Hole 1256D Research Articles

P30 高速拡大海嶺系の上部海洋地殻構造を決める要因について : 深海掘削第1256D孔からの考察(ポスターセッション)

P30 高速拡大海嶺系の上部海洋地殻構造を決める要因について : 深海掘削第1256D孔からの考察(ポスターセッション)

Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization of a dynamic melt lens at fast spreading ridges

The transition between the small melt lens observed on top of fast spreading ridge magma chambers and the overlying sheeted dike complex marks the interface between magma and the hydrothermal convective system. It is therefore critical to our understanding of fast spreading ridge accretion processes. We present maps of two areas of the Oman ophiolite where this transition zone is observed as continuous outcrops. Our observations, which include the base of the sheeted dike being crosscut by gabbros, are consistent with episodic dike injections in a steady state model but also suggest that the root of these dikes is commonly erased by vertical movements of the top of the melt lens. Dike assimilation is a possible mechanism for incorporating hydrated phases, which result from hydrothermal alteration, to the melt lens during upward migrations of its upper boundary. Upward migrations are also responsible for a granoblastic overprint of the root of the dikes that is also observed in the stoped diabase xenoliths. This granoblastic overprint attests to reheating of previously hydrothermally altered lithologies which can even trigger hydrous partial melting due to the lowering of the solidus of mafic lithologies by the presence of a water activity. Clinopyroxenes present in these granoblastic lithologies are typically low in Ti and Al content, thus strongly contrasting with corresponding magmatic clinopyroxene. This may attest to the recrystallization of clinopyroxenes after amphiboles under the peculiar conditions present at the root zone of the sheeted dike complex. Downward migrations of the top of the melt lens result in the crystallization of the isotropic gabbros at its roof, which represent the partly fossilized melt lens. Melt lens fossilization eventually occurs when magma supply is stopped or at the melt lens margins where the thermal conditions become cooler. Melt lens migration, recrystallization of hydrothermally altered sheeted dikes during reheating stages, and assimilation processes observed in the Oman ophiolite are consistent with the observations made in IODP Hole 1256D. We propose a general dynamic model in which the melt lens at fast spreading ridges undergoes upward and downward movements as a result of either eruption/replenishment stages or variations in the hydrothermal/magmatic fluxes.

Deformation pattern in a massive ponded lava flow at ODP‐IODP Site 1256: A core and log approach

A complete “in situ” section of upper oceanic crust, from extrusive lavas, through dikes into gabbros has been recently drilled for the first time in 15 Ma old crust created at the superfast spreading East Pacific Rise (EPR). The upper igneous basement cored at Ocean Drilling Program (ODP)–Integrated Ocean Drilling Program (IODP) Holes 1256C and 1256D consists of sheet flows and massive flows, including a ponded lava flow unit (“lava pond”) interpreted as a thick lava flow delivered off‐axis and accumulated in a topographic depression. Fine‐scale structural analysis shows that the lava pond is much less fractured than the deeper lavas, probably as the effect of structure spacing that is larger than in the underlying lava flows. In Hole 1256D, structures are mainly subhorizontal in the lava pond and become steeper downhole, toward the sheeted dike complex. This reflects the distance of the lava pond from the stress field produced by the intrusion of dikes at depth, inducing mostly subvertical eruptive fractures and fracture network, and the distance of the site from active rifting at the EPR. Contrasting structural patterns in the lava pond and in the deeper parts of the drilled section are reflected in contrasting physical properties patterns, as obtained by shipboard and downhole logging data. Electrical resistivity, compressional velocity, and bulk density are generally higher, whereas natural radioactivity and neutron porosity are lower in the lava pond than in the deeper hole. The occurrence of a thick massive lava flow at the top part of the volcanic section affects the deformation pattern of the entire drilled crust and, consequently, its permeability regime which, in turn, controls the geometry and distribution of hydrothermal circulation and basalt alteration.

