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doi:10.2204/iodp.pr.335.2011

Principal results

Hole opening, remediation operations, and the comprehensive destruction of a C-9 hard formation coring bit resulted in a major loss of time from the coring and wireline activities planned for Expedition 335. Coring during this expedition deepened Hole 1256D only modestly, from 1507.1 to 1521.6 mbsf (Cores 335-1256D-235R through 239R), at low rates of penetration (0.9 m/h) and recovery (11%) (Table T2; Fig. F25). However, the availability of the Expedition 312 archive- and working-half sections from the lowermost granoblastic dikes and the plutonic section of Hole 1256D (Table T5; Fig. F26) allowed the detailed redescription of these cores and some discrete sample measurements during the extended first phase of hole cleaning. Further fishing and hole cleaning operations at the bottom of Hole 1256D, particularly those runs that deployed the RCJB (Runs 12, 13, 19, 20, and 21) (Table T3) brought back a unique collection of large cobbles (as heavy as 4.5 kg; e.g., 335-1256D-Run 12-RCJB-Rock-A), angular rubble, and fine cuttings of principally strongly to completely recrystallized granoblastic basalt with minor gabbroic rocks and evolved plutonic rocks (Table T6). The large blocks exhibit intrusive structural and textural relationships and overprinting and crosscutting hydrothermal alteration and metamorphic paragenetic sequences that hitherto have not been observed because of the one-dimensional nature of drill cores and the very low rates of recovery (<7%) of the granoblastic dikes during Expedition 312. Some of the rubble and ~30% of the fine cuttings recovered by fishing and cleaning operations clearly came from the lava sequences at the top of the hole on the basis of igneous textures and low-temperature alteration minerals (Mg saponite and amorphous silica). These rocks will not be described further. In contrast, the high extent of metamorphic recrystallization exhibited by the granoblastic basalts, along with operational factors (e.g., pipe movements), provide strong evidence that the granoblastic basalt, minor gabbros, and evolved plutonic rocks were sourced from the lowermost reaches of Hole 1256D (1494.9–1521.6 mbsf), most probably from below Gabbro 2. Hence, the rocks recovered during Expedition 335 represent a ~15 m interval of the upper crust–lower crust transition, occurring below the ~90 m section, recovered during Expedition 312, of two gabbroic bodies (Gabbro 1 and Gabbro 2) separated by Dike screen 1.

Igneous petrology

The interval sampled during Expedition 335 comprises predominantly fine-grained granoblastic aphyric basalt. Although all samples have granoblastic textures (Fig. F27), indicative of high-temperature metamorphism, the degree of recrystallization varies, with strongly recrystallized rocks dominant over completely recrystallized rocks. Some samples preserve dike/dike contacts, and many preserve cores of former plagioclase (micro)phenocrysts (Fig. F27), indicating that the granoblastic rocks represent a metamorphosed sheeted dike complex. This interval has been designated Dike screen 2 (Fig. F28).

Approximately half of the granoblastic basalts contain small, irregular patches (<5 cm × 3 cm), veins (~1–2 mm wide), and dikelets (<1.5 cm wide) of evolved plutonic rocks (oxide gabbro, diorite, and tonalite) (Fig. F29A, F29B). The veins are observed to be offshoots of the igneous patches, and diffuse patches are issued from the dikelets. These two observations indicate that the igneous veins, dikelets, and patches form part of the same network, marking a single generation of intrusion of melts into the granoblastic basalts. Their magmatic textures (subhedral to euhedral shapes of several phases, along with poikilitic textures) contrast strongly with the granoblastic textures of the host rocks, demonstrating that intrusion occurred after the granoblastic recrystallization of the host rock. The ubiquitous occurrence of primary magmatic amphibole in the veins and patches suggests high water activities during their formation. Moreover, the presence of quartz, as well as accessory apatite and zircon, implies that the patches, veins, and dikelets crystallized from highly evolved melts.

A small number of gabbroic rocks was recovered (Fig. F29C); the rocks range in composition from disseminated oxide gabbro to orthopyroxene-bearing olivine gabbro. Expedition 335 gabbroic rocks have a more “salt and pepper,” equigranular appearance and less textural variability compared to the gabbroic rocks recovered during Expedition 312 (Fig. F19C). Hence, although the contact relationships with the granoblastic basalts were not recovered, it is likely that at least some of the gabbroic rocks occur intercalated with the granoblastic basalt, perhaps forming small intrusions.

