TY - JOUR
T1 - Dacitic ocelli in mafic lavas, 3.8-3.7 Ga Isua greenstone belt, West Greenland: Geochemical evidence for partial melting of oceanic crust and magma mixing
AU - Appel, Peter W.U.
AU - Polat, Ali
AU - Frei, Robert
N1 - Funding Information:
J.C. Ordóñez-Calderón is acknowledged for their help during geochemical analyses. R. Kerrich is acknowledged for reviewing the initial version of the manuscript. We thank two anonymous reviewers for their constructive comments on the manuscript that have made significant improvements to the paper. Field work was supported by the Bureau of Minerals and Petroleum in Nuuk and the Geological Survey of Denmark and Greenland (GEUS). This is a contribution of the Isua Multi-disciplinary Research Project coordinated by P.W.U Appel. This is a contribution of NSERC grants 250926 to AP. R. Frei is supported by FNU (Forskningsrådet for Natur of Univers) grant nr. 21-01-0492 56493.
PY - 2009/1/30
Y1 - 2009/1/30
N2 - Mafic volcanic rocks in the 3.8-3.7 Ga Isua greenstone belt, southern West Greenland, contain randomly distributed 1 to 10-centimeters long white spheroidal structures. In this study, these structures are called 'ocelli'. In the western part of the belt, ocelli-bearing lavas are enclosed in basaltic to picritic flows (MgO = 9-21 wt.%) with a subduction zone geochemical signature. The ocelli are composed predominantly of polycrystalline Na-plagioclase and quartz, with minor hornblende and biotite, whereas the surrounding amphibolite matrix (basaltic host) is composed mainly of hornblende, Ca-plagioclase, and quartz. The ocelli are devoid of radial or concentric internal structure, and display all stages of coalescence. Contacts between the ocelli and surrounding amphibolite matrix are sharp to gradational. Compositionally, the ocelli are calc-alkaline dacites (SiO
2 = 62.9-72.0 wt.%; MgO = 0.60-3.50 wt.%; Ni = 58-143 ppm; Cr = 250-510 ppm), whereas the surrounding matrix is tholeiitic basalt (SiO
2 = 46.6-50.6 wt.%; MgO = 8.70-12.30 wt.%; Ni = 119-175 ppm; Cr = 330-600 ppm). In terms of major element composition, the Isua ocelli closely resemble plagiogranites in Phanerozoic supra-subduction zone ophiolites. Field and petrographic observations, and geochemical data (SiO
2 = 54.2-60.7 wt.%; MgO = 3.95-7.72 wt.%; Ni = 127-158 ppm; Cr = 500-570 ppm) on the transitional areas between the ocelli and the matrix suggest magma mixing between dacitic and basaltic melts. On a chondrite-normalized diagram, the basaltic host is characterized by variably depleted LREE patterns (La/Sm
cn = 0.30-0.94; Gd/Yb
cn = 1.03-1.45), whereas the dacitic ocelli display LREE-enriched patterns (La/Sm
cn = 1.30-2.60; Gd/Yb
cn = 1.32-2.58). The strongly depleted REE patterns in the basaltic host are attributed to LREE loss during carbonate alteration. Partial melting of a forearc mantle wedge is favoured for the origin of the protolith of the basaltic host. The geochemical characteristic of the ocelli cannot be explained by post-magmatic alteration, slab melting, fractional crystallization of tholeiitic melts, or liquid immiscibility. We suggest that the dacitic ocelli might have been derived from hydrous melting of the fragments of oceanic crust (high-Mg volcanic rocks) that fell into the magma chamber, suggesting magma-crust interaction in the early Earth. Formation of dacitic volcanic rocks by partial melting of altered oceanic crust may have played an important role in the generation of felsic crust in the early Archean.
AB - Mafic volcanic rocks in the 3.8-3.7 Ga Isua greenstone belt, southern West Greenland, contain randomly distributed 1 to 10-centimeters long white spheroidal structures. In this study, these structures are called 'ocelli'. In the western part of the belt, ocelli-bearing lavas are enclosed in basaltic to picritic flows (MgO = 9-21 wt.%) with a subduction zone geochemical signature. The ocelli are composed predominantly of polycrystalline Na-plagioclase and quartz, with minor hornblende and biotite, whereas the surrounding amphibolite matrix (basaltic host) is composed mainly of hornblende, Ca-plagioclase, and quartz. The ocelli are devoid of radial or concentric internal structure, and display all stages of coalescence. Contacts between the ocelli and surrounding amphibolite matrix are sharp to gradational. Compositionally, the ocelli are calc-alkaline dacites (SiO
2 = 62.9-72.0 wt.%; MgO = 0.60-3.50 wt.%; Ni = 58-143 ppm; Cr = 250-510 ppm), whereas the surrounding matrix is tholeiitic basalt (SiO
2 = 46.6-50.6 wt.%; MgO = 8.70-12.30 wt.%; Ni = 119-175 ppm; Cr = 330-600 ppm). In terms of major element composition, the Isua ocelli closely resemble plagiogranites in Phanerozoic supra-subduction zone ophiolites. Field and petrographic observations, and geochemical data (SiO
2 = 54.2-60.7 wt.%; MgO = 3.95-7.72 wt.%; Ni = 127-158 ppm; Cr = 500-570 ppm) on the transitional areas between the ocelli and the matrix suggest magma mixing between dacitic and basaltic melts. On a chondrite-normalized diagram, the basaltic host is characterized by variably depleted LREE patterns (La/Sm
cn = 0.30-0.94; Gd/Yb
cn = 1.03-1.45), whereas the dacitic ocelli display LREE-enriched patterns (La/Sm
cn = 1.30-2.60; Gd/Yb
cn = 1.32-2.58). The strongly depleted REE patterns in the basaltic host are attributed to LREE loss during carbonate alteration. Partial melting of a forearc mantle wedge is favoured for the origin of the protolith of the basaltic host. The geochemical characteristic of the ocelli cannot be explained by post-magmatic alteration, slab melting, fractional crystallization of tholeiitic melts, or liquid immiscibility. We suggest that the dacitic ocelli might have been derived from hydrous melting of the fragments of oceanic crust (high-Mg volcanic rocks) that fell into the magma chamber, suggesting magma-crust interaction in the early Earth. Formation of dacitic volcanic rocks by partial melting of altered oceanic crust may have played an important role in the generation of felsic crust in the early Archean.
KW - Dacite
KW - Eoarchean
KW - Hydrous melting
KW - Isua
KW - Magma mixing
KW - Ocelli
KW - Pillow basalt
KW - Plagiogranite
UR - http://www.scopus.com/inward/record.url?scp=58149471773&partnerID=8YFLogxK
U2 - 10.1016/j.chemgeo.2008.09.011
DO - 10.1016/j.chemgeo.2008.09.011
M3 - Article
SN - 0009-2541
VL - 258
SP - 105
EP - 124
JO - Chemical Geology
JF - Chemical Geology
IS - 3-4
ER -