V21B-0600
Geochemical constraints on the origin of high-Mg andesites in the southernmost Okinawa Trough
The Okinawa Trough, extending from SW Kyushu to NE Taiwan, is a backarc basin of the Ryukyu arc-trench system due to subduction of the Philippine Sea plate under the Eurasian plate. The southernmost part of the Okinawa Trough (SPOT), however, does not situate in a simple backarc setting but is an embryonic rift zone in which early arc volcanism takes place. Kueishantao that consists mainly of andesitic flows dated to be ~7000 yr old is an emerged volcanic islet thus formed in SPOT. Here we report whole-rock major and trace element, and Sr-Nd-Pb-Hf isotope compositions of the Kueishantao andesites. Some of the samples have unexpectedly high magnesium, with MgO ≥ 5 wt.% and Mg# > 0.5, relative to their silica contents (SiO2 ~ 60 wt.%), so can be coined as high-Mg andesites (HMAs). These HMAs display enrichments in Cs, Rb, Ba, Th, U, LREE and Pb, and depletions in HFSE, in the incompatible element variation diagram. Their overall geochemical compositions are similar to those of the mean continental crust proposed by Rudnick and Fountain (1995). The HMAs have uniform radiogenic isotope ratios, with low ƒÕεNd (-4.3 to -5.0), low ƒÕεHf (-0.9 to -2.4), and high 87Sr/86Sr (~0.706) and 206Pb/204Pb (~18.75). In contrast to previous notion that calls for significant contamination of upper continental crust in the magma chamber, we propose the Kueishantao HMAs to have resulted from partial melting of the subducted sediments and altered Philippine Sea crust followed by melt-mantle interaction in the mantle wedge. This interpretation is consistent with seismic tomographic results under the SPOT region marking with a combination of collision/extension/subduction tectonic context off NE Taiwan.
V21B-0601
Variable crustal structure along the Juan de Fuca Ridge; influence of on-axis hotspots and absolute plate motions
Observations of crustal structure along the Juan de Fuca Ridge (JdFR) reveal influence of on-axis hotspots and absolute motion of the spreading ridge on axial melt distribution. Multi-channel reflection seismic and bathymetric data are used to constrain axial structure and spreading history for past 4-8 Ma within 3 spreading corridors crossing Cleft, Northern Symmetric (NSymm) and Endeavour segments. Along-axis data reveal south-to-north gradients in seafloor relief, depth and presence of the crustal magma lens which indicate a warmer axial regime at Cleft segment than at the northerly NSymm and Endeavour segments. South-to-north gradients are also observed within individual ridge segments with shallower ridge axis and crustal magma lens located to the south within most segments. Cross-axis lines reveal differences in inferred crustal thickness with higher average two- way travel times (twtt) to Moho found at Cleft and Endeavour segments (2300 and 2200 msec) coincident with distinct plateau, 32 and 40 km wide. Further on the ridge flanks, Moho twtt are similar at all 3 segments (~2100 +/- 100 msec) indicating little difference in inferred crustal thickness prior to ~0.6-0.7 Ma. We attribute the recent increase in crustal production at Cleft and Endeavour segments to initiation of ridge axis-centered melt anomalies associated with the Cobb HS and the Heckle melt anomaly. Rapid along-axis channeling of the Cobb mantle anomaly, preferentially to the south to influence melt production at Cleft segment 160 km away is implied. The northwesterly absolute motion of the JdFR axis could account for preferential southward directed along-axis asthenospheric flow. The regional and within-segment scale south to north gradients in seafloor and sub- seafloor structure along the JdFR may also reflect the influence of absolute motion of the ridge axis on sub-axial melt distribution.
