Lithological, Chemical and Chronological Constraints on Melt Extraction from the Mantle Section of the ~492 Ma Shetland Ophiolite Complex, Scotland

Abstract:

The mantle sections of ophiolites offer a means of studying the composition and structure of the oceanic mantle. In particular, the relations between different lithologies can be established in the field, permitting an assessment of the relative timing of processes such as melt extraction and melt-rock reaction. The Shetland Ophiolite Complex (SOC) contains a well-preserved mantle section that is dominated by harzburgite (≥70 vol.%), with dominantly chondritic present-day ¹⁸⁷Os/¹⁸⁸Os compositions¹. Melt extraction and melt-rock reaction is evident in the form of dunite and chromitite layers and lenses, with thicknesses ranging from millimetres-to-metres. These lithologies are characteristic of supra-subduction zone processing and are considered to relate to closure of the Iapetus Ocean at ~492 Ma¹. However, evidence of much earlier melt extraction has been suggested for some SOC harzburgites, which have relatively unradiogenic ¹⁸⁷Os/¹⁸⁸Os compositions that yield T_RD model ages as old as ~1.4 Ga¹.

In order to assess the scales at which such compositional heterogeneities are preserved in the mantle, a small (45 m²) area of the SOC mantle section was selected for detailed lithological mapping and sampling. A selection of harzburgites (n=8), dunites (n=6) and pyroxenites (n=2) from this area has been analysed for their Os isotope and highly-siderophile element (HSE) compositions. Six of the harzburgites and four of the dunites have relative HSE abundances and gOs values that are approximately chondritic, with gOs ranging only from -0.6 to +2.7 (n=10). Two dunites have more radiogenic gOs (up to +7.5), that is correlated with enhanced concentrations of accessory base-metal sulphides, suggesting formation via melt percolation and melt-rock reaction. The two remaining harzburgites have less radiogenic gOs (-3.5 and -4), yielding Mesoproterozoic T_RD ages. The new data indicate that a comparable range of Os isotope compositions to that previously measured across the entire SOC mantle section is present in the mapped area, i.e., at the m² scale, revealing the modest scale of isotopic and chemical heterogeneity in the oceanic mantle.

¹O’Driscoll B, Day JMD, Walker RJ, Daly JS, McDonough WF, Piccoli PM (2012). Earth and Planetary Science Letters 333-334: 226-237.