Kevin T. M. Johnson

Open system melting and temporal and spatial variation of peridotite and basalt at the Atlantis II fracture zone

In situ ion microprobe analysis of trace and rare earth elements in discrete diopsides in abyssal peridotites from nine transform dredge hauls from the Atlantis II Fracture Zone (All FZ) shows that these samples have a wide range of trace element contents close to the total range found for the entire Southwest Indian Ridge. Though the spread in analyses is large, the average composition of the peridotites is close to that reported for the All FZ by Johnson et al. (1990) and lies at the relatively undepleted end of the spectrum for SW Indian Ridge residual mantle peridotites. A sharp break in peridotite diopside composition and modal mineralogy occurs across the transform, suggesting that it acts as a boundary for different melting regimes and initial mantle compositions. The difference in peridotite compositions is mirrored in spatially associated basalts, which lie on separate parallel liquidus trends in the normative ternary pyroxene-olivine-plagioclase. Basalts from the east side of the transform have higher normative plagioclase contents, indicating that they may be products of lower degrees of mantle melting than basalts from the western side, consistent with greater depletion of peridotites from the western wall or a more depleted initial composition. Basalts from the eastern wall also have consistently lower Fe8.0 and higher Na8.0 than basalts from the western wall and lie parallel to the global along-ridge Fe8.0 − Na8.0 trend (Klein and Langmuir, 1987) and orthogonal to the local melting paths of Klein and Langmuir (1989). Our data provide strong evidence for segmentation of the melting regime, with major mantle discontinuities occurring at transform offsets at slow spreading ridges. Peridotites analyzed along the eastern wall of the fracture zone also show a systematic change in composition with latitude and, with the older peridotites from the median tectonic ridge, define a systematic change in the degree of melting of the mantle occurring beneath the paleoridge axis over the last 11 m.y. Emplacement of mantle showing the lowest degree of melting, or the least depleted parental mantle composition, corresponds roughly to the time of crystallization of Ocean Drilling Program site 735B gabbros. Melting is modeled as a non-steady state, discontinuous process with 0.1–0.5 vol % aggregated melt retained in the porous residue (open system melting). The range in degree of open system melting for the combined suite of All FZ peridotites is 8–20%. Such a large systematic variation would appear to require a dynamically significant change with time, either in the initial temperature and/or a large compositional difference of the mantle beneath the paleoridge axis. This in turn suggests that in the relative reference frame of the ridge axis, mantle flow was non-steady state. This could reflect episodic mantle diapirism beneath the ridge axis or, alternatively, that the ridge axis has moved over a zone of enhanced upflow in the underlying mantle that was fixed in the absolute hotspot mantle reference frame.

Anomalous seafloor spreading of the Southeast Indian Ridge near the Amsterdam-St. Paul Plateau

The Amsterdam-St. Paul Plateau is bisected by the intermediate-rate spreading Southeast Indian Ridge, and numerous geophysical and tectonic anomalies arise from the interactions of the Amsterdam-St. Paul hotspot and the spreading center. The plate boundary geometry on the hotspot platform evolves rapidly (on timescales <1 Myr), off-axis volcanism is abundant, the seafloor does not deepen away from the axis, and transform faults do not have fracture zone extensions. Away from the hotspot platform the ridge-transform geometry is typical of mid-ocean ridges globally. In contrast, the Amsterdam-St. Paul Plateau spreading segments are shorter, they often overlap each other significantly, and the intervening discontinuities are smaller, more ephemeral, and more migratory. Abyssal hills are smaller and less uniform on the hotspot platform than on neighboring spreading segments. From gravity and isostasy analysis the average thickness of the platform crust is ∼10 km, approximately 50% thicker than that of typical oceanic crust. Most of the isostatic compensation of the hotspot plateau occurs at the Moho or within the lower crust, and the effective elastic thickness of the plateau lithosphere is ∼1.6 km, less than half that of adjacent spreading segments. Away from the platform some transform faults contain intratransform spreading centers; on the platform the two transform faults have valleys which may be depocenters for abundant axial or off-axis volcanism and mass wasting. Although not wellconstrained by magnetics coverage, the Amsterdam-St. Paul hotspot appears to have been “captured” by the Southeast Indian Ridge, enhancing crustal production at the ridge since about 3.5 Ma. Prior to this time the hotspot formed a line of smaller, isolated volcanoes on older Australian plate. The underlying cause for the present-day crustal accretion anomalies is the effect of melt generation from separate sources of mantle upwelling (due to plate spreading and the hotspot) which has a consequent effect of weakening the lithosphere.