V21B-0602
Frequently Shifting Magma Sources at Endeavour Segment, Juan de Fuca Ridge
Different mantle sources and magma batches contribute to crustal growth over short temporal and spatial scales at the 10-km-long summit of the Endeavour segment of the intermediate spreading rate Juan de Fuca Ridge. Based on analyses of >275 basalts collected by submersible, about twenty "chemo-stratigraphic units" have been identified and mapped. Each reflects a different combination of mantle sources, differentiation path, and mixing history. Each represents a different filling of a magma chamber and most include a range in differentiation. Most occur within the <1 km-wide axial valley that is thought to be <10 Ka in age. The maximum along-strike length of any one unit is ~2 km. The flanking ridges differ from each other and from most of the axis in their uppermost basalts. The maximum extent of fractional crystallization within any single chemostratigraphic unit is about 30%; more fractionated magmas do not erupt, or mix in the chamber with subsequent magma batches. Two types of enriched basalts are recognized. E-MORB is enriched in K, LREE, and Nb, and depleted in Y+HREE, but is similar isotopically to axial N-MORB. We attribute it to low degree deep melting of damp peridotite. In contrast, although T-MORB has K/Ti ratios intermediate between N- and E-MORB, this is because of preferential enrichment in HFSE. Its Pb, Sr, Nd, and Hf isotope ratios are maxima, approaching those of FOZO. It is attributed to low degree melting of a chemically distinct component, perhaps pyroxenitic. Both enriched components lie on the same Pb isotope chords so are thought to be similar in age and origin.
V21B-0603
Effects of Triple Junction Plate Boundary Geometry on Mantle Thermal Structure and Crustal Production
Plate boundary geometry likely has an important influence on crustal production at mid-ocean ridges. Many studies have explored the effects of geometrical features such as transform offsets and oblique ridge segments on mantle flow and melting. This investigation calculates how triple junction geometry may influence the generation of oceanic crust. An earlier study (Georgen and Lin 2002) suggested that the effects of a ridge-ridge- ridge configuration are most pronounced under the branch with the slowest spreading rate. Thus, we create a three-dimensional, finite element, variable viscosity numerical model that incorporates thermal buoyancy and focuses on the slowest-diverging ridge. First, we employ a model geometry similar to the Southwest Indian Ridge near the Rodrigues Triple Junction in the central Indian Ocean. Within 100 km of the triple junction, axial temperatures at depths within the partial melting zone are predicted to increase by ~40 deg. C, and crustal thickness is calculated to increase by 1 km. We also explore how varying spreading rate magnitude affects triple junction dynamics. Consistent with Georgen and Lin (2002), variable-viscosity flow models for calculations where all spreading rates are greater than the Rodrigues-like case (i.e., with plate kinematics similar to those observed around the Galapagos Triple Junction) predict little influence of the triple junction geometry on the thermal structure of the slowest-spreading ridge. However, when ridge divergence rates are all relatively slow (i.e., with plate kinematics similar to those observed around the Azores Triple Junction), significant along-axis increases in mantle temperature and crustal thickness are calculated. At depths within the partial melting zone, temperatures are predicted to increase by ~150 deg. C, similar to the excess temperatures associated with mantle plumes. Likewise, crustal thickness is calculated to increase by approximately 6.5 km over the 200 km of ridge closest to the triple junction. These results could imply that some component of the excess volcanism observed in geologic settings such as the Terceira Rift may be attributed to the effects of triple junction geometry, although the important influence of features like nearby hotspots (e.g., the Azores hotspot) cannot be evaluated without additional numerical modeling.
V21B-0604
Discovery of hydrothermal plumes at the Rodoriguez segment, Mid-Indian Ridge
Geochemical surveys for the Rodriguez Segment of the Central Indian Ridge have been quite few so far. One of the principal objectives of this cruise is to locate hydrothermally active sites along the plate spreading center of the segments 15 and 16 of the Central Indian Ridge. We conducted water column observations using a CTDT (a package of Conductivity, Temperature, Depth, and light transmission sensors) + CMS (Carousel Multi-Sampling system) attached with an in situ Mn analyzer GAMOS (Okamura et al., 2001) in order to detect any anomalies due to hydrothermal plumes. Summary of some highlighted results: (1) Two typical hydrothermal active stations have been recognized; one is located east of East Brigitte Protrusion in the Roger Plateau (on the segment 15), and the other is close to Beak Rocks on the Great Dodo Lava Plain (on the segment 16). (2) Both sites are characterized by light transmission (LT) anomalies of ˜0.2 % at maximum, suggesting the existence of black (or white) smoker activity. (3) While the DMn(nM)/DLT(%) at the former station is ˜100, in a similar trend as that observed during KH-93-3 cruise (Kairei Field), the ratio at the latter station is only 10, an order of magnitude lower than that at station 20. There may be a significant difference in hydrothermal fluid chemistry at these two locations.
