Douglas G. Pyle

Geochemical characteristics of lavas from Broken Ridge, the Naturaliste Plateau and southernmost Kerguelen Plateau: Cretaceous plateau volcanism in the southeast Indian Ocean☆

Lavas from several major bathymetric highs in the eastern Indian Ocean that are likely to have formed as Early to Middle Cretaceous manifestations of the Kerguelen hotspot are predominantly tholeiitic; so too are glass shards from Eocene to Paleocene volcanic ash layers on Broken Ridge, which are believed to have come from eruptions on the Ninetyeast Ridge. The early dominance of tholeiitic compositions contrasts with the more recent intraplate, alkalic volcanism of the Kerguelen Archipelago. Isotopic and incompatible-element ratios of the plateau lavas are distinct from those of Indian mid-ocean ridge basalts; their Nd, Sr, 207Pb204Pb and 208Pb204Pb isotopic ratios overlap with but cover a much wider range than measured for more recent oceanic products of the Kerguelen hotspot (including the Ninetyeast Ridge) or, indeed, oceanic lavas from any other hotspot in the world. Samples from the Naturaliste Plateau and ODP Site 738 on the southern tip of the Kerguelen Plateau are particularly noteworthy, with ϵNd(T) = − 13 to −7, (87Sr86Sr)T=0.7090 to 0.7130 and high 207Pb204Pb relative to 206Pb204Pb. In addition, the low-ϵNd(T) Naturaliste Plateau samples are elevated in SiO2 (> 54 wt%). In contrast to “DUPAL” oceanic islands such as the Kerguelen Archipelago, Pitcairn and Tristan da Cunha, the plateau lavas with extreme isotopic characteristics also have relative depletions in Nb and Ta (e.g., ThTa, La Nb > primitive mantle values); the lowest ϵNd(T) and highest ThTa and La Nb values occur at sites located closest to rifted continental margins. Accepting a Kerguelen plume origin for the plateau lavas, these characteristics probably reflect the shallow-level incorporation of continental lithosphere in either the head of the early Kerguelen plume or in plume-derived magmas, and suggest that the influence of such material diminished after the period of plateau construction. Contamination of asthenosphere with the type of material affecting Naturaliste Plateau and Site 738 magmatism appears unlikely to be the cause of low-206Pb04Pb Indian mid-ocean ridge basalts. Finally, because isotopic data for the plateaus do not cluster or form converging arrays in isotope-ratio plots, they provide no evidence for either a quickly evolving, positive ϵNd, relatively high-206Pb204Pb plume composition, or a plume source dominated by mantle with ϵNd of −3 to ∼ 0.

Resolving an isotopic boundary within the Australian-Antarctic discordance

Abstract New Sr, Nd and Pb isotopic analyses of MORB glasses from the Australian-Antarctic Discordance (AAD) confirm the presence of an abrupt boundary between ‘Indian’ type and ‘Pacific’ type MORB mantle. The transition between these two upper mantle reservoirs is gradational along ∼ 40 km of the easternmost AAD spreading center (i.e. segment B5W) and terminates at its western ridge-transform intersection. Axial lavas dredged immediately west of the B4/B5 transform are unequivocally derived from an ‘Indian’ MORB source, whereas axial lavas dredged east of this transform have a ‘Pacific’ type signature with evidence of an ‘Indian’ MORB imprint. Off-axis sampling of the easternmost AAD spreading segment show these lavas to have been derived from an ‘Indian’ type source; indicating that the isotopic boundary has migrated westward into the AAD in the last 3–4 Myr. Pacific mantle must flow beneath the B5 spreading axis at a rate of ∼ 25 mm/yr in order for it to displace Indian mantle as the source of melt for this segment. The gradational character of the boundary suggests that, as the westward migration of ‘Pacific’ mantle progresses, an increasing memory of a remnant ‘Indian’ signature is incorporated into lavas near the leading edge of the boundary zone. At present, approximately equal proportions of ‘Pacific’ and ‘Indian’ mantle contribute to the isotopic signature of lavas erupted 10 km east of the B4/B5 spreading axis offset at ∼ 126°E. Migration of the boundary could reflect a a continuous, large-scale, westward outflow of upper mantle which has recently arrived beneath the AAD from a shrinking Pacific basin, or, alternatively, the displacement may reflect a small-scale perturbation of a long-term isotopic discontinuity, created and maintained by the mantle dynamics producing the AAD.

Chaotic topography, mantle flow and mantle migration in the Australian–Antarctic discordance

Oceanic crust formed over the past 30 million years at the Australian–Antarctic discordance (AAD) is characterized by chaotic sea-floor topography, reflecting a weak magma supply from an unusually cold underlying mantle. During the past 3–4 million years, however, a source of increased magma supply, coinciding with the known Indian–Pacific mantle isotopic boundary, has propagated into the eastern AAD, displacing the chaotic terrain and replacing it with normal sea floor. Pacific mantle reached the eastern boundary of the AAD at least 7 million years ago, but it was not until 3–4 million years ago that lavas derived from Pacific mantle were first erupted within the AAD. This long hiatus, combined with the ridge–transform geometry across the AAD boundary, constrains the locus of mantle migration to a narrow, relatively shallow region, directly beneath the spreading axis of the Southeast Indian ridge.

