T53B-1293
What is an Oceanic Core Complex?
The Mid-Atlantic Ridge (MAR) 75km north and south of the 15-20 Fracture Zone (FZ) has produced upper oceanic lithosphere composed dominantly of mantle peridotite with gabbro intrusions. In the absence of diapirism, mantle peridotite can only be exposed on the seafloor by extensional faulting, thus the sea floor geology and bathymetry provide widespread evidence for extensive low-angle faulting. However, only 3% of the seafloor in this region has the domal morphology characteristic of features that have been termed oceanic core complexes; suggesting that other processes, in addition to low-angle faulting, are responsible for the generation of domal core complexes. Most low-angle faults near the 15-20 FZ form gently dipping (10-15°), 10-15km-wide dip slopes on the flanks of 2000m relief bathymetric ridges that are up to 15-40km long (parallel to the MAR). Core recovered from ODP Leg 209 drill holes in these ridges is dominantly peridotite with small (<50m thick) gabbro intrusions. The peridotite is cut by a very high density of brittle faults dipping at both steep and gentle angles. Several holes also contain long-lived shear zones/faults in their upper reaches in which strain was localized at granulite facies, indicated by mylonitic olivine and cpx, and remained active during cooling to sub-greenschist grade, indicated by cross-cutting of progressively lower-grade syn-deformation mineral assemblages. These observations suggest that seafloor spreading is largely accommodated here by slip on low-angle faults, and that these faults are correctly termed detachment faults. Holes drilled into a domal oceanic core complex north of the 15-20 FZ during Leg 209 (ODP Site 1275) recovered dominantly gabbro and not mantle peridotite. This hole is cut by significantly fewer brittle and ductile faults than the peridotite drilled at the non-core-complex detachment fault sites. The detachment fault in the upper reaches (50m) of Site 1275 was localized at temperatures near feldspar's ductile-to-brittle transition, indicated by cataclasis with minor crystal plastic flow in plagioclase, and a lack of pervasive pure-ductile deformation. Amphibole-plagioclase thermometry in the fault yields equilibrium temperatures from 600-650°C, compared to equilibrium temperatures of 750-850°C for the gabbro outside the fault. The presence of talc- chlorite schists and cataclasites cutting the higher-temperature deformation textures indicate fault activity down- temperature from amphibolite through greenschist facies. This core-complex-bounding fault contrasts with the fault that bounds the Atlantis Bank Core Complex on the Southwest Indian Ridge (SWIR). There, the fault is 100m thick and strain was initially localized at granulite grade (>800°C) (Mehl & Hirth, 2007); significantly hotter than the Site 1275 fault. Therefore, the formation of core-complex morphology does not seem to depend on the initial faulting conditions. Both oceanic core complexes that have been drilled besides Site 1275, Atlantis Massif at 30°N (IODP Hole 1309D) on the MAR and Atlantis Bank on the SWIR (ODP Hole 735B), are also comprised dominantly of gabbro. This suggests that magma supply may be an essential requirement for core complex formation and raises the question whether all domal oceanic core complexes are cored by gabbro? We also ask whether the term ‘oceanic core complex' should be restricted to these domal features and not applied to detachment-bound, non- domal, peridotite-cored ridges; or if these should be considered two sub-classes of oceanic core complexes.
T53B-1294
Dikes Intrusions and the Formation of Mid-Ocean Ridge Faults
The first numerical model that treats faulting and dike intrusions in a mechanically consistent way is used to simulate mid-ocean ridge topography, fault patterns and extrusive thicknesses. The observed range of these features are reproduced by varying three variables: the effective radius of the axial partial melt or mush zone, R; the time interval between diking episodes , T; and the axial lithosphere thickness, D. The mush zone radius R controls the amount of magma that can be supplied to fill dikes and erupt for a given driving pressure. For reasonable values of the diking time interval it is the ratio of R to D that controls the height to which dikes may build topography. For R/D > ~1 an axial high can result. If the diking time interval T is greater than the time to develop lithosphere scale faults, Tf, then a valley will result if R/D < ~1. The observed spreading rate dependence of axial relief is reproduced if the axial lithospheric thickness is inversely related to spreading rate and the effective magma mush region is directly related to spreading rate. The maximum depth of an axial valley with little or no magma input is limited by the value of D. Fault offsets, similar to those seen in association with axial valleys, result when T>Tf. Fault offset increases with increasing dike time interval. Oceanic core complex style, unlimited fault offset for a restricted range of parameters: the dike time interval, T must be close to twice the time for stresses to build the level of faulting Tf; the effective magma source dimension R must be less than the axial lithospheric thickness D.
T53B-1295
Evolution of oceanic core complex domes and corrugations
In regions of the oceans where detachment faulting is developed widely, individual core complex domes (elevated massifs capped by corrugated detachment surfaces) show a consistent morphology. At their outward sides, most core complex domes are attached to a planar slope, interpreted (Smith et al., 2006) as an originally steep inward-facing normal fault that has been rotated to shallower angles. We suggest that the break in slope where the originally steep normal fault meets the domal corrugated surface marks the trace of the brittle-ductile transition at the base of the original normal fault. The steep faults originate within a short distance of the spreading axis. This means that the arcuate shape of the intersection of the steep fault with the dome must indicate the shape of the brittle-ductile transition very close to the spreading axis. The transition must be very shallow close to the summit of the dome and deeper on each flank. Evidence from drilling of some core complexes (McCaig et al, 2007) shows that while the domal detachment faults are active they may channel hydrothermal flow at black smoker temperatures and may be simultaneously injected by magma from below. This indicates a close link between igneous activity, hydrothermal flow and deformation while a core complex is forming. Once the shape of the core complex dome is established, it persists as the ductile footwall mantle rising from below is shaped by the overlying brittle hanging wall that has been cooled by the hydrothermal circulation. The corrugations in the footwall must be moulded into it by irregularities in the brittle hanging wall, as suggested by Spencer (1999). The along-axis arched shape of the hanging wall helps to stabilise the domal shape of the footwall as it rises and cools.