Determination of the volcanostratigraphy of oceanic crust formed at superfast spreading ridge: Electrofacies analyses of ODP/IODP Hole 1256D

The objective of this study is to construct a representative volcanostratigraphy of Ocean Drilling Program Hole 1256D, the first complete penetration of intact upper oceanic crust formed at a superfast spreading rate. An accurate knowledge of the volcanostratigraphy is vital to understand processes of crustal construction and submarine magmatism and to estimate chemical exchange with seawater, but this is rarely achieved due to very low recovery rates in most basement holes. We used two approaches to determine the rock types that form the wall rocks in the basement sections of Hole 1256D: (1) user guided interpretations of electrofacies acquired by imaging tools combined with other wireline tools; and (2) the use of an artificial neutral network to objectively classify the responses of all available logging information. Great availability of formation microscanner (FMS) images provided superior coverage of the borehole wall compared to previous attempts at core‐log integration. This has resulted in more confident and detailed lithologic classifications, such as with the distinction between pillows and different styles of breciation. Ten lithology types are suggested for a volcanostratigraphy model: massive flows, ponded lava, fractured massive flows, fragmented flows, thin flows or thick pillows, pillows, breccias, dikes in dike complex, isolated dikes, and gabbros. Three major lithology types in the extrusive section are massive flows (both massive and fragmented massive flow, 22%), fragmented flows (32%), and breccias (19%). Pillow lavas make up only 1.9% of the volcanic section and are confined to a 100 m interval. Below the extrusive section, subvertical contacts interpreted to be dike margins are typically observed every 1 to 2 m with brecciated zones along the contacts. The dikes dip steeply to the northeast indicating slight rotation away from the ridge axis. We used an artificial neural network (ANN) approach to determine a quantitative lithostratigraphy. The ANN is most strongly influenced by porosity and alteration degrees and the resulting stratigraphy most closely resembles the above classifications when clustered by FMS texture as opposed to lithologic interpretation. The ANN thus provides a porosity‐based stratigraphy of the basement rather than the traditional lithology‐based stratigraphy.

Velocity and attenuation in young oceanic crust: New downhole log results from DSDP/ODP/IODP Holes 504B and 1256D

To characterize the acoustic properties of oceanic crust and investigate the nature of the oceanic layer 2/3 seismic boundary, we analyze sonic logging waveforms recorded in the two deepest holes ever drilled in intact oceanic crust: Ocean Drilling Program (ODP)/Integrated Ocean Drilling Program (IODP) Hole 1256D and Deep Sea Drilling Program (DSDP)/ODP Hole 504B. Hole 1256D penetrated 15 Ma old crust on a superfast spreading segment of the Cocos plate, while Hole 504B was drilled in a ∼6 Ma old section of an intermediate spreading segment of the Nazca plate. Careful reprocessing of different sets of waveforms allows us to draw depth profiles of the primary parameters controlling wave propagation in the crust: compressional and shear velocities (Vp and Vs), and attenuation. The results underline the importance of proper filtering and processing parameters for the acquisition of acoustic logs in hard rocks. While the reprocessed Vp logs improve previous data, particularly at the bottom of Hole 1256D, the Vs logs represent significant enhancements in quality and in extent of previous profiles in the two holes. Attenuation results provide the first measure of intrinsic attenuation in these holes. The velocity and attenuation logs confirm that Hole 504B had reached oceanic layer 3 without recovering gabbros. Conversely, while gabbros were recovered in Hole 1256D, no layer 3 velocity values were recorded in the interval logged. However, the reprocessed data suggest that the transition to layer 3 might occur within the deepest 87 m of the hole that were not logged or immediately below. Accordingly, the layer 2/3 seismic boundary does not appear to be related to the top of the plutonic crust. Despite different ages, spreading rates, and mineralogy, however, the two sites display remarkably similar velocity and attenuation profiles through the entire crustal sections logged, suggesting that they can be reliably considered representative of the acoustic properties of the Nazca/Cocos spreading system and more generally of undisturbed oceanic crust.

