Emily M. Klein

Geochemistry of basalts from the southeast Indian Ridge, 115°E–138°E

The ocean basin south of Australia contains the Australian-Antarctic Discordance, an anomalously deep portion of the Southeast Indian Ridge that marks a boundary between isotopic provinces characteristic of the Indian and Pacific oceans. Samples recovered from the ridge within the discordance display unusual chemical compositions compared to normal mid-ocean ridge basalt (N-MORB) of the same MgO contents, including low iron, high silica, and high sodium abundances and elevated abundances of highly incompatible trace elements. In contrast, samples from the ridge east of the discordance, where the ridge is of average axial depth, display major and trace element systematics more typical of N-MORB. Major and moderately incompatible trace elements show no evidence of a discontinuity in source composition corresponding to the location of the known isotopic discontinuity within the discordance. Ratios of highly incompatible trace elements, however, reveal a gradational change in the range of values across the location of the isotopic discontinuity. Modelling of along-strike variations in major element chemistry suggest they may result from systematic variations in the extent and pressure of melting. The lowest solidus pressures and least extents of melting occur in the mantle beneath the discordance, supporting geophysical inferences based on bathymetric, gravity, and seismic evidence that the discordance overlies a region of cooler mantle temperatures.

Structure of uppermost fast-spread oceanic crust exposed at the Hess Deep Rift: Implications for subaxial processes at the East Pacific Rise

The uppermost 2 km of the oceanic crust created at the fast spreading (135 mm yr−1, full rate) equatorial East Pacific Rise (EPR) is exposed for tens of kilometers along escarpments bounding the Hess Deep Rift. Mosaics of large-scale digital images from the remotely operated vehicle (ROV) Argo II and direct observations from the submersible Alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading. Dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units. These rock units are characterized by extensive faulting, fine-scale fracturing, and rotations of coherent crustal blocks meters to tens of meters across. The uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces. Through most of the basaltic lava unit, however, lava flow contacts dip (20°–70°W) toward the EPR and generally increase in dip downward in the section. Dikes cutting the lavas and in the underlying sheeted dike unit generally dip (90°–40°E) away from the EPR. Deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units. We interpret this upper crustal structure as the result of subaxial subsidence within 1–2 km of the EPR that accommodated the thickening of the basaltic lava unit to ∼500 m. Variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply. These results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the Hess Deep Rift. If this type of structure is typical of uppermost oceanic crust generated at the EPR, these processes may be common along fast spreading mid-ocean ridges.

Subduction zone geochemical characteristics in ocean ridge basalts from the southern Chile Ridge: Implications of modern ridge subduction systems for the Archean

The southern portion of the Chile Ridge is one of few sites where active subduction of a spreading center and its consequences for ridge axis magmatism can be investigated. New major element, trace element, and isotopic data for lavas recovered from the ridge axis between 43 °S and 46 °20′S of the southern Chile Ridge have revealed a suite of mid-ocean ridge basalts which possess typical major element variations, but diverse and sometimes unusual trace element characteristics. For several Chile Ridge lavas, key trace element ratios, such as RbCs, CePb, NbU, LaTa, HfTh and NbLa, extend well outside the fields for normal MORB or ocean island basalts and have values more commonly associated with arc volcanics and continental crust. This hybrid mixture between MORB-like major elements and arc-like trace element signatures has only previously been seen in back-arc basins, and is considered to primarily reflect contamination of a depleted MORB source mantle with slab-derived components. Along the southern Chile Ridge, contamination with slab components is occurring in advance of the subduction zone, possibly as a result of slab break-up or shearing in conjunction with subduction of young, buoyant lithosphere, and subsequent entrainment of these slab components into the sub-ridge mantle. Interestingly, many Archean greenstone basalts share the unusual hybrid MORB-arc geochemical characteristics found along the southern Chile Ridge. On the basis of theoretical modeling, it has been suggested that the mantle was hotter, plate motions were more rapid and ridge-trench interactions were more frequent during the Archean. Although use of geochemical signatures to discriminate tectonic setting must be approached with caution, the observed geochemical affinity of modern lavas from the southern Chile Ridge and some Archean greenstone lavas lends support to the idea that ridge subduction may have been an important mechanism in the formation of Archean greenstone basalts.

The biogeochemistry of phosphorus after the first century of soil development on Rakata Island, Krakatau, Indonesia

This study examined the accumulation of organic carbon (C) and fractions ofsoil phosphorus (P) in soils developing in volcanic ash deposited in the1883 eruption of Krakatau. Organic C has accumulated at rates of 45 to 127g/m2/yr during 110 years of soil development, resulting inprofiles with as much as 14 kgC/m2. Most soil P is found inthe HCl-extractable forms, representing apatite. A loss of HCl-extractableP from the surface horizons is associated with a marked accumulation ofNaOH-extractable organic P bound to Al. A bioassay with hill rice suggeststhat P is limiting to plant growth in these soils, perhaps as a result ofthe rapid accumulation of P in organic forms.

