13301甲第4056号博士（理学）金沢大学博士論文要旨Abstract 以下に掲載予定：Journal of Petrology. Oxford Journals. 共著者：Norikatsu Akizawa, Shoji Arai, Akihiro Tamura, Kazuhito Ozawa, Katsuyoshi Michibayash

Akizawa Norikatsu

秋澤 紀克

13301甲第4056号博士（理学）金沢大学博士論文本文Full 以下に掲載予定：Journal of Petrology. Oxford Journals. 共著者：Norikatsu Akizawa, Shoji Arai, Akihiro Tamura, Kazuhito Ozawa, Katsuyoshi Michibayash

Kanazawa University Repository for Academic Resources

Along-ridge diversity of melt migration processes in the mantle:implications from the northern Oman ophiolite

  1 学   位   論   文   要	 旨  学位論文題名 Along-ridge diversity of melt migration processes in the mantle:  implications from the northern Oman ophiolite  学位論文題名 (邦訳) マントルにおけるメルト輸送過程の海嶺方向の多様性：北部オマーンオフィオライトの例  金沢大学大学院	 自然科学研究科 環境科学専攻	 自然計測講座 氏名: 秋澤 紀克 主任指導教員名: 荒井 章司     2 要約 The Oman ophiolite is the most extensively exposed ophiolite the world over, 500 km in NW-SE and 50-100 km in EW. It allows us to observe three-dimensional oceanic lithosphere on land, and provides information on the whole picture from its generation at the mid-ocean ridge to the end of magmatism when obducted on land. Dunite bands and dikes in the mantle section of the Oman ophiolite are interpreted as fossil melt channels within the suboceanic mantle. In particular, concordant dunite bands are of possible importance as the melt channels through which parental melts of MORB (mid-ocean ridge basalt) were transported to shallower mantle beneath the paleoridge axis. However, no comprehensive petrological data of concordant dunite bands and their ambient peridotites along a ridge segment have been published. I conducted sampling, field and microscopic observations, and chemical analysis of concordant dunite bands and their ambient peridotites from various “stratigraphic levels” in the mantle section along a paleoridge segment at the northern Oman ophiolite. I also conducted numerical modeling by using these data to clarify the diversity of melt-migration processes within suboceanic upper mantle beneath a fast spreading mid-ocean ridge. I took the segment structure into consideration because of difference in geotherm along the ridge segment, which possibly induces various melt migration processes.  学位論文要旨 Dunite bands and dikes in the ophiolitic mantle peridotite are interpreted as fossil melt channels within the suboceanic mantle. In particular, concordant dunite bands are of possible importance as the melt channels through which parental melts of MORB (mid-ocean ridge basalt) were transported to shallower mantle beneath the paleoridge axis. However, no comprehensive petrological data of concordant dunite bands and ambient peridotites along a ridge segment have been published. I conducted sampling of concordant dunite bands and their ambient peridotites from various “stratigraphic levels” in the mantle section along a paleoridge segment at the northern Oman ophiolite (Fig. 1). The Oman ophiolite is a fragment of Tethyan oceanic lithosphere formed at a fast spreading center. It allows us to observe three-dimensional oceanic lithosphere on land, and provides information on the whole picture from its generation at the mid-ocean ridge to the end of magmatism when obducted on land. The concordant dunite bands here are various both in thickness (few millimeters to few meters) and in frequency of appearance, and their boundaries with peridotites are sharp. The dunites are almost free from clinopyroxenes, which, if any, are tiny and interstitial to olivines and spinels.   