Boron and chlorine contents of upper oceanic crust: Basement samples from IODP Hole 1256D

Boron, Cl, Ti, K, Sm, and Gd concentrations of oceanic crust samples were determined by prompt gamma neutron activation analysis. The samples include 63 basalts and 5 gabbros obtained at various depths (369–1503 m below seafloor) from Hole 1256D during Ocean Drilling Program Leg 206 and Integrated Ocean Drilling Program Expedition 309 and 312. The results show stepwise downhole variations, B content and B/K decrease, and Cl content and Cl/K increase with increasing depth. High B content (>1.5 ppm) in shallower rocks (extrusive lavas) is probably achieved by low‐temperature seawater alteration, and high Cl contents (up to 1400 ppm) in the deeper rocks (plutonic section) would show result of high‐temperature hydrothermal alteration. Average B and Cl values for the upper oceanic crust in Hole 1256D are 1.5 ppm and 334 ppm, respectively.

Velocity structure of upper ocean crust at Ocean Drilling Program Site 1256

We examine shipboard physical property measurements, wireline logs, and vertical seismic profiles (VSP) from Ocean Drilling Program/Integrated Ocean Drilling Program Hole 1256D in 15 Ma ocean crust formed at superfast spreading rates to investigate lateral and vertical variations in compressional velocity. In general, velocities from all methods agree. Porosity is inversely related to velocity in both the logging and laboratory data. We infer that microfracturing during drilling is minor in the upper 1 km of basement, probably due to connected pores and, thus, low effective stress. The closure of porosity to very low values coincides with the depth below which laboratory velocities diverge from logging velocities. We infer that porosity controls velocity in layer 2, lithostatic pressure controls the thickness of seismic layer 2, and the distribution of flow types determines seismic velocity in the upper 200 m of basement. In the sheeted dikes, changes in physical properties, mineralogy, and chemistry define clusters of dikes.

Depth‐shifting cores incompletely recovered from the upper oceanic crust, IODP Hole 1256D

Seafloor drilling operations, especially those in crustal rocks, yield incomplete recovery of drilled sections, and depths of the recovered core pieces are assigned with some uncertainty. Here we present a new depth‐shifting method that is simple and rapid, requires little subjective input, and is applicable to any core‐log integration problem where sufficient comparable data have been collected in both the open hole and from the recovered core. Over the depth range for which both core and log data have been collected, an automatic algorithm selected the best new depth for each piece. The criteria for determining the best depth were as follows: (1) find new depths for as many pieces as possible, and (2) minimize the difference between core density and log density. In this study, depth‐shifting is applied at Integrated Ocean Drilling Program (IODP) Hole 1256D, which is our first opportunity to study a section of intact, in situ upper ocean crust drilled down to gabbro. The new depths significantly improve the agreement between an independent data set and the logging record.

海洋地殻の構造と形成プロセス

Recent progress of studies on oceanic crust is reviewed focusing on the architecture and its formation process. Although crustal thickness does not vary with spreading rates with the exception of very-slow spread oceanic crust, architectures vary with spreading rates. The thickness of the upper oceanic crust (lava and sheeted dike complex) decreases with increasing spreading rates at intermediate to ultra-fast spread oceanic crusts. This implies that the gabbroic section thickens with spreading rates. The lava layer seems to become thicker, while sheeted dike complex becomes thinner with spreading rates. Thus, a systematic change of the crustal structures is expected with spreading rates for intermediate to ultra-fast spread oceanic crusts. However, such systematics are not followed at a slow to ultra-slow spread oceanic crust, where the sheeted dike complex might occur only locally. Therefore, the magmatic systems controlling the formation of the crustal architecture differ significantly between slow to ultra-slow and intermediate to ultra-fast spread oceanic crusts. The difference is also shown in lower oceanic crusts, i.e., small and isolated numerous gabbroic intrusions at slow to ultra-slow spread crusts, and thick successive gabbroic layer at intermediate to ultra-fast spread oceanic crusts. Recent dense sampling from extensive areas near the East Pacific Rise and drilling at Hole 1256D show that off-axis magmatism plays an important role to in making the lava layer thicker. It should be emphasized that the upper succession in the lava layer might correspond to magmatism from on-axis to off-axis toward the upside. Therefore, vertical variations in the upper portion of the lava layer show a lateral igneous variation from on-axis to off-axis magmatism. Another important result obtained at Hole 1256D is the appearance of granoblastic dikes that are recrystallized under high temperature conditions up to pyroxene hornfels facies. Petrographical and petrological observations suggest that dehydration partial melting occurred in the metamorphosed dikes due to invasion by gabbros. Gabbroic rocks recently obtained from Hole U1309D of the Atlantis massif near the Mid-Atlantic Ridge, give a new constraint for the formation process of slow spread oceanic crusts. Down hole variation at this site confirms that the lower crusts of slow spread crusts are composed of many gabbroic intrusions. However, it is noted that the most primitive gabbroic rocks such as troctolite occur in the upside of the drill hole as was the case for Hole 735B of the Atlantis bank. It is not understood yet what mechanism produces such primitive gabbros at the upside. The accretion fashion of the lower crust beneath intermediate to ultra-fast spreading ridges is still controversial with two end models proposed, gabbro glacier model and sheeted sill model. Hybrid models based on thermal analyses and petrological data have also been proposed. Segmentation structures might also affect also for the accretion style of the lower crust. Much deeper drillings and detailed studies of ophiolites are required to solve the accretion style of lower oceanic crusts beneath intermediate to ultra-fast spreading ridges.