V21B-0605
The Azores Triple Junction zone: a highly detailed multibeam bathymetry survey
We present here the results of a high precision bathymetric mapping of the Azores Triple Junction zone, close to the Mid-Atlantic Ridge. The survey was made with an EM300 on water depths between 1000 and 2000m, assuring its optimal behavior and hence the production of a high resolution grid of 50 m x 50 m. The STRIPAREA cruise was sailed on 2006 on board the N/O Suroit, as a result of a international collaboration involving CIMA (UALG), CGUL (ULIS) and the French Research lab: UMR6538/CNRS from Brest. Analysis of the data is ongoing but several observations can be outlined. We show that the TJ area closet to the MAR is a complex tectonic area, largely depleted from volcanic constructs. This is in clear contrast with the much higher magmatic budget observed in the adjoining MAR axis to the West and Faial Ridge to the East. The fault pattern is organised into three main directions: WSW-ENE, NW-SE, WNW- ESE disrupting nearly N-S MAR generated Abyssal hill fabric and controlling several basins. The East side of the North Azores Fracture Zone is not a continuous structure but it is interrupted at one location by ridge parallel constructions.
V21B-0606
Origin of Volcanic Seamounts Offshore California Related to Interaction of Abandoned Spreading Centers with the Continental Margin
The numerous NE-SW trending volcanic seamounts at the continental margin offshore central to Southern California owe their existence to the complex tectonics that resulted when small spreading ridge segments intersected and partly subducted beneath the continental margin during the Miocene plate reorganization. A limited number of dredged samples had indicated multiple episodes of coeval, alkalic volcanism at geographically widely separated sites (Davis et al., 2002, GSA Bull. 114, 316-333). 450 new samples were collected from 8 seamounts from 37. 5°N to 32.3°N with MBARI's ROV Tiburon. Ar-Ar ages for 50 of these samples extend the ages of volcanism from 18 Ma to 2.8 Ma. The dominant whole rock compositions are differentiated alkalic basalt, hawaiite, and mugearite, but include minor benmoreite, trachyte, and rare tholeiitic basalt. This entire range of compositions is also present in glassy margins or in volcaniclastic breccias, except for the trachyte, which had no glassy margins. Trace element abundances and ratios (e.g. REE, Zr, Nb, Ta, Th, Ba, etc.) are typical for ocean island basalt, whether the seamount is located on the Pacific plate (e.g. Pioneer, Gumdrop, Guide, Davidson, San Juan, San Marcos) or on the continental slope (Rodriguez) or within the Southern Continental Borderland (Northeast Bank). Nine samples, predominantly from Rodriguez Seamount, show a calc-alkaline trend with lower Nb, Ta, and higher Th. These samples may be erratics (Paduan et al., 2007, Marine Geology, in press). Sr, Nd, and Pb isotopic compositions plot within the Pacific N-MORB field for the northern seamounts (Pioneer, Gumdrop, Guide) but suggest progressively more radiogenic sources southward. There is considerable scatter at each site, especially with regard to 87Sr/86Sr, despite severe acid-leaching of the samples. Isotopic and trace element compositions indicate sources that are heterogeneous at a small scale. Chondrite-normalized Ce/Yb suggest smaller degree of melting and more alkalic compositions with decreasing age, although there is again considerable scatter. Chondrite-normalized La/Sm versus Zr/Nb form a continuum from the seamount lavas to depleted N-MORB and E-MORB suggesting a common origin by decompression melting of a mantle source with randomly distributed enriched heterogeneities, which are incorporated to a greater degree with decreasing degree of melting. Based on symmetric magnetic anomalies, only Davidson Seamount has been identified as straddling a fossil spreading center (Lonsdale, 1991, AAPG Mem. 47, 87-125). However, the other seamounts along the continental margin with the same NE-SW orientation and similar geochemical characteristics probably originated in a similar setting, erupting lavas along zones of weakness in the ocean floor fabric related to past seafloor spreading. Small volumes of magma can apparently rise long after spreading ceases if there is enough enriched source component to facilitate melting combined with zones of weakness in the underlying ocean crust fabric and/or extensional tectonics.
