title: Primitive layered gabbros from fast-spreading lower oceanic crust.
abstract: Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks--in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas--provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.
authors.affiliation: School of Earth and Ocean Sciences, University of Victoria, PO Box 1700 Station CSC, Victoria, British Columbia V8W 2Y2, Canada.
authors.forename: Kathryn M
authors.initials: KM
authors.lastname: Gillis
authors.affiliation: Earth and Atmospheric Sciences, University of Houston, 312 Science and Research Building 1, Houston, Texas 77204-5007, USA.
authors.forename: Jonathan E
authors.initials: JE
authors.lastname: Snow
authors.affiliation: United States Implementing Organization, Integrated Ocean Drilling Program, Texas A&M University, 1000 Discovery Drive, College Station, Texas 77845, USA.
authors.forename: Adam
authors.initials: A
authors.lastname: Klaus
authors.affiliation: Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan.
authors.forename: Natsue
authors.initials: N
authors.lastname: Abe
authors.affiliation: Institute of Geoscience, Rio Grande do Sul Federal University, Avenida Bento Gonçalves, 9500 Bloco I Prédio, 43113 Sala 207, Porto Alegre/RS 91501-970, Brazil.
authors.forename: Alden B
authors.initials: AB
authors.lastname: Adrião
authors.affiliation: Department of Earth Sciences, Kanazawa University, Kakuma-machi, Kanazawa Ishikawa 920-1192, Japan.
authors.forename: Norikatsu
authors.initials: N
authors.lastname: Akizawa
authors.affiliation: Observatoire Midi-Pyrénées (UMS 831), CNRS, Université Paul Sabatier, 14 Avenue Edouard Belin, Toulouse Cedex 31400, France.
authors.forename: Georges
authors.initials: G
authors.lastname: Ceuleneer
authors.affiliation: Department of Geology and Geophysics, University of Wyoming, 1000 University Avenue, Department 3006, Laramie, Wyoming 82071, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Cheadle
authors.affiliation: 1] School of Earth and Ocean Sciences, University of Victoria, PO Box 1700 Station CSC, Victoria, British Columbia V8W 2Y2, Canada [2] Institut für Geologie, Mineralogie, and Geophysik, Ruhr-Universität Bochum, Universitätsstrasse 150, 44780 Bochum, Germany.
authors.forename: Kathrin
authors.initials: K
authors.lastname: Faak
authors.affiliation: Institute for Marine and Antarctic Studies and School of Earth Sciences, University of Tasmania, Hobart, Tasmania 7001, Australia.
authors.forename: Trevor J
authors.initials: TJ
authors.lastname: Falloon
authors.affiliation: Department of Geology, Southern Illinois University at Carbondale, Carbondale, Illinois 62901, USA.
authors.forename: Sarah A
authors.initials: SA
authors.lastname: Friedman
authors.affiliation: Géosciences Montpellier, Université Montpellier 2, CNRS UMR5243, CC 60, 34095 Montpellier Cedex 5, France.
authors.forename: Marguerite
authors.initials: M
authors.lastname: Godard
authors.affiliation: Borehole Research Group, Lamont-Doherty Earth Observatory of Columbia University, PO Box 1000, 61 Route 9W, Palisades, New York 10964, USA.
authors.forename: Gilles
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authors.lastname: Guerin
authors.affiliation: Institute of Geology and Geoinformation, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 7, 1-1-1 Higashi, Tsukuba Ibaraki 305-8567, Japan.
authors.forename: Yumiko
authors.initials: Y
authors.lastname: Harigane
authors.affiliation: Department of Geology, Oberlin College, Oberlin, Ohio 44074, USA.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Horst
authors.affiliation: Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Hoshide
authors.affiliation: Géosciences Montpellier, Université Montpellier 2, CNRS UMR5243, CC 60, 34095 Montpellier Cedex 5, France.
authors.forename: Benoit
authors.initials: B
authors.lastname: Ildefonse
authors.affiliation: Department of Geology and Environmental, Geosciences, Northern Illinois University, Davis Hall 312, Normal Road, DeKalb, Illinois 60115, USA.
authors.forename: Marlon M
authors.initials: MM
authors.lastname: Jean
authors.affiliation: Department of Geology and Geophysics, University of Wyoming, 1000 University Avenue, Department 3006, Laramie, Wyoming 82071, USA.
authors.forename: Barbara E
authors.initials: BE
authors.lastname: John
authors.affiliation: Institut für Mineralogie, University of Hannover, Callinstrasse 3, Hannover 30167, Germany.
authors.forename: Juergen
authors.initials: J
authors.lastname: Koepke
authors.affiliation: Department of Earth Sciences, Kanazawa University, Kakuma-machi, Kanazawa Ishikawa 920-1192, Japan.
authors.forename: Sumiaki
authors.initials: S
authors.lastname: Machi
authors.affiliation: Department of Natural History Sciences, Hokkaido University, North 10, West 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan.
authors.forename: Jinichiro
authors.initials: J
authors.lastname: Maeda
authors.affiliation: Chemistry and Material Sciences Department, Lawrence Livermore National Laboratory, PO Box 808, L-231, Livermore, California 94551, USA.
authors.forename: Naomi E
authors.initials: NE
authors.lastname: Marks
authors.affiliation: School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK.
authors.forename: Andrew M
authors.initials: AM
authors.lastname: McCaig
authors.affiliation: Centre for Geobiology and Department of Earth Science, University of Bergen, Allegaten 41, Bergen 5007, Norway.
authors.forename: Romain
authors.initials: R
authors.lastname: Meyer
authors.affiliation: School of Geography, Earth and Environmental Sciences, Plymouth University, Drake Circus, Plymouth PL4 8AA, UK.
authors.forename: Antony
authors.initials: A
authors.lastname: Morris
authors.affiliation: Department of Earth Sciences, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan.
authors.forename: Toshio
authors.initials: T
authors.lastname: Nozaka
authors.affiliation: Department of Natural History Sciences, Hokkaido University, North 10, West 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan.
authors.forename: Marie
authors.initials: M
authors.lastname: Python
authors.affiliation: Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata 700 019, India.
authors.forename: Abhishek
authors.initials: A
authors.lastname: Saha
authors.affiliation: Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, Indiana 47405, USA.
authors.forename: Robert P
authors.initials: RP
authors.lastname: Wintsch