Significance of the Concentration Gradients Associated with Dunite Bodies in the Josephine and Trinity Ophiolites

Article Geochemistry 3 Volume 9, Number 7 Geophysics 29 July 2008 Q07025, doi:10.1029/2008GC001954 GeosystemsG G ISSN: 1525-2027 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Click Here for Full Article Significance of the concentration gradients associated with dunite bodies in the Josephine and Trinity ophiolites Zachary Morgan and Yan Liang Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA ([email protected]) Peter Kelemen Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA [1] Detailed transects were sampled across dunite bodies in the Josephine and Trinity ophiolites. The major peridotite lithologies sampled in the Josephine transect are a sequence of dunite and harzburgite and in the Trinity transect a sequence of dunite, harzburgite, lherzolite, and plagioclase lherzolite (DHL-PL). Major, minor, and selected trace element abundances in olivine, orthopyroxene, clinopyroxene, and spinel were measured. The composition profile from the Josephine transect has revealed a concentration gradient near the dunite-harzburgite contact. The composition profile from the Trinity transect has revealed several concentration gradients: two within the dunite, one in the harzburgite, and at least two in the plagioclase lherzolite. The composition profiles record complex histories of melt transport, melt-rock reaction, and subsequent subsolidus reequilibration. Analyses of closure distance suggest that compositional variation trends for a majority of major and minor elements in olivine, clinopyroxene, orthopyroxene, and spinel reported in this study were magmatic in origin. Subsolidus reequilibration may reduce the range or magnitude of variations for the 2+ cations such as Fe and Mg in olivine and spinel and significantly redistribute Ca and Li in coexisting minerals. Numerical simulations exploring the coupling of diffusion and advection in a porous matrix were used to explain compositional variations across the peridotite sequences. Melt flow from the host harzburgite into the dunite produces composition gradients near the dunite-harzburgite contacts similar to those from the Josephine transect. In contrast, melt flow from the dunite into the surrounding peridotite lithologies can produce concentration gradients similar to those observed in the Trinity transect. At least two chemically distinct episodes of melt flow within the same dunite channel system are proposed. Results from this study show that concentration gradients developed around the dunite-harzburgite and DHL-PL sequences can be used to infer part of the melt flow history of the dunite channel systems in the mantle. Results from this study and those from other ophiolite studies also demonstrate that concentration profiles in dunite and the surrounding peridotite lithologies are highly variable even among differing peridotite sequences within the same ophiolite, suggesting that the composition of instantaneous melt flowing through individual dunite channels is quite variable and the mantle source regions are heterogeneous. Components: 14,711 words, 20 figures. Keywords: dunite; harzburgite; lherzolite; melt-rock reaction; reactive dissolution; concentration gradient. Index Terms: 8412 Volcanology: Reactions and phase equilibria (1012, 3612); 8416 Volcanology: Mid-oceanic ridge processes (1032, 3614); 8434 Volcanology: Magma migration and fragmentation. Copyright 2008 by the American Geophysical Union 1 of 28 Geochemistry 3 morgan et al.: concentration gradients in ophiolites Geophysics 10.1029/2008GC001954 Geosystems G Received 22 January 2008; Revised 4 June 2008; Accepted 6 June 2008; Published 29 July 2008. Morgan, Z., Y. Liang, and P. Kelemen (2008), Significance of the concentration gradients associated with dunite bodies in the Josephine and Trinity ophiolites, Geochem. Geophys. Geosyst., 9, Q07025, doi:10.1029/2008GC001954. 1. Introduction dunite, harzburgite, and lherzolite [Liang, 2003; Morgan and Liang, 2003, 2005]. The growth of [2] Dunites are commonly observed in the mantle the dunite layer in these diffusive dissolution sections of ophiolite and peridotite massifs around experiments is controlled by the rates of diffusion the world [e.g., Boudier and Nicolas, 1985]. They of major elements in the melt and the extent of occur as veins, and tabular or sometimes irregular undersaturation of opx with respect to the dissolv- shaped bodies in the host harzburgite or plagio- ing magma. clase-bearing lherzolite and are interpreted as rem- [4] In the presence of grain-scale melt flow, which nants of conduits or channels for melt migration in is likely for melt migration in at least part of the the mantle [e.g., Kelemen et al., 1997, and