Petrology of Ultramafic Rocks from Site 395

Arai, Shoji; Fujii, Toshitsugu

doi:10.2973/dsdp.proc.45.134.1979

Cited by 22 publications

(21 citation statements)

References 13 publications

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“…This is consistent with the discussion of mineralogical variability for the Mid-Atlantic Ridge peridotites (e.g., Michael and Bonatti, 1985;Bonatti et al, 1992). It is noted that the spinel compositions around Cr# = 29 are slightly higher than those from Site 670 in the MARK area (Fujii, 1990;Juteau et al, 1990;Komor et al, 1990), and lower than those from Site 395 in the western flank of Mid-Atlantic Ridge in the MARK area (Arai and Fujii, 1979;Sinton, 1979), Site 334 (DSDP Leg 37) in the FAMOUS area (Clarke and Loubat, 1977;Sigurdsson, 1977;Symes et al, 1977), and some fracture zones around Mid-Atlantic Ridge (Prinz et al, 1976;Bonatti et al, 1992;Cannat et al, 1992).…”

Section: Characteristics Of Chemical Composition Of Primary Mineralssupporting

confidence: 88%

“…The TiO 2 contents are commonly less than 0.15 wt%. The compositional ranges of the Hole 920D clinopyroxenes are comparable with those from Site 670 (Leg 109) in the MARK area (Fujii, 1990;Hebert et al, 1990;Juteau et al, 1990;Komor et al, 1990) (Arai and Fujii, 1979;Sinton, 1979). The Hole 920D clinopyroxenes are extremely poor in both Ti and Na (lOOTi < 0.7, lOONa < 1.2), suggesting a highly depleted peridotite signature, compared with other suboceanic peridotites (Fig.…”

Section: Clinopyroxenesupporting

confidence: 52%

“…Abbreviations as in Table 1. Clarke and Loubat, 1977;Sigurdsson, 1977;Symes et al, 1977;Arai and Fujii, 1979;Sinton, 1979;Fujii, 1990;Juteau et al, 1990;Komor et al, 1990;Bonatti et al, 1992;Cannat et al, 1992). A highly uniform composition of primary spinel cores (Mg# = 70-75 and Cr# = 27-31) is conspicuous in Hole 920D spinel harzburgites and lherzolites (Fig.…”

Section: Spinelmentioning

confidence: 99%

“…Data sources for clinopyroxenes from Mid-Atlantic Ridge (Prinz et al, 1976;Clark and Loubat, 1977;Symes et al, 1977;Arai and Fujii, 1979;Sinton, 1979;Fujii, 1990;Hebert et al, 1990;Juteau et al, 1990;Komor et al, 1990;Bonatti et al, 1992;Cannat et al, 1992); American-Antarctic Ridge (AAR) and Southwest Indian Ridge (SWIR; Dick, 1989); and those related to the pre-oceanic rifting during the early stage of continental breakup (preoceanic rifts) from Saint Paul's Rocks (Melson et al, 1972), Galicia Margin (Evans and Girardeau, 1988;Kornprobst and Tabit, 1988), Zabargad (Bonatti et al, 1986;Piccardo et al, 1988), southwest Australia (Nicholls et al, 1981), and Antarctic Seamount B (Niida and Yuasa, 1995). …”

Section: Modification Of Peridotites By Magma Channeling Magma Channementioning

confidence: 99%

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Mineralogy of MARK peridotites: replacement through magma channeling examined from Hole 920D, MARK area

Nilia¹

1997

Proceedings of the Ocean Drilling Program, 153 Scientific Results

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Peridotites drilled at Site 920 of Ocean Drilling Program Leg 153 in the Mid-Atlantic Ridge near the Kane Transform (MARK) area were examined to understand the mode of magma channeling and modification of the upper mantle peridotites. This paper presents mineralogical data from a slow-spreading ocean ridge for peridotites that have been modified by magma channeling and for those that are free from the modification.The chemical compositions of primary minerals in spinel harzburgite and lherzolite are highly uniform. Olivine core compositions concentrate around Fo = 91.0 and NiO = 0.37 wt%. Spinel cores also have a small compositional range of Mg# = 70-75 and Cr# = 27-31. Orthopyroxenes with Mg# = 90.5-91.6 have A1 2 O 3 contents of 3.17 to 4.77 wt%. TiO 2 and Na 2 O contents in primary clinopyroxenes are extremely low, less than 0.15 and 0.11 wt%, respectively. These results suggest that the MARK peridotites represent moderately depleted upper mantle peridotite. The chemical compositions of primary minerals are modified where the peridotites are crosscut by gabbro and pyroxenite dikes and veins. Such peridotites are enriched in FeO* and TiO 2 , and depleted in MgO, NiO, and A1 2 O 3 .Spatial variations in chemical composition of minerals were examined across channels in Samples 153-920D-10R-2 (Piece 4, 35-41 cm) and 153-920D-13R-2 (Piece 7, 136-143 cm). Primary minerals in spinel harzburgite are enriched in FeO* and TiO 2 within narrow replacement bands next to websterite veins. No evidence for melt extractions was detected in the harzburgite wall, indicating that the replacement occurred as a one-way modification process by melt addition from the channeling magma. The Fo-NiO systematics for the modified olivines support a model of open-system reaction between the wall harzburgite and the infiltration melt with a highly evolved composition. All of the results emphasize that the final magma channeling occurred in "cold" peridotites at a shallow level of the upper mantle.

