Ubiquitous old depleted mantle in the oceanic mantle

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Author(s): Alard, O.
Author Affiliation(s): Primary:
Université de Montpellier II, Géosciences Montpellier, Montpellier, France
Volume Title: AGU 2011 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2011; American Geophysical Union 2011 fall meeting, San Francisco, CA, Dec. 5-9, 2011. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Abyssal peridotites (AP) are believed to be the residue of recent partial melting of the MORB source mantle, thus their petrologic and geochemical characteristics were interpreted within the framework of recent melting of a fertile convective mantle. However, a number of studies have shown that the "magmatic" history of AP are much complex and record multiple episodes of melt percolation/reaction. Recent isotopic data (e.g. Re-Os) have prompted question on whether AP are direct residues of recent MORB melting. Magmatic sulfides are the main carrier of highly siderophile elements in the mantle, including Os and Re. Recent studies have shown that several sulphide populations characterized by different micro-structural occurrences and compositions coexist at the thin section scale and record the various episodes of melting and melt/rock reaction events. Therefore the decay of 187Re to 187Os provides an exceptional tool to unravel the sequence of melt extraction and percolation. Petrographic and geochemical charaterization of sulfides coupled with in-situ Re-Os in abyssal peridotites (or related rocks) from the MAR (Kane Fracture Zone, Leg 209), the SWIR, Hess Deep and from the Ligurides and Oman Ophiolites indicate that typically 3 sulfide populations coexist in AP. Hydrothermal sulfides occur as veins, however as shown by mass balance calculation their bearings on the whole rocks Os budget are trivial. Upon microstructural observation and mineralogical compositions broadly two types of magmatic sulfides can be distinguished. Type 1 magmatic sulfides (M1) are either hosted in Opx or occur as relicts within the serpentine matrix. They consist of pentlandite (Pn) ± pyrrhotite (Po) and small amounts (<5%) of chalcopyrite (Cp) (with Pn>Po>Cp). These features along with their PGE systematic (i.e., low Pd/Ir) indicate that they are residual after melting. Further they have unradiogenic Os and low Re/Os indicative of long-term evolution in a Re-depleted environment. They yield TRD ages in excess of 2.2 Ga in most localities. Type 2 magmatic sulfides (M2) are spatially associated with Cpx2±Spl2 clusters. They have convoluted form (low dihedral angle) and occur as large plage (≥100µm) or as swarm of smaller sulfide blebs (≤50µm) and networked veinlets. They show a high abundance of Cu-rich phases (Cp ± Bornite (Bo)≥5%). These features along with their high Pd/Ir ratio indicate that M2 sulfides were precipitated during melt-rock reaction. In most cases M2 sulfides show radiogenic composition (>0.13) and higher Re/Os ration than M1-sulfides, however in leg 209, M2 sulfides show constant and unradiogenic 187Os/188Os ≈ 0.106, despite large variation in Re/Os. Along with their association with extremely depleted cpx they suggest that they are derived form an old (2.4Ga) depleted mantle also sampled through melt inclusion in MORBs (e.g., Sobolev and Shimizu, 199X). Together with previous findings these results suggest that contrary to the accepted geodynamic model, the oceanic mantle beneath active mid-oceanic ridge contains significant proportion of old depleted mantle blobs (reminiscent of Archean to Proterozoic sub-continental mantle) and survived convection resetting for time ≥ 2 Ga.
Year of Publication: 2011
Research Program: ODP Ocean Drilling Program
Key Words: 18 Geophysics, Solid-Earth; Abyssal plains; Archean; Atlantic Ocean; Basalts; Convection; Depletion; East Pacific; Hess Deep; Igneous rocks; Isotope ratios; Isotopes; Kane fracture zone; Leg 209; Mantle; Melting; Metals; Mid-Atlantic Ridge; Mid-ocean ridge basalts; North Atlantic; North Pacific; Northeast Pacific; Ocean Drilling Program; Ocean floors; Oceanic mantle; Osmium; Pacific Ocean; Partial melting; Pentlandite; Peridotites; Platinum group; Plutonic rocks; Precambrian; Proterozoic; Radioactive isotopes; Re-187/Os-188; Rhenium; Semail Ophiolite; Siderophile elements; Stable isotopes; Sulfides; Ultramafics; Upper Precambrian; Volcanic rocks
Record ID: 2016107495
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by, and/or abstract, Copyright, American Geophysical Union, Washington, DC, United States

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