IODP Expeditions 309 and 312 Drill an Intact Section of Upper Oceanic Basement into Gabbros

The Integrated Ocean Drilling Program’s (IODP) Expeditions 309 and 312 successfully completed the first sampling of an intact section of upper oceanic crust, through lavas and the sheeted dikes into the uppermost gabbros. Hole 1256D, which was initiated on the Ocean Drilling Program’s (ODP) Leg 206, now penetrates to > 1500 mbsf and > 1250 m sub-basement. The first gabbroic rocks were encountered at 1407 mbsf. Below this, the hole penetrates ~100 m into a complex zone of fractionated gabbros intruded into contact metamorphosed dikes. doi:10.2204/iodp.sd.4.01.2007

IODP Expeditions 309 and 312 Drill an Intact Section of Upper Oceanic Basement into Gabbros

The Integrated Ocean Drilling Program’s (IODP) Expeditions 309 and 312 successfully completed the first sampling of an intact section of upper oceanic crust, through lavas and the sheeted dikes into the uppermost gabbros. Hole 1256D, which was initiated on the Ocean Drilling Program’s (ODP) Leg 206, now penetrates to > 1500 mbsf and > 1250 m sub-basement. The first gabbroic rocks were encountered at 1407 mbsf. Below this, the hole penetrates ~100 m into a complex zone of fractionated gabbros intruded into contact metamorphosed dikes. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.4.01.2007" target="_blank">10.2204/iodp.sd.4.01.2007</a>

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.

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.

ODP-IODP SITE 1256 (EAST PACIFIC RISE): AN IN-SITU SECTION OF UPPER OCEAN CRUST FORMED AT A SUPERFAST SPREADING RATE

The igneous rocks cored in Holes 1256C and 1256D during Ocean Drilling Program (ODP) Leg 206 and Integrated Ocean Drilling Project (IODP) Expedition 309 are dominated by thin (10’s of cm to 3 m) basaltic sheet flows separated by chilled margins, with several massive flows (> 3 m thick), minor pillow basalt and hyaloclastites, and rare small dikes. In Hole 1256D, the percentage of massive flows decreases downhole. One notable feature of both holes cored is the presence of a very thick massive lava (~35 m thick in Hole 1256C and ~75 m thick in Hole 1256D) near the top of each hole. The structural aspect of this thick massive unit are here described. It has been interpreted as a lava pond emplaced in a topographic depression. Although the ocean crust drilled at Hole 1256C and Hole 1256D partly fits the “Penrose” model for showing the superposition of volcanics on sheeted dikes and intrusives, the relative thickness of the lava-dike sequence could reflect a combination of two or more of the following processes: local (spatial) heterogeneity along ridge-axis, (temporal) variability, essentially linked to a more or less intense activity of the magma chamber, and off-axis eruptions.

