Nick W. Rogers

The geochemistry of potassic lavas from Vulsini, central Italy and implications for mantle enrichment processes beneath the Roman region

Major and trace element and 143Nd/144Nd (0.51209–0.51216) and 87Sr/86Sr (0.70879–0.71105) isotope analyses are presented on a representative group of lavas from the Vulsini district of the Roman magmatic province. Three distinct series are identified; the high-K and low-K series are similar to those described from other Italian volcanoes, while the third is represented by a group of relative ly undifferentiated leucite basanites which are thought to be near-primary mantle melts. Major and trace element variations within the high-K series are consistent with fractional crystallisation from a parental magma similar to the most magnesian leucitites. Crustal contamination resulted in an increase in 87Sr/86Sr with increasing fractionation, but it was superimposed on magmas which had already inherited a range of incompatible element and isotope ratios from enrichment processes in the sub-continental mantle. These are reviewed using the available results from Vulsini, Roccamonfina and Ernici. Transition element abundances and Ta/Yb ratios indicate that the pre-enrichment mantle was similar to that of E-type MORB, and that these elements were not mobilised by the enrichment process. Mixing calculations suggest that three components were involved in the enrichment process; mantle comparable with the source of MORB, and two other components rich in trace elements. One, the low-K component, had high Sr/Nd, Th/Ta and Ba/Nb and no europium anomaly while the second had lower Sr/Nd, a negative europium anomaly and very high Th/Ta. It was also characterised by low Nb/Ba and high Rb/Ba ratios, similar to those reported from phlogopite-rich peridotite xenoliths. The trace element enrichment processes are therefore thought to have occurred in the mantle wedge above a subduction zone with the trace element characteristics of the high-K end-member reflecting the subduction of sediments and the stabilisation of mantle phlogopite.

Continental mantle lithosphere, and shallow level enrichment processes in the Earth's mantle

The continental mantle lithosphere may be a significant reservoir for incompatible elements, and it remains a key natural laboratory in which to investigate element fractionation processes in the upper mantle. Major, trace element and isotope data on mantle xenoliths, lamproites and kimberlites, and on selected continental flood basalts are integrated to develop a model for the continental mantle lithosphere. It is argued that the composition of such mantle lithosphere, and hence its density, thickness and capacity to generate basalt, varies with age. Archaean mantle lithosphere is characterised by relatively low FeO abundances, which are attributed to komatiite extraction, and thus it is intrinsically less dense than the surrounding asthenosphere. In contrast the post-Archaean mantle lithosphere may be compositionally similar to that sampled recently as spinel peridotite inclusions in alkali basalts. It is therefore sufficiently fertile to contribute in the generation of continental flood basalts, and dense enough to be more readily delaminated and incorporated into the asthenosphere source regions of ocean basalts. Combining the available data on mantle xenoliths and continental flood basalts suggests that the continental mantle lithosphere contains less than 10% of the K, and 3.5% of the Sr and Nd in the crust/mantle system. Many continental mafic rocks have distinctive isotope ratios with low eNd, variable eSr and often low 206Pb/204Pb. In particular the combination of slightly elevated 87Sr/86Sr at low 206Pb/204Pb is increasingly regarded as a feature of the continental mantle lithosphere. Elemental data on lamproites, kimberlites, melilitites and oceanic basalts demonstrate that potassic phase(s) strongly influence(s) U/Pb fractionation in the upper mantle. Locally, as in the western U.S., there is evidence for amphibole control, but in most cases the potassic phase appears to be phlogopite, consistent with the negative Rb/SrU/Pb arrays inferred from Sr- and Pb-isotopes. The significance of such arguments is that both amphibole and phlogopite are restricted to relatively shallow levels ( < 250 km), and thus they have a key role in distinguishing shallow from deep level enrichment processes in the Earths mantle.

Earliest magmatism in Ethiopia : evidence for two mantle plumes in one flood basalt province

Tertiary magmatism in Ethiopia has been linked to the thermal influence of the Afar mantle plume. However, new laser 40Ar/39Ar ages for the volcanic succession in southern Ethiopia confirm the presence of two distinct magmatic phases at 45–35 Ma and 19–12 Ma. The earliest phase predates both extension and magmatism in northern Ethiopia by 15 m.y. and cannot be related to any simple model of melting in response to extension over a single mantle plume. We propose a model in which the Ethiopian province was initially related to the thermal influence of the Kenyan, and subsequently, the Afar mantle plume during northward movement of the African plate in the Tertiary. Support for this model comes from paleogeographic evidence that places southern Ethiopia ∼1000 km farther south than its current position during the early melting event at 45 Ma. Moreover, the rate of migration of the onset of magmatism from southern Ethiopia to Tanzania is similar to the rate of migration of the African plate over the same period. Comparable eruption rates in southern Ethiopia and Kenya further strengthen this link. In the light of this evidence, eruption rates ascribed to melting of the Afar mantle plume may be overestimated, which calls into question the potential for the Afar mantle plume to have had a significant effect on the biosphere.

