Spinel-facies mantle xenoliths from Cerro Redondo, Argentine Patagonia: Petrographic, geochemical, and isotopic evidence of interaction between xenoliths and host basalt

Schilling M., Conceicao R.V., Mallmann G., Koester E., Kawashita K., Herve F., Morata D. and Motoki A. (2005) Spinel-facies mantle xenoliths from Cerro Redondo, Argentine Patagonia: Petrographic, geochemical, and isotopic evidence of interaction between xenoliths and host basalt. Lithos, 82 3-4 SPEC. ISS.: 485-502. doi:10.1016/j.lithos.2004.09.028


Author Schilling M.
Conceicao R.V.
Mallmann G.
Koester E.
Kawashita K.
Herve F.
Morata D.
Motoki A.
Title Spinel-facies mantle xenoliths from Cerro Redondo, Argentine Patagonia: Petrographic, geochemical, and isotopic evidence of interaction between xenoliths and host basalt
Journal name Lithos   Check publisher's open access policy
ISSN 0024-4937
Publication date 2005-01-01
Sub-type Article (original research)
DOI 10.1016/j.lithos.2004.09.028
Open Access Status
Volume 82
Issue 3-4 SPEC. ISS.
Start page 485
End page 502
Total pages 18
Language eng
Subject 1906 Geochemistry and Petrology
Abstract Cerro Redondo is an ancient cinder cone now almost completely eroded, sited over a sill that corresponds to a sub-volcanic magma chamber, in Santa Cruz province, Patagonia, Argentina. It is composed of Pliocene-Pleistocene alkaline basalt containing spinel-facies lherzolite and harzburgite mantle xenoliths. Core compositions of pyroxenes indicate temperatures of 823 °C to 1043 °C and pressures of 12.4 kb to 21.4 kb. Based on P-T estimates, petrographic, geochemical, and isotopic characteristics, we propose that Cerro Redondo xenoliths come from a thick homogeneous mantle column (36 km to 63 km depth), and present different degrees of basalt infiltration. A simple mixing model based on Sr isotopes was used to quantify the host basalt infiltration, and contamination values of 0.0%, 0.2%, 3%, and 12% were obtained for samples X-F, X-D, X-C, and X-B, respectively. For unknown reasons, samples X-G and X-E suffered selective isotopic and trace element modification, respectively, associated with ∼1% of basalt infiltration. Sample X-F best represents the sub-continental lithospheric mantle column, conserving primary equilibrium textures with sharp grain boundaries, and having TiO 2, CaO, Na 2O, K 2O, and P 2O 5 contents lower than average spinel lherzolite, flat chondrite-normalized REE pattern, and 87Sr/ 86Sr and 143Nd/ 144Nd ratios of 0.70519 and 0.51297, respectively. This sample records a decoupling of the Sr-Nd system where Sr ratios increase at constant Nd ratios, possibly caused by chromatographic processes. Its 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb ratios are 17.987, 15.556, and 37.959, respectively. As the interaction with the host basalt increases, xenoliths show a gradual increase of disequilibrium textures such as reaction rims and exsolution lamellae in orthopyroxene and clinopyroxene, and increase of TiO 2, CaO, Al 2 O 3, Na 2O, K 2O, P 2 O 5, LREE, and incompatible element concentrations. The Sr-Nd system shows an unusual positive trend from the unmodified sample X-F toward the host basalt isotope composition with 87Sr/ 86Sr and 143Nd/ 144Nd ratios of 0.70447 and 0.51279, respectively, while 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb ratios tend to increase toward those of the host basalt (18.424, 15.648, and 38.728, respectively) as the xenolith-basalt interaction increases. The basalt-xenolith reaction probably started during the transport of the xenoliths to the surface, and continued during the residence of xenoliths in the sub-volcanic magma chamber of Cerro Redondo.
Keyword Alkaline basalt
Basalt infiltration
Santa Cruz Province - Argentina
Spinel lherzolite-harzburgite
Sub-continental lithospheric mantle
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Unknown

Document type: Journal Article
Sub-type: Article (original research)
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