The Northern Samail Ophiolite: An Oxygen isotope, microprobe, and field study

Geological, petrological, and oxygen isotopic data are presented for 228 whole rock and mineral samples collected from a 100×20 km area of the northern Samail ophiolite in Oman. Most of these samples are from three detailed profiles through the pillow lavas, sheeted dikes, and layered gabbros of this laterally heterogeneous fragment of Cretaceous oceanic crust, down to and across the petrologic Moho. The profiles encompass a range of petrologic and tectonic styles, and each profile exhibits distinctive ^(18)/^(16)O variations compared to one another and to mid-ocean ridge basalts, as a result of pervasive seawater-hydrothermal interaction that varied in intensity along strike in the ophiolite. In general, ^(18)O depletions are observed in the layered gabbros and ^(18)O enrichments in most of the sheeted dikes and pillow lavas, similar to results previously observed in the southern part of the ophiolite (Ibra area), where ^(18)O depletions within the gabbroic section are quantitatively balanced by ^(18)O enrichments in the shallower parts of the oceanic crust. The Wadi Hilti profile, selected as an example of relatively intact crust, differs from Ibra in having more uniform and slightly higher δ^(18)O in the gabbros (+5.4 to +6.3), as well as in containing more hydrous alteration minerals (amphibole, epidote, chlorite, and prehnite). The profiles in the Wadi Kanut-Shafan and Wadi Rajmi sections are much more complex and reveal the impact of off-axis intrusions and deep crustal shearing. Plagiogranite-wehrlite intrusions in the Shafan-Kanut area superimposed a local hydrothermal aureole on the ophiolite, evident in dikes highly depleted in ^(18)O, quartz-sulfide veins, abundant epidote, thullite, and chlorite in shallower rocks, and low-temperature hydrous alteration of deeper gabbroic rocks; the latter produced an overall increase in whole rock δ^(18)O (+6.2 to +6.9). Such late stage intrusions are found throughout the northern half of the Samail ophiolite. The Wadi Rajmi area, which is a possible fossil transform or propagating rift, represents the most complex of the three profiles; it also contains the most abundant highly deformed and hydrothermally altered rocks, together with the deepest and largest zone of ^(18)O depletion yet found in any ophiolite (locally δ^(18)O < +2.0). Conduits for large volumes of high-temperature hydrothermal fluids were provided by fractures now occupied by low-^(18)O gabbro pegmatites and low-^(18)O dikes. Material balance estimates for the regional samples and from the various transects through the ophiolite give crustal bulk δ^(18)O averages (+5.9 to +6.3) that are, within sampling error, almost identical to the average MORB basalt value of about +5.8, if both vertical and lateral crustal heterogeneities are integrated into a three-dimensional model. This supports and amplifies the conclusion of earlier workers that the δ^(18)O of seawater is buffered and controlled by hydrothermal interaction with oceanic crust, as long as the cumulative effects (both spatial and temporal) of all seafloor magmatic/hydrothermal processes are considered. The very slight out-of-balance enrichment of the integrated crustal average δ^(18)O compared to MORB may be explained by the ubiquitous mineralogical and isotopic evidence for a late, low-temperature alteration event in the basal gabbros; these effects are prominent in the vicinity of the petrologic Moho and may indicate exchange with low-temperature aqueous fluids during or after detachment of the obducted slab

The Cleft revealed: geologic, magnetic, and morphologic evidence for construction of upper oceanic crust along the southern Juan de Fuca Ridge

Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 7 (2006): Q04003, doi:10.1029/2005GC001038.The geology and structure of the Cleft Segment of the Southern Juan de Fuca Ridge (JdFR) have been examined using high-resolution mapping systems, observations by remotely operated vehicle (ROV), ROV-mounted magnetometer, and the geochemical analysis of recovered lavas. Bathymetric mapping using multibeam (EM300) coupled with in situ observations that focused on near-axis and flank regions provides a detailed picture of 0 to 400 ka upper crust created at the southern terminus of the JdFR. A total of 53 rock cores and 276 precisely located rock or glass samples were collected during three cruises that included sixteen ROV dives. Our observations of the seafloor during these dives suggest that many of the unfaulted and extensive lava flows that comprise and/or cap the prominent ridges that flank the axial valley emanate from ridge parallel faults and fissures that formed in the highly tectonized zone that forms the walls of the axial valley. The geochemically evolved and heterogeneous nature of these near-axis and flank eruptions is consistent with an origin within the cooler distal edges of a crustal magma chamber or mush zone. In contrast, the most recent axial eruptions are more primitive (higher MgO), chemically homogeneous lobate, sheet, and massive flows that generate a distinct magnetic high over the axial valley. We suggest that the syntectonic capping volcanics observed off-axis were erupted from near-axis and flank fissures and created a thickened extrusive layer as suggested by the magnetic and seismic data. This model suggests that many of the lavas that comprise the elevated ridges that bound the axial valley of the Cleft Segment were erupted during the collapse of a magmatic cycle not during the robust phase that established a new magmatic cycle.This research has been partially supported by a NSF grant to M. Perfit (OCE-0221541). M. Tivey acknowledges support from WHOI’s Mellon grant for Independent Study. Support for D. Stakes, T. Ramirez, D. Caress, and N. Maher and for the entire field program was provided by funds to MBARI from the Lucille and David Packard Foundation