Calc-alkaline magmatism, lithospheric thinning and extension in the Basin and Range

Most of the volcanic rocks in the Colorado River Trough (CRT) and the Mogollon-Datil Volcanic Field (MDVF) in the Basin and Range exhibit calc-alkaline major element trends and relatively low high field strength element abundances, similar to those erupted from the volcanoes of Aso and Towada in Japan. Such features are widely regarded as characteristic of subduction-related magmatism, and yet the rocks in the Basin and Range were generated in response to lithospheric extension. The preextensional to synextensional rocks of the CRT and the MDVF have higher Na2O, K2O, and TiO2, in the range 47–55% SiO2, and relatively low Al2O3, and overall, they tend to have higher Sr contents and Zr/Y and La/Nb ratios than those from Aso and Towada. In addition, the basalts in the Basin and Range tend to be more aphyric than those in Japan, consistent with more rapid movement of magma through the crust during extension in the Basin and Range, and the rate of melt generation appears to have been significantly less in the Basin and Range than along recent destructive plate margins. The geochemical differences are attributed to smaller degrees of partial melting in the Basin and Range and to source regions that had been enriched in incompatible elements since the Proterozoic, resulting in parental magmas with higher alkali contents than those commonly observed in subduction-related calc-alkaline suites. Within the CRT the subsequent calc-alkaline trend was due at least in part to mixing with crustal derived melts, whereas in the MDVF such trends reflect both crustal contamination and fractional crystallization involving magnetite and amphibole. The small volumes of magma with minor and trace element features similar to oceanic basalts indicate that relatively little melt was generated in underlying asthenosphere. Thus it is inferred that magmatism in the Basin and Range was not associated with a significant increase in temperature, such as might be attributed to a mantle plume, but rather it was in response to lithospheric extension. Calculations are presented which demonstrate that the magma volumes and inferred source regions, extension, present-day heat flow, and topography are consistent with a model of convective lithospheric thinning after thickening in the Laramide and Sevier orogenies.

Two mantle plumes beneath the east African rift system: Sr, Nd and Pb isotope evidence from Kenya Rift basalts

Major and trace element and radiogenic isotope ratios (Sr, Nd and Pb) are presented for a suite of Neogene to Recent basalts (MgO>4 wt%) from the axial regions of the Kenya Rift. Samples have compositions ranging from hypersthene-normative basalt through alkali basalt to basanite and are a subset of a larger database in which compositions extend to nephelinite. A broadly negative correlation between Zr/Nb ( 20, broadly similar to values from OIB. The Kenya Rift cuts through basement of different ages and aspects of the composition of mafic magmas reflect the anisotropy of the underlying lithosphere. Specifically, those basalts from that part of the rift underlain by the Tanzanian craton (TC) have higher Ce/Y and lower Zr/Nb ratios than those erupted through the Panafrican Mozambique belt (MB) implying an origin either at greater depth or from a more trace element-enriched source region. Samples erupted through the zone of reactivated craton margin (RCM) share the characteristics of mafic lavas from both the craton and the mobile belt. MB samples have 143Nd/144Nd=0.5130–0.5127, 87Sr/86Sr=0.7030–0.7035 and 206Pb/204Pb=18.3–19.8, defining a steep negative trend on the Nd–Sr diagram and plotting close to the NHRL on conventional Pb isotope diagrams. By contrast TC and RCM samples have 143Nd/144Nd=0.5124–0.51275, 87Sr/86Sr=0.7035–0.7056 and 206Pb/204Pb=17.6–21.2, defining flat-lying arrays on Nd–Sr plots and a much greater scatter and spread on Pb isotope diagrams, with many analyses plotting above the NHRL. Both groups of analyses trend towards a common end member on a plot of 143Nd/144Nd against 87Sr/86Sr, at 143Nd/144Nd0.51275 and 87Sr/86Sr0.7035. These values are suggested to reflect the isotopic characteristics of the sub-lithospheric Kenyan mantle, inferred to be the Kenya mantle plume. Comparison with data from Afar suggest that the Kenya plume is distinct from the Afar plume, implying that the east African Rift is underlain by at least two distinct mantle plumes. Eocene and Oligocene basalts from southern Ethiopia bear a closer resemblance to the Kenyan basalts than to those from Afar and the Ethiopian plateau, suggesting that the Kenya plume has been active for at least 45 Ma. Migration of magmatism from southern Ethiopia southwards is consistent with the northeastward migration of the African plate over the Kenya plume over the past 50 My.

