Christine Laverne

Drilling to gabbro in intact ocean crust

Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D.

Downhole variation of lithium and oxygen isotopic compositions of oceanic crust at East Pacific Rise, ODP Site 1256

Bulk rock lithium and oxygen isotope compositions from ODP Site 1256 were analyzed to investigate the seawater circulation in the upper oceanic crust formed at the East Pacific Rise (EPR). The upper extrusive basalts have ?18O values from +6.1‰ to +9.2‰, reflecting alteration of oceanic crust by seawater at low temperatures (<200–250°C). Bulk rocks from the sheeted dike complex and plutonic section have overall lower ?18O values (+3.0‰–+5.5‰). In the sheeted dike complex bulk rock ?18O values gradually decrease with depth, and then increase toward the fresh MORB ?18O value after reaching a minimum of +3.0‰ at ?1350 m below seafloor (mbsf). The entire sampled crust is dominated by rocks with low lithium contents relative to fresh MORBs except for a few localized Li enrichment. The upper volcanic zone is characterized by a spread of ?7Li from low to high values relative to average unaltered MORB values (?7Li = +3.4 ± 1.4‰). The presence of rocks with low ?7Li values in the upper crust most likely indicates zones of upwelling of relatively hot (?200–250°C) hydrothermal fluids. In the sheeted dike complex, bulk rock ?7Li values show wide range of variation, but exhibit a general trend from enriched to depleted values at ?1280 mbsf and then return to that for fresh MORB within the upper tens of meters of the plutonic section at the bottom of the after reaching a minimum at ?1350 mbsf (?7Li = ?1.6‰). The downhole pattern of ?7Li principally reflects variations in water-rock ratio (w/r) together with a downhole increase of temperature. Seawater flow in the upper volcanic zone is likely to be channeled with generally small but variable w/r ratios. The w/r ratios increase rapidly with depth in the lower volcanic section into the sheeted dike complex indicating water dominated pervasive hydrothermal flow due to intensive upwelling of hydrothermal fluids.

Nitrogen content and isotopic composition of oceanic crust at a superfast spreading ridge: A profile in altered basalts from ODP Site 1256, Leg 206

The present paper provides the first measurements of both nitrogen content and isotopic composition of altered oceanic basalts. Samples were collected from Ocean Drilling Program Site 1256 located at the eastern flank of the East Pacific Rise. Twenty-five samples affected by low temperature alteration were analyzed. They include moderately altered basalts together with veins and related alteration halos and host rocks, as well as unique local intensely altered basalts showing green (celadonite-rich) and red (iron oxyhydroxide-rich) facies. Nitrogen contents of moderately altered basalts range from 1.4 to 4.3 ppm and are higher than in fresh MORB. Their d 15 N values vary in a large range from +1.6 to +5.8%. Veins, halos, and host rocks are all enriched in N relative to moderately altered basalts. Notably, veins show particularly high N contents (354 and 491 ppm) associated with slightly low d 15 N values (+0.4 and A2.1%). The intensely altered red and green facies samples display high N contents of 8.6 and 9.7 ppm, respectively, associated with negative d 15 N values of A3.8 and A2.7%. Detailed petrological examination coupled with N content suggests that N of altered basalts occurs as ammonium ion (NH 4 +) fixed in various secondary minerals (celadonite, K-and Na-feldspars, smectite). A body of evidence indicates that N is enriched during alteration of oceanic basalts from ODP Site 1256, contrasting with previous results obtained on basalts from DSDP/ODP Hole 504B (Erzinger and Bach, 1996). Nitrogen isotope data support the interpretation that N in metasomatizing fluid occurred as N 2 , derived from deep seawater and likely mixed with magmatic N 2 contained in basalt vesicles.