V21B-0607
Constraints on Melting Beneath the Slow-Spreading Kolbeinsey Ridge from 238U, 230Th, and 231Pa Excesses
We report measurements of U-Th and U-Pa disequilibria, radiogenic isotopes, and major and trace element compositions for young basalts (<10 ka) from the slow-spreading Kolbeinsey Ridge north of Iceland (u1/2~1 cm/yr; 67°05'- 70°26'N). These basalts display both (230Th/238U)<1 and (230Th/238U)>1. While the (230Th/232Th) in the samples are essentially constant, there is a large range in (238U/232Th), generating a horizontal array on a (230Th/232Th) - (238U/232Th) isochron plot. Kolbeinsey rocks have highly depleted incompatible element abundances and ratios and (231Pa/235U)>1. Samples with 238U excesses have the lowest Th concentrations, consistent with either shallow mantle melting or shallow mantle-melt rock reaction. Nd, Hf, and Sr isotopic compositions of these samples are not correlated with (230Th/238U) and fall into two groups relative to their proximity to Iceland: samples south of the Spar Fracture Zone (nearer to the Iceland hotspot) generally are isotopically enriched, while samples north of the Spar FZ are isotopically depleted, reflecting a change in mantle source composition north and south of the fracture zone. Southern Kolbeinsey samples also tend to have higher (231Pa/235U) values than those from north of the Spar FZ, which could reflect source heterogeneity, differences in the melting process, or a combination thereof.
V21B-0608
Davidson Seamount: A Volcano Slowly Built on an Abandoned Spreading Center
Davidson Seamount is located 80 km off Big Sur, California, and rises from the 3500 m abyssal plain to 1254 m depth. The elongated volcanic edifice consists of a series of parallel ridges serrated with steep cones, built over millions of years above an abandoned spreading center. It has been explored and sampled with the ROV Tiburon, and the lithologic distribution, glass chemistry, and ages of the rocks are presented here. Large, bulbous pillow lavas are common deep on the seamount. The shallower cones are mainly composed of blocky flows that provide substrate for large corals and sponges. The cones are draped with volcaniclastic rocks ranging from sandstone to breccia as thick, layered pavements that are now eroded with pits and potholes. This fragmental material is evidence of explosive eruptions. A perched lava pond was discovered in high-resolution maps made by MBARI's Mapping AUV and explored with the ROV Tiburon. Nothing like it has been found elsewhere on Davidson or the other seamounts off the California continental margin. The pond lies between high ridges near the summit. It was a vigorous flow that overtopped its levees with elongate pillows, and then drained, leaving collapse pits a few meters deep veneered with "bathtub rings" and no lava pillars. Deeper than 2000 m, glass from pillow rinds and breccias are basalt and hawaiite. Shallower than 2000 m depth, the rocks include basalt and hawaiite, and also fractionated lavas of mugearite and trachyte. The lavas were all submarine erupted, even the fragmental material, as inferred from high sulfur content in the glasses. Ages of the lavas range from 9.8 to 14.8 Ma. The oldest rocks are along the central ridge, and the youngest rocks are on the flanks and southern end of the edifice. The volcano erupted onto much older crust, which is inferred to be 20 Ma from magnetic anomalies. The numerous small cones of disparate chemistry and long eruptive period suggest episodic growth of the volcano over 5 to 10 million years.
