David W. Peate

The petrogenesis of Mesozoic Gondwana low-Ti flood basalts

Abstract New major, trace element and isotopic data for Jurassic basalts from SE Australia indicate that they are strikingly similar to the Jurassic tholeiitic rocks of Tasmania and the Transantarctic Mountains. These rocks are all characterised by low TiO2, P2O5, Na2O, Fe2O3, Ti/Zr, Ti/Y and eNd, and high SiO2, Rb/Ba, Rb/Sr, 87Sr/86Sr and 207Pb/204Pb, relative to oceanic basalts. They therefore comprise a major province, termed the Ferrar magmatic province, which extended for 3000–4000 km across the Gondwana supercontinent. A review of the other Mesozoic low-Ti CFBs suggests that the Ferrar rocks are an extreme example of these magma types. It is striking that both the major and trace element compositions are different from oceanic basalts, which suggests that these features are linked, and it is argued that they were derived from distinctive source regions in the sub-continental mantle. Such source regions were variably depleted in major and minor elements, and then relatively enriched in highly incompatible elements and Sr and Pb isotopes, which is best explained by the introduction of a small amount of subducted sediment. The tectonic setting of the Ferrar magmatism is poorly constrained, but at present there is no clear geochemical evidence for the involvement of asthenospheric plume material in the petrogenesis of these low-Ti CFBs.

Chemical stratigraphy of the Paraná lavas (South America): classification of magma types and their spatial distribution

A new classification scheme has been developed to assign the lava flows of the Paraná continental flood basalt province (South America) into geochemically distinct magma types, with six basaltic major and trace element abundances and/or ratios. By mapping out the spatial distribution of these magma types within the lava sequences, it has been possible to determine the internal stratigraphy of the lava pile on a regional scale. Previous studies on road profiles traversing the well-exposed coastal Serra Geral escarpment of southern Brazil are summarised together with results from some new sampled sections. More widespread stratigraphical investigations of the Paraná lavas have been hampered by the lack of sufficient topographic relief and the cover of sedimentary rocks. However, access to drill-core chippings from nine boreholes in the central Paraná region has provided a unique opportunity to investigate the stratigraphy of the otherwise inaccessible deeper levels of the lava pile and to map out stratigraphic variations in three dimensions. The borehole samples have indicated cated a stacking of units of different magma types all overlapping towards the north, which suggests that the main locus of magmatism moved northwards with time within the Paraná basin. This migration could be related to the northward propagation of rifting during the initiation of the South Atlantic Ocean. Maps of the surface distribution of samples of each magma type show a pattern consistent with the stratigraphy inferred from the boreholes, although suggesting that the shift in magmatism may have been towards the northwest. On the basis of geochemical similarities between magma types and their inferred stratigraphical relationships, it is proposed that the Paraná can be divided into two principal magmatic centres: (1) an older one in the south, comprising the Gramado, Esmeralda and Urubici magma types; and (2) a younger one, developed about 750 km to the north, formed by the Pitanga, Paranapanema and Ribeira magma types.

Paraná magmatism and the opening of the South Atlantic

Abstract New chemical and isotope results are presented on dyke rocks associated with the Paraná CFB, together with preliminary laser 40Ar/39Ar analyses on selected Paraná basalts. Dyke rocks from the Ponta Grossa Arch are similar to the Pitanga and Paranapanema magma types in the Paraná lavas, but dykes from the Santos-Rio de Janeiro section include samples with compositions not observed in the overlying lavas. Rather their minor and trace elements are strikingly similar to basalts recently erupted on Tristan de Cunha, and thus these late stage dykes may represent the first direct evidence for the involvement of typical plume-related OIB in the Paraná province. Laser 40Ar/39Ar analyses of two Gramado low Ti basalts have yielded preferred isochron ages of 132.4 ± 1.4 and 132.9 ± 2.8 Ma. These indicate a short eruption time for at least the Gramado magma type, and that magmatism took place several million years after the species extinction in the Tithonian (c. 141 Ma). The majority of basalts and basaltic andesites in the Paraná CFB have distinctive trace elements ratios (low Nb/La and Nb/Ba), and relatively enriched Sr, Nd, and Pb isotope compositions. Since such features are not commonly observed in oceanic basalts, and they occur in CFBs which have been screened for the effects of crustal contamination, they are typically attributed to old, incompatible element enriched source regions in the continental mantle lithosphere. In some models the minor and trace element ‘mantle lithosphere’ component was introduced in small degree melts (lamproites) added to asthenosphere derived magmas. However, such models appear to be inconsistent with the data from low Ti CFB, and they also require that the asthenosphere derived magmas have very low incompatible element contents, in marked contrast to the high Nb/La late stage dykes in the Paraná. Alternatively some CFBs may have been generated within the mantle lithosphere in the presence of small amounts of water. The results of preliminary calculations indicate that in the presence of a mantle plume up to 5 km of melt may be generated entirely from within the mechanical boundary layer, for β values of less than 1.2.

