Superfast Spreading Rate Research Articles

Interactions of the Greater Ontong Java mantle plume component with the Osbourn Trough.

The Ontong Java-Manihiki-Hikurangi plateau (OJMHP) is considered to have originated from a starting mantle plume, and have been rifted apart by two spreading ridges. However, the ages of these spreading ridges and their possible interactions with the presumed mantle plume are unclear. The Manihiki-Hikurangi plateau has been rifted apart by the Osbourn Trough which formed the southwestern Pacific crust to the east of the Tonga-Kermadec trench. Here we report Pb-Hf-Os isotopes of the basaltic crust (Site U1365 of IODP Expedition 329) formed by the Osbourn Trough. Linear regression of Re-Os isotopes results in an age of 103.7 ± 2.3 Ma for Site U1365 basalts, indicating that the Manihiki-Hikurangi plateau was rifted apart by the Osbourn Trough with a spreading rate of ~190 mm/yr. The superfast spreading rate supports the Osbourn as an abandoned segment of the early Pacific spreading ridge, which initially overlapped with the giant starting plume. Moreover, the Pb-Hf isotopes of some of Site U1365 basalts show distinct differences from those of the Pacific mid-ocean ridge basalts, while they are similar to the basalts of the Ontong Java and Manihiki plateaus. We suggest that the OJMHP mantle plume components has been involved by the Osbourn spreading center.

IODP Expedition 335: Deep Sampling in ODP Hole 1256D

Abstract. Observations of the gabbroic layers of untectonized ocean crust are essential to test theoretical models of the accretion of new crust at mid-ocean ridges. Integrated Ocean Drilling Program (IODP) Expedition 335 (Superfast Spreading Rate Crust 4) returned to Ocean Drilling Program (ODP) Hole 1256D with the intention of deepening this reference penetration of intact ocean crust a significant distance (~350 m) into cumulate gabbros. Three earlier cruises to Hole 1256D (ODP 206, IODP 309/312) have drilled through the sediments, lavas, and dikes and 100 m into a complex dike-gabbro transition zone. Operations on IODP Expedition 335 proved challenging throughout, with almost three weeks spent re-opening and securing unstable sections of the hole. When coring commenced, the comprehensive destruction of the coring bit required further remedial operations to remove junk and huge volumes of accumulated drill cuttings. Hole-cleaning operations using junk baskets were successful, and they recovered large irregular samples that document a hitherto unseen sequence of evolving geological conditions and the intimate coupling between temporally and spatially intercalated intrusive, hydrothermal, contact-metamorphic, partial melting, and retrogressive processes. Hole 1256D is now clean of junk, and it has been thoroughly cleared of the drill cuttings that hampered operations during this and previous expeditions. At the end of Expedition 335, we briefly resumed coring before undertaking cementing operations to secure problematic intervals. To ensure the greatest scientific return from the huge efforts to stabilize this primary ocean lithosphere reference site, it would be prudent to resume the deepening of Hole 1256D in the nearest possible future while it is open to full depth. doi: 10.2204/iodp.sd.13.04.2011

IODP Expedition 335: Deep Sampling in ODP Hole 1256D

Observations of the gabbroic layers of untectonized ocean crust are essential to test theoretical models of the accretion of new crust at mid-ocean ridges. Integrated Ocean Drilling Program (IODP) Expedition 335 ("Superfast Spreading Rate Crust 4") returned to Ocean Drilling Program (ODP) Hole 1256D with the intention of deepening this reference penetration of intact ocean crust a significant distance (~350 m) into cumulate gabbros. Three earlier cruises to Hole 1256D (ODP 206, IODP 309/312) have drilled through the sediments, lavas, and dikes and 100 m into a complex dike-gabbro transition zone. <br><br> Operations on IODP Expedition 335 proved challenging throughout, with almost three weeks spent re-opening and securing unstable sections of the hole. When coring commenced, the comprehensive destruction of the coring bit required further remedial operations to remove junk and huge volumes of accumulated drill cuttings. Hole-cleaning operations using junk baskets were successful, and they recovered large irregular samples that document a hitherto unseen sequence of evolving geological conditions and the intimate coupling between temporally and spatially intercalated intrusive, hydrothermal, contact-metamorphic, partial melting, and retrogressive processes. <br><br> Hole 1256D is now clean of junk, and it has been thoroughly cleared of the drill cuttings that hampered operations during this and previous expeditions. At the end of Expedition 335, we briefly resumed coring before undertaking cementing operations to secure problematic intervals. To ensure the greatest scientific return from the huge efforts to stabilize this primary ocean lithosphere reference site, it would be prudent to resume the deepening of Hole 1256D in the nearest possible future while it is open to full depth. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.13.04.2011" target="_blank">10.2204/iodp.sd.13.04.2011</a>

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.