T53B-1296
The use of Mid-Ocean Ridge Basalts to Infer Subsurface Processes Along Slow and Ultraslow Spreading Ridges: a Cautionary Tale
Mid-ocean ridge basalts (MORB) are commonly used to infer melting and crystallization processes in the underlying subsurface. Here, we draw attention to two caveats, which may be particularly important at the lower end of the global range in spreading rate. First, in areas where crustal accretion is focused, as appears to be the case along most slow- and ultraslow- spreading ridge segments, MORB may not originate from the immediately underlying crust and/or mantle. This is demonstrated by gabbroic rocks exposed in the Kane Core Complex (Mid-Atlantic Ridge), which are primitive in the ancient segment center, compatible with being residues of MORB generation, but too evolved along most of the segment to have been fractionated from MORB. Along most of the segment, then, the basalts are unrelated to the underlying lower crust. The fractionation process appears to have been restricted to the segment center, followed by extensive along-axis intrusion of melts. Second, it is often assumed that most crystallization of melts generated by mantle melting occurs in deep parts of the lithosphere at slow and ultraslow spreading ridges. This is generally based on MORB liquid lines of descent (LLD), which are offset to lower CaO and higher Al2O3 at a given MgO from low-pressure LLD. This can be explained by earlier onset of clinopyroxene crystallization, combined with a smaller role for plagioclase fractionation, both of which are expected at higher pressures because the clinopyroxene stability field increases, and that of plagioclase decreases, with increasing pressure. In addition, gabbros often contain high Mg# clinopyroxene, also taken as evidence for pressure-induced early clinopyroxene crystallization. However, the offset of MORB LLD, as well as high-Mg# clinopyroxene, can be explained by reactions between ascending melts and lower crustal primitive cumulates. Thus if melt-rock reaction is an important process in the lower crust, as is suggested by many gabbro suites, MORB compositions cannot be used to infer fractionation pressure.
T53B-1297
Geochemistry of a long in-situ section of intrusive slow-spread crust: Results from IODP Site U1309 (Atlantis Massif, 30°N Mid-Atlantic-Ridge)
IODP Site U1309 was drilled at Atlantis Massif (western rift flank of Mid-Atlantic Ridge (MAR) 30°N; Expeditions 304 and 305), a 1.5-2 Myr old oceanic core complex. The main hole, Hole U1309D, is the second deepest hole drilled into an intrusive slow-spread oceanic lithosphere: it penetrated 1415.5 mbsf (75% recovery). We present here the results of a bulk rock geochemical study (major and trace elements – ICPMS -) carried out on 234 samples representative of the different lithologies sampled at Site U1309. Over 96% of Hole U1309D is made up of gabbroic rocks, cross-cut by late diabases and basaltic dykes in the upper part of the section. Diabases and basalts have depleted MORB compositions (La/Yb ~0.8 and Yb ~ 3.5 ppm) similar to basalts sampled at MAR 30°N. Relics of mantle were recovered at shallow depth. Mantle peridotites show petrographic evidence of melt impregnation. They have relatively fertile compositions, similar to MARK peridotites, with Mg# (100xMg/(Mg+Fe)) of 89-90, Ni>2400ppm and Yb 0.03-0.11 ppm. Gabbroic rocks span a wide range of lithologies and geochemical compositions. They comprise olivine-rich troctolites (>70% modal olivine), troctolites, olivine and troctolitic gabbros (5 to 50% modal olivine), gabbros (including gabbronorites) and oxide gabbros (>2% modal Fe-Ti oxides), which represent respectively 5.4 %, 2.7%, 25.5%, 55.7% and 7% of the core recovered at Hole U1309D. Minor felsic ("leucocratic") dikes cross- cutting gabbros were also sampled. Troctolites and olivine-rich troctolites have high Mg# (82-89), high Ni (up to 2300 ppm) and low trace element contents (Yb 0.06-0.8 ppm). They overlap in composition with peridotites sampled at Atlantis Massif and with impregnated peridotites drilled along the MAR (e.g., ODP Site 1271 (MAR 15°20'N)). Gabbros and olivine gabbros have high Mg# (60-86) and low trace element contents (Yb 0.125-2.5 ppm – (La/Yb)CN ~ 0.4-0.7); these gabbros are among the most primitive and depleted yet sampled along slow spreading ridges (e.g., MAR 23°N and 15°20'N and ODP Hole 735B on the Southwest Indian Ridge). Oxide gabbros and leucocratic dykes represent the most evolved end-members of the gabbroic suite with low Mg# (<50), low Ni (~65 ppm) and high trace element contents (Yb up to 26 ppm). Troctolites and gabbros have Eu positive anomalies (Eu/Eu* ~1.4) whereas oxide gabbros and leucocratic dykes have Eu negative anomalies (Eu/Eu* ~0.75). The former are interpreted as cumulates of a common parental magma, while the later precipitated after the differentiated melts. We note that gabbros and gabbronorites overlap in composition, thus precluding precipitation after a significantly more evolved melt for orthopyroxene crystallization. The bulk composition of gabbroic rocks from Hole 1309D shows no Sr or Eu anomalies; although there may have been local separation of melt and solids, there was no large scale removal of melts from this gabbro section. It is somewhat depleted in the most incompatible elements compared to parental MORB with Ba (40%), Nb (40%) and Rare Earth Elements (REE: Ce 70%, Yb 95%), but enriched in compatible and refractory elements such as Mg (120%) or Ni (200%) and Cr (200%). This may indicate strong chemical interaction between the parental melt and the intruded (assimilated?) shallow depleted mantle, probably during the earliest stages of the formation of Site U1309 gabbroic body. Olivine-rich troctolites may represent the ultimate residue of this reaction- accumulation process.
T53B-1298
'Hydromagmatic' epidote in in-situ deep oceanic diorites from IODP Hole U1309D, Atlantis Massif, Mid-Atlantic Ridge, 30°N
Epidote is one of the key minerals for controlling hydrothermal vent fluid composition at the 'reaction zone' of mid- ocean ridge hydrothermal system. In slow-spreading ridges (SSR), there has been very few geophysical evidences yet to indicate the presence of a magma chamber. Therefore, short-lived small melt lens and/or near- solidus hot gabbroic rocks have been assumed as heat sources of high-temperature hydrothermal circulation at SSR. Epidote-bearing plutonic rocks in SSR are typically oxide gabbros, diorites, and trondhjemitic rocks. Why occurrence of epidote is rather restricted in such differentiated-magma derived oceanic plutonic rocks is a fundamental question concerning petrogenetic relationship between magmatic and alteration processes. We report euhedral, discrete and zoned epidotes in hydrothermally altered leucocratic diorites, and discuss petrological features of transition from the epidote-bearing leucocratic diorites to adjacent disseminated oxide gabbros recovered from IODP Hole U1309D, Atlantis Massif at Mid-Atlantic Ridge 30°N. Constituent mineral compositions and texture suggest that the diorites crystallized from incompatible element- and fluid-rich late- stage melt segregated from oxide gabbros at low pressure (~0.6 kbar). Zoned, euhedral epidotes in leucocratic diorites are essentially interpreted as magmatic product and their pistacite contents (= octahedral (Fe3+/(Fe3+ + AlVI) × 100) <33) suggest crystallization from oxidized, fluid-rich melt or high-temperature fluid itself. Sr isotopic initial ratio of the epidote-bearing leucocratic diorites (87Sr/86SrI = 0.70312) and associated disseminated oxide gabbro (87Sr/86SrI = 0.70262) are relatively low. Calculated water/rock ratio of those rocks (0.5-3.0) suggests involvement of relatively rock dominant fluids in the alteration. Overall our assessment suggests that high-temperature alteration started at the latest stage of magmatic processes, and could attain the W/R ratio required to explain common hydrothermal fluid composition. It implies epidote-bearing leucocratic diorites and related lithologies are substantial materials in the 'reaction zone' of hydrothermal circulation system at SSR. The autometamorphism reasonably explain restricted occurrence of epidotes in leucocratic plutonic rocks at SSR.