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.

UTh isotope disequilibria and ocean island basalt generation in the Azores

Abstract A suite of 23 alkalic lavas from the Azores islands of Sao Jorge, Pico, Faial, Terceira and Sao Miguel have been analysed for their major and tram element and Sr, Nd, Pb and Th isotopic compositions. Previous studies have shown that lavas along the MAR show increases in La/Sm and 87Sr/86 Sr coupled with decreasing Fe towards the Azores platform inferred to reflect increasing dominance of a plume component over a depleted MORB-mantle component. The La/Sm and Sr, Nd and Pb isotope data of lavas from Sao Jorge, Pico, Faial and Terceira overlap the high La/Sm, 87Sr/86Sr end of the MAR lava array inferred to represent the deep seated plume and they have low Fe8.0∗ compared with for example, Iceland basalts. When compared with the MAR array, higher Fe8.0∗ and Na8.0 and lower Si8.0 probably reflect lower degrees of melting at higher average pressures beneath a lithospheric lid. The combination of enrichment in incompatible elements (including volatiles) with depletion in Fe in the regional plume component is a feature most likely to have developed within the lithospheric mantle. Lavas from Sao Miguel require an additional high 87Sr/86Sr, 207Pb/204Pb, low 143Nd/144Nd component with high Th/Nb which is suggested to result from incorporation of sediment in the source of these lavas. UTh isotope data indicate 9–29% 230Th excesses similar to those observed in alkalic basalts from Iceland or Hawaii. This, combined with the observation that lavas from Sao Miguel at the periphery of the inferred plume have smaller 230Th excesses (average 14%) than those nearer the centre, including the recent measurements from the MAR-ridge (average 23%), suggests that there is no simple correlation between buoyancy flux or lithospheric lid thickness and 230Th excesses. Long-term source Th/U ratios inferred from Pb isotopes are greater than those measured indicating U addition some time prior to partial melting. The large variations in (230Th/232Th) and (238U/232Th) are negatively correlated with 87Sr/86Sr indicating that much of the variation in U/Th reflects long-lived source heterogeneity. Moreover, the degree of 230Th excess is also negatively correlated with 87Sr/86Sr suggesting a link between melting process and source heterogeneity. The variations in 230Th excess are unlikely to result from thermally controlled variations in solidus pressure since this would require temperature differences (250°C) that are improbable at the length scale between Sao Miguel and Terceira (200 km). Instead, it is suggested that the incorporation of sediment and volatiles beneath Sao Miguel has led to increased rates of melting, resulting in smaller 230Th excesses, consistent with the higher volcanic output on Sao Miguel compared with Terceira.

Mantle plume and lithosphere contributions to basalts from southern Ethiopia

Major, trace element and radiogenic isotope analyses are presented for a suite of basaltic rocks from two transects across the Main Ethiopian Rift in southern Ethiopia. The northern transect crosses the MER within the limits of the Ethiopian Plateau, regarded as the topographic expression of the underlying Afar mantle plume, while the southern transect crosses the Turkana Depression to the south of the plateau. The samples are generally transitional between tholeiitic and alkaline, although, in the northern samples, there is a tendency for compositions to become more alkaline and enriched in incompatible elements with time. 87Sr86Sr ratios vary between 0.7032 and 0.7043, 143Nd144Nd from 0.51261 to 0.51287 and 206Pb204Pb from 18.11 to 20.28, and there is a marked tendency for 206Pb204Pb to increase and ZrNb to decrease with time in the northern samples. 206Pb204Pb and ZrNb show a marked negative correlation, which is interpreted as reflecting mixing between plume-derived small melt fractions with high 206Pb204Pb and low ZrNb and larger melt fractions from the mantle lithosphere with low 206Pb204Pb and high ZrNb. Plume contributions to the pre-rift basalts in the northern transect are < 30% but increase in the syn-rift basalts, which are predominantly plume derived. All but the most recent alkali basalts from the southern transect are predominantly lithosphere derived. Extrapolating these results to the whole Ethiopian Province suggests that melt production rates in the mantle plume increased from < 0.01 km3a−1 in the pre-rift to ∼ 0.03 km3a−1 in the syn-rift basalts, which is similar to that of plume-related ocean islands. These results imply that lithospheric thinning was achieved initially by thermo-mechanical erosion by the underlying plume and subsequently by tectonic extension during rifting.

