Yaoling Niu

A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge

Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron- and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite- to amphibolite-facies alteration is most important, coinciding with brittle^ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift low-temperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high- and low-temperature alteration are their respective

Trace element evidence from seamounts for recycled oceanic crust in the Eastern Pacific mantle

We present trace element data for 80 samples from about 50 seamounts in the east equatorial Pacific near the East Pacific . Rise. These data indicate that the heterogeneous mantle source that supplies the seamounts consists of two components: 1 . an extremely depleted component, much more depleted than estimates of the source of depleted MORB; and 2 an enriched component even more enriched than average OIB. The depleted component shows large variations in ZrrHf, NbrTa, RbrCs, CerPb, and ThrU that are correlated with each other and with LarSm, indicating that these paired elements do fractionate from each other in some oceanic basalts. The order of incompatibility of trace elements we find differs slightly from that found elsewhere. For example, for seamounts, we find that D f D - D f D. In comparison with Th and U, Nb Th Ta U the enriched component shows anomalous enrichments of Ta and Nb. Since such fractionations are characteristic of subduction zones, we suggest that the most likely ultimate source of the enriched component is recycled ocean crust.

An empirical method for calculating melt compositions produced beneath mid‐ocean ridges: Application for axis and off‐axis (seamounts) melting

We present a new method for calculating the major element compositions of primary melts parental to mid-ocean ridge basalt (MORB). This model is based on the experimental data of Jaques and Green (1980), Falloon et al. (1988), and Falloon and Green (1987, 1988) which are ideal for this purpose. Our method is empirical and employs solid-liquid partition coefficients (Di) from the experiments. We empirically determine Di = ƒ(P,F) and use this to calculate melt compositions produced by decompression-induced melting along an adiabat (column melting). Results indicate that most MORBs can be generated by 10–20% partial melting at initial pressures (P0) of 12–21 kbar. Our primary MORB melts have MgO = 10–12 wt %. We fractionate these at low pressure to an MgO content of 8.0 wt % in order to interpret natural MORB liquids. This model allows us to calculate Po, Pƒ, To, Tƒ, and F for natural MORB melts. We apply the model to interpret MORB compositions and mantle upwelling patterns beneath a fast ridge (East Pacific Rise (EPR)8°N to 14°N), a slow ridge (mid-Atlantic Ridge (MAR) at 26°S), and seamounts near the EPR (Lament seamount chain). We find mantle temperature differences of up to 50°–60°C over distances of 30–50 km both across axis and along axis at the EPR. We propose that these are due to upward mantle flow in a weakly conductive (versus adiabatic) temperature gradient. We suggest that the EPR is fed by a wide (−100 km) zone of upwelling due to plate separation but has a central core of faster buoyant flow. An along-axis thermal dome between the Siqueiros transform and the 11°45′ Overlapping Spreading center (OSC) may represent such an upwelling; however, in general there is a poor correlation between mantle temperature, topography, and the segmentation pattern at the EPR. For the Lament seamounts we find regular across-axis changes in Po and F suggesting that the melt zone pinches out off axis. This observation supports the idea that the EPR is fed by a broad upwelling which diminishes in vigor off axis. In contrast with the EPR axis, mantle temperature correlates well with topography at the MAR, and there is less melting under offsets. The data are consistent with weaker upwelling under offsets and an adiabatic temperature gradient in the sub axial mantle away from offsets. The MAR at 26°S exhibits the so-called local trend of Klein and Langmuir (1989). Our model indicates that the local trend cannot be due solely to intracolumn melting processes. The local trend seems to be genetically associated with slow-spreading ridges, and we suggest it is due to melting of multiple individual domains that differ in initial and final melting pressure within segments fed by buoyant focused mantle flow.

Origin of enriched-type mid-ocean ridge basalt at ridges far from mantle plumes: The East Pacific Rise at 11°20′N