Overall, a picture emerges of a section of metamorphosed, granoblastic sheeted dikes that underwent small-scale intrusion by both gabbroic and evolved plutonic rocks (Fig. F30).

Geochemistry

Only a limited number of whole-rock geochemical analyses were undertaken during Expedition 335 (Fig. F31). Major and trace element and volatile concentrations were determined on three granoblastic basalts from Cores 335-1256D-235R through 238R, on one basalt lava, and on five granoblastic basalt and two gabbroic rocks recovered during junk basket runs. These samples were chosen from the least altered parts of the core and rock samples and as far as possible from hydrothermal veins and magmatic intrusions to obtain the best estimate of primary compositions.

The basalt and granoblastic dikes have mid-ocean-ridge basalt (N-MORB) compositions similar to that of the variably altered basaltic lavas, dikes, and granoblastic dikes cored in the overlying crust in Hole 1256D (e.g., Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, et al., 2006). The two gabbros (olivine gabbronorite and olivine gabbro) have high loss on ignition (LOI) (~1.5–2 wt%), consistent with those samples being more affected by hydrothermal alteration processes than the neighboring granoblastic basalts. The gabbros are also quite distinct from the granoblastic dikes with respect to major and trace element concentrations. Expedition 335 gabbro compositions are typical of gabbroic cumulate suites sampled in oceanic environments but are also similar to those of the less evolved members of Gabbro 1 previously sampled in Hole 1256D. However, they are more primitive than any analysis from the lower Gabbro 2. Expedition 335 gabbroic rocks have relatively high MgO (~12 wt%) and Ni (~300 ppm) concentrations but low concentrations of incompatible trace elements compared to granoblastic dikes (Fig. F31). The relatively high Mg# (70–72) and Ni concentrations of the Expedition 335 gabbros principally reflect their modal olivine contents.

One characteristic of the granoblastic basalts sampled during Expedition 335 is relatively low concentrations of Cu, Zn, Zr, and Y compared to previously sampled basaltic samples in Hole 1256D (Fig. F31). Compared to all granoblastic and sheeted dike analyses, the Expedition 335 granoblastic basalt yields the most depleted compositions for these elements.

The lower part of Hole 1256D, below 1340 mbsf, is characterized by strong chemical variations, with Mg# ranging from 42 to 72 and Zr from 23 to 117 ppm. These changes in composition mainly reflect the changes in rock types from the low Mg#, trace element–rich sheeted dikes and granoblastic dikes to the higher Mg# and trace element–depleted gabbroic rocks of Gabbro 1 and Gabbro 2. There is a general downhole trend of decreasing incompatible element (e.g., Zr and Y) contents in the granoblastic dikes (Fig. F31). At a more localized scale, the granoblastic basalt sampled within Dike screen 1 and below Gabbro 2 in Dike screen 2 defines trends of increasing Zr with depth. The lowest Zr values occur directly at the upper interface between the granoblastic basalt and the overlying gabbroic intrusion. This pattern is particularly marked at the bottom of Hole 1256D, where the most trace element–poor granoblastic basalt of Hole 1256D was sampled at 1502 mbsf, just below Gabbro 2 (Yamazaki et al., 2009). Expedition 335 samples show a near doubling of Zr contents from ~30 ppm at 1507–1512 mbsf to 58 ppm at 1518 mbsf. These downhole variations in incompatible element contents are mimicked by changes in Zr/Y, with granoblastic basalt from directly below Gabbro 2 having low Zr/Y (~1.5) compared to the sheeted dikes and granoblastic dikes above Gabbro 1 (2–3). We interpret the systematic depletion observed in the granoblastic basalt just below both Gabbro 1 and Gabbro 2 as indicative of small degrees of partial melting, probably caused by gabbroic intrusions into the partially hydrated dikes (e.g., Miyashita et al., 2007; Koepke et al., 2007, 2008). The degree of partial melting is probably minor, as the effects on dike major element compositions are undetectable. We also observe a general decrease in Cu and Zn contents in the granoblastic basalt sampled at the bottom of Hole 1256D, consistent with mobilization of Cu and Zn by high-temperature hydrothermal alteration (>400°C). At the bottom of Hole 1256D, low base metal concentrations appear to be a signature of only the granoblastic basalt. This may imply that the intrusion of Gabbro 1 and Gabbro 2 occurred after this stage of high-temperature hydrothermal alteration, consistent with the remelting process suggested by downhole Zr/Y variations.