Melting in the lithospheric mantle: Inverse modelling of alkali-olivine basalts from the Big Pine Volcanic Field, California

The variations in trace element abundances of a suite of alkali-olivine basalts from the Big Pine volcanic field, California, have been ‘inverted’ following the method of Hofmann and co-workers to obtain source concentration and distribution coefficient data. The high Mg-numbers and ne-normative mineralogy of these lavas allow a simple correction to be made for fractional crystallisation, and together with a limited range in 87Sr/86Sr (0.7056–0.7064), suggest derivation from a relatively homogeneous source region. Negative correlations between SiO2 and P2O5, and SiO2 and Rb in the calculated primary magmas imply that both major and trace elements vary in a coherent fashion as a function of the degree of partial melting. The Big Pine lavas are characterised by high ratios of large-ion lithophile to high-field strength elements (Ba/Nb>60), and the inverse procedure demonstrates that this reflects source concentrations, as opposed to a mineralogical control. The calculated mantle source is further characterised by generally high abundances of Sr, Ba, K, and Th relative to Nb and Ta which imply that incompatible element enrichment of the source occurred above a subduction zone. A model Sm/Nd age of 1.8 Ga for this enrichment coincides with the regional crustal formation age. Such features imply that both the major and trace element components of the Big Pine lavas are derived from within lithospheric mantle, perhaps mobilised by the high geothermal gradients which characterise the extensional environment of the Basin and Range Province. A comparison with other Cenozoic mafic lavas throughout the western United States suggests that a substantial proportion of the mantle lithosphere in this area has similar chemical characteristics to the source of the Big Pine lavas. If this is the case, then it implies that convergent margins represent an important tectonic environment for the formation of lithospheric mantle.

Late Cenozoic basaltic magmatism in the Western Great Basin, California and Nevada

Alkali-rich basaltic rocks have been erupted in small volumes throughout the Western Great Basin (WGB) since 17 Ma. SiO2 ranges from 46 to 53% in samples with >4% MgO, whereas Fe2O3 ranges from 8 to 10% and TiO2 from 1 to 2% in the most magnesian rocks. They have high incompatible element contents, except for relatively low high field strength elements abundances, and thus their minor and trace element patterns closely resemble subduction-related rather than intraplate magmas. Their radiogenic isotope ratios are also enriched with 87Sr/86Sr = 0.7040–0.7078, 143Nd/144Nd = 0.5129–0.5120 and variable Pb isotopes (206Pb/204Pb = 18.0–19.2), generally displaced above the Northern Hemisphere Reference Line in both the 207Pb/204Pb and 208Pb/204Pb diagrams. The WGB straddles the isotopic and tectonic boundary between continental North America, underlain by Proterozoic basement, and the younger accreted terrain that forms the basement to most of California. There are no discernible differences in the major and trace element contents of the basalts either side of this boundary except that those erupted through Proterozoic basement tend to lower silica contents and have higher 87Sr/86Sr and lower 143Nd/144Nd ratios. Nd model ages and a secondary Pb isochron from basalts east of the boundary suggest an age of 1.6–1.8 Ga, significantly younger than the age of crust generation inferred from Nd model ages of granites. Correlated trace element and isotope ratios in basaltic rocks to the west of the boundary indicate a contribution from recent subduction, recognized by a decrease in 87Sr/86Sr as Nb/Sr and Zr/Sr decrease. A second, less well defined trend to higher Nb/Sr and Zr/Sr and lower 87Sr/86Sr is interpreted to reflect a minor asthenospheric contribution to contemporary magmatism. Relative to basalts from the Basin and Range, the basaltic rocks from the WGB have higher SiO2 and lower TiO2 and Fe2O3 at similar degrees of fractionation. These differences are interpreted to reflect different pressures of segregation from their mantle source regions, the WGB basalts being derived from shallower depths than the Basin and Range basalts, within the lithospheric mantle. This conclusion is consistent with their lithospheric trace element and isotope characteristics and trace element models that allow minimal garnet in the source region. By contrast the lower silica contents of the Basin and Range alkali basalts are more consistent with derivation from depths of 90 km or more, which may place their source regions in the asthenosphere, also consistent with their trace element and isotope variations.