Ocean floor metamorphism in the Betts Cove ophiolite, Newfoundland

Coish, R. A.

doi:10.1007/bf01166800

Cited by 114 publications

(37 citation statements)

References 30 publications

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“…We are therefore hesitant to use H 2 O + to index the rocks as "altered" or "fresh" and to use this as a basis to reject samples as not representative of the original fresh rock. This would be in agreement with the general observations concerning elements like Ti, Zr, Y, Nb, Cr, and Ni, which indicate that they are usually stable during alteration and low-grade metamorphism (e.g., Cann, 1970;Hart, 1970;Frey et al, 1973;Thompson, 1973;Coish, 1977;Seyfried and Mottl, 1982;Staudigel and Hart, 1983;Valsami and Cann, 1992).…”

supporting

confidence: 91%

Site 504

1992

Proceedings of the Ocean Drilling Program

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Principal results: The primary objective of Leg 148 was to revisit Hole 504B in the eastern equatorial Pacific in order to deepen the hole into oceanic Layer 3 and to determine the nature of the transition from Layer 2 to Layer 3. Site 504 is located on 5.9-m.y.-old crust 201 km south of the Costa Rica Rift, the easternmost segment of the Galapagos Spreading Center (at 1°13.61 l'N, 83°43.818'W, at a water depth of 3460 m). Prior to Leg 148, Hole 504B was the deepest scientific drill hole in the oceans, extending 2000.4 meters below seafloor (mbsf; 1725.9 m sub-basement). It is the only hole to penetrate through the volcanic section into the underlying sheeted dike complex, and the site has become an important in-situ reference section for the physical and chemical structure of the upper oceanic crust. Results from recent drilling in Hole 504B during Leg 140 in late 1991, as well as seismic evidence, suggested that the bottom of the hole lay within the lower portion of the sheeted dike complex, close to the seismic Layer 2/Layer 3 boundary. Many scientists believe that the Layer 2/Layer 3 transition coincides with the change downward from sheeted dikes to underlying gabbros as is observed in ophiolites, but the seismic boundary may be a metamorphic transition within gabbros or within the lower sheeted dikes. Although the transition from sheeted dikes to gabbros has been observed by submersible (Auzende et al., 1989;Francheteau et al., 1992) in tectonic exposures of both Atlantic and Pacific ocean crust, this transition has never been observed in situ in undisturbed ocean crust, and its relation to the Layer 2/Layer 3 boundary remains unproven.Upon re-entering Hole 504B on 28 January, 1993, temperatures were logged from seafloor to just above the total depth of the hole (2000.4 mbsf) with somewhat ambiguous results caused by a calibration error or instru-1 Alt, J.C., Kinoshita, H., Stokking, L.B., et al., 1993. Proc. ODP, Init. Repts., 148: College Station, TX (Ocean Drilling Program).2 Shipboard Scientific Party is as given in the list of participants preceding the contents. mentation problem. The measured gradient through the cased section was close to the background value in the sediments, indicating that presently there is no downhole flow like that observed in the past. A slight dip in temperatures immediately below the bottom of the casing is similar to that recorded during Leg 140, indicating some hydrologic activity in the uppermost basement. Deep in the hole the gradient is generally linear and consistent with logs taken during Legs 137 and 140, indicating a maximum recorded temperature of about 180°C at the bottom of the hole. Following temperature measurements, borehole waters were sampled on 28-29 January. Water was collected in 6 of 8 runs, at 560, 796, 1031, 1266, 1501, and 1970 mbsf. One sample (1501 mbsf) clearly displays chemical characteristics of seawater that has reacted with basalt at elevated temperatures, whereas the compositions of other samples are close to seawater. Following drilling ...

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confidence: 91%

Site 504

1992

Proceedings of the Ocean Drilling Program

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“…Some elements are relatively mobile, whereas others are relatively immobile (Guilmette et al, 2009). For example, K, Na, Si, and Ca are mobile under conditions of greenschist-and amphibolite-facies meta morphism, while Ti, Al, Mn, and P are relatively stable and Zr, Sc, and Y are very immobile under such conditions (Ghazi et al, 2012;Coish, 1977;Winchester and Floyd, 1977;Pearce and Cann, 1973). In addition, some researchers have sugges ted that elements with high ionic potential (e.g., K, Ba, Sr, Cs, and Rb) are mobile, while Ta, Nb, trace elements (Cr, Co, Ni, V), and REEs are stable during greenschist-and amphibolite-facies metamorphism (Mullen, 1983;Seewald et al, 1990).…”

Section: Metamorphism and Elemental Mobilitymentioning

confidence: 99%

Petrology and geochemistry of metabasalts from the Taoxinghu ophiolite, central Qiangtang, northern Tibet: Evidence for a continental back-arc basin system

Wu¹,

Cai²,

Xu³

et al. 2016

AJES

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Newly discovered ophiolitic metabasalts in the Taoxinghu area of central Qiangtang on the Qinhai-Tibet Plateau, here described for the first time, have important implications for reconstructions of the tectonic history of the Paleo-Tethys Ocean. Most of the metabasalts belong to the tholeiitic basalt series and most have undergone greenschist-facies metamorphism. The distribution of rare earth elements and trace elements shows that the rocks are typical of sub-continental margin arc-basin lavas, similar to the environment of formation of the present-day Okinawa lava, suggesting that the Taoxinghu metabasalts represent the upper portions of a supra-subduction zone (SSZ)-type ophiolite that formed in a continental back-arc basin tectonic environment. The Taoxinghu metabasalts are mainly sourced from a depleted spinel mantle-source region, with a spinel lherzolite content equivalent to partial melting of 6 %-25 %. In addition, lava compositions were likely affected by melting of sediments during subduction, while the influence of aqueous fluids was minor. Combining with the existing knowledge on the ophiolites of Longmuco-Shuanghu-Lancang suture zone (LSLSZ), an evolutionary model is proposed. The LSLSZ Paleo-Tethys Ocean basin may have started to form during the Cambrian or earlier, and subducted in the early Carboniferous. As subduction proceeded, a continental back-arc basin was developed, the site of generation of most of the Taoxinghu metabasalts. The LSLSZ Paleo-Tethys Ocean finally closed in the Triassic.

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“…Evidence for alteration of oceanic crust at temperatures from O· to 900·C comes from drill core, dredge samples and ophiolite sequences (Coish, 1977;Humphris and Thompson, 1978;Bohlke et al, 1981;Oudin and Constantinou, 1984;Mevel, 1987Mevel, , 1988Schiffman et al, 1987;Delaney et al, 1987;Vanko, 1988;Schiffman and Smith, 1988;Silant'yev et al, 1988;ScMps and Herzig, 1990;Dick et al, 1991; among many others). Fig.…”

Section: Mineral Alterationmentioning

confidence: 95%

Marine Hydrothermal Systems and the Origin of Life

Holm¹

1992

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Ocean floor metamorphism in the Betts Cove ophiolite, Newfoundland

Cited by 114 publications

References 30 publications

Site 504

Site 504

Petrology and geochemistry of metabasalts from the Taoxinghu ophiolite, central Qiangtang, northern Tibet: Evidence for a continental back-arc basin system

Marine Hydrothermal Systems and the Origin of Life

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