Role of upwelling hydrothermal fluids in the development of alteration patterns at fast spreading ridges: Evidence from the sheeted dike complex at Pito Deep

Alteration of sheeted dikes exposed along submarine escarpments at the Pito Deep Rift (NE edge of the Easter microplate) provides constraints on the crustal component of axial hydrothermal systems at fast spreading mid-ocean ridges. Samples from vertical transects through the upper crust constrain the temporal and spatial scales of hydrothermal fluid flow and fluid-rock reaction. The dikes are relatively fresh (average extent of alteration is 27%), with the extent of alteration ranging from 0 to >80%. Alteration is heterogeneous on scales of tens to hundreds of meters and displays few systematic spatial trends. Background alteration is amphibole-dominated, with chlorite-rich dikes sporadically distributed throughout the dike complex, indicating that peak temperatures ranged from 450°C and did not vary systematically with depth. Dikes locally show substantial metal mobility, with Zn and Cu depletion and Mn enrichment. Amphibole and chlorite fill fractures throughout the dike complex, whereas quartz-filled fractures and faults are only locally present. Regional variability in alteration characteristics is found on a scale of <1–2 km, illustrating the diversity of fluid-rock interaction that can be expected in fast spreading crust. We propose that much of the alteration in sheeted dike complexes develops within broad, hot upwelling zones, as the inferred conditions of alteration cannot be achieved in downwelling zones, particularly in the shallow dikes. Migration of circulating cells along rides axes and local evolution of fluid compositions produce sections of the upper crust with a distinctive character of alteration, on a scale of <1–2 km and <5–20 ka.

Uranium-series age constraints on lavas from the axial valley of the Mid-Atlantic Ridge, MARK area

Abstract Mass spectrometric measurements of 230 Th– 226 Ra, 235 U– 231 Pa and 238 U– 230 Th disequilibria are used to determine eruption ages for four mid-ocean ridge basalts from the median valley of the Mid-Atlantic Ridge south of the Kane Fracture Zone (MARK area). Three samples were collected across-axis on the Axial Volcanic Ridge (i.e. the Neovolcanic Ridge) near the Snake Pit hydrothermal mound, and one sample was collected near the crest of Serocki Volcano ∼50 km south of Snake Pit. ( 226 Ra)/( 230 Th) and ( 230 Th)/( 238 U) activity ratios are low and uniform for all four samples, while ( 231 Pa)/( 235 U) activity ratios are elevated and somewhat more variable. Age constraints suggest that these lavas, from the most robust volcanic edifices in the MARK area, are 10 000–20 000 yr old. The age data are used to evaluate the efficacy of commonly used age estimate scales based on qualitative indicators (e.g. sediment cover, glass quality) and to begin to quantify the temporal and spatial dependence of volcanic, tectonic and hydrothermal processes at slow-spreading oceanic ridges.

Age constraints on crustal recycling to the mantle beneath the southern Chile Ridge: He‐Pb‐Sr‐Nd isotope systematics

Basalts from the four southernmost segments of the subducting Chile Ridge (numbered 1-4 stepping away from the trench) display large variations in Sr, Nd, Pb, and He isotope and trace element compositions. Klein and Karsten [1995] showed that segments 1 and 3 display clear trace element evidence for recycled material in their source (e.g., low Ce/Pb). The uniformly mid-ocean ridge basalt (MORB)-like 3 He/ 4 He and modest variations in Pb, Sr, and Nd isotopes of segment I (nearest the trench) suggest recent (<20 Ma) introduction of a contaminant into its source, consistent with recycling of material from the adjacent subduction zone. In contrast, segment 3 lavas display a dramatic southward increase in enrichment, extending to highly radiogenic Pb and Sr isotopic compositions (e.g., 206 Pb/ 204 Pb = 19.5) and the lowest 3 He/ 4 He yet measured in MORB (3.5RA). The segment 3 variations are most readily explained by ancient (∼2 Ga) recycling of terrigenous sediment and altered crust, but we cannot rule out more recent recycling of material derived from a distant continental source. The similarity in isotopic signatures of segment 4 lavas to Indian Ocean MORB extends the Dupal anomaly to the Chile Ridge. Like Indian Ocean MORB, the segment 4 isotopic variations are consistent with contamination by anciently recycled pelagic sediment and altered crust and require a complex history involving at least three stages of evolution and possibly a more recent enrichment event. Southern Chile Ridge MORB reflect the extensive degree of heterogeneity that is introduced into the depleted upper mantle by diverse processes associated with recycling. These heterogeneities occur on a scale of ∼50-100 km, corresponding to transform- and propagating-rift-bounded segmentation, and attest to the presence of distinct chemical domains in the mantle often bounded by surficial tectonic features that maintain their integrity on the scale sampled by melting.