refer- upper mantle, there exists a positive feedback ences therein]. On the basis of dominant lithology, between flow and dissolution as olivine normative Boudier and Nicolas [1985] divided ophiolites into basalt generated at greater depth percolating harzburgite subtype and lherzolite subtype. Dunite through a porous harzburgite or lherzolite matrix in lherzolite subtype ophiolite is surrounded by a [e.g., Daines and Kohlstedt, 1994; Aharonov et al., layer of harzburgite that, in turn, is surrounded by a 1995; Kelemen et al., 1995b, 1997; Spiegelman et layer of plagioclase-free lherzolite, and finally the al., 2001]. The extent of melt-peridotite interaction plagioclase-bearing lherzolite, forming the dunite, also depends on the rates and directions of melt harzburgite, lherzolite, and plagioclase lherzolite flow with respect to lithological contact. As in (DHL-PL) sequence (Figure 1a). Dunite in harz- peridotite dissolution experiments, records of burgite subtype ophiolite is simply surrounded by melt-peridotite interaction can be preserved or harzburgite (Figure 1b). partly preserved in the dunite-harzburgite or [3] The distinct peridotite sequences have also been DHL-PL sequence. Indeed, mineral compositional observed in high-temperature and high-pressure variations across dunite-harzburgite or DHL-PL reactive dissolution experiments involving melt- sequences from ophiolites or peridotite massifs have bearing peridotite and basalt [Daines and Kohl- been reported in a number of studies [e.g., Quick, stedt, 1994; Morgan and Liang, 2003, 2005; Beck 1981a; Obata and Nagahara, 1987; Kelemen et al., et al., 2006]. Depending on starting peridotite 1992; Takahashi, 1992; Takazawa et al., 2000; mineralogy, these dissolution experiments produce Kubo, 2002; Suhr et al., 2003; Braun, 2004; either a melt-bearing dunite-harzburgite or dunite- Lundstrom et al.,2005;Zhou et al., 2005]. In harzburgite-lherzolite sequence. The melt-bearing general, mineral compositional variations observed dunites produced in these experiments are ortho- in the field are more complicated than those pyroxene-free, while the melt-bearing harzburgites observed in peridotite dissolution experiments, produced in the lherzolite dissolution experiments reflecting more complicated geological processes are clinopyroxene-free. Another interesting feature involved in the nature. Figure 1c illustrates some of of the harzburgite or lherzolite dissolution experi- the variability of NiO in olivine from the Bay of ments is the presence of simple concentration Islands ophiolite [Suhr et al., 2003]. Three different gradients in minerals across the peridotite sequen- types of NiO in olivine profiles were observed: the ces. Figure 2 is a summary of recent experimental NiO content of olivine in the dunite is (1) lower than observations illustrating the lithological and chem- that in the surrounding harzburgite (Figure 1c, solid ical variations. The broad compositional variations line), (2) greater than that in the host harzburgite across dunite-harzburgite contact and harzburgite- (dotted line), and (3) approximately the same as that lherzolite contact result from a combined effect in the harzburgite (dashed line). Compositional of orthopyroxene (opx) dissolution at the dunite- trends similar to cases 2 and 3 were also observed harzburgite interface, clinopyroxene (cpx) dissolu- for the Mg# in olivine from the Bay of Islands tion at the harzburgite-lherzolite interface, olivine ophiolite [Suhr et al., 2003]. Explanations for the precipitation within the dunite, and diffusive ex- observed compositional variations include different change between the crystals and the melts in the degrees of partial melting, and/or various extents of 2of28 Geochemistry 3 morgan et al.: concentration gradients in ophiolites Geophysics 10.1029/2008GC001954 Geosystems G melt-rock reaction. Another interesting feature associated with compositional variations across a dunite-harzburgite or DHL-PL sequence is the spatial range over which mineral and bulk rock composition vary with respect to the nearest lithological contact: the latter, in general, is not centered at the former. In the case of NiO abundance in olivine shown in Figure 1c, spatial locations of the main compositional variations are shifted to the dunite side of the dunite-harzburgite interface. As will be shown in this study, the shift in concentration gradient is likely resulted from melt flow across lithological boundaries. [5] Motivated by our harzburgite and lherzolite dissolution experiments, we systematically sampled a number of dunite-harzburgite sequences at the Josephine ophiolite (in southern Oregon) and Figure 1. (a) A hypothetical dunite-harzburgite-lherzolite-plagioclase lherzolite sequence with sharp litho-

Load more