Melt-Fluid Evolution in Gabbroic Rocks from Hess Deep

Abstract

Mineralogical and fluid inclusion analyses of gabbroic rocks recovered from Site 894 on the intrarift ridge at Hess Deep indicate that the gabbroic sequence is a product of a complex history involving multiple magmatic and hydrothermal events. Pegmatitic patches throughout the core that contain high concentrations of apatite, zircon, oxide minerals, and coarse-grained amphibole are believed to have crystallized under relatively high fθ 2 from a hydrous magma. The patches of micropegmatite represent the most evolved compositions sampled at Site 894 and may reflect reequilibration with evolved, volatile-rich fluids that percolated through the gabbroic sequence late in the magmatic history. Analyses of fluid inclusions entrapped within these zones, indicate that 30-40 wt% NaCl ± Fe ± CO 2 -rich brines formed either under immiscible conditions in association with the evolved patches, or were exsolved in the absence of a vapor phase directly from the compositionally evolved melts. Migration of the high-salinity fluids may have resulted in the crystallization of apatites that contain up to ~6 wt% chlorine.The transition from magmatic to hydrothermal seawater-dominated conditions in the plutonic sequence is marked by penetration of fluids that exhibit equivalent fluid salinities that range from -0.1% to 200% of seawater values (3.2 wt% NaCl) and exhibit uncorrected homogenization temperatures that cluster at ~250°C. The extremely low-salinities were most likely formed during supercritical phase separation of seawater at temperatures >407°C. Their preservation requires that the vapor phase remained isolated from the associated brine and that subsequent mixing with seawater did not occur. The compositions of the plutonic-hosted fluids are similar to those of fluids exiting hydrothermal vents, indicating that the fracture networks may represent the feeder systems for sustained hydrothermal flow. Migration of these fluids along microfracture and vein networks resulted in the heterogeneous replacement of the gabbroic rocks by greenschist facies mineral assemblages. This alteration is most intense adjacent to vein networks and in cataclastically deformed zones, in which brittle failure facilitated enhanced fluid flow. Sealing of the microfracture and fracture networks with zeolite facies mineral assemblages marks the cessation of fluid circulation in the plutonic sequence. The plutonic sequence may have undergone as much as 2 km of uplift attendant with crustal thinning and propagation of the Cocos-Nazca spreading center.

Volatiles in mid-ocean ridge environments

“…However, in isolated pockets, the chemistry and occurrence of accessory minerals such as apatite, zircon, oxide minerals, and some amphiboles are consistent with crystallization of these phases under relatively high f (O 2 ) (oxygen fugacity) conditions and from a hydrous magma (Pedersen et al, 1996). The patches of micropegmatite represent the most evolved compositions sampled at Site 894 and may reflect re-equilibration with evolved, volatile-rich fluids that percolated through the gabbroic sequence late in the magmatic history (Pedersen et al, 1996;Kelley and Malpas, 1996). The complex suite of dunites, harzburgites, and gabbroic rocks recovered from Site 895 are thought to represent late-stage melt migration through residual mantle tectonites at the transition zone between the crust and mantle (Gillis, 1995;Dick and Natland, 1996).…”

“…With the exception of local cataclastic shear zones at shallow depths in the Site 894 gabbroic sequence, alteration is typically static and is reflected by pseudomorphic textures after primary igneous phases (Gillis, 1995;Früh-Green et al, 1996b). Metamorphism occurred through several stages of magma injection and hydrothermal alteration during progressive transport away from the axis of the East Pacific Rise and ultimate intersection with the Cocos-Nazca spreading center (Früh-Green et al, 1996a, 1996bKelley and Malpas, 1996;Manning and MacLeod, 1996;MacLeod et al, 1996).…”

“…Analyses of submarine basaltic glasses, and experimental data on CO 2 and H 2 O in mafic systems indicate that owing to the low solubility of CO 2 in basaltic melts, mid-oceanic- Honnorez et al, 1985; (2) Kelley and Delaney, 1987; (3) Delaney et al, 1987; (4) Schops and Herzig, 1990;(5) Nehlig, 1991;(6) Kelley et al, 1992; (7) Kelley et al, 1993;(8) Kelley et al, 1995;(9) Saccocia and Gillis, 1995; (10) Gu and Vanko, 1996;(11) Kelley, 1996;(12) Kelley and Malpas, 1996;(13) Kelley, 1997;(14) Kelley and Frü h-Green, 1999. † Plag-plagioclase, Cpx-clinopyroxene.…”

Evolution of gabbroic rocks of the Northern Apennine ophiolites (Italy): Comparison with the lower oceanic crust from modern slow-spreading ridges

“…The exotic component could be related to the upward migration-as a result of buoyancy in the gabbroic cumulate pile-of (1) trondhjemite-type, Si-Al-Na-rich liquids (Dick et al, 1991;Natland and Dick, 1996;Pedersen et al, 1996) or (2) exsolved igneous fluids (Kelley and Malpas, 1997;Kelley, 1996). In the ophiolites of the Northern Apennines and Western Alps, the trondhjemite-type rocks that locally crosscut the gabbroic rocks show slightly LREE-enriched patterns (La N /Sm N ϭ 2.1-3.7, for Sm N ϭ 50-90; Borsi et al, 1996).…”

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“…The compositional range is slightly narrower at Hole 894G (Mg# mean = 0.65; range = 0.51-0.68) and becomes slightly more mafic along the slope immediately south of the summit (mean Mg# = 0.74; range = 0.65-0.85). Similar to the Northern Escarpment, plagioclase displays extensive zoning and a large range in composition (An 44-87 ), whereas the range of Mg # for mafic phases is less broad (olivine Fo 61-71 , orthopyroxene Mg# = 64-74, and clinopyroxene Mg# = 62-81) Natland and Dick, 1996;Kelley and Malpas, 1996).…”