The East Pacific Rise (EPR) at 11°20′N erupts an unusually high proportion of enriched mid-ocean ridge basalts (E-MORB) and thus is ideal for studying the origin of the enriched heterogeneities in the EPR mantle far from mantle plumes. These basalts exhibit large compositional variations (e.g., [La/Sm]N = 0.68–1.47, 87Sr/86Sr = 0.702508–0.702822, and 143Nd/144Nd = 0.513053–0.513215). The 87Sr/86Sr and 143Nd/144Nd correlate with each other, with ratios of incompatible elements (e.g., Ba/Zr, La/Sm, and Sm/Yb) and with the abundances and ratios of major elements (TiO2, Al2O3, FeO, CaO, Na2O, and CaO/Al2O3) after correction for fractionation effect. These correlations are interpreted to result from melting of a two-component mantle with the enriched component residing as physically distinct domains in the ambient depleted matrix. The observation of [Nb/Th]PM > 1 and [Ta/U]PM > 1, plus fractionated Nb/U, Ce/Pb, and Nb/La ratios, in lavas from the northern EPR region suggests that the enriched domains and depleted matrix both are constituents of recycled oceanic lithosphere. The recycled crustal/eclogitic lithologies are the major source of the enriched domains, whereas the recycled mantle/peridotitic residues are the most depleted matrix. On Pb-Sr isotope plot, the 11°20′N data form a trend orthogonal to the main trend defined by the existing EPR data, indicating that the enriched component has high 87Sr/86Sr and low 206Pb/204Pb and 143Nd/144Nd. This isotopic relationship, together with mantle tomographic studies, suggests that the source material of 11°20′N lavas may have come from the Hawaiian plume. This “distal plume-ridge interaction” between the EPR and Hawaii contrasts with the “proximal plume-ridge interactions” seen along the Mid-Atlantic Ridge. The so-called “garnet signature” in MORB is interpreted to result from partial melting of the eclogitic lithologies. The positive Na8-Si8/Fe8 and negative Ca8/Al8-Si8/Fe8 trends defined by EPR lavas result from mantle compositional (vs. temperature) variation.

Lhasa terrane in southern Tibet came from Australia

The U-Pb age and Hf isotope data on detrital zircons from Paleozoic metasedimentary rocks in the Lhasa terrane (Tibet) defi ne a distinctive age population of ca. 1170 Ma with e Hf (t) values identical to the coeval detrital zircons from Western Australia, but those from the western Qiangtang and Tethyan Himalaya terranes defi ne an age population of ca. 950 Ma with a similar e Hf (t) range. The ca. 1170 Ma detrital zircons in the Lhasa terrane were most likely derived from the Albany-Fraser belt in southwest Australia, whereas the ca. 950 Ma detrital zircons from both the western Qiangtang and Tethyan Himalaya terranes might have been sourced from the High Himalaya to the south. Such detrital zircon connections enable us to propose that the Lhasa terrane is exotic to the Tibetan Plateau system, and should no longer be considered as part of the Qiangtang‐Greater India‐Tethyan Himalaya continental margin system in the Paleozoic reconstruction of the Indian plate, as current models show; rather, it should be placed at the northwestern margin of Australia. These results provide new constraints on the paleogeographic reconstruction and tectonic evolution of southern Tibet, and indicate that the Lhasa terrane evolved as part of the late Precambrian‐early Paleozoic evolution as part of Australia in a different paleogeographical setting than that of the Qiangtang−Greater India−Tethyan Himalaya system.

Direct geological evidence for oceanic detachment faulting: The Mid-Atlantic Ridge, 15°45′N

From a detailed survey and sampling study of corrugated massifs north of the Fifteen-Twenty Fracture Zone on the Mid-Atlantic Ridge, we demonstrate that their surfaces are low-angle detachment fault planes, as proposed but not previously verified. Spreading-direction–parallel striations on the massifs occur at wavelengths from kilometers to centimeters. Oriented drill-core samples from the striated surfaces are dominated by fault rocks with low-angle shear planes and highly deformed greenschist facies assemblages that include talc, chlorite, tremolite, and serpentine. Deformation was very localized and occurred in the brittle regime; no evidence is seen for ductile deformation of the footwall. Synkinematic emplacement of diabase dikes into the fault zone from an immediately subjacent gabbro pluton implies that the detachment must have been active as a low-angle fault surface at very shallow levels directly beneath the ridge axis. Strain localization occurred in response to the weakening of a range of hydrous secondary minerals at a very early stage and was highly efficient.

Geochemistry of near-EPR seamounts: importance of source vs. process and the origin of enriched mantle component