All granoblastic basalt recovered during Expedition 335 has trace element compositions similar to the granoblastic rocks cored directly below Gabbro 2 during Expedition 312 (>1500 mbsf). This is further strong evidence that Expedition 335 granoblastic basalt comes from the lowermost levels of Hole 1256D, probably below 1494 mbsf.

Alteration and metamorphism

Rocks recovered during Expedition 335 are mainly dark gray, fine-grained basalt that was recrystallized by contact metamorphism. They are essentially identical to the granoblastic basalt from Dike screen 1 between Gabbro 1 and Gabbro 2 cored during Expedition 312. Coarser grained plutonic material occurs as separate rock fragments or as 1 mm to 1 cm sized dikelets and veins, as well as irregular intrusions into the dike rocks.

Dike screen 2 basalt is recrystallized to granoblastic assemblages of clinopyroxene, orthopyroxene, plagioclase, magnetite, ilmenite, and rare brown hornblende and quartz, with accessory sulfide minerals (pyrrhotite, chalcopyrite, and pyrite) (Fig. F32). Veins of granoblastic orthopyroxene ± plagioclase ± clinopyroxene, 100–200 µm wide, are common and are likely recrystallization products of hydrothermal vein protoliths. Dike samples include a chilled intrusive dike contact and a brecciated intrusive dike margin that were hydrothermally altered and then recrystallized to granoblastic assemblages during contact metamorphism.

The granoblastic rocks typically exhibit at most only slight postcontact-metamorphism hydrothermal alteration (generally <15%), mainly to amphibole. Clinopyroxene is locally partly altered to amphibole, orthopyroxene is variably altered to amphibole and local talc and smectite, and plagioclase is locally slightly altered to trace chlorite, actinolite, secondary plagioclase, and smectite. Fe-Ti oxides are partly altered to titanite.

The dikes are cut by common 0.1–0.5 mm thick amphibole veins, with ~1–3 mm wide amphibole-rich alteration halos. These veins cut across the intrusive dike contact, granoblastic veins, and coarser grained igneous intrusive rocks. Also present are rare later veins containing actinolite, chlorite, quartz, epidote, and prehnite.

Coarser grained rocks (olivine gabbronorite, oxide gabbro, and quartz diorite) are more highly altered, with clinopyroxene highly altered to amphibole + fine magnetite and plagioclase partly altered to secondary plagioclase, amphibole, minor chlorite, and local epidote. Olivine exhibits coronitic alteration to amphibole, talc, magnetite, pyrrhotite, smectite, and rare iron oxyhydroxides. The intruded granoblastic host rocks are commonly highly altered to amphibole for as much as 1 cm around the intrusions. These observations are consistent with evidence from Expedition 312 that these intrusive boundaries are zones of enhanced fluid flow and fluid-rock exchange.

The cores and rocks recovered during Expedition 335 sample the transition from sheeted dikes to the gabbroic section of oceanic crust. The dikes underwent hydrothermal alteration in a mid-ocean-ridge hydrothermal system at the spreading axis. The altered dikes were then intruded by the two gabbro bodies cored during Expedition 312 and other magmatic bodies near the bottom of the hole and underwent contact metamorphism at temperatures of ~900°–1000°C. The effects of prior hydrothermal alteration influenced the degree of recrystallization. Cooling and fracturing allowed further penetration of fluids and hydrothermal alteration of these rocks, with formation of amphibole veins and later retrograde minerals (actinolite, quartz, epidote, chlorite, prehnite, and late smectite and iron oxyhydroxides).

The granoblastic dikes and underlying dike screens represent the conductive boundary layer between mafic magma and the overlying hydrothermal system, and the rocks from Hole 1256D are similar to those observed in ophiolites and elsewhere in oceanic crust (e.g., Gillis and Roberts, 1999; Gillis, 2008; France et al., 2009). The granoblastic basalt sampled beneath Gabbro 2 during Expedition 335 is part of a dike screen within the transition from sheeted dikes to gabbros, consistent with the presence of a significant underlying gabbro heat source.

