Ophiolites as faithful records of the oxygen isotope ratio of ancient seawater: the Solund-Stavfjord Ophiolite Complex as a Late Ordovician example

Abstract

Fragments of the Ordovician sea floor preserved in the Solund-Stavfjord Ophiolite Complex in Western Norway serve as proxies for the δ18O of Ordovician seawater. The pillow basalt sections at Oldra and Strand are both enriched in 18O, recording their alteration by seawater at low temperature on the sea floor. In contrast, the sheeted dykes and gabbros generally are depleted of 18O, reflecting the modal proportion of secondary, low-18O chlorite and epidote formed from seawater at high temperature. These isotopic contrasts simply reflect the high water to rock ratio of sea-floor alteration and the temperature dependence of the 18O partitioning between minerals and water. Superposition of high-δ18O pillows on low-δ18O dykes and gabbros is a necessary consequence of alteration at both low and high temperatures by a fluid near 0‰ and is easily recognized in well-preserved ophiolites. Also, the δ18O of seawater can be independently calculated from 18O fractionations among secondary minerals. Older, dismembered and highly metamorphosed segments of the oceanic crust may still retain the original seawater imprint because their subsequent obduction and metamorphism was relatively closed to external fluids. Suites of diamond-bearing nodules from kimberlites still have contrasting high- and low-δ18O eclogites, proving that even subduction into the mantle is not sufficient to erase the seawater fingerprint. Inspection of the sea-floor, ophiolite and eclogite data reveals no secular trend in δ18O, indicating that the δ18O of seawater has not changed with geological age. Because the δ18O of seawater itself is fixed by sea-floor-seawater exchange, the constancy of δ18O of seawater implies that the scale and style of sea-floor-seawater interactions has not changed over time.

Low δ18O in the Icelandic mantle and its origins: Evidence from Reykjanes Ridge and Icelandic lavas

“…Our knowledge of the oxygen isotope composition of oceanic crust (Table 5) comes from studies of ophiolites (Gregory and Taylor, 1981;Cocker et al, 1982;Agrinier et al, 1988;Lecuyer and Fourcade, 1991;Stakes and Taylor, 1992;Holmden and Muehlenbachs, 1993;Muehlenbachs et al, 2004), drill cores (Alt et al, 1986;Alt et al, 1995;Staudigel et al, 1995;Hart et al, 1999), xenoliths (Hansteen and Troll, 2003), and dredged material at fracture zones (Agrinier et al, 1995). Most of these are whole-rock or even composite studies, in order to achieve representative sampling of heterogeneous alteration.…”

“…This might explain the low d 18 O of Icelandic mantle sources, given the Pb isotopic evidence that they incorporate Lower Palaeozoic ocean crust (Thirlwall et al, 2004). However, there is no evidence whatsoever of unusually low d 18 O in the two Lower Palaeozoic ophiolites analyzed (Lecuyer and Fourcade, 1991;Muehlenbachs et al, 2004), and these are direct representatives of material that has most probably been recycled into the Icelandic mantle.…”

Combined Li–He isotopes in Iceland and Jan Mayen basalts and constraints on the nature of the North Atlantic mantle

“…Instead, Kokfelt et al (2006) have suggested that d 18 O reflects ancient hydrothermal alteration and that low d 18 O would require unrealistic degrees of assimilation and fractionation (AFC) when local altered Icelandic crust would be considered, consistent with independent models (Stracke et al, 2003a). It is more plausible that large-scale hydrothermal alteration with meteoric fluids provided source rocks with low d 18 O (Gautason and Muehlenbachs, 1998) that were formed in, for example, Palaeozoic ophiolites (Muehlenbachs et al, 2004) and these were subsequently recycled.…”

Effect of paleoseawater composition on hydrothermal exchange in midocean ridges

“…Epidote forms at temperatures greater than ∼300°C, and is likely to precipitate from fully evolved vent fluids. Furthermore, epidosites and epidote-quartz veins in particular are thought to form preferentially along concentrated flow paths where water/rock ratios are high (29,(45)(46)(47) and are thus likely to be the best records of high-temperature fluids through time. Epidosites and epidotequartz veins from the two youngest oceanic crustal sections [Pito Deep and Site 504B (33,48)] have 87 Sr/ 86 Sr values that are close to typical average modern vent fluids, reinforcing the hypothesis that they record vent fluid chemistry in older rocks.…”