Chemical fluxes during hydrothermal alteration of a 1200-m long section of dikes in the oceanic crust, DSDP/ODP Hole 504B

Abstract Chemical interactions between seawater and the oceanic crust have been widely investigated during recent years. However, most of these studies concern the uppermost volcanic part of the crust. The contribution of the underlying sheeted dike complex to the global budget of the oceans is inferred solely from some ophiolite studies and from the 500-m high-level dike section of DSDP/ODP 504B which was drilled in 1981. Hole 504B is the only place where a continuous and long (1260 m) section in the sheeted dike complex has been cored, and it is now regarded as a reference section for the upper oceanic crust. Many petrological and chemical data from these dolerites are available, including the relative proportions of veins, extensively altered adjacent rocks, and less altered “host-rocks”. For these three reasons, considering the entire dike section penetrated by Hole 504B is a unique chance to study chemical fluxes related to hydrothermal alteration of this part of the oceanic crust. The calculation of any chemical flux implies knowledge of the chemical composition of the fresh precursor (protolith). Previously, mean compositions of glasses (=P1a) or basalts from the Hole 504B volcanics have been used as protoliths. In this paper, we calculate and discuss the use of various protoliths based on dolerites from Hole 504B. We show that the most adequate and realistic protolith is the mean of individual protoliths that we calculated from the acquisition, by automatic mode, of about 1000 microprobe analyses in each thin-section of dolerite from the Hole 504B lower dikes. Consequently, PFm is further used to calculate chemical fluxes in the dike section of Hole 504B. The chemical compositions of the host-rocks adjacent to alteration halos tend to converge to that of PFm with depth, except for Fe2O3t and TiO2. Because the volume percent of alteration halos increases with depth, the total fluxes related to these halos increase with depth. This explains why the mean flux (host-rocks+halos+veins) of the upper dikes is roughly similar to the mean flux of the lower dikes. During the alteration of the entire Hole 504B dike section, the dolerites gained relatively large quantities of Fe2O3t (+4.0 g/100 cm3) and released much SiO2 (−6.8 g/100 cm3), CaO (−5.8 g/100 cm3), and TiO2 (1.6 g/100 cm3), and minor Al2O3 (−0.7 g/100 cm3) and MgO (−0.7 g/100 cm3). We show the importance of the choice of the protolith in the calculation of chemical budget, particularly for elements showing low flux values. In Hole 504B, the Mg uptake by the volcanics during low temperature alteration added to the Mg release by the dikes gives a net flux of −0.07×1014 g/year. We propose that part of the Mg uptake by the oceanic crust, which is necessary to compensate the rivers input (−1.33×1014 g/year), occurs in the underlying gabbros and/or in sections which are altered such as Trinity and Troodos ophiolites. Compared with ophiolites, fluxes calculated for elements other than Mg for the entire crust are generally similar (in tendency, if not in absolute value) to that we obtained from Hole 504B.

Hydroschorlomite in altered basalts from Hole 1256D, ODP Leg 206: The transition from low-temperature to hydrothermal alteration

Hydroschorlomite, a Ti-, Ca-, Fe-rich andraditic garnet present in the deepest cores of basalts (661–749 mbsf) drilled in Hole 1256D during Ocean Drilling Program (ODP) Leg 206 (equatorial east Pacific), is reported here for the first time in oceanic crust. Detailed petrological and mineralogical studies by optical microscope, electron microprobe, scanning and transmission electron microscope, and micro-Raman spectroscopy are used to characterize this hydrogarnet and its relationships with other minerals. Hydroschorlomite occurs in Hole 1256D as small (5–50 μm) anhedral or euhedral crystals associated either with celadonite in black halos adjacent to celadonite veins or with brown saponitic phyllosilicate in brown alteration halos adjacent to veins of saponite and iron oxyhydroxides. Both types of halos are formed at low temperature (less than about 100°C). Textural observations suggest that hydroschorlomite formation is contemporaneous with the phyllosilicates. Hydroschorlomite is rich in CaO (22.5–26.5 wt%), TiO2 (22.0–28.6 wt%), and FeOt (6.2–12.9 wt%) and contains significant F (up to 0.85 wt%) and Zr2O3 (up to 0.34 wt%). The presence of OH suggested by the low total percentages of oxides (95.2–97.3 wt%) is confirmed by the OH vibration at 3557 cm−1 in the micro-Raman spectrum. Chemical mapping indicates that hydroschorlomite is not zoned and is always associated with either celadonitic or saponitic phyllosilicates. Some hydroschorlomite crystals partly include tiny (<10 μm) skeletal titanomagnetite. The occurrence of hydroschorlomite in Hole 1256D basalts coincides with a general downward increase in temperatures and overall intensity of alteration manifest by the alteration of plagioclase and the occurrence of small amounts of mixed-layer chlorite-smectite. The titanium necessary to form hydroschorlomite is provided by the breakdown of primary tiny (<10 μm) titanomagnetite, while calcium is provided by the replacement of plagioclase by albite. Hydroschorlomite is thus an indicator of alteration of titanomagnetite under conditions transitional from low-temperature alteration to hydrothermal metamorphism with formation of titanite and may affect magnetic properties of the rocks.