Structural geology

Cores from the plutonic section drilled during Expedition 312 and cores, cuttings, and cobbles recovered during Expedition 335 contain important crosscutting relationships that illustrate the intimate interplay between magmatic, metamorphic, fluid flow, and brittle deformation processes. Unfortunately, because of the lack of oriented pieces, many structural investigations could not be completed.

Several vertically oriented Expedition 312 core pieces were tentatively azimuthally reoriented based on paleomagnetic data assuming normal polarity, although this remains to be definitively demonstrated. The reoriented azimuth of the upper contact between the granoblastic dikes and Gabbro 1 and between Dike screen 1 and Gabbro 2 were (dip/dip direction) 42°/260 and 81°/255, respectively. The orientations of the preserved contacts suggest that the gabbro intrusions dip at moderate to steep angles to the west-southwest, toward the paleospreading ridge. Veins from Expedition 312 cores show a bimodal orientation distribution, with a steeply dipping maximum (60°) and a second shallowly dipping maximum (10°). Using the same reorientation technique as for the contacts, all veins dominantly strike northwest–southeast.

A variable fracture density of subhorizontal irregular fractures is thought to be drilling-induced fractures. The boundaries between gabbros and granoblastic basalts have higher fracture densities.

Gabbroic rocks from the gabbro 1 interval are mineralogically and texturally heterogeneous and commonly display lighter color, leucocratic patches that result from a multistage magmatic history, with the percolation of more evolved melt through a partially crystallized gabbroic mush (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, et al., 2006). The two-dimensional distribution of these leucocratic patches was quantified using an image analysis technique on high-resolution photographs of the archive halves cut face. The shape-preferred orientation of the patches is very weak, and there is no obvious downhole trend. The image analysis also provides an accurate estimate of the modal distribution of the leucocratic patches, which shows some systematic trends downhole, correlated to distinct trends in magnetic susceptibilities, and revealing three possible distinct petrological units. These three zones are characterized by a broad upward increase in percentage of leucocratic patches and are delimited by limits that correspond to unit boundaries defined during Expedition 312 (85/86A) and Expedition 335 (89A/89C).

Paleomagnetism

Paleomagnetic analyses during Expedition 335 focused predominantly on a detailed investigation of samples from Gabbro 1 and Gabbro 2 because of the lack of oriented core recovered during Expedition 335. New data from shipboard thermal demagnetization experiments on discrete samples prepared during the expedition were augmented by unpublished shore-based data from the interval 1406–1503 mbsf that were analyzed by shipboard paleomagnetists. In addition, new data on the anisotropy of magnetic susceptibility (AMS) were acquired and reoriented using remanence data.

Samples are dominated by a steep, near-vertical drilling-induced remanent magnetization (DIRM) that represents an average of 66% of the total remanence (Fig. F33). Following removal of the DIRM by low–intermediate demagnetization treatments, shallow to moderate inclination components are typically isolated above 35 mT or 540°C and are considered to represent the characteristic remanent magnetization (ChRM) of the samples. The mean inclination of these components is 31°, significantly steeper than that expected for the paleoposition of Site 1256, which restores to an equatorial paleolatitude in the Miocene (Wilson, Teagle, Acton, et al., 2003). Potential causes of this apparent steepening of inclinations include the following:

1. Contamination of ChRM components by a residual DIRM persisting to high demagnetization levels. This is, however, unlikely to account fully for the observed data because it would require near complete overlap of the coercivity/unblocking temperature spectra of grains carrying the DIRM and ChRM at high fields/temperatures, and DIRM is predominantly carried by multidomain magnetite (Allerton et al., 1995) with generally low coercivity and distributed unblocking temperatures.

2. A present-day field thermoviscous overprint. This is unlikely, given the low ambient temperatures in the section and the high unblocking temperature of ChRMs, and is discounted entirely by the presence of multicomponent remanences in some interval.

3. The presence of a persistent nondipole field at Site 1256D during the Miocene. Little is known about the geometry of the field in the Pacific at 15 Ma, but analysis of anomalous skewness of younger marine magnetic anomalies in the Galapagos region (Schneider, 1988) suggests that nondipole field components may account for at most a few degrees of inclination anomaly.