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Section: Characteristics Of Chemical Composition Of Primary Mineralssupporting

confidence: 88%

Section: Clinopyroxenesupporting

confidence: 52%

Section: Spinelmentioning

confidence: 99%

Section: Modification Of Peridotites By Magma Channeling Magma Channementioning

confidence: 99%

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Mineralogy of MARK peridotites: replacement through magma channeling examined from Hole 920D, MARK area

Nilia¹

1997

Proceedings of the Ocean Drilling Program, 153 Scientific Results

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“…Sample DL^90 also has clinopyroxenes with very high A1 2 O 3 (7%), high TiO 2 (0.35-0.48%), and high Na 2 O (0.8%) contents suggesting that these are much less refractory materials than those in the drilled samples. These compositions are distinct from most pyroxene analyses from alpine or oceanic-type peridotites, which have Na 2 O contents in the range of 0.12-0.25% and A1 2 O 3 contents of 1.5-5.5% (Sinton, 1979;Arai and Fujii, 1979;Clarke and Loubat, 1977). However, they are not as high in Na as jadeitic clinopyroxenes in eclogite xenoliths (Shee and Gurney, 1979), and are similar in some ways to pyroxene compositions from Cr-diopsied megacrysts in alkalic basalts (Jagoutz et al, 1979;Nixon and Boyd, 1979).…”

Section: Mineral Compositionsmentioning

confidence: 67%

Petrology and Relict Mineralogy of Serpentinites from Deep Sea Drilling Project Leg 84

Bloomer¹

1985

Initial Reports of the Deep Sea Drilling Project

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Eleven serpentine samples from DSDP Leg 84 and four serpentinized ultramafic samples from Costa Rica and Guatemala were described and their relict mineral compositions measured by electron microprobe to try to determine the origin of the Leg 84 serpentinites and their relationship to the ultramafic rocks of the onshore ophiolites. The Leg 84 samples comprise more than 90% secondary minerals, principally serpentine, with hematitic and opaque oxides, and minor talc and smectites. Four distinct textural types can be identified according to the distribution of opaque phases and smectite. Remnants of spinel, olivine, orthopyroxene, and clinopyroxene occur variously in the samples; spinal occurs in all the samples. Textural evidence suggests that the serpentinites were originally clinopyroxene-bearing harzburgites. Relict mineral compositions are refractory and relatively uniform: olivine, Fo^ ^^ 9 ; orthopyroxene, En^^; clinopyroxene, Wo 47 En 50 Fs 3 ; spinels, Cr/Cr + Al = 0.4-0.6. 567A-29-2, 30-35 cm has slightly more magnesian olivines (Fo 92 ) and orthopyroxene, and more aluminous spinels (Cr/Cr + Al = 0.3). These compositions are similar to those inferred for refractory upper-mantle materials and also fall within the range of compositions for relict minerals in abyssal peridotites. They could be of oceanic origin. The onshore samples include serpentinites, a clinopyroxene-bearing harzburgite, and a clinopyroxenite. They too have magnesium-rich silicate assemblages, but relative to the drilled samples have more iron-rich olivines (Fogo) and more aluminous and sodic pyroxenes; spinels which are clearly relicts are very aluminum-rich (Cr/Cr + Al = 0.1-0.25). These samples are most likely mantle materials, but significantly less depleted. Their relationship to the drilled samples is unclear. Serpentinites were the most common basement materials recovered during Leg 84, and there appears to be a bimodal assemblage (basalt/diabase and serpentine) of igneous rocks sampled from the trench slope. Diapirism of serpentine throughout the trench slope and forearc is suggested as an explanation for this distribution of samples.

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Melt‐rock interactions and fabric development of peridotites from North Pond in the Kane area, Mid‐Atlantic Ridge: Implications of microstructural and petrological analyses of peridotite samples from IODP Hole U1382A

Harigane

Abe

Michibayashi

et al. 2016

Geochem Geophys Geosyst

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North Pond is an isolated sedimentary pond on the western flank of the Kane area along the Mid‐Atlantic Ridge. Drill‐hole U1382A of IODP Expedition 336 recovered peridotite and gabbro samples from a sedimentary breccia layer in the pond, from which we collected six fresh peridotite samples. The peridotite samples came from the southern slope of the North Pond where an oceanic core complex is currently exposed. The samples were classified as spinel harzburgite, plagioclase‐bearing harzburgite, and a vein‐bearing peridotite that contains tiny gabbroic veins. No obvious macroscopic shear deformation related to the formation of a detachment fault was observed. The spinel harzburgite with a protogranular texture was classified as refractory peridotite. The degree of partial melting of the spinel harzburgite is estimated to be ∼17%, and melt depletion would have occurred at high temperatures in the uppermost mantle beneath the spreading axis. The progressive melt–rock interactions between the depleted spinel harzburgite and the percolating melts of Normal‐Mid Ocean Ridge Basalt (N‐MORB) produced the plagioclase‐bearing harzburgite and the vein‐bearing peridotite at relatively low temperatures. This implies that the subsequent refertilization occurred in an extinct spreading segment of the North Pond after spreading at the axis. Olivine fabrics in the spinel and plagioclase‐bearing harzburgites are of types AG, A, and D, suggesting the remnants of a mantle flow regime beneath the spreading axis. The initial olivine fabrics appear to have been preserved despite the later melt–rock interactions. The peridotite samples noted above preserve evidence of mantle flow and melt–rock interactions beneath a spreading ridge that formed at ∼8 Ma.

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Petrology of Ultramafic Rocks from Site 395

Cited by 22 publications

References 13 publications

Mineralogy of MARK peridotites: replacement through magma channeling examined from Hole 920D, MARK area

Mineralogy of MARK peridotites: replacement through magma channeling examined from Hole 920D, MARK area

Petrology and Relict Mineralogy of Serpentinites from Deep Sea Drilling Project Leg 84

Melt‐rock interactions and fabric development of peridotites from North Pond in the Kane area, Mid‐Atlantic Ridge: Implications of microstructural and petrological analyses of peridotite samples from IODP Hole U1382A

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