Niu and Batiza [Earth Planet. Sci. Lett. 148 (1997) 471^483] show that lavas from the seamounts on the flanks of the East Pacific Rise (EPR) between 5‡ and 15‡N vary from extremely depleted tholeiites to highly enriched alkali basalts. The extent of depletion and enrichment exceeds the known range of seafloor lavas in terms of the abundances and ratios of incompatible elements. New Sr^Nd^Pb isotope data for these lavas show variations ( 87 Sr/ 86 Sr = 0.702362^0.702951; 206 Pb/ 204 Pb = 18.080^19.325 and 143 Nd/ 144 Nd 0.512956^0.513183) larger than observed in lavas erupted on the nearby EPR axis. These isotopic ratios correlate with each other, with the abundances and ratios of incompatible elements, with the abundances of measured major elements such as MgO, CaO, Na2O and TiO2 contents, and with the abundances and ratios of major elements corrected for crystal fractionation to Mg# = 0.72 (Ti72 ,A l 72 ,F e 72 ,C a 72 ,N a 72, and Ca72/Al72). These coupled correlations and the spatial distribution of seamounts require an EPR mantle source that has long-term (s 1 Ga) lithological heterogeneities on very small scales [Niu and Batiza, Earth Planet. Sci. Lett. 148 (1997) 471^483]. Mid-ocean ridge basalt (MORB) major element systematics are, to a great extent, inherited from their fertile sources, which requires caution when using major element data to infer melting conditions. The significant correlations in elemental and isotopic variability (defined as RSD% = 1c/ meanU100) between seamount and axial lavas suggest that both seamount and axial volcanisms share a common heterogeneous mantle source. We confirm previous interpretations [Niu and Batiza, Earth Planet. Sci. Lett. 148 (1997) 471^483; Niu et al., J. Geophys. Res. 104 (1999) 7067^7087] that the geochemical variability of lavas from the broad northern EPR region results from melting-induced mixing of a two-component mantle with the enriched (easily melted) component dispersed as physically distinct domains in a more depleted (refractory) matrix prior to the major melting events. The data also allow the conclusion that recycled oceanic crust cannot explain elevated abundances of elements such as Ba, Rb, Cs, Th, U, K, Pb, Sr etc. in enriched MORB and many ocean island basalts. These elements will be depleted in recycled oceanic crust that has passed through subduction zone dehydration reactions. We illustrate that deep portions of recycled oceanic lithosphere are important geochemical reservoirs hosting these and other incompatible elements as a result of metasomatism taking place at the interface between the low velocity zone and the cooling and thickening oceanic lithosphere. K 2002 Elsevier Science B.V. All rights reserved.

Basaltic liquids and harzburgitic residues in the Garrett Transform: A case study at fast-spreading ridges

Abstract The peridotite-basalt association in the Garrett Transform, ∼ 13°28′S, East Pacific Rise (EPR), provides a prime opportunity for examining mantle melting and melt extraction processes from both melts and residues produced in a common environment beneath fast-spreading ridges. The peridotites are highly depleted, clinopyroxene-poor, harzburgites. Residual spinel, orthopyroxene and clinopyroxene in these harzburgites are extremely depleted in Al2O3, and plot at the most depleted end of the abyssal peridotite array defined by samples from slow-spreading ridges (including samples from hotspot-influenced ridges), suggesting that these harzburgites are residues of very high extents of melting. The residual peridotites from elsewhere at the EPR (i.e., Hess Deep and the Terevaka Transform) also are similarly depleted. This suggests that the extent of melting beneath the EPR is similar to, or even higher than, beneath ridges influenced by hotspots (e.g., Azores hotspot in the Atlantic Ocean and Bouvet hotspot in the Indian Ocean), and is significantly higher than ≤ 10%, a value that has been advocated to be the average extent of melting beneath global ocean ridges. Many of these harzburgite samples, however, show whole-rock incompatible element abundances higher than expected. These same samples also have various amounts of excess olivine with forsterite contents as low as Fo85. The total olivine modes correlate inversely with olivine forsterite contents, and positively with whole-rock incompatible element abundances. These correlations suggest that the excess olivine and incompatible element enrichment are both the result of melt-solid re-equilibration. The buoyant melts that ascend through previously depleted residues crystallize olivine at shallow levels as a result of cooling. Entrapment of these melts leads to whole-rock incompatible element enrichment. These observations contrast with the notion that melts formed at depth experience little low pressure equilibration during ascent.

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.

Variations in the geochemistry of magmatism on the East Pacific Rise at 10?30'N since 800 ka

Samples of volcanic rock, collected from the flanks of the East Pacific Rise at 10o30 0 N, were used to investigate changes in the geochemistry of magmatism at the ridge axis, over the past 800 ka at this location. We show that there have been large variations in the major element chemistry of the lavas erupted at the spreading axis on this ridge segment over this period. For example, the average MgO content of lavas erupted at the ridge axis increased from about 3.0% at 600 ka, to about 7.0% at 300 ka. Since 300 ka the average MgO content has systematically decreased, and the average MgO content of lavas collected from within the neovolcanic zone at 10o30 0 N is 6.0%. These temporal changes in major element chemistry are not accompanied by systematic changes in isotope composition or incompatible trace element ratios, and are interpreted to reflect changes in the average rate of supply of melt to the ridge axis during this period. The data support previous arguments that changes in melt supply rate over periods of 100‐1000 ka have an important influence on the major element chemistry of the lavas erupted at fast spreading ridges. At 10o30 0 N, the melt supply rate appears to have been relatively low for much of the past 800 ka. Samples younger than 50 ka, collected from within 3 km of the ridge axis at 10o30 0 N (inside the neovolcanic zone), have a smaller range in major element chemistry compared to the samples dredged from the ridge flanks. Variations in the chemistry of lavas erupted over periods of less than about 100 ka may be controlled by the geometry of the magma plumbing system beneath the ridge axis.