4. Tectonic tilting of the section. Tilting of ~10°–20° is compatible with the observed dip of dike margins in the sheeted dike section of Hole 1256D (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, et al., 2006; Tominga et al., 2009) and may potentially account for ~10° of inclination steepening assuming a ridge-parallel rotation axis. Larger amounts of inclination change may be produced by rotation around nonridge-parallel axes but are difficult to reconcile with the tectonic setting of the section.

5. Deflection of remanence directions by a strong anisotropy of consistent orientation. This can result in ~10°–20° of inclination change (depending on the degree of anisotropy and its orientation relative to the remanence). It can only be quantified and assessed by analysis of the anisotropy of remanence (Potter, 2004), which will form a focus of postcruise research.

The new thermal demagnetization data reveal a previously unrecognized type of remanence structure in a sample from Gabbro 2. After removal of steep DIRM by low-temperature (liquid nitrogen) demagnetization, two well-defined intermediate and high unblocking temperature components with antipodal directions are identified (Fig. F33). These data represent the first multipolarity remanences seen in lower crustal rocks formed at the EPR and are similar to examples reported recently from sites along the Mid-Atlantic Ridge (Meurer and Gee, 2002; Morris et al., 2009). Sampling of the gabbroic section conducted during Expedition 335 for further shore-based analyses of these multipolarity remanences will allow their distribution and significance to be determined, potentially leading to new information on the thermal evolution of the section.

AMS tensors from discrete samples from Gabbro 1 and Gabbro 2 are shown to be randomly oriented in the core reference frame (Fig. F34). In the absence of independent reorientation of core pieces via analysis of FMS imagery, a basic, first-order reorientation of these data is possible by applying vertical axis rotations to AMS data to align corresponding ChRM directions to present-day north. This results in a preferred north–south alignment of AMS maximum principal axes. This is particularly apparent in data from samples with prolate (lineated) AMS fabrics and may indicate that a significant along-axis preferred mineral alignment is frozen into the gabbro section. Similar ridge-parallel magnetic lineations have been reported from the slow spreading rate Troodos ophiolite (Abelson et al., 2001) and have been interpreted to indicate along-axis migration of melt. Further detailed postcruise analyses of AMS and other forms of magnetic anisotropy (e.g., anisotropy of anhysteretic remanence) should allow this hypothesis to be tested.

Physical properties

Physical property measurements during Expedition 335 revealed that the granoblastic basalt samples generally have high magnetic susceptibility (~6200 × 10^–5 SI on average) (Fig. F35), whereas gabbroic rocks have lower average magnetic susceptibility (~3000 × 10^–5 SI) but display much larger variation. In the gabbro units, magnetic susceptibility and color reflectance data follow variations in oxide and olivine content. Natural gamma radiation shows peaks that coincide with occurrence of evolved plutonic rocks consistent with relatively high concentrations of K, U, Th, and other incompatible elements in these rocks.

A measuring technique using a seawater bath was developed that greatly improved the quality of shipboard P-wave velocity measurements of discrete samples. The velocities of gabbro range from 6298 to 6759 m/s, whereas measurements of granoblastic basalt range from 6610 to 6907 m/s. These relatively high velocities are consistent with the trends of downhole geophysical logs above 1400 mbsf and may indicate that the lower section of Hole 1256D is close to the seismic Layer 2/3 boundary (Fig. F36).

Downhole logging

Because of technical troubles, the triple combo was the only logging tool string deployed during Expedition 335. It recorded the density, porosity, gamma ray emission, and resistivity of the formation, as well as the temperature of the borehole fluid over the entire hole, reaching the maximum depth of 1520 mbsf, 80 m below the deepest logs recorded at the end of Expedition 312. In addition to measuring the properties of the gabbros and dike screens at the bottom of the hole, one of the objectives of the logging program was to record a full caliper log over the entire hole to assess the results of the previous cementing operations (Figs. F22, F23), to help plan the end-of-expedition cementing operations to stabilize Hole 1256D for future expeditions, and to provide information for potentially casing the uppermost part of Hole 1256D.