Multi-scalar structure at DSDP/ODP Site 504, Costa Rica Rift, III: faulting and fluid circulation. Constraints from integration of FMS images, geophysical logs and core data

Abstract Downhole geophysical logs and high-resolution electrical images (FMS) from DSDP/ODP Hole 504B are analysed in combination with core data to obtain an integrated description of oceanic faults met in the hole. About 34 500 fractures were mapped from FMS images over 1672 m of basement. The fracture distribution from FMS confirms the presence of a main fault zone between 800 and 1100 mbsf (metres below sea floor), elsewhere detected from seismic data as well as magnetic, acoustic, and electrical resistivity measurements. The fracture density profile reveals the presence of two other highly fractured zones, (1) between 400 and 575 mbsf and (2) close to the bottom of Hole 504B (1700 to 2100 mbsf). Consequently, we infer that Site 504 was submitted first to an extensional stress regime near the ridge axis, with circulation of high-temperature fluids and pervasive alteration of the basalts. This initial phase is associated with the main fault met in Hole 504B. Similar but less developed deformation was generated off-axis, with lower-temperature parageneses, such as that cored between 400 and 575 mbsf. The present compressional to strike-slip stress regime is expressed in subhorizontal fracturing detected in discrete zones, such as within the main fault zone and the lower fracture zone (1700 to 2100 mbsf) in Hole 504B.

Analyse de la déformation finie et signification des lentilles ultrabasiques des Maures occidentales (Var, France). Implications géodynamiques

Abstract The first structural study of ultramafic lenses from the Variscan Maures massif allows definition of the finite strain ellipsoid in relation to a tectonic event later than the Visean nappe tectonics and prior to the Namurian late-orogenic extension, which are characterized by a NW–SE-striking stretching lineation transverse to the belt. The ultramafites present a longitudinal NNE–SSW stretching lineation, associated with a sub-vertical flattening plate and essentially symmetrical structures. P–T conditions of deformation can be compared with those of the regional metamorphism. The en echelon structure of serpentinite sites can be related to a ductile strike-slip fault.

Multi-scalar structure at DSDP/ODP Site 504, Costa Rica Rift, II: fracturing and alteration. An integrated study from core, downhole measurements and borehole wall images

Abstract We used a database derived from the integration of core material and geophysical downhole measurements in order to investigate the relationships between fracturing and alteration in the volcanic section of DSDP/ODP Hole 504B. The studied crustal section (from top of the basement to 1000 mbsf (metres below sea floor)) consists of low resistivity/high porosity pillow lavas associated with breccias and rubble material, alternating with high resistivity/low porosity massive basalt flows. A positive correlation between DLL (Dual Laterolog)-derived porosity and occurrence of breccias in the core suggests that breccias more than fractures contribute to the electrical resistivity signal. A structural analysis performed from core suggests that most fractures and veins are steeply dipping, and may represent tectonic features or cracks due to contractional cooling of the crust, the latter being more abundant in pillows. Fractures and veins recorded on core tend to be clustered in massive units or thin flows. This result may derive from criteria adopted during structural measurements and must be taken with care. The natural radioactivity (GR) profile delineates two main alteration zones in the volcanic section: an oxidizing zone with increased potassium above, and a reducing one without K gain below. Most of the GR maxima are found to be correlated with celadonite-bearing alteration halos. GR minima are frequently located at the boundaries between domains of contrasting fracture orientation, where metasomatic reactions may have occurred due to contrasting permeability.