V21B-0609
Geochemistry of the Vance Seamount Chain: Off-axis Seamounts Along the Southern Juan de Fuca Ridge
The Vance Seamounts are a NW-SE trending chain of eight seamounts (Vance A through G) at approximately 45° 15'N and 130° 20'W on the flanks of the southern Juan de Fuca Ridge (SJdF). The chain consists of truncated cone-shaped volcanoes (A, C, E/D, and F) and large areas of disorganized volcanic constructs comprised of numerous small cones and volcanic ridges (B and G). Geologic evidence suggests that the seamounts progressively increase in age from the southeast (G) to the northwest (A). Eruptive products include pillow lavas, sheet flows, massive flows, and hyaloclastites. During July-August 2006, samples of the seamounts were collected using the ROV Tiburon; volcanic glass and whole rock samples were analyzed for major element, trace element, and isotopic compositions. Analyses show there are significant geochemical differences in lava composition between the seamounts in the chain, as well as within individual seamounts. Overall, Vance lavas are more primitive and exhibit greater diversity than samples from the adjacent SJdF. Major element data indicate that all samples are subalkaline-MOR basalts, but a range of depleted to moderately enriched varieties exist (0.02-0.31 wt.% K2O). Incompatible trace element data indicate samples range from very depleted to slightly enriched ([La/Sm]N =0.354-1.07; [Ce/Yb]N =0.498-1.59; Zr/Y=1.69-3.86). Variations in major and trace elements suggest differing sources and/or styles of partial melting, rather than shallow level processes like fractional crystallization as the reason for most of the geochemical diversity. Sr-Nd-Pb isotope data supports slight variations in source characteristics (87Sr/86Sr = 0.702477-0.702687; 206Pb/204Pb = 18.117-18.816; and 143Nd/144Nd = 0.513052-0.513181). Variations are just outside of analytical error for many of the seamounts, but fall along a general mixing trend between the Depleted Mantle (DM) and High μ (HIMU) mantle reservoirs. Mantle heterogeneity beneath the Vance Seamounts appears to be on a very short length scale, with each seamount showing significant incompatible trace element and isotopic variations that are somewhat correlated. The very depleted nature of many of the Vance samples compared to adjacent ridge samples is similar to observations made at other off-axis chains (e.g. Lamont Seamounts, Heck Seamounts), but the presence of E-MORB is unusual for both the SJdF ridge and other near-axis seamounts. These results support a "veined mantle'' model for the NE Pacific mantle. The off-axis seamount samples appear to record small scale heterogeneity of the underlying mantle because individual melt batches undergo only minor amounts of differentiation and mixing whereas the inherent mantle diversity is homogenized for on-axis ridge samples due to the more steady-state shallow magmatic processes that occur in sub-axial magma bodies.
V21B-0610
Preliminary Geophysical Results from the Ninetyeast Ridge Expedition
Geologic and geophysical data were collected along the Ninetyeast Ridge (NER) between 5.5°N and 26.1°S during the NSF-funded KNOX06RR cruise of R/V Roger Revelle during June to August 2007. Scientific objectives were to obtain site survey data for proposed drilling and to test the hotspot formation model of NER. During 48.6 days underway, 3861 km of high-resolution multichannel seismic reflection data were collected at seven sites, along with multibeam echo-sounder bathymetry, 3.5 kHz echo-sounder profiles, magnetic, and gravity data while underway. In addition, 33 dredges were collected along the ridge, recovering basalt for geochemical and geochronologic analysis at 24 sites. Bathymetric data show changes in morphology along NER from large, individual seamounts in the north, to smaller seamounts and ridges in the central NER, and to a high, narrow, continuous ridge in the south. Seismic and bathymetric data indicate that NER is pervasively faulted, indicating a greater role for tectonic deformation than is generally appreciated. The east side of the ridge exhibits a steep, linear escarpment, 1-2 km high, especially along the south NER. In contrast, western NER slopes are gentle, apparently unfaulted volcanic flanks. Morphology implies that the scarps resulted from strike-slip faulting nearly parallel to the ridge. Seismic data also show faulting of volcanic basement with trends highly oblique to the NER trend. This set of faults displays horst and graben structures, creates sediment-filled troughs, and sometimes nearly dissects the ridge. Many of these faults form half- grabens and may be related to seafloor spreading. Thick sediment covers northern NER volcanoes (typical thickness of 400-500 m) resulting from proximity to the Bengal fan. Sediment cover decreases from north to south, with typical thicknesses less than approximately 100 m at the south end of NER, where sediment thickness seems to be controlled mainly by topography.