I selected 28 outcrops from the mantle section of the northern Oman ophiolite (Fig. 1). I numbered them as T-1 ~ T-10 (Wadi Thuqbah), R-1 ~ R-3 (Wadi Rajmi), Z-1 (Wadi Zabin), F-1 ~ F-6 (Wadi Fizh), H-1 (Wadi Hilti), S-1 ~S-4 (Wadi Sarami), Ry-1 (Wadi Rayy), U-1 (Wadi Umm al Hasa) and K-1 (Wadi Khudayrah). The mantle section is classified into five subsets for   3 simplicity; “uppermost mantle section” is within few tens of meters below the MTZ, “upper mantle section” is within up to 5 km-wide zone in plan to the west of the MTZ, “middle mantle section” is from 5 km to 10 km-wide zone in plan to the west of the MTZ, “lower mantle section” is from over 10 km-wide zone in plan to the west of the MTZ, and “basal section” is located at the base of the Oman ophiolite, which is above the metamorphic sole. I assume the outcrops along Wadi Thuqbah (T-1 ~ T-10) are derived from the center of a second-order paleoridge segmentation (segment center), whereas other outcrops (R-1 ~ R-3, Z-1, F-1 ~ F-6, H-1 and S-1 ~ S-4) are uncertain which part of the paleoridge segmentation they are derived from, although I regard them as having been derived from a distal part of a segment, possibly a segment end. In the other outcrops (Ry-1, U-1 and K-1), the MORB-related magmatism had been overprinted by off-ridge magmatism.  Most of the concordant dunite bands overlap in major-element compositions with their ambient peridotites, however some of them show rather lower Cr/(Cr + Al) atomic ratio (< 0.50) of chromian spinels (Figs. 2a and b), and higher Al2O3 (up to 4.0 wt%) and Cr2O3 (up to 1.3 wt%) of clinopyroxenes (Figs. 2c and d) in the uppermost mantle section than in the lower mantle section. In the ambient peridotites of dunite bands, trace-element (rare earth elements: REE with Sr, Zr, Ti and Y) patterns of clinopyroxene incline from heavy-REE (HREE) to light-REE (LREE) abruptly, whereas they show various shapes in dunites, depending on the stratigraphic levels along the paleoridge segment, most of them are gently to steeply declining from HREE to LREE (Figs, 3a, b and f), some are spoon-shaped at the lower to middle mantle section in the segment center (Figs. 3c and d), and those of the concordant dunite band at outcrop F-5 are heterogeneous in LREE to MREE contents even within a thin section (about 4 cm long by 2 cm) (Fig. 3e).  I suggest various melt migration processes in the mantle section based on different appearances of the dunite bands in the field and their mineral chemical characteristics along the paleoridge segment. I conducted numerical modeling (1-D steady state modeling) for clarifying a variety of the melt migration processes in the mantle section by using trace-element compositions of clinopyroxene. The modeling duplicates simple fractional melting and influx melting processes. The results indicate that LREE-enriched melt generated at higher pressure conditions flowed through a melt channel until shallower mantle section (less than 10 km to the MTZ), where such LREE-enriched melt mingled with highly depleted melts generated at lower pressure conditions, and N-MORB-like melts were generated as a consequence. Some melt channels mainly transported the highly depleted melts at the shallower mantle section, which occasionally precipitated orthopyroxene-rich rocks like orthopyroxenites along the melt channel. Another numerical modeling (plate model) suggests the fracture-related melt transport with chromatographic melt percolation into the wall peridotite in the uppermost mantle section at the paleoridge segment end. These variations in the melt migration process resulted from a difference in mantle geotherm along   4 the paleoridge segment related to the active mantle upwelling, e.g., higher at the segment center and lower at the segment end at a given stratigraphic level.  