Melt viscosity, temperature and transport processes, Troodos ophiolite, Cyprus

Abstract

The lava section in the Troodos ophiolite, Cyprus, is chemically stratified and divided into a shallow lava sequence with low TiO 2 content and a deeper lava sequence with high TiO 2 content. We calculate the viscosity at magmatic temperature based on major element chemistry of lavas in Cyprus Crustal Study Project (CCSP) Holes CY-1 and 1A. We find that typical shallow low-Ti lavas have a magmatic viscosity that is two to three orders of magnitude lower than that of the deeper high-Ti lavas. This implies that, after eruption on-axis, Troodos low-Ti lavas would have been able to flow down the same slope faster and farther than high-Ti lavas. The calculated lava viscosity increases systematically from the lava^sediment interface to the bottom of the composite Hole CY-1/1A. This suggests that an efficient process of lava segregation by viscosity on the upper flanks of the paleo Troodos rise may have been responsible for the chemical stratification in the Troodos lava pile. Calculated magmatic temperature and molar Mg/ (Mg+Fe), or Mg#, decrease systematically down-section, while SiO 2 content increases. Correlation of Mg# in the lavas with Mg# in the underlying, lower crustal plutonic rocks sampled by CCSP Hole CY-4 shows that the shallow lavas came from a high-temperature, lower crustal magma reservoir which is now represented by high-Mg# pyroxenite cumulates, while the deeper lavas were erupted from a lower-temperature, mid-crustal reservoir which is now represented by gabbroic cumulates with lower Mg#. ß 2002 Published by Elsevier Science B.V.

Magmatic cycles and formation of the upper oceanic crust at spreading centers: Geochemical study of a continuous extrusive section in the Oman ophiolite

“…However, a continuum of magmatic activity can be inferred from the absence of volcanic breccias and deep‐water sediments within the Wadi Shaffan section. Therefore a scenario similar to that proposed by Schouten and Kelemen [2002] for Troodos lava emplacement may also apply to the Wadi Shaffan section. However, in contrast with Troodos, the transition between the two lava sequence in Wadi Shaffan and therefore the two different magma chamber processes (from an open to a closed system) is sharp and associated with a change in the degree of melting in the magma source.…”

“…In Troodos, data on chilled margins in the sheeted dyke complex, which include both low‐Ti and high‐Ti lavas, suggest that the two sequences are contemporaneous [ Baragar et al , 1990; Staudigel et al , 1999]. Schouten and Kelemen [2002] emphasize the effects of chemical variations on melt physical properties, the highly differentiated lower sequence (high SiO 2 ) being more viscous than the top low SiO 2 , low TiO 2 lavas [ Shaw , 1972]. These authors show that these viscosity variations may have resulted in different depocenters for the two lava types.…”

Numerical modeling of magmatic–hydrothermal systems constrained by U–Th–Pb–He time–temperature histories

“…Dynamic viscosity of lava at magmatic temperature is even lower (10 1 -10 4 Pa s; Schouten and Kelemen, 2002) and is sensitive to a variety of factors including: composition, temperature, pressure, volatile content, abundance and size of crystals, and thermal and mechanical history of the melt (McBirney and Murase, 1984).…”

Magmatic processes in developing oceanic crust revealed in a cumulate xenolith collected at the East Pacific Rise, 9°50′N

“…Currently, there is debate on whether normal mid‐ocean ridge basalt (NMORB) erupts principally from the uppermost melt lens or includes substantial volumes from melt‐rich zones within the underlying, stratigraphically high‐level crystal mush. In either case, typical NMORB and more chemically evolved compositions develop when primitive melts exit the upper mantle, ascend through the axial magma system [ Korenaga and Kelemen , 1997, 1998; Schouten and Kelemen , 2002] and undergo fractionation and magma mixing [ Langmuir et al , 1986; Dixon et al , 1986; Grove et al , 1992; Perfit et al , 1994; Perfit and Chadwick , 1998]. (Note: At the NEPR, NMORB is represented by a range of chemical compositions that can largely be explained by mixing of depleted (DMORB) and enriched (EMORB) end‐members from off‐axis locations, e.g., seamounts.)…”