Seismic and seafloor evidence for free gas, gas hydrates, and fluid seeps on the transform margin offshore Cape Mendocino

Seismic data and seafloor samples indicate the presence of free gas, gas hydrate, and fluid seeps south of the Gorda Escarpment, a topographic feature that marks the eastern end of the Gorda/Pacific transform plate boundary southwest of Cape Mendocino, California. In spite of high sedimentation rates and high biological productivity, direct or indirect indicators of gas hydrate presence had not previously been recognized in this region, or along transform margins in general. Gas is indicated by a bottom simulating reflection (BSR) observed near the Gorda Escarpment, by ‘‘bright spots’’ and ‘‘gas curtains’’ scattered throughout the sedimentary basin to the south, and by δ¹³C and δ¹⁸O isotopes of carbonates, which are similar to those recovered from other hydrate-bearing regions. The BSR reflection coefficient of -0.13 ± 0.04 and interval velocities as low as 1.38 km/s indicate that free gas is present beneath the BSR. Local shallowing of the BSR toward the north facing Gorda Escarpment and beneath a channel near the crest suggests fluid flow toward the seafloor. Integrating these various observations, we suggest a scenario in which methane is formed in thick Miocene and Pliocene deposits of organicrich sediments that fill the marginal basin south of the transform fault. Dissolved and free gas migrates toward the escarpment along stratigraphic horizons, resulting in hydrate formation and in channels, slumps and chemosynthetic communities on the face of the escarpment. We conclude that the BSR appears where hydrate-bearing sediments are uplifted because of current triple junction tectonics

Mineral and chemical composition of hydrothermally altered rocks and sediments from the Middle Valley, ODP Leg 139 data

The Middle Valley segment at the northern end of the Juan de Fuca Ridge is a deep extensional rift blanketed with 200-500 m of Pleistocene turbiditic sediment. Sites 857 and 858 were drilled during Ocean Drilling Program Leg 139 to determine whether these two sites were hydrologically linked end members of an active hydrothermal circulation system. Site 858 was placed in an area of active hydrothermal discharge with fluids up to 270°C venting through anhydrite-bearing mounds on top of altered sediment. The shallow basement of fine-grained basalt that underlies the vents at Site 858 is interpreted as a seamount that was subsequently buried by turbidites. Site 857 was placed 1.6 km south of the Site 858 vents in a zone of high heat flow and numerous seismically imaged ridge-parallel faults. Drilling at Site 857 encountered sediments that are increasingly altered with depth and that overlie a series of mafic sills at depths of 460-940 m below sea floor. Sill margins and adjacent baked sediment are highly altered to magnesian chlorite and crosscut with veins filled with quartz, chlorite, sulfides, epidote, and wairakite. The sill interiors vary from slightly altered, with unaltered plagioclase and clinopyroxene in a mesostasis replaced by chlorite, to local zones of intense alteration and brecciation. In these latter zones, the sill interiors are pervasively replaced by chlorite, epidote, quartz, pyrite, titanite, and rare actinolite. The most complete replacement is associated with brecciated horizons with low recovery and slickensides on fracture surfaces, which we interpret as intersections between faults and the sills. Geochemically, the alteration of the sill complex is reflected in significant whole-rock depletions in Ca, Sr, and Na with corresponding enrichments in Mg, Al, and most metals. The latter results from the formation of conspicuous sulfide poikiloblasts. In contrast, metamorphism of the Site 858 seamount includes incomplete albitization of plagioclase phenocrysts and replacement of sparse mafic phenocrysts. Much of the basement alteration at Site 858 is confined to crosscutting veins except for a highly altered and veined horizon at the contact between basaltic basement and the overlying sediment. The sill complex at Site 857 is more highly depleted in 18O (d18O = 2.4 per mil - 4.7 per mil) and more pervasively replaced by secondary minerals relative to the extrusives at Site 858 (d18O = 4.5 per mil - 5.5 per mil). There is no evidence of significant albitization of the plagioclase at Site 857, suggesting high Ca/Na in the pore fluids. Fluid-inclusion data from hydrothermal minerals in altered mafic rocks and veins at Sites 857 and 858 show a consistency of homogenization temperatures, varying from 245 to 270°C, which is within the range of temperatures observed for the fluids venting at Site 858. The consistency of the fluid inclusion temperatures, the lack of albitization within the Site 857 sills, and the apparently low water/rock ratio collectively suggest that the sill complex at Site 857 is in thermal equilibrium and being altered by a highly evolved Ca-rich fluid similar to the fluids now venting at Site 858. The alteration evident in these two deep crustal drillsites is a result of the ongoing hydrothermal circulation and is consistent with downhole logging results, instrumented borehole results, and hydrothermal fluid chemistry. The pervasive alteration of the laterally extensive sill-sediment complex at Site 857 determines the chemistry of the fluids that are venting at Site 858. The limited alteration of the Site 858 lavas suggests that this basement edifice acts as a penetrator or ventilator for the regional hydrothermal reservoir with much of the flow focussed at the highly altered and veined sediment-basalt contact