V21B-0611
Solid mantle upwelling rate beneath the Mid Atlantic Ridge
The upwelling velocity of the solid mantle beneath mid ocean ridges affects processes of melting and generation of the oceanic crust, and constrains models of spreading of oceanic plates. Models of passive flow require that the solid mantle rises beneath a ridge at a speed similar to the half spreading rate of the plates. However, increased buoyancy due to melt depletion and melt retention may cause the sub-ridge mantle to rise at a speed much faster than the half spreading rate. In order to constrain these models it is important to estimate the sub- ridge mantle up-welling rate. Such estimates have been attempted in a few cases by measuring in zero-age basalt disequilibria in short lived isotopes produced by the U-decay series. Melt upward migration from the sub-ridge melting zone to the crust is much faster than its parent solid mantle upward flow; therefore, a time lag is created between melt emplacement as basalt in upper lithosphere and emplacement of the parent residual peridotite in the lower lithosphere. We were able to estimate this time lag along a lithospheric sliver, generated at a 80 km long segment (EMAR) of the Mid Atlantic Ridge located between 10° N and 11°N, just south of the Vema transform. The northern edge of this lithospheric sliver exposes crustal and upper mantle units along a 320 km long spreading flowline equivalent to a 26 Myr long time interval. Comparing temporal variations of crustal thickness, inferred from the Residual Mantle Bouguer Anomaly along a flowline starting from the centre of the EMAR segment, to temporal variations of mantle degree of melting, estimated from mineral chemistry of peridotites exposed along the Vema lithospheric section, allowed us to estimate an average solid mantle rising rate of 16.5 mm/yr below the EMAR segment for a time interval from 26 to 2 Ma. This rate is slightly higher than the average half spreading rate of 15.6 mm/yr for the same period. The similarity between average up-welling rate and spreading rate at 11° N on the Mid Atlantic ridge is in line with up-welling estimates obtained by U-series disequilibria, supporting in general a mostly passive mantle flow model. However, temporal variations of spreading rate at 11° N for the last 26 Myr are decoupled from mantle degree of melting and geothermometry estimates, suggesting variations of mantle upwelling rates due to an active component.
V21B-0612
Compositional Comparison of Iceland Rift Zones and Adjacent Portions of the Mid-Atlantic Ridge.
Iceland is a portion of the Mid-Atlantic Ridge (MAR) that has been built by anomalous crustal production throughout the 55ma spreading history of the opening of the Atlantic Ocean. The anomalously thick crust of Iceland contains the subaerial traces of the MAR which are the volcanically active rift zones. From the south, the Reykjanes Ridge (RR) continues on land as the Western Volcanic Zone (WVZ). In the north, the Northern Volcanic Zone (NVZ) traces into the sea where it offset from the Kolbeinsey Ridge (KR) by the Tjornnes Fracture Zone (TVZ). We report the results of petrologic comparison of the WVZ, the EVZ, and the NVZ of Iceland and the adjacent portions of the MAR – the RR and the KR. The EVZ, WVZ, and NVZ have been shown to have similar crustal structures with ~20 km thick crust thickening toward the hotspot in central Iceland with magma chambers located at the base of the crust and at some depth in the upper crust. Likewise, the KR and RR have melt chambers at the base of and within the crust. Melt compositions have been compared using a filtered database of 588 glass analyses from 29 localities throughout the rift zones, 57 glass analyses from the KR, and 521 glass analyses from the RR. This is the first such study carried out with such an extensive data set. Compositions are similar between the NVZ and WVZ with SiO2 wt.% of 49.0, and 48.6, MgO wt.% of 7.9, and 7.5, and FeOT wt.% of 10.9, and 11.7 respectively. The EVZ which is considered to be a propagating rift is a bit different with SiO2 wt.% of 49.4, MgO wt.% of 5.9, and FeOT wt.% of 13.8. The NVZ and WVZ have also been compared with their respectively adjacent ridge segments, the KR (SiO2 50.3 wt.%, MgO 6.9 wt.%, and FeOT 12.0 wt.%), and the RR (SiO2 50.8 wt.%, MgO 6.9 wt.%, and FeOT 12.3 wt.%). Mg#s for the NVZ and the WVZ are 0.56, and .053 respectively while the Mg# for both the KR and RR is 0.50. For further comparison, a database of 9035 glass analyses from mid-ocean ridge basalts worldwide gives average SiO2 wt.% of 50.7, MgO wt.% of 7.3, FeOT wt.% of 10.3, and Mg# of 0.56.