John M. Sinton

Volcanic eruptions on mid‐ocean ridges: New evidence from the superfast spreading East Pacific Rise, 17°–19°S

uniform sediment cover were recovered from lava that buries older faulted terrain. The boundary in lava composition coincides with a change in depth to the top of an axial magma lens seismic reflector, consistent with magmas from two separate reservoirs being erupted in the same event. Chemical compositions from throughout the area indicate that lavas with identical compositions can be emplaced in separate volcanic eruptions within individual segments. A comparison of our results to global data on submarine mid-ocean ridge eruptions suggests consistent dependencies of erupted volume, activated fissure lengths, and chemical heterogeneity with spreading rate, consistent with expected eruptive characteristics from ridges with contrasting thermal properties and magma reservoir depths. INDEX TERMS: 3035 Marine Geology and Geophysics: Midocean ridge processes; 8414 Volcanology: Eruption mechanisms; 8439 Volcanology: Physics and chemistry of magma bodies; 3655 Mineralogy and Petrology: Major element composition; KEYWORDS: lava flow, chemical heterogeneity, erupted volume, lava morphology, side-scan sonar

Isotope and trace element characteristics of a super-fast spreading ridge: East Pacific rise, 13–23°S

Isotopic patterns of Nd, Sr, and Pb are remarkably coherent along the super-fast spreading portion of the East Pacific Rise from 13°S to 23°S. Between 15.8°S and 20.7°S, all three define a broad, smooth peak, which culminates at ∼ 17–17.5°S and is characterized by elevated 87Sr86Sr, 206Pb204Pb, and lower ϵNd (reaching values of 0.70271, 18.64, and +8.9, respectively). To the north and south this peak is flanked by ∼ 300 km long, isotopically homogeneous sections of ridge with higher ϵNd (∼ +10.9) and lower 87Sr86Sr (∼ 0.7024) and 206Pb204Pb (∼ 18.1). Although otherwise similar, these two sections differ from each other slightly in their 207Pb204Pb and 87Sr86Sr ratios. The isotopic peak corresponds to a region of greater axial cross-sectional area, but axial bathymetry and physical segmentation appear generally unrelated to mantle isotopic composition. However, an abrupt break in isotopic ratios does occur at the large, > 3 Ma, southward-propagating overlapping spreading center at 20.7°S, which marks the end of the south limb or flank of the isotopic peak. The peak itself appears to be a manifestation of large-scale binary mixing between material possessing at least mildly plume-like Nd, Pb, and Sr isotopic characteristics (most abundant at ∼ 17–17.5°S) and two slightly different high-δNd mantle end-members equivalent to those north of 15.8°S and south of 20.7°S. Helium isotopes also define a prominent along-axis peak, but it spans a much narrower range of latitude and is offset slightly to the north of those for Nd, Sr and Pb isotopes. The combined results suggest that a discrete mantle heterogeneity may be entering into the melt zone near 15.8°S and migrating southward as far as the 20.7°S overlapping spreading center. Isotopic variability at short length scales is very limited throughout the entire 13–23°S region. It cannot be solely a result of homogenization by petrogenetic processes, because there is a lack of corresponding uniformity in ratios of highly to moderately incompatible elements; also, isotopes do not correlate with major element indicators of degree of partial melting or differentiation, or, in general, with the secondary magmatic segmentation thought to reflect different partial melting domains. Therefore, the subaxial mantle must be isotopically well-mixed relative to the scale of melting. In part, this probably reflects: (1) a larger volume of melting per unit length of ridge; and (2) a greater flow of mantle into the subaxial melt zone at super-fast spreading; but also must represent (3) a reduced amount of real isotopic variability in the shallow asthenosphere, as emphasized by the regional isotopic uniformity north and south of the isotopic peak. Such large-scale homogeneity could be a result of enhanced convective asthenospheric mixing over a long period of time. It could also reflect a low, long-term input of continental, lithospheric, recycled slab, or plume-type material into the regional asthenosphere. Largely independent of the north-south isotopic patterns is a fairly regular, southward depletion in highly incompatible elements such as Rb and Nb, superimposed on which is sizable local variability. Because ratios of highly to moderately incompatible elements show little or no correlation with major-element indicators of degree of melting, much of the variation in highly incompatible elements must be caused by a different (probably larger) volume of mantle than that conferring the major element signatures, or by one (or more) event that preceded the main melting episode in the not too ancient past.

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.