LITHOSPHERIC TO ASTHENOSPHERIC TRANSITION IN LOW-TI FLOOD BASALTS FROM SOUTHERN PARANA, BRAZIL

Two geochemically distinct Low-Ti magma types (Gramado and Esmeralda) are distinguished within the flood basalt sequences of southern Parana on the basis of certain element abundances and ratios. Esmeralda magmas have lower Ce/Sm and (87Sr86Sr),i and higher TiZr and ϵNd, than Gramado magmas. Detailed stratigraphical work indicates that there was a temporal progression from Gramado- to Esmeralda-type magmas. This compositional shift cannot simply be explained in terms of a declining extent of crustal contamination of an asthenosphere-derived melt with time, and instead it seems that the two magma types evolved from distinct parental magmas. Although compositional variations of Gramado magmas are dominated by crustal assimilation and fractional crystallisation, geochemical features of the inferred parental magmas [low TiY of + 4 and a Gramado-type magma, at relatively shallow crustal levels, followed by fractional crystallisation. The asthenospheric component has relatively depleted incompatible element characteristics comparable to MORB. Thus, Low-Ti lava sequences of southern Parana record a shift from a predominantly lithospheric to an asthenospheric signature with time. Parana flood basalts appear to have been generated in response to lithospheric extension associated with the opening of the South Atlantic over a region of anomalously hot mantle, attributed to the presence of the Tristan mantle plume. However, the distinctive geochemical signature of the modern Tristan plume, seen on the islands of Tristan da Cunha, Gough and Inaccessible, is not evident in the compositions of Low-Ti magmas in southern Parana.

Elemental U and Th variations in island arc rocks: implications for U-series isotopes

Isotope and trace element data from well characterised arc suites are used to identify contributions from fluids and subducted sediments in arc rocks. U-series isotopes are then used to argue that the fluid and sediment components are characterised by different transfer times through the mantle wedge. U, but not Th, is readily mobilised in the fluid component. Th behaves as a high field strength element (HFSE), and the negative array between Th/Ce and Nd isotopes indicates that significant amounts of the Th and by implication other HFSE, in arc rocks are derived from sediments in the subducted slab. These elements may therefore only be regarded as conservative in the sense that they are not mobilised in the fluid component. As the fluid component contains U, but little if any Th, the resultant (238U/ 230Th) disequilibria may be used to estimate transfer times for the fluid component, and these are typically 30–120 ka. In contrast, rocks with a greater contribution from subducted sediments (high Th/Ce and low 143Nd/144Nd) tend to have (238U/ 230Th) ∼ 1, and low 10Be, suggesting transfer times of several million years. Overall, the Th, LREE and Ta contents of many arc rocks are dominated by the sediment component which is probably partial melts of sediments in the subducted slab. It is estimated that ∼ 30% of the Th in subducted sediments is returned to the crust in arc magmas. The fractionation of U/Th and Sm/Nd requires that average continental crust was generated in the presence of residual garnet, presumably by processes early in Earth history that were different from those at recent plate margins.