T53B-1299
Evolved Melt Migration in Primitive Lower Crustal Gabbros at IODP Site U1309, 30N, Mid- Atlantic Ridge
IODP Hole U1309D (Legs 304/305) penetrated 1415 m into the core-complex of the Atlantis Massif at 30°N, Mid-Atlantic Ridge. Of the 75% recovered rock, 96% is gabbroic, comprising olivine-rich troctolite, (olivine-) gabbros, (olivine-) gabbronorite and evolved ferrogabbro. Possibly, a larger-scale magmatic cycle is preserved between 600 und 1240 mbsf, as marked by olivine-rich troctolites at the base and more common gabbronorites near the top. In detail, however, internal magmatic contacts are numerous with more evolved gabbros intruding into less evolved ones. Here we present a microprobe overview of the 800-1200 mbsf interval and a detailed study of the transition from evolved rocks at 1240 mbsf to the overlying primitive gabbros. Geochemically, there is a good correlation between the REE in cpx, the Mg# in cpx, and lithological evolution. The data can be modeled as simple incremental batch fractionation from 5 to >80% of a primitive average N- MORB. The need for replenishment of the melt body seems not present. In the more evolved gabbros, the fractionation of apatite is indicated and observed. The rims of nearly all clinopyroxene grains were overprinted by an evolved melt. A three-stage model is required to explain the data set: (1) a cumulus phase; (2) displacement of residual melt bodies and formation of new cumulate bodies. These two processes can explain the presence of well-equilibrated, but differently evolved cores of clinopyroxene. (3) Compaction of a residual melt and differentation of it as it migrated through the cumulus pile. This stage explains the late overprint of the rims of clinopyroxene. In a following cycle, a new pluton would dock to the base of the earlier body. We are still somewhat uncomfortable with what is a rather abrupt transition between the olivine rich troctolites (containing high Cr in clinopyroxene and high Ni in olivine) and the overlying olivine gabbros and the large volume of melt (without replenishment) required in the model.
T53B-1300
Origin of Olivine-Rich Troctolites From IODP Hole U1309D in the Atlantis Massif (Mid-Atlantic Ridge) : Petrostructural and Geochemical Study
IODP Hole U1309D sampled the Atlantis Massif (western flank of Mid-Atlantic Ridge, 30°N; IODP Expeditions 304 and 305), an oceanic core complex, through an exposed detachment fault. Core U1309D is mostly made of gabbroic rocks; olivine-rich troctolites (Ol >70%) represent 5.4% of the recovered rocks. These troctolites are the most primitive gabbroic rocks ever drilled at mid-ocean ridges. We present a petrostructural (EBSD) and in-situ geochemical (EPMA, LA-ICPMS and LA-HR-ICPMS) study of olivine-rich troctolites and associated gabbros from Hole U1309D. Olivine-rich troctolites from Hole U1309D display poikilitic textures, with olivine ranging from coarse-grained subhedral crystals to medium-grained rounded crystals, embedded in large, undeformed clinopyroxene and plagioclase poikiloblasts. Olivine crystallographic preferred orientations are weak, indicating deformation by dislocation creep with activation of the high-temperature (010) [100] slip system. Analysis of misorientation angle distribution in olivine reveals well-developed (100) subgrain boundaries in some grains. Plagioclase has homogeneous composition with La ranging from 0.63 to 2.68 × chondrites, La/Sm of 1.49 and Eu/Eu* of 8.26 in all analysed samples. In contrast, Cpx display strong elemental variations at the scale of the mineral and of the thin section, but these variations do not correspond to changes in the sample lithology. A first group of Cpx (Cpx type 1) has depleted compositions with Yb ranging from 5.37 to 12.25 × chondrites, La/Yb of 0.069. The second group (Cpx type 2) are distinguished by enriched trace element composition (Yb 9.32-17.33 × chondrites), Eu negative anomaly (Eu/Eu* of 0.74) and similar REE fractionation (La/Yb of 0.074) than cpx type 1, which suggests co-precipitation of Cpx 2 and Plag. Olivine in olivine- rich troctolites has low trace element contents (Yb 0.23-0.83 × chondrites) and has strongly fractionated REE patterns (La/Yb of 0.47). Cpx 1 and Plag-Cpx2 assemblages are in equilibrium with depleted MORB, both in olivine-rich troctolites and in neighbouring gabbros. Olivine is characterized by highly variable compositions, displaying a linear correlation between Mg# (82 to 88) and Ni content (1479 to 2294), atypical of cumulate trends. In addition, olivine has highly depleted REE compositions in disequilibrium with both Plag and Cpx. Our results suggest a complex crystallization history in an open system with (i) Cpx and Plag precipitated after the same melt during the same stage of the magmatic evolution that produced Site U1309 gabbroic suite, in both olivine-rich troctolites and in neighbouring gabbros, (ii) olivine in olivine-rich troctolites predate this magmatic process. Hole U1309D olivine-rich troctolites could have been either (i) ultramafic cumulates with a very primitive character suggesting very magnesian source melts, so far unknown at mid-ocean ridges, or (ii) mantle peridotites. We favor an inherited mantle origin, the mantle composition being strongly modified by interaction with, and impregnation by large volumes of MORB-type melt.
T53B-1301
Comparison of Analytical Results from MIR Spectroscopy, Micro-Raman Spectrography, and Petrographic Examination of Serpentinized Samples from the Atlantis Massif: IODP Expedition 304/305, Hole U1309D
The composition and texture of serpentinized dunite and troctolite intervals in gabbroic rocks from IODP Hole U1309D vary downhole, with the most altered, olivine–rich sequences generally in proximity to more highly fractured intervals in the cores. Detailed analysis of samples from the heavily serpentinized intervals, using middle infrared (MIR) spectroscopy and micro-Raman spectrographic analysis shows the utility of these techniques for rapid determination of modal mineralogy, effects of foliation (from fractures, vein-development, and/or mineral alignment), and secondary alteration affecting the rocks. Although spectra of serpentines in the MIR exist, there are essentially none aimed at discrimination of serpentine phases, determination of bulk serpentinite modal mineralogy, or bulk serpentine orientation. Comparison between MIR spectroscopy results and the petrographic and micro-Raman analysis of the samples show that, to a first order, MIR spectroscopy gives indication of the effect of bulk rock foliation originating from preferred serpentine orientation and fracture patterns in the rocks. MIR spectroscopy also provides a method for quickly and inexpensively analyzing large numbers of samples as triage to maximize scientific return from more detailed analysis. We have performed micro-Raman analysis, using polished thin sections, of serpentinized olivine-rich samples from IODP Expedition 304/305 and are able to map not only primary and minor phases, but are able to identify regions within serpentinized portions of the samples that show intergrowths of serpentine with secondary alteration phases on a scale of a few microns.