引用文献 Adachi, Y. & Miyashita, S. (2003) Geology and petrology of the plutonic complexes in the Wadi Fizh area: multiple magmatic events and segment structure in the northern Oman ophiolite. Geochemistry Geophysics Geosystems 4, 8619, doi: 10.1029/2001GC000272. Girardeau, J. Monnier, C., Launeau, P. & Quatrevaux, F. (2002) Kinematics of mantle flow beneath a fossil overlapping spreading center: The Wuqbah massif case, Oman ophiolite. Geochemistry Geophysics Geosystems 3, doi: 10.1029/2001GC000228. Lippard, S. J., Shelton, A. W. & Gass, I. G. (1986) The ophiolite of northern Oman. Memoir 11, Geological Society, London, 178 pp. Ministry of Petroleum and Minerals (1992a) Geological map of Buraymi, scale 1:250,000, Sultanate of Oman, Muscat. Ministry of Petroleum and Minerals (1992b) Geological map of Ibri, scale 1:250,000, Sultanate of Oman, Muscat. Miyashita, S., Adachi, Y. & Umino, S. (2003) Along-axis magmatic system in the northern Oman ophiolite: Implications of compositional variation of the sheeted dike complex. Geochemistry Geophysics Geosystems 4, 8617, doi: 10.1029/2001GC000235. Suetake, A. & Takazawa, E. (2012) Spatial compositional variability and origin of dunites in the Fizh mantle section, the Oman ophiolite. The Earth Monthly 134, 247-251 (in Japanese). Sun, S. S. & McDonough, W. F. (1989) Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A. D. & Norry, M. J. (eds.) Magmatism in the Ocean Basins, Geological Society Special Publication 42, Geological Society, London, 313-345. Tamura, A. & Arai, S. (2006) Harzburgite-dunite-orthopyroxenite suite as a record of supra-subduction zone setting for the Oman ophiolite mantle. Lithos 90, 43-56. Umino, S. Miyashita, S. Hotta, F. & Adachi, Y. (2003) Along-strike variation of the sheeted dike complex in the Oman ophiolite: Insights into subaxial ridge segment structure and the magma plumbing system. Geochemistry Geophysics Geosystem 4, 8618, doi: 100.1029/2001GC000233.      5  Figure 1 Geological maps. (a) Simplified geological map of the Oman ophiolite modified from Lippard et al. (1986). (b) Distribution of rocks in the northern Oman ophiolite with distribution of the outcrops that were examined here. Modified from the 1:250,000 geological maps of Buraymi and Ibri (Ministry of Petroleum and Minerals, 1992a, b). Paleoridge segment structures are quoted from Adachi & Miyashita (2003), Miyashita et al. (2003) and Umino et al. (2003). Area of highly depleted peridotites is enclosed by dashed line (Suetake & Takazawa, 2012). Abbreviations: Center = segment center, End = segment end, MTZ = Moho transition zone, CDB = concordant dunite band, and DDD = discordant dunite dike. 25KmOman   24°00’N56°00’E 56°30’E24°30’N Segment end (OSC, 2nd) Segment center (2nd)Highly depleeted area Upper extrusivesLower extrusivesSheeted dike complexPlagiograniteUpper gabbroLower gabbroGabbronoriteWehrliteDunite Peridotite56°E 57°E 58°E25°NMuscat50kmSegment end (DEVAL, 3rd) 24°NSoharKhawr FakkanAswadFizhHiltiSaramiHaylaynRustaqWuqbahMiskinBahlaSumail(a)(b) (b)Sheeted dikes & extrusivesGabbro & cumulate peridotitesTectonite peridotite = Mantle sequenceT-2: 20 cm (CDB)T-3: 3 cm (CDB)T-4: 250 cm (CDB)T-5: 25 cm (CDB)T-6: 6 cm (CDB)T-7: 80 cm (CDB)T-8: 8 cm (CDB)T-9: 30 cm (CDB)T-10: 40 cm (DDD)F-2: 20 cm (CDB)F-3: 8 cm (CDB)F-4: 25 cm (CDB)F-5: <1 cm (CDB)F-6: 5 cm (CDB)T-1 ~ 4T-5T-6T-7T-8T-9T-10H-1S-1 & 2S-3S-4F-1 & 2F-3 ~ 5F-6Z-1R-1R-2R-3U-1K-1Ry-1T-1<1000 mF-1≒10 mS-1>10 mMTZDunite at CenterW. ThuqbahDunite at Basal sectionDunite at EndR-1: 25 cm (CDB) R-2: 20 cm (CDB)R-3: 15 cm (CDB)Ry-1: 40 cm (CDB)S-2: 5 cm (CDB)S-3: 4 cm (CDB)S-4: 2 cm (CDB)H-1: 20 cm (CDB)Uppermost mantle sectionUpper mantle sectionMiddle mantle sectionLower mantle sectionBasal sectionW. SaramiW. HiltiW. RajmiW. RayyW. KhudayrahW. Umm al Hasa W. ZabinW. FizhW. ThuqbahW. Bani Umar al GharbiW. SalahiW. AhinW. SudumOutcropsW. FizhW. ZabinZ-1: 5 cm (CDB)W. RajmiW. SaramiW. HiltiU-1: 3 cm (CDB)K-1: 3 cm (CDB)W. RayyW. KhudayrahW. Umm al HasaWadi Tayin  6  Figure 2 Mineral chemical characteristics of dunites (du) and their ambient harzburgites (hz). (a and