transition zone (DTZ) is still debated. It is viewed either as a pile of cumulus olivine from high-MgO melts, or as former mantle peridotite pervasively percolated by melts undersaturated with pyroxene (e.g., as mid-oceanic ridge basalts [MORBs] at low pressure), and transformed into dunite. We show that the two hypotheses are not mutually exclusive, although they do not account for the same parts of the DTZ. We determined a petrological profile through a 330-m-thick DTZ that developed at the top of a mantle diapir in the Oman ophiolite. The lowermost 280 m have a reactional origin: olivine and Cr-spinel record the complex percolation and interaction history between mantle peridotite and MORB. In the uppermost 50 m, chemical trends become consistent with a cumulus origin of the dunite, olivine crystallization being a prelude to the crystallization of the overlying gabbros. The DTZ develops largely in response to melt-rock reaction, consistent with the “reactive filter” hypothesis, but the proportion of cumulate dunite is high enough to require parent melts with a significantly higher Mg content than the most primitive MORB erupted on the seafloor.