TY - JOUR

T1 - A Reference Section Through Fast-Spread Lower Oceanic Crust, Wadi Gideah, Samail Ophiolite (Sultanate of Oman)

T2 - Trace Element Systematics and REE Crystallization Temperatures—Implications for Hybrid Crustal Accretion

AU - Müller, Samuel

AU - Garbe-Schönberg, Dieter

AU - Koepke, Jürgen

AU - Hoernle, Kaj

N1 - Funding Information: The authors highly appreciate the support of the Oman Public Authority For Mining for permission to conduct fieldwork in Oman. Special thanks go to Dr. Ali Al Rajhi, Dr. Mohamed Al Batashi, and Dr. Mohamed Al Araimi. The authors kindly thank Prof. Dr. Sobhi Nasir from the geological department of the Sultan Qaboos University for assistance in planning and conducting fieldwork and shipping samples. The authors are thankful for detailed reviews by Michael Perfit and an anonymous colleague. Comments and thoughts by John Lassiter significantly advanced the manuscript—thank you very much. The authors acknowledge the work of the Hanover preparation lab for producing excellent thin sections. Special thanks go to Ulrike Westernströer and the Kiel University ICP‐MS Lab team for their great support in conducting LA‐ICP‐MS analysis. The authors also thank the Kiel University and Hannover University EPMA lab teams for support with micro‐analysis. This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) by a grant given to Dr. Dieter Garbe‐Schönberg and Prof. Dr. Jürgen Koepke, project no.: 214851514. Open access funding enabled and organized by Projekt DEAL.

PY - 2022/3/17

Y1 - 2022/3/17

N2 - Oceanic gabbros are the most abundant rocks close to Earth's surface. Here we present new data from a consistent profile through the paleocrust of the Samail Ophiolite (Oman), which is thought to provide the best analog for modern fast-spreading oceanic crust. Incompatible trace elements of co-existing plagioclase and clinopyroxene fractionate from the mineral core to rim and up section from layered to foliated to varitextured gabbros. Layered gabbro parental melts correspond to mid-ocean-ridge basalts (MORB), and plagioclase Ca# shows a pronounced inverse zonation. Likely, they crystallized in situ from hydrous melts, compositionally buffered by replenishment at equilibrium to MORB and near steady-state boundary conditions. Further upsection, the compositional variability increases. Foliated gabbro rim and core compositions indicate increased fractionation and disequilibrium to MORB, triggered by open-system fractional crystallization within a heterogeneous magma plumbing structure, characterized by magma mixing, varying ambient water activities, and boundary conditions. Varitextured gabbros are chemically diverse with parental melts partially more primitive than MORB, suggesting that primitive melts directly reach the axial melt lens (AML). REE-in-plagioclase-clinopyroxene thermometry compared to and supported by anorthite-in-plagioclase thermometry reveals a relationship of (Formula presented.) [°C] = 6.1 ± 0.2 × An + 706 ± 19. Crystallization temperatures of the layered gabbros cover a narrow range of 1216 ± 14°C. Considerable temperature variability of 1077–1231°C is observed further upsection, featuring a thermal minimum within the foliated gabbros. This minimum is assumed to represent a zone where the fractionated descending crystal mushes originating from the AML meet with evolved liquids expelled from deeper crustal levels. Our findings suggest hybrid accretion of fast-spread crust.

AB - Oceanic gabbros are the most abundant rocks close to Earth's surface. Here we present new data from a consistent profile through the paleocrust of the Samail Ophiolite (Oman), which is thought to provide the best analog for modern fast-spreading oceanic crust. Incompatible trace elements of co-existing plagioclase and clinopyroxene fractionate from the mineral core to rim and up section from layered to foliated to varitextured gabbros. Layered gabbro parental melts correspond to mid-ocean-ridge basalts (MORB), and plagioclase Ca# shows a pronounced inverse zonation. Likely, they crystallized in situ from hydrous melts, compositionally buffered by replenishment at equilibrium to MORB and near steady-state boundary conditions. Further upsection, the compositional variability increases. Foliated gabbro rim and core compositions indicate increased fractionation and disequilibrium to MORB, triggered by open-system fractional crystallization within a heterogeneous magma plumbing structure, characterized by magma mixing, varying ambient water activities, and boundary conditions. Varitextured gabbros are chemically diverse with parental melts partially more primitive than MORB, suggesting that primitive melts directly reach the axial melt lens (AML). REE-in-plagioclase-clinopyroxene thermometry compared to and supported by anorthite-in-plagioclase thermometry reveals a relationship of (Formula presented.) [°C] = 6.1 ± 0.2 × An + 706 ± 19. Crystallization temperatures of the layered gabbros cover a narrow range of 1216 ± 14°C. Considerable temperature variability of 1077–1231°C is observed further upsection, featuring a thermal minimum within the foliated gabbros. This minimum is assumed to represent a zone where the fractionated descending crystal mushes originating from the AML meet with evolved liquids expelled from deeper crustal levels. Our findings suggest hybrid accretion of fast-spread crust.

KW - crystallization temperatures

KW - Gabbro mineral composition

KW - igneous geochemistry

KW - lower oceanic crustal accretion

KW - Oman ophiolite

KW - REE-in-plagioclase-clinopyroxene thermometer

UR - http://www.scopus.com/inward/record.url?scp=85127279735&partnerID=8YFLogxK

U2 - 10.1029/2021JB022699

DO - 10.1029/2021JB022699

M3 - Article

AN - SCOPUS:85127279735

VL - 127

JO - Journal of Geophysical Research: Solid Earth

JF - Journal of Geophysical Research: Solid Earth

SN - 2169-9313

IS - 3

M1 - e2021JB022699

ER -