Evolution of abyssal lavas along propagating segments of the Galapagos spreading center

Abstract The unusual petrological diversity of abyssal lavas erupted along some segments of the Galapagos spreading center is a direct consequence of the propagation (elongation) of these segments into older oceanic crust. With increasing distance behind propagating rift tips, relatively unfractionated MORB erupted close to the tips are joined first by FeTi basalts (bimodal assemblage) and then by a wide range of basaltic and siliceous lavas. Further behind propagating rift tips, this broad range diminishes again, approaching the narrow compositional range of adjacent normal ridge segments. These compositional variations reflect the evolution of the subaxial magmatic system beneath the newly forming spreading center as it propagates through a pre-existing plate. We envisage this evolution as proceeding from small, isolated, ephemeral magma chambers through increasing numbers of larger, increasingly interconnected chambers to the steady-state buffered system of a normal ridge. Throughout this evolution, magma supply rates gradually increase and cooling rates of crustal magma bodies decrease. High degrees of crystal fractionation are favored only when a delicate balance between cooling rate and resupply rate of primitive magma is achieved. At other propagating and non-propagating ridge-transform intersections the degree to which the balance is achieved and the length of ridge over which it evolves control the distribution of fractionated lavas. These effects may be evaluated provided a number of tectonic variables including transform length, spreading and propagation rates are taken into account.

Petrologic consequences of rift propagation on oceanic spreading ridges

Abstract The production of anomalously differentiated lava compositions at several mid-ocean spreading centers can be attributed to magmatic processes associated with propagating rifts. The degree of differentiation attained by magmas beneath oceanic spreading ridges depends mainly on the balance between cooling rate and the supply rate of new magma to shallow chambers. Low supply rates and moderate cooling rates allow advanced degrees of closed-system fractionation to occur. High supply rates result in open systems in which magma compositions are buffered by frequent replenishment with new hot magma. Propagating rift tips are a special class of ridge-transform intersection in which the balance between cooling and supply rates is conducive to the development of advanced degrees of differentiation over an expanded length of ridge. This balance is affected by the spreading rate, the propagation rate of the rift, the length of the bounding transform and proximity to hotspots. Maximum compositional variability and maximum degree of differentiation occur within 50 km of propagating rift tips and subsequently diminish with increasing distance. Rifts that propagate through plates in directions approximating their absolute motion relative to the lower mantle are characterized by the presence of anomalously differentiated lavas over longer ridge segments than are rifts that propagate against their absolute motion. Geochemical anomalies may persist, though changing in degree and extent, for several million years on ridge segments that stop propagating. The concept of “magnetic telechemistry” is generally supported by our study, but in the vicinity of hotspots, magnetic anomaly amplitude may be controlled more by bathymetric and/or thickened magnetic layer effects than by geochemistry.

Mantle source heterogeneity and melting processes beneath seafloor spreading centers: The East Pacific Rise, 18°–19°S

We present new major and trace element and Nd-Pb-Sr isotopic data on samples from the East Pacific Rise (EPR) axis and nearby seamounts in the Hump area, 18°–19°S. Most samples studied are normal mid-ocean ridge basalt (N-MORB); four samples from the southern seamounts are enriched MORB (E-MORB). Dredge 52 samples from a southern seamount are depleted in incompatible elements yet possess “enriched” isotopic signatures. Except for the dredge 52 samples, all the samples show significant correlations between isotopic ratios and ratios of incompatible elements; that is, incompatible elements and isotopes are coupled. Sr and Nd isotopic ratios correlate significantly better with ratios of moderately incompatible elements than with ratios involving highly incompatible ones (e.g., Rb, Nb, and K) which appear to be “overenriched”. Both isotopic and incompatible element ratios also correlate with the extent of melting calculated from major elements. We interpret these correlations as mixing trends resulting from melting of a heterogeneous source containing enriched (“plume-like”) domains of variable sizes. Overenrichment of highly incompatible elements in E-MORB appears to be recent and is best explained by low-degree-melt infiltration in the source region prior to major melting events. The low-degree melts are primarily derived from isotopically N-MORB mantle. This low-degree melt process also explains the incompatible element-isotope decoupling throughout the EPR between 13° and 23°S. The dredge 52 samples too are consistent with such a process, but their immediate source is a site of low-degree melt generation.

Postglacial eruptive history of the Western Volcanic Zone, Iceland

New field observations, age constraints, and extensive chemical analyses define the complete postglacial eruptive history of the 170-km-long Western Volcanic Zone (WVZ) of Iceland, the ultraslow-spreading western boundary of the south Iceland microplate. We have identified 44 separate eruptive units, 10 of which are small-volume eruptions associated with the flanking Grimsnes system. Overall chemical variations are consistent with very simplified models of melting of a source approximating primitive mantle composition. The 17 eruptions in the first 3000 years of postglacial time account for about 64% of the total postglacial production and are incompatible-element depleted compared to younger units, consistent with enhanced melting as a consequence of rebound immediately following deglaciation. Steadily declining eruption rates for the last 9000 years also correlate with changes in average incompatible element ratios that appear to reflect continued decline in melting extents to the present day. This result is not restricted to the WVZ, however, and may herald a decline in melting throughout all of western Iceland during later postglacial time. Lavas from the northern part of the WVZ are depleted in incompatible elements relative to those farther south at all times, indicating either a long-wavelength gradient in mantle source composition or variations in the melting process along axis. We find no evidence in the postglacial volcanic record for current failure of the WVZ, despite evidence for continued propagation of the eastern margin of the microplate. The dominance of lava shields in the eruptive history of the WVZ contrasts with the higher number of fissure eruptions in other Icelandic volcanic zones. WVZ shields represent long-duration, low-effusion rate eruptions fed by recharge magma arising out of the mantle. Average effusion rate is the key variable distinguishing shield and fissure eruptions, both within the WVZ and between different volcanic zones. High effusion rate, large-volume eruptions require the presence of large crustal magma reservoirs, which have been rare or absent in the WVZ throughout postglacial time.