T53B-1302
Strontium and Neodymium Isotope Systematics of Plutonic Rocks From the Atlantis Massif, Mid-Atlantic Ridge
Gabbroic rocks and associated basaltic dikes from Atlantis Massif, Mid-Atlantic Ridge at 30°N (IODP Exp. 304 & 305, Hole U1309D), have been examined in order to understand magmatic processes, water rock interactions and source components at the slow-spreading Mid-Atlantic Ridge. The recovered rocks range from plagioclase-bearing peridotite, troctolite, gabbro and gabbronorite to evolved oxide gabbro, and diabase. One sample about every 20-30 m from 43 to 1410 mbsf of different lithologies, plus discrete sampling of intrusive doleritic and basaltic dikes and a closer sampling (about every half a meter) through an ultramafic section, has been done. 88 samples have been analysed for Nd and Sr isotope compositions. The 143Nd/144Nd ratio of the plutonic rocks range from 0.513141 to 0.513215 with an average of 0.513187. There seems to be no systematic variation in the Nd-isotope compositions through the core. However, repeated analyses of single samples are needed to decide if the observed variations are larger than can be explained by analytical errors alone.The 87Sr/86Sr ratios vary from 0.70250 to 0.70351. The Sr-isotopic composition has been shifted slightly more towards to seawater values in the upper 800m than in the lower 600m of the cored section, showing that fluid-rock interactions decrease downward. The water-rock induced shift in Sr-isotope compositions in these layer 3 rocks is however significantly less then the shift reported from the sheeted dike complex sampled by ODP Hole 504B. The associated doleritic and basaltic dykes show on average 87Sr/86Sr ratios that are slightly higher and 143Nd/144Nd ratios that slightly lower than the plutonic rocks. This indicates that the plutonic rocks and the associated dikes were derived from slightly different mantle sources.
T53B-1303
Compressional-Wave Velocities of Discrete Samples at In-Situ Pressures From IODP Hole U1309D
One of the exciting objectives of IODP Expeditions 304 and 305 was to recover a seismically fast section of rock within the Atlantis Massif on the Mid-Atlantic ridge 30°N. Initial refraction analyses suggest that velocities greater than 7.5 km/s, which would correspond to essentially fresh peridotite, occur within the upper km at this site. After drilling 1.4 km at Hole U1309D and measuring compressional-wave velocities of individual samples shipboard, no single sample measurement was found to be greater than 6.8 km/s. P-wave velocities in individual samples may not accurately represent regional velocity structure because of the effect of increasing pressure down-hole, which was not accounted for in shipboard measurements. Sonic logging data do measure velocities at in-situ pressure but hole conditions can affect these data and in Hole U1309D they were obtained only to 800 mbsf. To compensate for these deficiencies we measured discrete sample velocities at elevated pressures. Two intervals (300 - 400 mbsf and 700 - 900 mbsf) were selected, based on available logging data and petrologic analysis, as likely intervals for a rapid change in physical properties. Samples from these intervals (~ 30 from 300-400 mbsf and 70 from 700-900 mbsf) were pressurized and velocity measurements were recorded at regular pressure increments up to 200 MPa. Pressure down-hole is expected to increase at a rate between 20 and 30 MPa/km, depending on the pore pressure. Velocities of pressurized samples from Hole U1309D match closely with pressurized velocity measurements from ODP Hole 735B, which sampled similar lithology (gabbro). In-situ pressure velocities are higher than shipboard lab-pressure measurements by ~ 1 km/s (300-400 mbsf interval: min 5.8 km/s; max 7.5 km/s, ave 6.7 km/s. 700-900 mbsf interval: min 6.5 km/s, max 7.5 km/s, ave 7.0 km/s). Even including the effect of increasing pressure down-hole, we do not find evidence to support a high velocity (greater than 7.5 km/s) reflective layer on the scale of core samples in either interval. This suggests that regional reflectivity must come from another source, such as larger scale changes in porosity and/or alteration.
T53B-1304
Shallow seismic structure of the Kane core complex, Mid-Atlantic Ridge 23°30'N
We present high-resolution travel-time seismic tomography models obtained along and across the Kane core complex (KCC), a proposed IODP drilling target located off-axis on the Mid-Atlantic Ridge (MAR) near 23°30'N south of the Kane Fracture Zone. Together with existing geological studies, our results characterize the lateral variability in structure and composition of this well developed oceanic core complex. The KCC is a large (~23 km by ~35 km in the dip and strike directions, respectively) megamullion formed by a long-lived detachment fault between ~3.3 and 2.1 Ma. The detachment is cut along-strike by a high- angle, west-dipping normal fault (East fault). Extensive ROV and dredge sampling indicates that the northern part of East fault is covered by in situ pillow basalts, but at the central dome the fault exposes the underlying lithosphere, which is dominated by highly altered harzburgites. In contrast, the northern dome to the east of East fault appears to consist largely of gabbros. We derived the two-dimensional, shallow P-wave velocity structure of the KCC along six ~20- to 40-km-long profiles, including three strike lines and three dip lines. The seismic data were acquired in 2001 using the 6-km- long hydrophone streamer and air-gun array of R/V Ewing (cruise EW0102). The dense sampling (shots spaced every 37.5 m, with receivers spaced every 12.5 m) and the shallow seafloor of the area (<3000 m) allows us to image lateral variations in velocity structure at scales of 1 km or less within the upper ~0.5-1.5 km of the lithosphere. Our results show significant lateral variations in velocity structure in both strike and dip directions, and these variations to first order correlate with sampled lithologies. The lowest observed velocities (~3.3 km/s at seafloor increasing to ~5.1 km/s at ~1 km depth) correlate with the zone of volcanics found along the northern part of East fault. Low velocities also occur beneath the volcanic terrain of the remnant hanging wall to the east of the detachment termination. The northern dome near Kane Fracture Zone, and the eastern, younger part of the central dome are characterized by relatively high velocities and high vertical velocity gradients near the seafloor. Seafloor velocities there are ~4.1-4.4 km/s and rapidly increase to ~6.0 km/s within the upper ~450 meters. In contrast, the southern dome and the central and western (older) part of the central dome show more moderate near-seafloor velocities and vertical gradients (~3.7 km/s at the seafloor, increasing to ~5.0 km/s at ~580 meters below the seafloor). We interpret the relatively high velocities of the northern dome and the eastern part of the central dome as representing primarily gabbroic rocks. This is based on available rock samples but also on a striking similarity in velocity structure between these parts of the KCC and the Atlantis core complex on the MAR at 30°N [Canales et al., this meeting], where drilling at IODP Hole U1309D recovered a ~1.5-km-thick section of gabbros. Geological samples at the KCC also suggest that the more moderate seismic velocities beneath the southern dome and the western and central part of the central dome probably represent highly serpentinized mantle peridotites. In summary, our results show that the shallow sub-seafloor velocity structure of the KCC is highly heterogeneous, indicating that lower crustal gabbros are dominant beneath some parts of the detachment surface while serpentinized mantle peridotites predominate in other areas.