MICROSTRUCTURAL FEATURES OF A SUBAQUEOUS LAVA FROM BASALTIC CRUST OFF THE EAST PACIFIC RISE (ODP SITE 1256 , COCOS PLATE)

This work examines a massive basaltic lava emplaced in a subaqueous environment and drilled at ODP Site 1256. Site 1256 was drilled on the eastern flank of the East Pacific Rise during ODP Leg 206 (6°44N, 91°56W; Guatemala Basin), located in 15 Ma old oceanic crust created by superfast seafloor spreading (ca. 220 mm/yr). The massive lava lies between thin sheet flows and caps pillow lavas, sheet flows with minor hyaloclastite, breccia and dikes. The massive basalt was encountered in two holes, 1256C and 1256D, which are 30 m apart, and has a thickness of 35 m in Hole 1256C and 75 m in Hole 1256D. It can be interpreted as a ponded lava, originated by rapidly erupted lava accumulated in a off axis >3-5 km depression of a steep paleotopography (Teagle et al., 2004). The Hole 1256 lava pond has been divided into distinct petrographic units and structural units. Five main structural units with different key flow-related textures and syn-magmatic or late magmatic structures were recognized. Ductile and brittle-ductile structures attributed to the flow gives constraints about the emplacement mechanism of the lava and possibly seafloor topography. Unusual textural features related to the flow kinematics were recognized mainly in the top and in the bottom parts of the lava pond, whereas brittle-ductile and brittle deformations occur throughout the whole ponded body. Microstructures in the base may be interpreted as flow-related deformation of hot ductile coalesced spatter clasts erupted during the first stages of emplacement. Alternatively, they may have been formed during lava drain-back, in the final emplacement stages.

The “road to the MoHole” four decades on: Deep drilling at Site 1256

Ocean Drilling Program (ODP) Hole 1256D, drilled 500 m into oceanic basement in the Guatemala Basin, is the first hole prepared with the infrastructure desirable for drilling a moderately deep hole into the oceanic basement (1.5–2 km). A second ocean drilling leg to this site should finish the recovery of the first complete section of upper oceanic crust from lavas down through the dikes into the upper‐most gabbros. Although Site 1256 has many advantageous features for a program to drill to the mantle, planning such an operation now would be premature, due in part to lack of engineering experience. In addition, there is significant scientific knowledge to be gained from drilling to depths of 1–3 km. Only through the characterization of the heterogeneity of the ocean crust by moderate‐depth drilling at this and other sites will the essential information necessary to even choose a location for a full crustal penetration be acquired. The information learned on the “road to” selecting a MoHole site is at least as important as succeeding with that inspirational goal itself [see also Murray et al, 2002].

Imbalance in the oceanic strontium budget

Palmer and Edmond [Earth Planet. Sci. Lett. 92 (1989) 11–26] indicated that thermally plausible oceanic hydrothermal inputs of strontium to the oceans are not sufficient to balance the riverine input. It has recently been suggested that off-axis low-temperature hydrothermal circulation may reconcile this discrepancy [e.g. Butterfield et al., Geochim. Cosmochim. Acta 65 (2001) 4141–4153]. Strontium isotope alteration profiles are compiled for sampled in situ ocean and ophiolite crust to calculate a sustainable cumulative hydrothermal flux to the oceanic strontium budget. High-temperature circulation contributes ∼1.8×109 mol yr−1 of basaltic strontium to the oceans. Enhanced hydrothermal systems in arc-related spreading environments (10% of the crust) may increase this to ∼2.3×109 mol yr−1. It is shown that low-temperature flow cannot supply the remaining flux required to reconcile the oceanic strontium budget (∼8.7×109 mol yr−1) because this would require 100% exchange of seawater strontium for basaltic strontium over an 820 m section of MORB-like crust. Currently sampled in situ ocean crust is not altered to this extent. The isotopic alteration intensity of 120 Myr crust sampled in DSDP Holes 417D and 418A indicates that off-axis low-temperature flow may contribute up to ∼8×108 mol yr−1 of basaltic strontium (9% of that required). The ocean crust can sustain a total basaltic strontium flux of ∼3.1±0.8×109 mol yr−1 (87Sr/86Sr ∼0.7025) to the oceans. This is consistent with hydrothermal flux estimates, but remains less than a third of the flux required to balance the oceanic strontium budget. The ocean crust cannot support a higher hydrothermal contribution unless the average ocean crust is significantly more altered than current observation.