Crustal thickness along the western Galápagos Spreading Center and the compensation of the Galápagos hotspot swell

Wide-angle refraction and multichannel reflection seismic data show that oceanic crust along the Galapagos Spreading Center (GSC) between 97‡W and 91‡25PW thickens by 2.3 km as the Galapagos plume is approached from the west. This crustal thickening can account for V52% of the 700 m amplitude of the Galapagos swell. After correcting for changes in crustal thickness, the residual mantle Bouguer gravity anomaly associated with the Galapagos swell shows a minimum of 325 mGal near 92‡15PW, the area where the GSC is intersected by the Wolf^ Darwin volcanic lineament (WDL). The remaining depth and gravity anomalies indicate an eastward reduction of mantle density, estimated to be most prominent above a compensation depth of 50^100 km. Melting calculations assuming adiabatic, passive mantle upwelling predict the observed crustal thickening to arise from a small increase in mantle potential temperature of V30‡C. The associated thermal expansion and increase in melt depletion reduce mantle densities, but to a degree that is insufficient to explain the geophysical observations. The largest density anomalies appear at the intersection of the GSC and the WDL. Our results therefore require the existence of compositionally buoyant mantle beneath the GSC near the Galapagos plume. Possible origins of this excess buoyancy include melt retained in the mantle as well as mantle depleted by melting in the upwelling plume beneath the Galapagos Islands that is later transported to the GSC. Our estimate for the buoyancy flux of the Galapagos plume (700 kg s 31 ) is lower than previous estimates, while the total crustal production rate of the Galapagos plume (5.5 m 3 s 31 ) is comparable to that of the Icelandic and Hawaiian plumes.

Pliocene and Pleistocene alkalic flood basalts on the seafloor north of the Hawaiian islands

Abstract The North Arch volcanic field is located north of Oahu on the Hawaiian Arch, a 200-m high flexural arch formed by loading of the Hawaiian Islands. These flood basalt flows cover an area of about 25, 000 km 2 ; the nearly flat-lying sheet-like flows extend about 100 km both north and south from the axis of the flexural arch. Samples from 26 locations in the volcanic field range in composition from nephelinite to alkalic basalt. Ages estimated from stratigraphy, thickness of sediment on top of the flows, and thickness of palagonite alteration rinds on the recovered lavas, range from about 0.75–0.9 Ma for the youngest lavas to somewhat older than 2.7 Ma for the oldest lavas. Most of the flow field consists of extensive sheetflows of dense basanite and alkalic basalt. Small hills consisting of pillow basalt and hyaloclastite of mainly nephelinite and alkalic basalt occur within the flow field but were not the source vents for the extensive flows. Many of the vent lavas are highly vesicular, apparently because of degassing of CO 2 . The lavas are geochemically similar to the rejuvenated-stage lavas of the Koloa and Honolulu Volcanics and were generated by partial melting of sources similar to those of the Koloa Volcanics. Prior to eruption, these magmas may have accumulated at or near the base of the lithosphere in a structural trap created by upbowing of the lithosphere.

Recent tectonic, magmatic, and hydrothermal activity on the East Pacific Rise between 17°S and 19°S: Submersible observations

The objective of the Naudur cruise (December 1993) of the submersible Nautile was to study the interaction among magmatic, tectonic, and hydrothermal processes at a very fast spreading mid-ocean ridge axis. Twenty-three dives were completed, both along and across the axis, in four areas located between 17°10′ and 18°45′S on the East Pacific Rise. Rock, sulfides, water, and biological samples have been collected along each of the segments. Two main types of segments have been distinguished, characterized either by the predominance of present-day volcanic activity or by predominant tectonic activity. Linked to both types of activity, 69 hydrothermal sites have been discovered and sampled. They comprise four types, interpreted as successive evolutionary stages. The first are shimmering water sites which occur immediately after the formation of lava lakes and are characterized by large surface area and poorly developed associated fauna. The second, in areas dominated by recent volcanic activity, have waters venting directly from lava fissures and more focused discharge areas through black smoker chimneys. The third stage is represented by more mature hydrothermal vents and deposits, along the faults bounding the eastern side of the axial graben in tectonic-dominated areas. The associated fauna is well developed. The fourth stage corresponds to the reactivation of volcanic activity with lava flows, young black smokers, and diffuse venting associated with the faults bounding the axial graben. Fluids collected range from 200° to 340°C and show a wide variability in chemical and gas composition. Within each of the explored areas, evidence of recent volcanic activity has been observed.