T53B-1305
Geometry of a Polarity Reversal Boundary in Lower Crust and Upper Mantle at Kane Megamullion
Magnetic polarity reversal (C2r/C2An,~2.581 Ma) is coherently recorded in lower crustal gabbro and serpentinized upper mantle peridotites exposed on the seafloor of the northern and central domes, respectively, of Kane megamullion, which formed by detachment faulting at ~3.3-2.1 Ma. Rock magnetic results suggest that both lithologies contribute to the magnetic signal. Serpentinized peridotites have higher mean natural remanent magnetization (NRM) (4.7±5.8 A/m) than the gabbros (1.5±2.5 A/m), but gabbro NRM is more stable and has higher median destructive field and Koenigsberger ratio values. Paleomagnetic data identify both normal and reverse polarity samples, with most of the reverse polarity samples located in polarity transition zones. Polarity boundary geometry is estimated from two near-bottom magnetic profiles acquired across the northern and central domes. The analytic signal approach is used to calculate a range of dip angle solutions for varying rotations of the magnetic source layer, assuming an initial remanence direction parallel to the geocentric axial dipole. The amount of footwall rotation is estimated to be at least ~35° away from the ridge axis, as interpreted from the shape of the anomaly across a basalt ridge in the northern region. Based on this rotation, the dip of the polarity boundary is 46°W±14° (away from the ridge axis) in the northern region and 41°E±17° (towards the ridge axis) in the central region. The opposing dip angles in the two regions correspond to observed differences in seafloor sample lithology. We interpret the polarity boundary in the northern region to be a rotated cooling isotherm in a predominantly gabbroic layer. Numerical modeling of core complex thermal structure indicates that ≤45° outward rotation of a 580°C cooling isotherm with an initial dip of ~15°W at the ridge axis is possible. In contrast, the polarity boundary in the central serpentinized peridotite region likely reflects a rotated alteration front that was initially close to vertical. The geometry of this boundary may result from perturbed isotherms that were steeply dipping towards the ridge axis in the detachment footwall due to seawater penetration. The continuity of the polarity boundary across Kane megamullion implies that both gabbros and serpentinites acquired their magnetization close to the ridge axis and at approximately the same time.
T53B-1306
Further Geological Sampling Around the Rainbow Hydrothermal Site, Mid-Atlantic Ridge
The Rainbow hydrothermal site, at 36°14'N on the Mid-Atlantic Ridge, is one of the few known site hosted in ultramafic basement. The MOMAR DREAM cruise (July 2007, R/V Pourquoi Pas ?) combined biological and geological objectives to study the role of abundant iron in controlling geological, biological and hydrological active processes at all scales. Two Nautile dives and a dredging program were achieved to further constrain the lithology and geological structures on the seafloor at the scale of the massif that hosts Rainbow. This massif is an inside corner high of the non-transform offset between the AMAR and South AMAR second-order ridge segments, and presents the characteristic dome morphology of oceanic core complexes. The abundant sediment cover of the massif precludes continuous geological mapping and completely successful dredging. However, our limited sampling is consistent with the lithological variability encountered in other oceanic core complexes along the Mid-Atlantic Ridge. The Rainbow serpentinite basement was continuously observed to a distance of about 1 km to the south of the hydrothermal site, with serpentinites sampled along N-S trending, fault planes steeply dipping to the West. Serpentinites were also found on the northwestern, northern, and northeastern flanks of the massif. Approximately 800 m the North of the hydrothermal site, the most prominent outcrop, cut by a family of subvertical, ~ E-W faults, is at least partly made of olivine- orthopyroxene bearing gabbro. Basalts and fresh basaltic glass were also recovered in talus and sediments on the Southwest and Northeast flanks of the massif.
T53B-1307
Deep-Towed Sidescan Sonar Studies of Amagmatic Spreading Centres: the Mid-Atlantic Ridge at 13N
In areas of sparse magmatism, plate separation is accommodated predominately by tectonic processes, which is often observed in the form of long-lived detachment faults exhuming lower crustal rocks and mantle peridotites to the seafloor. During research cruise JC007, a number of these oceanic core complexes (OCCs) recently identified by Smith et al. (2006) along the Mid-Atlantic Ridge (MAR) were imaged using the National Oceanography Centres Towed Ocean Bottom Instrument (TOBI). Sidescan sonar data were collected across two active OCCs at 1320N and 1330N and one inactive OCC at 1350N, including the intervening regions. We made extensive use of 3-D Fledermaus visualisations during our interpretations, and will include these in our presentation. The sidescan sonar data show distinct similarities between the two active OCCs. They both appear as large domal detachment surfaces being exhumed from beneath the ridge axis that are capped by a complexly deformed central massif and volcanic ridge at the breakaway (as suggested by Smith et al, 2006). The structures extend for c.20km off axis and c.10km along axis, and the detachment faults are characterised by large-scale spreading-parallel bathymetric corrugations and fine scale striations (interpreted as streams of basalt derived from tectonic erosional screes imaged at the terminations). The hanging walls above the zones of emergence of the detachment faults exhibit faulting that appears to trend from the ridge axis (N and S of the OCC) towards the spreading direction (at the OCC). The neo-volcanic zones along the ridge axis are apparently absent opposite active OCC formation, which is likely indicative of predominately tectonic spreading. In the regions between the active OCCs, sidescan sonar imagery shows a clear neo-volcanic zone at the ridge axis, indicating robust magmatic plate separation and accretion. The extinct OCC is characterised by heavy sediment cover over a much shallower detachment surface, presumably due to rotation and subsequent termination of the footwall. References Smith, D.K., Cann, J.R. & Escartin, J. Widespread active detachment faulting and core complex formation near 13N on the Mid-Atlantic Ridge, Nature, v.442/27, p.440-443, 2006.
T53B-1308
Mid-Atlantic Ridge at 13-14N: Evidence of Unstable Seafloor Spreading Processes From Deep-Towed Magnetic Measurements
During cruise JC007 in March-April 2007 we recorded total magnetic field anomalies over two active and one defunct oceanic core complex (OCC) and the intervening seafloor. Measurements were made by towed magnetometer at the sea surface, and by the TOBI deep-towed vehicle approximately 400 m above seafloor, along 13 E-W lines about 60 km long and spaced 3 to 6 km apart. Sea-surface data show a fairly coherent central anomaly on most lines, though on some it is significantly displaced from the spreading axis as indicated by bathymetry and side-scan sonar data. Modelling in terms of a standard, simple (but probably unrealistic), continuous reversal sequence requires total spreading rates ranging from about 15 to 40 km/Myr with offsets of the axis up to 20 km and highly asymmetric spreading. The deep-towed data were corrected for the heading-dependent magnetic effects of the TOBI vehicle before inversion to crustal magnetisation using the 2D Parker & Huestis (1974) procedure. These results were checked by comparing with inversions of the sea-surface field, which shows similar features at reduced resolution. The deep-towed inversion results show a rather incoherent magnetisation pattern. The central magnetisation high is nowhere more than 13 km wide, only 70% of the expected width of the Brunhes here, and several profiles yield apparently negative magnetisation over areas we expect to be of Brunhes age based on sonar and bathymetry data. This may due to a combination of destruction of magnetisation by faulting (Hussenoeder at al., 1996), departure from the 2D geometry assumed for the inversions, and departure (via tectonic rotation) from the assumed constant magnetisation direction. We are now carrying out fully 3D inversions and forward modelling guided by the structural evidence provided by sidescan and bathymetry. These results will be presented and discussed in relation to the seafloor spreading history and structure of the region.