Chlorine in submarine glasses from the Lau Basin: seawater contamination and constraints on the composition of slab-derived fluids

Measurements of chlorine concentrations in matrix glasses from 18 primitive (>6 wt% MgO) and eight evolved lavas from active spreading centers in the Lau Basin back-arc system provide insight into the processes which control chlorine concentrations in subduction-related magmas, and can be used to investigate chlorine enrichment related to fluids derived from the underlying subducted slab. Chlorine contents of the glasses are highly variable (0.008–0.835 wt%) and generally high with respect to uncontaminated mid-ocean ridge basalt. Chlorine contents are highest in fractionated lavas from propagating ridge tips and lowest in more primitive basaltic lavas. Two different styles of enrichment in chlorine (relative to other incompatible elements) are recognized. Glasses from the Central Lau Spreading and Eastern Lau Spreading Center typically have low Ba/Nb ratios indicating minimal input of slab-derived components, and high to very high ratios of chlorine relative to K2O, H2O, and TiO2. This style of chlorine enrichment is highest in the most fractionated samples and is consistent with crustal assimilation of chlorine-rich altered ocean crust material. Data from the literature suggest that contamination by chlorine-rich seawater-derived components also characterizes the Woodlark Basin and North Fiji Basin back-arc systems. The second style of chlorine enrichment reflects input from slab-derived fluid(s) to the mantle wedge from the adjacent Tonga subduction zone. Basaltic glasses from the Valu Fa Ridge and Mangatolu Triple Junction show correlations between ratios of chlorine and K2O, H2O, and TiO2 and indices of slab-derived fluid input such as Ba/Nb, Ba/Th and U/Th, consistent with chlorine in these lavas originating from a saline fluid added to the mantle wedge. Within the Valu Fa Ridge the measured range of chlorine contents equates to a chlorine flux of 224–1120 kg/m/yr to the back-arc crust. Using a simple melting model and additional data from other back-arc and arc sample suites we conclude that chlorine is a major component within the slab fluids that contribute to many arc and back-arc melting systems, and probably plays an important role in regulating trace element transport by slab fluids in the mantle wedge. For the back-arc suites we have examined the estimated Cl/H2O and Cl/K2O ratios in the slab fluid component correlate with proximity to the arc front, suggesting that progressive dehydration of the slab and/or re-equilibration and transport within the mantle wedge may influence the overall degree of chlorine enrichment within the slab fluid component. The degree of chlorine enrichment observed in most back-arc lavas also appears too great to be explained solely by melting of amphibole, phlogopite or apatite within the mantle source and suggests that chlorine must be present in another phase, possibly a chlorine-rich fluid.

Mantle plumes, flood basalts, and thermal models for melt generation beneath continents: assessment of a conductive heating model and application to the Paraná

Growing evidence suggests that there is more than one type of continental flood basalt (CFB). Many CFB, such as the Deccan, were probably derived by decompression melting of asthenospheric peridotite in a mantle plume resulting in high eruption rates (∼1 km3 yr−1) and geochemical signatures of uncontaminated basalt which are similar to ocean island basalts. However, several geochemical studies have concluded that other CFB were derived from the subcontinental lithospheric mantle. This requires that melting occurs via heating of lithospheric peridotite above a mantle plume instead of melting within the plume itself. The feasibility of this model is tested with a simple one-dimensional, time-dependent thermal model. The results show that 1–2 km thick CFB may be derived from the lithospheric mantie without melting occurring within the underlying mantle plume if (1) the mantle potential temperature of the plume is between 1380 and 1580°C; (2) the lithospheric mantle is composed of volatile-enriched peridotite; (3) the overlying lithosphere is >100 km thick; (4) eruption occurs over 10–15 Myr; and (5) melting occurs over an area similar to the surface distribution of basalt. Small volumes of alkalic basalt may precede the eruption of tholeiites and if higher plume temperatures prevail, or extension/thermal erosion leads to lithospheric thicknesses ≤100 km, melts from the plume will rapidly dominate those from the lithosphere and eruption rates will increase (magmatic underplating may reduce the strength of the lithosphere sufficiently to initiate or focus rifting if the lithosphere is already under tension). These predictions can be used to discriminate between different CFB and are illustrated by application to the Parana CFB using published data along with new geochemical data from a borehole through the thickest section of basalt.

