TY - JOUR

T1 - ICDP Oman Drilling Project

T2 - varitextured gabbros from the dike-gabbro transition within drill core GT3A

AU - Engelhardt, Artur

AU - Koepke, Jürgen

AU - Zhang, Chao

AU - Garbe-Schönberg, Dieter

AU - Jesus, Ana Patrícia

N1 - Funding Information: This research has been supported by the Deutsche Forschungsgemeinschaft (DFG; KO 1723/21-1 and KO 1723/26-1).The publication of this article was funded by the open-access fund of Leibniz Universität Hannover. Funding Information: This research used samples and/or data provided by the Oman Drilling Project. The Oman Drilling Project (OmanDP) has been possible through co-mingled funds from the International Continental Scientific Drilling Project (ICDP; Peter B. Kelemen, Jürg M. Matter, Damon A. H. Teagle lead PIs), the Sloan Foundation – Deep Carbon Observatory (grant 2014-3-01, Peter B. Kelemen PI), the National Science Foundation (NSF-EAR-1516300, Peter B. Kelemen lead PI), NASA Astrobiology Institute (NNA15BB02A, Alexis Templeton PI), the German Research Foundation (DFG; KO 1723/21-1, Jürgen Koepke PI), the Japan Society for the Promotion of Science (JSPS; no. 16H06347, Katsuyoshi Michibayashi PI, and KAKENHI 16H02742, Eiichi Takazawa PI), the European Research Council (Adv: no. 669972, Bjørn Jamveit PI), the Swiss National Science Foundation (SNF; 20FI21_163073, Gretchen Früh-Green PI), JAMSTEC, and the TAMU JR Science Operator and contributions from the Sultanate of Oman Ministry of Regional Municipalities and Water Resources, the Oman Public Authority for Mining, Sultan Qaboos University, CNRS Univ. Montpellier, Columbia University in the City of New York, and the University of Southampton. This study was funded by DFG projects KO 1723/26. Ana Patrícia Jesus acknowledges the Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement no. 894599.

PY - 2022/12/12

Y1 - 2022/12/12

N2 - The Oman ophiolite (Samail massif, Sultanate of Oman) is the largest sub-aerial exposure of oceanic lithosphere on Earth and provides the opportunity to study the accretion and alteration of oceanic lithosphere formed under fast-spreading conditions. Drill hole GT3A (23g 06′50.7′gN, 58g 12′42.2′gE) of the ICDP (International Continental Scientific Drilling Program) Oman Drilling Project with a length of 400gm aimed at penetrating the dike-gabbro transition of the Samail ophiolite paleocrust in order to shed light on the role of the axial melt lens (AML) during accretion of the lower plutonic crust. AMLs beneath fast-spreading mid-ocean ridges are sandwiched between the sheeted dike complex and the uppermost gabbros and are believed to feed the upper crust and, at least partially, the underlying crystal mush. Typical gabbroic rocks from dike-gabbro transitions of fast-spreading systems are the so-called "varitextured gabbros", often showing considerable variations in mineral mode, texture and grain size, which are regarded as the frozen fillings of axial melt lenses. Here, we present a detailed petrographic, microanalytical and bulk-chemical investigation of 36 mafic rocks from the drill hole GT3A, which represent mostly varitextured gabbros, revealing a complex formation with several evolution stages. Poikilitic domains formed first, corresponding to an early crystallization stage, where only plagioclase and clinopyroxene of more primitive composition crystallized. Later, domains of granular textures containing also interstitial amphibole and Fe-Ti oxide were formed. This stage is characterized by a magma evolution that underwent crystal fractionation established by lower temperatures due to more efficient hydrothermal cooling at the margin of the AML. A last stage is characterized by pervasive hydrothermal alteration, where all primary minerals have been altered under temperature conditions, varying from the magmatic regime down to greenschist facies. A highlight of this stage is amphiboles showing noticeable compositional zoning. The observation of peculiar microgranular domains, representing relics of stoped exogenic material from the sheeted dike complex, documents the upward migration of an AML in a replenishment event, forcing the AML to burn through previously altered sheeted dikes. This process is responsible for significant assimilation of hydrothermally altered components, indicated by a marked Cl enrichment in the outer zones of magmatic amphiboles. Petrological modeling involving gabbros and basalts revealed that the GT3A rock suite followed a fractional crystallization evolution trend, with a primitive MORB as parental melt with an estimated water content of 0.2gwtg% to 0.8gwtg%. The modeled liquid lines of descent suggest a magmatic evolution via fractional crystallization, where the basalts correspond to frozen liquids, while the gabbros, especially the more primitive ones, show a significant cumulate component.

AB - The Oman ophiolite (Samail massif, Sultanate of Oman) is the largest sub-aerial exposure of oceanic lithosphere on Earth and provides the opportunity to study the accretion and alteration of oceanic lithosphere formed under fast-spreading conditions. Drill hole GT3A (23g 06′50.7′gN, 58g 12′42.2′gE) of the ICDP (International Continental Scientific Drilling Program) Oman Drilling Project with a length of 400gm aimed at penetrating the dike-gabbro transition of the Samail ophiolite paleocrust in order to shed light on the role of the axial melt lens (AML) during accretion of the lower plutonic crust. AMLs beneath fast-spreading mid-ocean ridges are sandwiched between the sheeted dike complex and the uppermost gabbros and are believed to feed the upper crust and, at least partially, the underlying crystal mush. Typical gabbroic rocks from dike-gabbro transitions of fast-spreading systems are the so-called "varitextured gabbros", often showing considerable variations in mineral mode, texture and grain size, which are regarded as the frozen fillings of axial melt lenses. Here, we present a detailed petrographic, microanalytical and bulk-chemical investigation of 36 mafic rocks from the drill hole GT3A, which represent mostly varitextured gabbros, revealing a complex formation with several evolution stages. Poikilitic domains formed first, corresponding to an early crystallization stage, where only plagioclase and clinopyroxene of more primitive composition crystallized. Later, domains of granular textures containing also interstitial amphibole and Fe-Ti oxide were formed. This stage is characterized by a magma evolution that underwent crystal fractionation established by lower temperatures due to more efficient hydrothermal cooling at the margin of the AML. A last stage is characterized by pervasive hydrothermal alteration, where all primary minerals have been altered under temperature conditions, varying from the magmatic regime down to greenschist facies. A highlight of this stage is amphiboles showing noticeable compositional zoning. The observation of peculiar microgranular domains, representing relics of stoped exogenic material from the sheeted dike complex, documents the upward migration of an AML in a replenishment event, forcing the AML to burn through previously altered sheeted dikes. This process is responsible for significant assimilation of hydrothermally altered components, indicated by a marked Cl enrichment in the outer zones of magmatic amphiboles. Petrological modeling involving gabbros and basalts revealed that the GT3A rock suite followed a fractional crystallization evolution trend, with a primitive MORB as parental melt with an estimated water content of 0.2gwtg% to 0.8gwtg%. The modeled liquid lines of descent suggest a magmatic evolution via fractional crystallization, where the basalts correspond to frozen liquids, while the gabbros, especially the more primitive ones, show a significant cumulate component.

UR - http://www.scopus.com/inward/record.url?scp=85145585756&partnerID=8YFLogxK

U2 - 10.5194/ejm-34-603-2022

DO - 10.5194/ejm-34-603-2022

M3 - Article

AN - SCOPUS:85145585756

VL - 34

SP - 603

EP - 626

JO - European journal of mineralogy

JF - European journal of mineralogy

SN - 0935-1221

IS - 6

ER -