Paleomagnetic inclinations in DSDP Hole 417D reconsidered: Secular variation or variable tilting?

In Deep Sea Drilling Program (DSDP) Hole 417D in the western central North Atlantic, dips of pillow margins measured in the upper and lower part of basement correlate with tilts predicted from paleomagnetic inclinations in the corresponding parts of the hole. A ∼40° westward dip of the seafloor around 417D matches the tilt predicted from inclinations and pillow margin dips in the uppermost lavas. The correlation suggests that most of the large long‐wavelength inclination variation in Hole 417D is caused by variable tilting about an axis more or less parallel to the paleo rift rather than by secular variation. The variable tilt in Hole 417D is interpreted as the sum of rotation due to lava loading along the Mesozoic paleo rift and a later block rotation of ∼40° in the rift valley walls. This interpretation allows a reconstruction of the paleo rift environment in which the 417D upper crust formed.

The boron isotopic composition of altered oceanic crust

Samples of drilled oceanic crust, from DSDP Holes 417A, 417D and 418A and ODP Hole 735B, and oceanic crust from the Oman and Cyprus ophiolites, were analyzed for B contents and δ 11B. Composite samples from DSDP Holes 417A, 417D and 418A were used to represent the upper 550 m of altered oceanic crustal Layer 2A. Whole-rock samples from the Troodos ophiolite, Cyprus, and the Oman ophiolite wer selected to represent crustal Layer 2B dikes. Composite samples from ODP Hole 735B were used to represent crustal Layer 3. The B content of the DSDP composites ranges from 7.2 ppm to 104 ppm and correlates with both δ 18O and K, showing that it is a good indicator of the extent of low temperature alteration. The δ 11B of the DSDP composites varies between −2.5‰ and 5.4‰. The B content of the samples from the Troodos ophiolite ranges from 2.4 ppm to 8.1 ppm; δ 11B varies from −0.9‰ to 7.8‰. The B content of the Oman ophiolite samples ranges from 5.0 ppm to 11.1 ppm; δ 11B varies from −1.6‰ to 16.9‰. The B content of the samples from ODP Hole 735B ranges from 1.1 ppm to 7.1 ppm; δ 11B varies from −4.3‰ to 24.9‰. The general pattern displayed by these samples is one of greatest (and most variable) B enrichment at the top of the crust and least enrichment at the bottom of the section. All of these samples are enriched compared to unaltered MORB, which is believed to have a B content of approximately 0.5 ppm. The δ 11B values of deeper samples, from Layers 2B and 3, are more variable and generally higher than those from Layer 2A. Boron contents and δ 11B are not correlated. The data from the DSDP Site 417 418 composites indicate that the δ 11B of fluid circulating in the upper crust changes only slightly during alteration, increasing by an average of 5.1‰ with an accompanying decrease in B concentration of 7%. Low temperature alteration appears to be a water-dominated process resulting in minor modification of circulating seawater. A minimum water-rock ratio of 400 is calculated for these samples, implying a minimum low-temperature seawater flux through the upper oceanic crust of 3.4 × 10 14 l/y. The average B content of altered oceanic crust, as represented by these samples, is 5.2 ± 1.7 ppm and the average δ 11B is 3.4 ± 1.1‰. This average isotopic composition is measurably different from the apparent average of oceanic sediments, supporting the idea that δ 11B could be useful for identifying the source(s) of B in island arcs.

Characteristics of magnetite in submarine basaltic glass

SUMMARY We have compared the rock-magnetic characteristics of Cretaceous submarine basaltic glass (SBG) from DSDP/ODP Holes 417D, 418A, 807C and 543A to data from Holocene glasses from the East Pacific Rise. Both groups are very similar in that they contain single-domain, low-Ti magnetite as the carrier of a strong and stable remanent magnetization. The behaviour of magnetic hysteresis in SBG is, however, dominated by superparamagnetic grain-size populations. Transmission electron microscopy revealed spherically shaped grains of 10-20 nm diameter with crystal lattice properties consistent with magnetite. These grains could be responsible for the magnetic behaviour of SBG.