Sea-Level Highstand 81,000 Years Ago in Mallorca

Standing High Sea-level rises and falls as Earths giant ice sheets shrink and grow. It has been thought that sea level around 81,000 years ago—well into the last glacial period—was 15 to 20 meters below that of today and, thus, that the ice sheets were more extensive. Dorale et al. (p. 860; see the Perspective by Edwards) now challenge this view. A speleothem that has been intermittently submerged in a cave on the island of Mallorca was dated to show that, historically, sea level was more than a meter above its present height. This data implies that temperatures were as high as or higher than now, even though the concentration of CO2 in the atmosphere was much lower. Measurements from the island of Mallorca indicate that past sea levels were much higher than had been assumed. Global sea level and Earth’s climate are closely linked. Using speleothem encrustations from coastal caves on the island of Mallorca, we determined that western Mediterranean relative sea level was ~1 meter above modern sea level ~81,000 years ago during marine isotope stage (MIS) 5a. Although our findings seemingly conflict with the eustatic sea-level curve of far-field sites, they corroborate an alternative view that MIS 5a was at least as ice-free as the present, and they challenge the prevailing view of MIS 5 sea-level history and certain facets of ice-age theory.

Mantle plumes and flood-basalt stratigraphy in the Paraná, South America

Geochemical studies of the Parana continental flood basalts in Brazil have led to the recognition of distinct magma types, which have been used to infer the internal stratigraphy of the lava pile. The overstepping of stratigraphic units toward the north and a similar compositional change in sills within the underlying Parana basin sedimentary rocks imply that the site of magnatism migrated ∼750 km toward the north during this volcanic event. This has important implications for recent plume-related models that have argued whether fithospheric rifting must accompany the presence of a mantle plume in order to generate continental flood-basalt magmatism. It is difficult to ascribe the magnitude and direction of the shift in the locus of the Parana magmatism to movement of the Brazilian lithosphere relative to the underlying Tristan mantle plume. Instead, it is suggested that the observed lava distribution and internal structure of the Parana-Etendeka flood-basalt province were imposed by the intracacies of the rifting process during the initial opening of the South Atlantic Ocean.

Crystallisation ages in coeval silicic magma bodies: 238U-230Th disequilibrium evidence from the Rotoiti and earthquake flat eruption deposits, Taupo volcanic zone, New Zealand

Abstract The timescales over which moderate to large bodies of silicic magma are generated and stored are addressed here by studies of two geographically adjacent, successive eruption deposits in the Taupo Volcanic Zone, New Zealand. The earlier, caldera-forming Rotoiti eruption (>100 km3 magma) at Okataina volcano was followed, within months at most, by the Earthquake Flat eruption (∼10 km3 magma) from nearby Kapenga volcano; both generated non-welded ignimbrite and coeval widespread fall deposits. The Rotoiti and Earthquake Flat deposits are both crystal-rich high-silica rhyolites, with sparse glass-bearing granitoid fragments also occurring in Rotoiti lag breccias generated during caldera collapse. Here we report 238U–230Th disequilibrium data on whole rocks and mineral separates from representative Rotoiti and Earthquake Flat pumices and the co-eruptive Rotoiti granitoid fragments using TIMS and in situ zircon analyses by SIMS. Multiple-grain zircon-controlled crystallisation ages measured by TIMS from the Rotoiti pumice range from 69±3 ka ( 350 ka, with a pronounced peak at 70–90 ka. The weighted mean of isochrons is 83±14 ka, in accord with the TIMS data. One glass-bearing Rotoiti granitoid clast yielded an age of 57±8 ka by TIMS (controlled by Th-rich phases that, however, are not apparently present in the juvenile pumices). Another glass-bearing Rotoiti granitoid yielded SIMS zircon model ages peaking at 60–90 ka, having a similar age distribution to the pumice. Age data from pumices are consistent with a published 64±4 ka eruptive age (now modified to 62±2 ka), but chemical and/or mineralogical data imply that the granitoid lithics are not largely crystalline Rotoiti rhyolite, but instead represent contemporaneous partly molten intrusions reflecting different sources in their chemistries and mineralogies. Similarly, although the Earthquake Flat eruption immediately followed (and probably was triggered by) the Rotoiti event, age data from juvenile material are significantly different. A multiple-grain zircon-controlled crystallisation age measured by TIMS from a representative pumice is 173±5 ka, while SIMS model ages range from 70−26+34 ka to >350 ka, with a peak at 105 ka. These age data coupled with previously published geochemical and isotopic data show that the Rotoiti and Earthquake Flat deposits were erupted from independent, unconnected magma bodies.