Logging results

The hole size (Fig. F37) shows that the bottom was significantly enlarged after several weeks of junk basket runs dedicated to cleaning the hole. The hole is irregular below ~1410 mbsf, and the low density and high porosity readings below this depth are a direct consequence of the hole size. The decoupling between the shallow and resistivity logs is also a consequence of the hole size. However, the deepest resistivity measurement should not be affected by hole size and indicates a decrease in resistivity with depth starting below Gabbro 1 (~1460 mbsf) that becomes more apparent by Gabbro 2. In contrast with the increase with depth expected in the plutonic section, this suggests that the deepest section might be fractured, possibly part of a fault, which could explain some of the difficulties encountered while coring.

Hole size

Higher up in the hole, hole cleaning and cementing operations around ~920 mbsf considerably changed the shape of the hole (Fig. F38). Although the several days spent trying to pass this interval contributed to the enlargement above it, the cement reduced the hole size and its roughness below. Between 930 and 970 mbsf the hole is large but without asperities and should not present any difficulty for future reentries, as shown by the 15 smooth reentries following the cementing operation during Run 8.

Temperature logs

Comparison between the temperature logs recorded by the two temperature tools during Expedition 335 and the temperatures measured during previous expeditions in Hole 1256D (Fig. F39) shows similar trends as the borehole fluid recovers from the disturbance of the drilling operations. Several excursions to lower temperatures, in particular around 925 mbsf and 1060 mbsf, coincide with intervals with lower resistivity, indicating more permeable intervals where the formation might have been invaded by the drilling fluid and is consequently recovering more slowly from the drilling process. The kick at ~1300 mbsf that was also observed during Expedition 312 coincides with lower resistivity and is probably also associated with fluid exchange with the formation. These anomalies will be the object of numerical modeling, which in combination with other logs should provide estimates of the permeability in these intervals.

Conclusions

When the textural and contact relationships exhibited by the large rocks recovered from the junk baskets are placed in the geological context of the Hole 1256D stratigraphy, a vision emerges of a complex, dynamic thermal boundary layer zone. This region of the crust between the principally hydrothermal domain of the upper crust and the intrusive magmatic domain of the lower crust is one of evolving geological conditions. There is intimate coupling between temporally and spatially intercalated magmatic, hydrothermal, partial melting, intrusive, metamorphic, and retrograde processes.

With only a minor depth advance in Hole 1256D, we have yet to recover samples of cumulate gabbros required to test models of ocean ridge magmatic accretion and the intensity of hydrothermal cooling at depth. Nor have we crossed the Layer 2/3 boundary at Site 1256. The total vertical thickness of granoblastic basalts is >114 m, and Dike screen 2 is now about the same thickness (so far) as Dike screen 1. High perched, isolated gabbro intrusions are uncommon in ophiolites. The energy requirements for the granoblastic recrystallization at granulite facies condition of a >114 m thick zone of sheeted dikes massively exceeds the thermal capacity of Gabbros 1 and 2 (e.g., Koepke et al., 2008; Coggon et al., 2008) if a simple subhorizontal arrangement of these layers is assumed. The enormous heat requirements for such extensive granulite facies recrystallization, the evidence for partial melting, together with the tantalizing presence of minor but not uncommon gabbroic rocks and felsic intrusive, dikelets, and veins, strongly indicates that the layer of purely plutonic rocks should be at most only a few tens of meters deeper in the hole.

Although the extensive remedial operations on Expedition 335 precluded significant deepening of Hole 1256D, significant progress was made in improving the borehole. The most problematic out-of-gauge zone at ~920–960 mbsf that caused reentry problems during Expeditions 312 and 335 has been stabilized with cement. The bottom of the hole has been cleared of rubble and junk, and there appears to be only a short, slightly undergauge zone (<1 m). Importantly, the regular, large sweeps of high-viscosity mud (as much as 200 bbl sepiolite, every ~2 h), have finally overcome and expunged the vast amount of fine cuttings recirculating in the hole, some most likely resident since Leg 206. This progress is shown by the absence of soft fill in the final ~5 reentries compared to >50 m of soft fill at the end of Expedition 312. The engineering efforts during Expedition 335 have repaired and prepared Hole 1256D for further deep drilling, following 5 years of neglect. Hole 1256D is 1500 m of hard rock coring closer to cumulate gabbros than any other options in intact ocean crust. It is once more poised to answer fundamental questions about the formation of new crust at fast-spreading mid-ocean ridges, best achieved by a timely return to the site.

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