The origin of bathymetric highs at ridge-transform intersections: A multi-disciplinary case study at the Clipperton Fracture Zone

Bathmetric highs on the old crust proximal to ridge-transform intersections (RTIs), termed “intersection highs”, are common but poorly understood features at offsets of fast to intermediate rate spreading centers. We have combined new reflection seismic, photographic, and geochemical data with previously published Seabeam, SeaMARC I, and SeaMARC II data to address the nature of the intersection highs at the Clipperton Fracture Zone. The Clipperton Intersection Highs are both topped by a carapace of young lavas at least 100 m thick. These lavas, which were erupted on the intersection highs, are chemically similar to their adjacent ridge segments and different from the surrounding older crust. At least some of the erupted magma traveled directly from the adjacent ridge at a shallow crustal level. Ridge-related magma covers and intrudes at least the upper 500 m of the transform tectonized crust at the RTI. We suspect that additional magma enters the intersection highs from directly below, without passing through the ridge. The young oceanic crust near the western Clipperton RTI is not thin by regional comparison. The 1.4 m.y. old crust near the eastern Clipperton RTI thickens approaching the transform offset. If the thermal effects of the proximal ridge were negligible, the eastern intersection high crust would appear to be in isostatic equilibrium. We believe that thermal effects are significant, and that the intersection high region stands anomalously shallow for its crustal thickness. This is attributable to increased temperature in the mantle below the ridge-proximal crust. Although ridge magma is injected into the proximal old crust, plate boundary reorganization is not taking place. Intersection high formation has been an ongoing process at both of the Clipperton RTIs for at least the past 1 m.y., during which time the plate boundary configuration has not changed appreciably. We envision a constant interplay between the intruding ridge magma and the disrupting transform fault motion. In addition, we envision a nearly constant input of magma from below the high, as an extension of the magma supply to the ridge from the mantle. Because the proximal ridge profoundly affects the juxtaposed crust at the RTI, sea floor fabric along the aseismic extensions of this fast-slipping transform fault is primarily a record of processes at work at the RTI rather than a record of transform tectonism.

Petrogenesis of axial lavas from the southern Chile Ridge: Major element constraints

We present major element glass data for 163 rock samples collected from four ridge segments of the southern Chile Ridge between the Chiloe Fracture Zone and the Chile Margin Triple Junction, including the segment currently being subducted at the Chile Trench (segment 1). The subridge mantle is heterogeneous at small spatial scales. Normal mid-ocean ridge basalts (N-MORB), recovered from all four ridge segments, have experienced variable extents of low pressure fractionation but have been generated by relatively uniform extents (F) and initial pressures (Po) of melting of a slightly heterogeneous depleted source. Type 1 E-MORB, found only on segment 4, have trace element affinities to some ocean island basalts, display a large range of major element variations at constant and high MgO, and are spatially associated with N-MORB. Type 2 E-MORB have trace element affinities with suprasubduction zone settings. They are found at two segment 1 sites and along most of segment 3. In order to minimize fractionation and source heterogeneity effects and assess melting conditions, E-MORB compositions were double-backtracked to 8 wt % MgO and a K/Ti ratio of 0.1. Although the magnitudes of F and Po are model-dependent, we find that N-MORB and both types of E-MORB were generated under similar melting conditions. These observations indicate that spreading rate and mantle temperature exert primary control on the southern Chile Ridge thermal regime. We see no influence of ridge subduction on the major element systematics and melting conditions of segments closest to the trench.

Tectonic and magmatic segmentation of the Global Ocean Ridge System: a synthesis of observations

Abstract Mid-ocean ridges display tectonic segmentation defined by discontinuities of the axial zone, and geophysical and geochemical observations suggest segmentation of the underlying magmatic plumbing system. Here, observations of tectonic and magmatic segmentation at ridges spreading from fast to ultraslow rates are reviewed in light of influential concepts of ridge segmentation, including the notion of hierarchical segmentation, spreading cells and centralized v. multiple supply of mantle melts. The observations support the concept of quasi-regularly spaced principal magmatic segments, which are 30–50 km long on average at fast- to slow-spreading ridges and fed by melt accumulations in the shallow asthenosphere. Changes in ridge properties approaching or crossing transform faults are often comparable with those observed at smaller offsets, and even very small discontinuities can be major boundaries in ridge properties. Thus, hierarchical segmentation models that suggest large-scale transform fault-bounded segmentation arises from deeper level processes in the asthenosphere than the finer-scale segmentation are not generally supported. The boundaries between some but not all principal magmatic segments defined by ridge axis geophysical properties coincide with geochemical boundaries reflecting changes in source composition or melting processes. Where geochemical boundaries occur, they can coincide with discontinuities of a wide range of scales.