Evidence for variable upper mantle temperature and crustal thickness in and near the Australian-Antarctic Discordance

Abstract The Southeast Indian Ridge (SEIR) in and near the Australian-Antarctic Discordance (AAD) exhibits, at a constant spreading rate, almost the full range of the many geophysical and geochemical parameters characteristic of the ‘slow’ Mid-Atlantic Ridge and ‘fast’ East Pacific Rise. We used satellite-derived gravity data, in combination with SeaMARC II bathymetry in and near the AAD, to examine regional density variations in the upper mantle beneath the AAD. Through three-dimensional gravity analysis, we found that at least two end-member models satisfy the gravity observations: regional crustal thickness variations of at least 3 km along the SEIR near the AAD or a temperature anomaly of the order of 150°C in the upper mantle beneath the SEIR. These new observations, combined with other geophysical and geochemical characteristics of the Australian-Antarctic Discordance, provide further evidence that the temperature structure of a mid-ocean ridge is a controlling factor, in addition to spreading rate, in the crustal accretionary process. Numerical models of mantle flow beneath mid-ocean ridges offer one means of investigating the dynamic effect of a variable upper mantle temperature on the accretionary process. Our results indicate that temperature is important, especially at intermediate and slower spreading rates, where thermal effects can dominate mantle flow beneath a mid-ocean ridge and result in increasing crustal production with decreasing spreading rate. At the constant, intermediate spreading rate of 37 mm/yr, characteristic of the SEIR in and near the AAD, our numerical models show that significant crustal thinning (2–4 km) can occur with relatively small variations in upper mantle temperature, all else being equal. Thus, combined with our end-member gravity models, these observations and results suggest that both anomalously cool upper mantle and thin crust exist beneath the AAD.

Along-axis dynamic topography constrained by major-element chemistry

Abstract Variations in thickness and density of both the crust and the associated uper mantle have been derived from a compilation of zero-age major-element composition along the Mid-Atlantic Ridge, the East Pacific Rise and the Southeast Indian Ridge. Assuming isostatic compensation, the axial depth computed from major-element data correctly agrees with observed axial depth. Discrepancies are essentially located near hotspots such as Iceland and Azores. The residual topography, expressed as the difference between observed and compensated axial depth has a root-mean-square of 426 m along the three spreading axes, which is below the resolution power of the method. This insignificant topography, which is assumed to contain the dynamic surface topography associated with mantle convection, bears an important constraint on the relative variations of the dynamic topography predicted by models of mantle circulation.

Ka‘ena Volcano—A precursor volcano of the island of O‘ahu, Hawai‘i

Ka‘ena and Wai‘alu Ridges form prominent submarine ridges NW of the island of O‘ahu, Hawai‘i. We evaluate whether or not either one of these ridges represents a submarine extension of Wai‘anae Volcano on O‘ahu using new bottom observations, geophysical surveys, and geochemical data acquired on new samples from the region. Wai‘alu Ridge has the morphology of a submarine rift zone but is too shallow for its distance from the O‘ahu shoreline; Ka‘ena Ridge also is unusually shallow and is surmounted by two topographic shields. Ka‘ena and Wai‘alu Ridges have similar magmatic and volcanic evolutionary histories, beginning ca. 5 Ma with a submarine, shield phase of volcanism that produced high-SiO 2 , low-FeO* tholeiites with higher 208 Pb/ 204 Pb than in the adjacent Wai‘anae Volcano. Late-shield volcanism included transitional and alkalic rock types, with lower SiO 2 and enrichment in incompatible elements, especially P 2 O 5 , Nb, Zr, Ti, and light rare earth elements. The transition from shield to late-shield stage occurred as the edifice was beginning to emerge from the sea. Geological observations and K/Ar ages indicate that Ka‘ena emerged above sea level ca. 3.5 Ma, reaching a maximum height of ∼4000 m above the abyssal ocean floor and 1000 m above sea level. Relatively weak gravity anomalies, topographic lineaments, and the orientation of dike complexes indicate a volcanic structure that is independent of Wai‘anae Volcano. Thus, volcanic structure, geochemistry, and age all indicate a precursor volcano to the island of O‘ahu, which we call Ka‘ena Volcano. After emergence, Ka‘ena Volcano tilted ∼2° to the south. We estimate a total volume of 20–27 × 10 3 km 3 for Ka‘ena Volcano, taking into account overlapping geometry of concurrently active volcanoes. Sample compositions from the Ka‘ena landslide deposit are entirely consistent with derivation from Ka‘ena, whereas most samples from the Wai‘anae slump are likely derived from Wai‘anae Volcano. Uniformly oriented dikes in the Wai‘anae NW rift zone likely reflect buttressing by a preexisting Ka‘ena Volcano. Unusual isotopic compositions of some Wai‘anae samples, including unique hydrous silicic lavas, probably reflect interaction with underlying Ka‘ena crust. A newly recognized lava flow field on the southern flank of Ka‘ena Ridge extends the previously known distribution of secondary volcanism in the Kaua‘i Channel. Putative submarine volcanic activity in the region in 1956 cannot have built a large edifice and is unlikely to have produced pumice that was found on O‘ahu shores. This eruptive activity therefore remains unconfirmed.