Influence of the Amsterdam/St. Paul hot spot along the Southeast Indian Ridge between 77° and 88°E: Correlations of Sr, Nd, Pb, and He isotopic variations with ridge segmentation

The submarine Amsterdam-St. Paul (ASP) Plateau, bisected by the Southeast Indian Ridge (SEIR), is a bathymetric high rising ∼2 km above the surrounding seafloor that includes the islands of Amsterdam and St. Paul; this excess volcanism is attributed to a mantle hot spot. We obtained new Sr, Nd, and Pb (n = 37) and He isotopic (n = 10) ratios for basalt glasses from 11 SEIR segments on and adjacent to the plateau and from three seamounts on the plateau. The results show systematic spatial variations in these isotopic ratios that correlate with physical segmentation of the ridge. Specifically, lavas from the four ridge segments on the ASP Plateau have higher 208Pb/204Pb at a given 206Pb/204Pb than SEIR basalts distant from ASP Plateau. Surprisingly, lavas from the ridge segment 100 km north of the ASP Plateau are distinguished by the most radiogenic 206Pb/204Pb (up to 19.4) and highest 3He/4He ratios (up to 14.1 RA). These are characteristics of lavas erupted at Amsterdam and St Paul Islands. The isotopic data for SEIR basalts erupted on or adjacent to the ASP Plateau provide equivocal evidence for a mantle component derived from the distant Kerguelen hot spot. Overall, the Pb-Nd-Sr-He isotope variations within this data set are explained well by three mantle end-members: (1) depleted mantle having relatively low 206Pb/204Pb and 87Sr/86Sr and high 143Nd/144Nd, which has been variably mixed with (2) material having relatively high 208Pb/204Pb and 87Sr/86Sr and low 143Nd/144Nd, a signature commonly ascribed to detached or eroded metasomatized continental lithosphere, and (3) hot spot–related mantle having elevated 3He/4He and 206Pb/204Pb but intermediate 87Sr/86Sr and 143Nd/144Nd, similar to the common or C material observed in hot spots globally. These results suggest either that the ASP hot spot is isotopically heterogeneous or that the shallow mantle or lithosphere beneath the ASP Plateau contains more continentally derived material than the SEIR mantle ≥500 km away. Perhaps, like the 39°–41°E section of the Southwest Indian Ridge, beneath the ASP Plateau there are rafts of continental material stranded within a local “tectonic corridor,” possibly present since the opening of the Indian Ocean basin.

Petrology and Geochemistry of Submarine Lavas from the Lau and North Fiji Back-Arc Basins

Basaltic lavas in seven new dredges from the Lau and North Fiji basins comprise eight distinct chemical groups, representing three magma types. Type I lavas from three sites including one dredge of abyssal ferrobasalt are low-Kmid-ocean ridge basalt (MORB) similar to those of mid-ocean spreading centers, but with slightly higher 87Sr/86Sr = 0.7034. Lavas from two sites are slightly enriched in K, Rb, Zr, Ba, and possibly water, relative to normal MORB; these Type II lavas evolved from primary magmas formed by very high degrees of melting and have many similarities to lavas from the Mariana Trough back-arc basin. Type II lavas appear to be older than Type I, suggesting that the Lau and North Fiji basins evolved from initial eruption of a back-arc basin basalt (BABB) magma type, followed by later production of MORB magmas. Two sites on the South Pandora Ridge yielded alkali- and other incompatible-element enriched “transitional” basalts (Type III). The extremely fresh appearance of these lavas suggests that they are very young. We propose that the South Pandora Ridge represents a recently activated, pre-existing lineament that includes Rotuma Island. New isotope data from the North Fiji Basin suggest that a previously proposed southern-hemisphere, globe-encircling mantle anomaly may not be continuous west of the Samoa volcanic province.

Petrology of seamounts northwest of Samoa and their relation to Samoan volcanism

Petrological and geochemical data on dredged samples from five submarine volcanos northwest of Samoa indicate that three of these volcanos belong to the Samoan volcanic province (Field, Lalla Rookh, and Combe banks), and two belong to separate magmatic zones (Wallis Islands and Alexa Bank). The Samoan volcanic province increases in age westward and both shield-building tholeiitic and alkalic lavas (Combe Bank) and strongly undersaturated (post-erosional?) melilitites or nephelinites and ankaramites (Field and Lalla Rookh banks) are present. The age progression and petrochemical character of these rocks is consistent with a fixed hotspot beneath eastern Samoa. Slightly askew from this trend is Alexa Bank where dredged lavas are ocean-island tholeiites; however, its radiometric age and compositional characteristics apparently preclude its association with Samoa by a fixed-hotspot model. Dredged volcanic rocks from near the Wallis Islands are geochemically, petrologically, and temporally different from Samoan volcanism, but are similar in these respects to Quaternary volcanism in Rotuma and Fiji and may be related to plate reorganization accompanying opening of the North Fiji Basin.