T53B-1309
Fluid flow at active oceanic core complexes, 13°N Mid-Atlantic Ridge
Oceanic core complexes (OCCs) are the result of long-lived, large displacement, low-angle detachment faults that expose lower crust and mantle rocks at slow-spreading mid-ocean ridges (MOR's). While OCCs share many structural and lithological features indicating some common tectonic processes of formation, until now there has been little constraint on whether fluid flow is related to their activity. Here, we describe recently acquired water column data and rock samples from several active OCCs (from near 13°N on the Mid-Atlantic Ridge) that reveal a history of high and low-temperature fluid flow. At the toe of the active OCC's, where the footwall emerges from beneath an uplifted wedge of fissured volcanics that forms the trailing edge of the hanging wall, massive sulphide chimneys and large volumes of mineralised talc mud indicate the passage of high-temperature hydrothermal fluids. The sulphides are a mixture of iron and copper sulphide in association with chalcedony and talc. The host rock is usually serpentinised peridotite mantle material although some greenschist diabase is also present in the form of dykes. Above the highest point of one of the active OCC's, CTD data revealed a plume of low salinity water. The plume was ~200m wide, had normal background temperature and was devoid of any particulates. We speculate that the origin of the high-temperature mineralization is hydrothermal circulation in the hanging wall, driven by intrusive volcanism injected from the subjacent neovolcanic accretion zones. The origin of the low-salinity plume is more elusive and could be a result of dehydration reactions of serpentinite to either fosterite or, with silicification, to talc. The latter mechanism would accord with the extensive outcrops of talc mud found near the OCC toe, but implies excessive volumes of rock in the reaction zone. Either way, the presence in unusual settings of fluid flow at OCC's indicates that fluid-rock reaction plays an important role at active OCC's.
T53B-1310
High Resolution Mapping of the Ashadze and Logachev Hydrothermal Fields, Mid Atlantic Ridge 13-15°N.
During the SERPENTINE cruise (feb 25 to apr. 5, 2007) on board the french research vessel Pourquoi Pas? a new multibeam bathymetric system (RESON 7125 echo sounder) was used on the ROV Victor 6000 to gather real time fine-scale bathymetry. The resolution is 5% of the altitude above the seafloor (h), and the footprint at the seafloor is 0.2% of h. The Ashadze and Logachev hydrothermal fields are located in the 13-15°N region of the Mid-Atlantic Ridge, on outcrops of serpentinized mantle-derived peridotite with interspeded gabbroic bodies. Maps at 50 m above the seafloor were done to investigate the relationships between the vent fields and their tectonic/volcanic environnement. Higher resolution mapping, 20 m above the seafloor, was done at the scale of the vent fields. These maps, which have resolutions of a few tens of centimetres, are unique tools to understand the local geological control on the vents. Our observations also emphasize the role of slope failure, and spreading-parallel or oblique structural lineaments on the fine scale topography of MAR axial valley walls. Ashadze area comprises two active vent fields located at two different levels on the western wall of the axial valley near 13°N. The Ashadze1 and 2 sites, 5 km apart, are respectively 4 km and 9 km off-axis. Active vents at Azhadze1 (4100 m) are distributed over an area about 150 m-long, along an EW-trending south-facing scarp. High resolution mapping at 20 m (450x450 m area) reveals the fine structure of sulfide mounds, as well as complex fissure arrays near the vents. Away from the vents, topography appears dominated by slope failure, with prominent landslides. Black smokers at Ashadze 2 (3260 m) are located in a crater-shaped depression, about 25 m in diameter, which lies in a narrow (about 70 m), N-S trending trough. On the high resolution maps at Ashadze 2 (800x450 m) the N-S trending trough appears bounded to the east by a faulted gabbroic body. To the west, it is limited by a narrow N-S trending ridge, 20 to 50 m-high, that bears numerous extinct hydrothermal chimneys. Logachev comprises two active vent fields located east of the ridge near 14°45"N. Logachev 1 and 2 sites, 5 km apart, are located 8 km and 12 km off-axis. We only mapped Logatchev 1, which is a large and well studied vent field on the eastern axial valley wall. It comprises many vents in a NW-trending elongated area about 400 m-long. High resolution mapping at 20m (550X750 m) reveals the circular shape of the main sulfide mounds, as well as complex arrays of scarps and fissures, oriented predominantly E-W, and NE- SW. Numerous lens-shaped slump features are also revealed near the vents.
T53B-1311
Magma Starvation, Extensive Development of Oceanic Core Complexes, and Evidence of High Degrees of Melting in a Region of Low Magmatic Production on the Mid-Atlantic Ridge at 13°-14°N
The region between 12\textdegree{}N and 16\textdegree{}N on the Mid-Atlantic is characterized by at least 10 core complexes exposing ultramafic and gabbroic rocks. We have extended sampling programs from previously surveyed northern regions from 14\textdegree{} to 16\textdegree{}N to four newly discovered core complexes (Smith et. al., 2006) and adjacent ridge segments between 14 and 12\textdegree{}30'N on a cruise of the research vessel James Cook in March-April 2007. We examine the distribution of lithologies sampled in the entire region, the newly discovered core complexes, the ridge and core complex morphotectonics along the segments, and the results of geochemical analysis of basalts, gabbroic rocks and mantle assemblages. Geochemical results from these assemblages may help to explain why the basalt characteristics can show variations symptomatic of high degrees of melting and why the mantle compositions are among the most strongly depleted along the MOR (also indicating high degrees of melting), yet the evidence of mantle unroofing and a thin magmatic crust persist throughout such a broad area. Integrated studies of major element, trace element, and isotopic variations among basalts, gabbroic rocks and igneous and residual ultramafic rocks in the region indicate that 1) the enriched basalts have positive Ta-Nb anomalies, enriched relative to La and Th, 2) basalts have relatively high SiO2 abundances compared to the global average, 3) basalts show a HIMU isotopic influence, and 4) bulk major element abundances and mineral chemistry in mantle rocks indicate that they are among the most depleted, although variably refertilized, residual mantle assemblages sampled to date along MORs. We suggest that much of the regional variation in major and trace element data, as well as isotopic data and the unusual regional geology (multiple core complexes and ridge morphotectonics) reflecting magma supply can be explained by melting of a sub-axial mantle that contains two end members, one highly depleted and the other enriched. These components appear to involve ancient recycled ocean crust and lithospheric mantle.