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.

Magmatic processes at superfast spreading mid‐ocean ridges: Glass compositional variations along the East Pacific Rise 13°–23°S

Major and minor element analyses of 496 natural volcanic glass samples from 141 locations along the superfast spreading (150 mm/yr) East Pacific Rise (EPR), 13°–23°S, and near‐ridge seamounts comprise 212 chemical groups. We interpret these groups to represent the average composition of individual lava flows or groups of closely related flows. Groups slightly enriched in K2O (T‐MORB) are distributed variably along the axis, in contrast to the Galapagos Spreading Center where T‐MORB are extremely rare. This result is consistent with the interpretation that T‐MORB magmas arise from low‐melting temperature, K‐rich heterogeneities in the subaxial EPR mantle. The Galapagos Spreading Center, which is migrating to the west in an absolute reference frame, is underlain by mantle previously processed and depleted in the T‐MORB component during melting events giving rise to earlier EPR magmas. Excluding T‐MORB, there are nearly monotonic, twofold increases in K/Ti and K/P of axial lavas from 23°S to 13°S. From 22°S to 17°S these gradients correlate with isotopic ratios, but north of 17°S there is a reversal of isotopic gradients, indicating (recent?) decoupling of the isotopic and minor element ratios in the subaxial mantle. A strong, southward increase in degree of differentiation for approximately 200 km north of the large offset at 20.7°S correlates with a gradient in bathymetry, consistent with previous interpretations that this offset is propagating to the south. Samples from recently abandoned ridges associated with this dueling propagator mainly carry the distinctive, evolved fractionation signatures of rift propagation, suggesting that propagating rift tips have been abandoned preferentially to failing rift tips. Glass compositional variations south of this offset are consistent with rift failure on the southern limb within 40 km of the offset, and possibly also south of 22°S; the latter region may be affected by deformation accompanying northward growth of the Easter Microplate. Near‐ridge seamounts on the Pacific Plate between 18°–19°S comprise two distinct populations: those aligned approximately parallel to the spreading direction are extremely variable in major element composition, but consistently enriched in Sr relative to nearby axial lavas; smaller seamounts aligned approximately parallel to the direction of absolute plate motion are uniformly depleted in minor elements and Sr relative to axial lavas. The degree of differentiation of axial lavas between 18°–19°S can be related to the structural development of the rift axis and/or vigor of hydrothermal activity of individual segments. Glass compositional variations indicate that magmatic segmentation occurs on several different scales at the superfast spreading rate of this area. Primary magmatic segmentation mainly reflects mantle source variations, the boundaries of which correlate with the largest physical offsets in the rise axis between the Easter Microplate and Garrett Transform Zone. A secondary magmatic segmentation, defined by the along‐axis continuity of similar parental magma compositions or liquid lines of descent, occurs with a length scale varying from 11 to 185 km, with an average of 69 ±57 (1σ) km. The boundaries of these segments mainly occur at overlapping spreading centers. All first‐, second‐ and third‐order physical offsets correspond to secondary magmatic segment boundaries, but some secondary magmatic segment boundaries also occur at small, fourth‐order ridge axis discontinuities. The secondary magmatic segments define the length scale of mantle melting variations, mainly variations in extent of melting, but not the scale of melt extraction processes that feed the axis. This scale must be smaller than that of the secondary magmatic segments and probably corresponds to the length scale of fourth‐order physical discontinuities along axis. There is a good positive correlation of average secondary magmatic segment length with spreading rate for four well‐sampled areas varying from 20 to 150 mm/yr. Secondary magmatic segments also become more variable in axial length with increasing spreading rate. The average lengths of secondary magmatic segments are smaller than those predicted by gravitational instability considerations at all spreading rates. Superposed on the axial magmatic segmentation are variations reflecting subaxial magmatic temperature, defined by extent of magmatic differentiation, which bears little systematic relation to physical or other kinds of magmatic segmentation. At 13°–23°S, the length scale of this variation is 217±60 (1σ) km, approximately corresponding to the wavelength of “rolls” in the gravity field observed off‐axis. Taken together, the various kinds and scales of magmatic variations observed for this superfast spreading ridge suggest that regional temperature of the upwelling asthenosphere, magma supply to the axis, and crustal magmatic temperature reflect independent, regionally decoupled processes.