TY - JOUR

T1 - Contamination of MORB by anatexis of magma chamber roof rocks

T2 - Constraints from a geochemical study of experimental melts and associated residues

AU - France, Lydéric

AU - Koepke, Juergen

AU - MacLeod, Christopher J.

AU - Ildefonse, Benoit

AU - Godard, Marguerite

AU - Deloule, Etienne

N1 - Funding Information: We express our warm thanks to the various people involved at different technical stages of this work: Otto Diedrich (Hannover) for his high quality thin sections, Bernard Boyer for his assistance during SIMS measurements at Geosciences Montpellier and, Sarah Cichy (Hannover) for her assistance with the experimental facilities. Constructive reviews by L. Coogan and an anonymous reviewer are gratefully acknowledged. This research used data provided by the Integrated Ocean Drilling Program (IODP). We gratefully acknowledge the Captain and shipboard crew of IODP Expedition 335 for their assistance in data collection at sea. We wish to thank the Scientific Party of IODP Expedition 335 for fruitful discussions during the cruise. We thank the Director General of Minerals, Ministry of Commerce and Industry of the Sultanate of Oman, for allowing us to conduct field work in the Oman ophiolite. This research was supported by CNRS-INSU programs 3F and SYSTER (AMISHADOq), and post-cruise funding (IODP Expedition 335) by the Région Lorraine (‘soutien aux projets de recherche’ program), and by the Université Franco–Allemande/Deutsch–Französische Hochchule . This is CRPG contribution number 2321.

PY - 2014/8

Y1 - 2014/8

N2 - Mid-ocean ridge basalts (MORBs) are the most abundant magmas produced on Earth. They are widely studied to infer mantle compositions and melting processes. However, MORB liquids are also the complex end-product of a variety of intra-crustal processes such as partial or fractional crystallization, melt-rock interaction, and contamination. Deciphering the relative contribution of these different processes is of first-order importance. Contamination at ocean crustal levels is likely, and may occur at magma chamber margins where fresh magmas can interact with previously hydrothermally altered rocks. Characterizing the composition of this crustal contaminant component is critical if we are to understand the relative importance of each component in the resulting MORB liquid.Here we present the results of experiments designed to reproduce the processes occurring at oceanic magma chamber roofs, where crustal contamination should be most extensive, by melting a representative sample of the sheeted dike complex. Anatectic melts thus produced are likely to represent the principal crustal contaminant in MORB. These melts were characterized for major and trace elements, showing B, Zr, Hf, and U enrichment, and Sr, Ti, and V depletion relative to original MORB liquids. In comparison to the starting material, relative element fractionations are observed in the anatectic melts, with enrichments of: U relative to Ba, Nb, and Th; LREE and MREE relative to Sr; and Zr-Hf relative to LREE. Bulk partition coefficients for element partitioning during magma chamber roof anatexis are derived and proposed as valuable tools for tracking MORB contamination.Comparison with natural samples from the East Pacific Rise and the Oman ophiolite shows that anatectic melts can crystallize in situ to form oceanic plagiogranite intrusions, and that residual assemblages associated with the hydrous partial melting stage are represented by hornfelsic dikes and enclaves (also named granoblastic basalts). We now recognize these as commonplace at the root of the sheeted dike complex both at present-day and fossil oceanic spreading centers.

AB - Mid-ocean ridge basalts (MORBs) are the most abundant magmas produced on Earth. They are widely studied to infer mantle compositions and melting processes. However, MORB liquids are also the complex end-product of a variety of intra-crustal processes such as partial or fractional crystallization, melt-rock interaction, and contamination. Deciphering the relative contribution of these different processes is of first-order importance. Contamination at ocean crustal levels is likely, and may occur at magma chamber margins where fresh magmas can interact with previously hydrothermally altered rocks. Characterizing the composition of this crustal contaminant component is critical if we are to understand the relative importance of each component in the resulting MORB liquid.Here we present the results of experiments designed to reproduce the processes occurring at oceanic magma chamber roofs, where crustal contamination should be most extensive, by melting a representative sample of the sheeted dike complex. Anatectic melts thus produced are likely to represent the principal crustal contaminant in MORB. These melts were characterized for major and trace elements, showing B, Zr, Hf, and U enrichment, and Sr, Ti, and V depletion relative to original MORB liquids. In comparison to the starting material, relative element fractionations are observed in the anatectic melts, with enrichments of: U relative to Ba, Nb, and Th; LREE and MREE relative to Sr; and Zr-Hf relative to LREE. Bulk partition coefficients for element partitioning during magma chamber roof anatexis are derived and proposed as valuable tools for tracking MORB contamination.Comparison with natural samples from the East Pacific Rise and the Oman ophiolite shows that anatectic melts can crystallize in situ to form oceanic plagiogranite intrusions, and that residual assemblages associated with the hydrous partial melting stage are represented by hornfelsic dikes and enclaves (also named granoblastic basalts). We now recognize these as commonplace at the root of the sheeted dike complex both at present-day and fossil oceanic spreading centers.

KW - Fast spreading mid-ocean ridge

KW - Hornfels and granoblastic dikes

KW - Hydrous partial melting

KW - Magma chamber processes

KW - Oceanic plagiogranites

KW - Trace elements

UR - http://www.scopus.com/inward/record.url?scp=84902179429&partnerID=8YFLogxK

U2 - 10.1016/j.lithos.2014.05.018

DO - 10.1016/j.lithos.2014.05.018

M3 - Article

AN - SCOPUS:84902179429

VL - 202-203

SP - 120

EP - 137

JO - LITHOS

JF - LITHOS

SN - 0024-4937

ER -