T53B-1312
Trace Element Study of MORB Glasses from 14°-16°N along Mid-Atlantic Ridge by LA-ICP- MS
A comparison of 20 MORB glasses from 14\textdegree{}-16\textdegree{} N along the Mid-Atlantic Ridge using both solution-based and in situ laser ablation-based ICP-MS trace element analyses on the same samples was conducted. Li, Be, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, Eu, Tb, Gd, Dy, Ho, Er, Tm, Yb,Lu, Hf, Ta, Pb, Th, and U were analyzed using the Varian 810 quadrupole ICP-MS. The instrument features a 90 degree ion mirror and low noise double-off-axis quadrupole that allows high sensitivity and low backgrounds. Precision in term of relative standard deviation (RSD) of the measurements for both methods based on repeated analyses of USGS BIR-1G and BHVO-2G glass standards and Max Planck KL-2G glass standard is within 5 % for all trace elements with the exception of Pb, which averaged 12 %. Measured trace element abundances are within 2% of recommended standard values using both solution and laser ablation methods. Comparison between the analyte concentrations obtained by solution-based ICP-MS and in situ microanalysis by laser ablation reveals little systematic differences in abundances(\begin{math}<\end{math}5% for all elements). The two-method correlation and strong repeatability of the results indicate that rapid in situ trace element analysis by laser ablation ICP-MS is likely to become a preferred method of trace element analysis for MORB glasses. Our geochemical results and previous studies of MORB glasses in the region of the MAR between 14\textdegree{}-16\textdegree{}N show that basalts are characterized isotopic and incompatible element enrichment.The nature of the enrichment has been the topic of significant discussion and speculation because a specific mantle plume is not well defined in the region. Likewise the magma supply is probably small in the region as the magmatic crust is interpreted to be very thin in most of the area studied. Integrated studies of major element, trace element, and isotopic variations among basalts, gabbroic rocks and igneous and residual ultramafic rocks in the region indicate that 1) the enriched basalts have positive Ta-Nb anomalies, enriched relative to U, Th, and La 2) basalts have relatively high SiO2 abundances compared to the global average, 3) basalts show a HIMU isotopic signature, and 4) bulk major element abundances and mineral chemistry in mantle rocks indicate that they are among the most depleted,although variably refertilized, residual mantle assemblages sampled to date along MORs.We suggest that much of the regional variation in major and trace element data, as well as isotopic data and the unusual regional geology (multiple core complexes) can be explained by melting of a sub-axial mantle that contains two end members, one highly depleted and the other enriched. These components appear to involve ancient recycled ocean crust and lithospheric mantle.
T53B-1313
Hydrothermal plume anomalies recorded during a 2007 Chinese cruise to the Indian Ocean
During January-May 2007 three legs of the Chinese research cruise DY115-19 have been carried out in the Indian Ocean on board the Chinese R/V ¡°Dayang Yihao" to investigate hydrothermal vents at mid-ocean ridges and to explore their mineral deposits. The cruise aimed to find new hydrothermal plumes that will lead to the discovery of new hydrothermal vent fields. Here we report observations of the Miniature Autonomous Plume Recorders (MAPRs), which were attached to the tow cables at designed water depths and have been proven to be a very effective tool for finding new hydrothermal plumes. At the SEIR near the St. Paul/Amsterdam plume we detected a similar hydrothermal anomaly, which was reported at the same location about ten years ago. The most significant hydrothermal plume anomalies were observed at the ultra-slow spreading Southwest Indian Ridge, where the largest turbidity readings were about 100 times the reported readings of a regular MAPR anomaly along mid- ocean ridges and up to 0.12°C temperature anomalies were also recorded by MAPRs. These observations play an important role in locating the new discovered hydrothermal vents at the site. A new hydrothermal plume anomaly was also detected near the vent site of Kairei at the Central Indian Ridge.
T53B-1314
IODP site survey for drilling Godzilla Mullion: preliminary report of R/V Hakuho KH07-2 Leg 2 & 4 cruise
Miocene seafloor in the Parece Vela Basin (PVB) in the Philippine Sea is characterized by rugged topography and mantle exposure, indicating a weak magma supply in an extinct backarc basin. The PVB is unique in that the worldfs largest oceanic core complex (OCC), Godzilla Mullion, occurs here along a fast/intermediate-spreading ridge. A proposal to drill Godzilla Mullion is currently in the IODP review system. A total of 13 expeditions were conducted in the PVB so far. These expeditions included the recent dredging cruise KR03-01, submersible dive cruise YK04-05-Leg 4, and the seismic study cruises by the Japanese government for the Law of the Sea project. An IODP site survey was conducted during August to September 2007 as a two-leg cruise (KH07-2-Leg 2 and 4) of R/V Hakuho in the PVB. The primary purpose of the cruise was to characterize Godzilla Mullion in more detail and to locate better drill sites on it. The primary survey item was multiple dredge hauls (12 hauls have been completed by the end of Leg 2) on Godzilla Mullion so as to map lithological variations of the OCC footwall in more detail. One dredge haul (D5) recovered fresh glassy basalts without gabbros and peridotites from a possible breakaway of an individual dome-like high on Godzilla Mullion, suggesting a possibility that Godzilla Mullion may have evolved through multiple detachment events, not a single large detachment. A deep-towed proton magnetometer survey along the flow line of Godzilla Mullion was also conducted to better characterize the exhumation history of the OCC.
T53B-1315
Boron in mid-Ocean Ridge Mantle Rocks
B is potentially a key element to study in mid-ocean ridge mantle-derived rocks, because it is thought to behave similarly to volatile species in the Earth's mantle (ie water) during subduction and recycling. The majority of studies of boron therefore have been carried out in convergent plate boundaries. The geochemical behavior and concentration of boron in the mid-ocean ridge upper mantle however, are not directly constrained. Boron is considered a moderately to highly incompatible element. Our knowledge of its mantle concentration and partitioning is largely derived from studies of basalt. Its concentration in the non-arc upper mantle is estimated to be 0.1ppm. MORB glasses average 0.34-0.72 ppm and 0.54-1.54 ppm for N-MORB and E-MORB respectively. Literature measurements on mantle minerals from orogenic peridotites show clinopyroxene to have the greatest affinity for B. We have measured concentrations of B, Li and Be on mantle minerals from abyssal peridotites and basalt glasses from Gakkel Ridge, Arctic Ocean. Preliminary data acquired using both quadrupole and sector field LA-ICP-MS seem to contradict many of the previously held assumptions about the concentration and behavior of B in the mid-ocean ridge upper mantle. Higher than expected bulk rock B concentrations of at least 5.6 ppm for the abyssal peridotites were estimated based on the B concentration in mantle silicates and their modal percentage in peridotites. Slightly higher than expected average B concentrations of 3.3 ppm in basalt glasses may be due to the relatively enriched nature of Gakkel Ridge. On both instruments B was also found to be more compatible in olivine than in clinopyroxene. Although these results agree with each other, measurements on additional samples and new data reduction methods will be tested to determine their validity.
