Oceanic and Continental Lithospheric Mantle in the 1.95-Ga Jormua Ophiolite Complex, Finland: Implications for Mantle and Crustal Evolution

Author:

Finlayson Valerie A1,Haller Mitchell2,Day James M D3,Ginley Stephen4,O'Driscoll Brian5,Kontinen Asko6,Hanski Eero7,Walker Richard J1

Affiliation:

1. University of Maryland, 8000 Regents Drive, College Park MD Department of Geology, , 20742 USA

2. Corning Inc , One Riverfront Plaza,14831 Corning NY, USA

3. Scripps Institution of Oceanography , 9500 Gilman Drive, 92093 La Jolla CA, USA

4. University of Ottawa Department of Earth and Environmental Sciences, , K1N 6N5, Ottawa, Ontario, Canada

5. University of Ottawa Department of Earth and Environmental Sciences, , K1N 9A7, Ottawa, Ontario, Canada

6. Finland Geological Survey (GTK) , FIN FI-7021 Kuopio, Finland

7. University of Oulu Oulu Mining School, , 90014 Oulu, Finland

Abstract

ABSTRACT The ca. 1.95-Ga Jormua Ophiolite Complex (JOC), Finland, is a rare Paleoproterozoic ophiolite that preserves a record of diverse upper mantle materials and melting processes. Meter-scale grid sampling of four JOC outcrops, as well as non-grid samples, permits evaluation of meter- to kilometer-scale mantle heterogeneity within the JOC. Significant heterogeneity is observed between the four grids, and also among a number of the non-grid samples examined. Variations in the concentrations of fluid-mobile elements are particularly large among different samples and locations. New whole-rock major, lithophile trace, and highly siderophile element data (HSE: Os, Ir, Ru, Pt, Pd, Re), including 187Re–187Os isotopic data, for serpentinized harzburgites indicate the presence of two distinct compositional types and probable modes of origin within the JOC. This is consistent with prior findings. Type 1 is similar to modern refractory abyssal-type mantle. Type 2 is more highly refractory than Type 1, and most likely represents samples from sub-continental lithospheric mantle (SCLM). Type 1 mantle is moderately heterogeneous with respect to major and trace element and Os isotopic compositions at both the meter and kilometer scales. By contrast, Type 2 mantle is considerably more homogeneous than Type 1 grids at the meter scale, but is more heterogeneous at the kilometer scale. The median initial γOs value for Type 1 mantle, calculated for 1.95 Ga, is ~ −2.0 (where γOs is the % deviation in 187Os/188Os relative to a chondritic reference calculated for a specified time). This isotopic composition is consistent with a moderate, long-term decrease in Re/Os relative to the estimate for primitive mantle, prior to JOC formation. The similarity in this γOs value to the value for the modern abyssal mantle, as well as the initial values for several Phanerozoic ophiolites, suggests that the upper mantle achieved a Re/Os ratio similar to the chondritic reference by ~2 Ga, then evolved along a subparallel trajectory to the chondritic reference since then. For this to occur, only limited Re could have been permanently removed from the upper mantle since at least the time the JOC formed. A localized secondary metasomatic event at ~2 Ga, concurrent with the estimated obduction age for the JOC and subsequent Svecofennian Orogeny, affected the HSE systematics of some Type 1 samples. By contrast, late Archean Os TRD model ages for Type 2 rocks indicate a depletion event superimposed upon the long-term Re depletion of the abyssal mantle. This event was established no later than ~2.6 Ga and may have occurred during a period of significant, well-documented crustal production in the Karelia craton at ~2.7 Ga.

Funder

NSF petrology and geochemistry

Natural Environment Research Council

Royal Society Research

NSF Petrology and Geochemistry

Publisher

Oxford University Press (OUP)

Subject

Geochemistry and Petrology,Geophysics

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