b) Variations of Cr# [Cr/(Cr + Al) atomic ratio] and Mg# [Mg/(Mg + Fe) atomic ratio]	 in chromian spinel. The boninite fields are modified from Tamura and Arai (2006). (c and d) Variations of Al2O3 and Cr2O3 in clinopyroxene and orthopyroxene. The ranges of chemical composition of orthopyroxene are shown in red for the lower to uppermost mantle section, and in blue for the basal section. The arrowed thick lines show a systematic change in chemistry from the lower mantle  (LM) section to the uppermost mantle (UMM) section through the middle mantle (MM) section and the upper mantle (UM) section of this study. Abbreviations are the same with Figure 1. 0.00.20.40.60.81.01.0 0.8 0.6 0.4 0.2 0.0(b) Spinel (harzburgite)0.00.20.40.60.81.01.0 0.8 0.6 0.4 0.2 0.0Mg#Cr#Mg#Cr#(a) Spinel (dunite)MTZ du (End)MTZ du (Center)Boninite BoniniteCDB at MM (Z-1)CDB at UM (F-6)DDD (T-10)Ambient hz at basal section (K-1)Aureole hz (T-10)Ambient hz of DDD (T-10)T-2 (UMM section)T-5 (UM section)CDB  associated with opx-rich rocks0.01.02.03.04.05.00.0 0.5 1.0 1.5Cr2O3 Cr2O3Al 2O30.01.02.03.04.05.00.0 0.5 1.0 1.5Al 2O3(c) Cpx (dunite) (d) Cpx (harzburgite)UMMLMUMMLMCDB at MM (Z-1)CDB at UM (F-6)DDD (T-10)Ambient hzat basal section (K-1)Opx (basal section)Opx (UMM ~ LM)MTZ du (End)MTZ du (Center)CDB  associated with opx-rich rocksT-2 (UMM section)T-5 (UM section)UMM sectionUM~MM sectionLM sectionBasal section  7  Figure 3 Chondrite-normalized trace-element patterns of clinopyroxene in dunites and their ambient harzburgites (hz). (a-d) Segment center. (e and f) Segment end. The harzburgite about 20 cm away from the dunite/harzburgite boundary (Hz 20 cm above band) is exceptionally enriched in trace elements of clinopyroxene at outcrop F-5 (e). Lithology and mineral chemistry are shown in rectangle boxes. Fo and Cr# are mean values. Chondrite values are from Sun and McDonough (1989). Symbols are not put for the data below the detection limits. Dunite band in SEGMENT CENTERCPX/Chondrite0.0010.010.1110La Ce PrSr Nd SmZr Eu Gd TbTi Dy HoY Er Tm Yb LuDunite (T-4)Ambient hz(a) W. Thuqbah (T-3 & 4)Dunite (T-3)CPX/Chondrite0.0010.010.1110La Ce PrSr Nd SmZr Eu Gd TbTi Dy HoY Er Tm Yb Lu(b) W. Thuqbah (T-6)DuniteAmbient hzDunite (6 cm): Fo=91.0, Cr#=0.58Ambient hz: Fo=91.9, Cr#=0.58CPX/Chondrite0.0010.010.1110La Ce PrSr Nd SmZr Eu Gd TbTi Dy HoY Er Tm Yb Lu(d) W. Thuqbah (T-8 & 9)Dunite (T-8)Dunite lensCPX/Chondrite0.0010.010.1110La Ce PrSr Nd SmZr Eu Gd TbTi Dy HoY Er Tm Yb Lu(c) W. Thuqbah (T-7)DuniteAmbient hzAureole hzDunite (80 cm): Fo=92.1, Cr#=0.62Aureole ha: Fo=91.6, Cr#=0.60Ambient hz: Fo=91.8, Cr#=0.49Uppermost mantle section Upper mantle sectionUppermost mantle section Upper mantle sectionMiddle mantle section Lower mantle sectionDunite (T-9)Ambient hz (T-8) Ambient hz (T-9)T-9T-8Dunite (8 cm): Fo=91.3, Cr#=0.66Aureole ha: Fo=91.2, Cr#=0.66Ambient hz: Fo=90.9, Cr#=0.49Dunite (30 cm): Fo=92.2, Cr#=0.57Ambient hz: Fo=91.5, Cr#=0.49Dunite (3 cm, T-3): Fo=91.1, Cr#=0.57Dunite (250 cm, T-4): Fo=91.1, Cr#=0.45Ambient hz: Fo=91.2, Cr#=0.57Dunite band at SEGMENT ENDCPX/Chondrite0.0010.010.1110La Ce PrSr Nd SmZr Eu Gd TbTi Dy HoY Er Tm Yb LuAureole hzAmbient hzHz 20 cm above bandDunite (< 1 cm): Fo=91.6, Cr#=0.57Hz 20 cm above band: Fo=91.5, Cr#=0.58Ambient hz: Fo=91.4, Cr#=0.53(e) W. Fizh (F-5)CPX/Chondrite0.0010.010.1110La Ce PrSr Nd SmZr Eu Gd TbTi Dy HoY Er Tm Yb Lu(f) W. Rajmi (R-2)<1cm<1cmAmbient ha20cmDunite (20 cm): Fo=90.7, Cr#=0.52Aureole hz (20 cm):Fo=91.0, Cr#=0.62Dunite (< 1 cm):Fo=91.6, Cr#=0.55Dunite (< 1 cm): Fo=91.8, Cr#=0.53Ambient hz: Fo=91.7, Cr#=0.52Aureole hz

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Along-ridge diversity of melt migration processes in the mantle:implications from the northern Oman ophiolite

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