T53B-1316
Cooling Rates of Mantle Peridotites Estimated from Lithophile Trace Element Diffusion in Orthopyroxene
Cooling rates of ocean floor mantle rocks from mid-ocean ridges can potentially provide important information about ridge dynamics, emplacement mechanisms and mantle uplift. There are a growing number of geospeedometric methods to retrieve such cooling rates in various settings. However, few exist for typical four- phase mantle peridotites and they only cover temperatures below 800° C. The down-temperature lithophile trace element exchange between clinopyroxene (cpx) and orthopyroxene (opx) can provide such a high- temperature spinel peridotite geospeedometer. Orthopyroxenes studied by SIMS from two fresh Gakkel Ridge peridotites are zoned in all trace elements while clinopyroxenes are homogeneous. This allows the calculation of equilibrium temperatures [1]. Several profiles in opx cover a range of 1250° C (opx core) to 800° C (opx rim) and are in agreement with straightforward diffusion and closure temperature models. The systematics of REE diffusion in opx deviate from the results of a recent experimental study [2]. The data allow us to estimate diffusion systematics of 16 elements (REE and TE) and their cation distributions in orthopyroxene. The data set is internally coherent as all elements were subjected to the same extrinsic parameters. 1. Decreasing ionic radius increases REE diffusion in opx (as it does in cpx). 2. M2-site diffusion is controlled more by ionic radius than by cationic charge. 3. M1-site diffusion is controlled by both ionic radius and cationic charge. 4. M1-site diffusion is generally slower than M2-site diffusion for isovalent cations, most likely because of higher M1- site energies compared to M2-site. The advantages of this geospeedometer should be its relatively good precision, use of standard analytical methods and its coverage of the important range between solidus temperatures and 800° C. In combination with other geospeedometers it will be possible to retrieve the continuous cooling history of a mantle rock from its solidus down to low temperatures. [1] G. Witt-Eickschen & H. O'Neil (2005): Chemical Geology, 65 – 101. [2] D.J. Cherniak & Y. Liang (2007): Geochimica et Cosmochimica Acta, doi 10.1016/j.gca.2006.12.001.
T53B-1317
The Generation of Oceanic Lithosphere in an Embryonic Oceanic Crust : the Example of the Chenaillet Ophiolite in the Western Alps
The Chenaillet Ophiolite exposed in the Franco-Italian Alps represents a well-preserved ocean-floor sequence that was only weakly affected by later Alpine convergence. Based on the similarity between rock types and structures reported from ultraslow spreading ridges and those observed in the Chenaillet Ophiolite, it may represent a field analogue for slow to ultraslow spreading ridges such as the Gakkel Ridge or the Southwest Indian Ridge. Mapping of the Chenaillet Ophiolite enabled to identify an oceanic detachment fault that extends over a surface of about 16 km2 capping exhumed mantle and gabbros onto which clastic sediments have been deposited. The footwall of the detachment is formed by mafic and ultramafic rocks. The mantle rocks are strongly serpentinized lherzolites and subordinate harzburgites and dunites. Microstructures reminiscent of impregnation, and cpx major and trace element chemistry indicate that spinel peridotite is (locally) replaced by plagioclase-bearing assemblages. Pyroxene thermometry on primary minerals indicates high temperatures of equilibration ( max 1200°C) for the mantle rocks. Gabbros range from troctolite and olivine-gabbros to Fe-Ti gabbros and show clear evidence of syn-magmatic deformation, partially obliterated by retrograde amphibolite and low-grade metamorphic conditions. In sections perpendicular to the detachment within the footwall, syn-tectonic gabbros and serpentinized peridotites grade over some tens of meters into cataclasites that are capped by fault gouges. Petro-structural investigations of the fault rocks reveal a syn-tectonic retrograde metamorphic evolution. Clasts of dolerite within the fault zone suggest that detachment faulting was accompanied by magmatic activity. Hydrothermal alteration is indicated by strong mineralogical and chemical modifications. Gabbro and serpentinized peridotite, together with serpentinite cataclasites occur as clasts in tectono-sedimentary breccias overlying directly the detachment fault. Across the whole Chenaillet Ophiolite, volcanic rocks directly overlie either the detachment fault or the sediments. In several places, N-S trending high-angle normal faults have been mapped. These faults truncate and displace the detachment fault leading to small domino-like structures. The basins, limited by these high-angle faults, are some hundreds to a few kilometres wide and few tens to some hundreds of meters deep. Because these high- angle faults are sealed locally by basalts and obliterated by volcanic structures, we interpret them as oceanic structures being active during the emplacement of the basalts. The alignment of porphyritic basaltic dykes parallel to, and their increasing abundance towards the high-angle faults suggest that they may have served as feeder channels for the overlying volcanic rocks. The complex poly-phase tectonic and magmatic processes observed in the Chenaillet Ophiolite are reminiscent of those reported from slow to ultraslow spreading ridges. The key result from our study is that mantle exhumation along detachment faults is followed by syn-magmatic normal faulting resulting in the emplacement of laterally variable, up to 300 meters thick massive lavas and pillow basalts covering the exhumed detachment fault. This implies that off-axis processes are more important as previously assumed and that large-scale detachment faults may be buried under massive volcanic sequences suggesting that detachment faulting is presumably more common than suggested by dredging or morpho-structural investigations of ultra- to slow- spreading oceanic crust.
T53B-1318
Oblique, ultra-slow Australian-Antarctic spreading mechanism explains lateral differences in transitional crust
A zone of transitional crust, up to 120 km in width, separates continental and oceanic crust on the Southern Australian and East Antarctic conjugate margins. Offshore southern Western Australia the transition zone is composed of the very rough E-W oriented ridges of the Diamantina Zone, while further east the crust changes to be composed of rough tilted basement blocks. A matching pattern exists in the conjugate Antarctic transition zone. The different types of transitional crust formed during a period of oblique northwest-southeast relative Australian-Antarctic plate motion that occurred until 50 Ma, ~30 million years longer than previously thought (Whitttaker et al., Major Australian-Antarctic plate reorganization at Hawaiian-Emperor bend time, Science, in press). During this period relative motion was slower and more oblique at the western end of the rift system compared to the east due to relative counter-clockwise motion of Australia. We have established a clear relationship between the rate and obliquity of plate motions and the roughness of the crust. The ultra-slow and very oblique motion in the west corresponds to the very rough E-W oriented ridges of the Diamantina Zone, while the slightly faster and less oblique motion in the east corresponds with the less rough, tilted fault block fabric of the central Australian Bight. Along the entire margin the boundary between the rough basement and normal ocean floor corresponds with the change in relative Aus-Ant plate motions from NW-SE to N-S at ~50 Ma.