Geochemistry of spinel-hosted amphibole inclusions in abyssal peridotite: insight into secondary melt formation in melt–peridotite reaction
- 579 Downloads
Spinel-hosted hydrous silicate mineral inclusions are often observed in dunite and troctolite as well as chromitite. Their origin has been expected as products associated with melt–peridotite reaction, based on the host rock origin. However, the systematics in mineralogical and geochemical features are not yet investigated totally. In this study, we report geochemical variations of the spinel-hosted pargasite inclusions in reacted harzburgite and olivine-rich troctolite collected from Atlantis Massif, an oceanic core complex, in the Mid-Atlantic Ridge. The studied samples are a good example to examine geochemical variations in the inclusions because the origin and geological background of the host rocks have been well constrained, such as the reaction between MORB melt and depleted residual harzburgite beneath the mid-ocean ridge spreading center. The trace-element compositions of the pargasite inclusions are characterized by not only high abundance of incompatible elements but also the LREE and HFSE enrichments. Distinctive trace-element partitioning between the pargasite inclusion and the host-rock clinopyroxene supports that the secondary melt instantaneously formed by the reaction is trapped in spinel and produces inclusion minerals. While the pargasite geochemical features can be interpreted by modal change reaction of residual harzburgite, such as combination of orthopyroxene decomposition and olivine precipitation, degree of the LREE enrichment as well as variation of HREE abundance is controlled by melt/rock ratio in the reaction. The spinel-hosted hydrous inclusion could be embedded evidence indicating melt–peridotite reaction even if reaction signatures in the host rock were hidden by other consequent reactions.
KeywordsSpinel Mineral inclusion Trace-element Harzburgite Olivine-rich troctolite Oceanic core complex
This study used samples and data of IODP Exp.304/305. We are grateful to the scientists, technicians, officers and crews aboard the JOIDES Resolution and in TAMU for their works. Kaori Hara is thanked for assistance with the electron microprobe work. The manuscript was greatly benefited from constructive comments by Riccard Tribuzio and Etienne Médard, and editorial comments by Timothy Grove. This study was supported by Grants-in-Aid for Scientific Research from Japan Society for the Promotion of Science to AT (18740336) and to SA (20244085).
- Arai S, Matsukage K (1996) Petrology of the gabbro–troctolite–peridotite complex from Hess Deep, equatorial Pacific: Implications for mantle-melt interaction within the oceanic lithosphere. In: Mével C, Gillis KM, Allan JF, Meyer PS (eds) Proceedings of the ocean drilling program, scientific results 147. Ocean Drilling Program, College Station, TX, pp 135–155Google Scholar
- Blackman DK, Ildefonse B, John BE, Ohara Y, Miller DJ, MacLeod CJ, the Expedition 304/305 Scientists (2006) Proceedings of the Integrated Ocean Drilling Program 304/305 expedition reports: oceanic core complex formation, Atlantis Massif. In: Proceedings of the Integrated Ocean Drilling Program. Integrated Ocean Drilling Program Management International, Inc., College Station, TX. doi:10.2204/iodp.proc.304305.2006
- Blackman DK, Karson JA, Kelley DS, Cann JR, Früh-Green GL, Gee JS, Hurst SD, John BE, Morgan J, Nooner SL, Ross DK, Schroeder TJ, Williams EA (2004) Geology of the Atlantis Massif (MAR 30°N): implications for the evolution of an ultramafic oceanic core complex. Marine Geophys Res 23:443–469CrossRefGoogle Scholar
- Blackman DK, Ildefonse B, John BE, Ohara Y, Miller DJ et al (2011) Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid-Atlantic Ridge 30°N. J Geophys Res 116:B07103Google Scholar
- Costa F, Dungan MA, Singer BS (2001) Magmatic Na-rich phlogopite in a suite of gabbroic crustal xenoliths from Volcán San Pedro, Chilean Andes: evidence for a solvus relation between phlogopite and aspidolite. Am Min 86:29–35Google Scholar
- Dick HJB, Natland JH (1996) Late-stage melt evolution and transport in shallow mantle beneath the East Pacific Rise. In: Mével C, Gillis KM, Allan JF, Meyer PS (eds) Proceedings of the ocean drilling program, scientific results, 147. Ocean Drilling Program, College Station, TX, pp 103–134Google Scholar
- Morishita T, Ishida Y, Arai S (2005b) Simultaneous determination of multiple trace element compositions in thin (< 30 μm) layers of BCR-2G by 193 nm ArF excimer laser ablation-ICP-MS: implications for matrix effect and elemental fractionation on quantitative analysis. Geochem J 39:327–340CrossRefGoogle Scholar
- Sun SS, McDonough WF (1989) Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders AD, Norry MJ (eds) Magmatism in the Ocean Basins. Geological Society Special Publication 42. Geological Society, London, pp 313–345Google Scholar
- Tribuzio R, Tiepolo M, Vannucci R (2000b) Evolution of gabbroic rocks of the Northern Apennine ophiolites (Italy): comparison with the lower oceanic crust from modern slow-spreading ridges. In: Dilek Y, Moores EM, Elthon D, Nicolas A (eds) Ophiolites and oceanic crust: new insights from field studies and the oceanic drilling program. Geological Society of America Special Paper 349. Geological Society of America, Boulder, Colorado, pp 129–138Google Scholar