Showing papers on "Basalt published in 1989"

Chemical and isotopic systematics of oceanic basalt : implications for mantle composition and processes

Abstract: Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.

Flood Basalts and Hot-Spot Tracks: Plume Heads and Tails

Abstract: Continental flood basalt eruptions have resulted in sudden and massive accumulations of basaltic lavas in excess of any contemporary volcanic processes. The largest flood basalt events mark the earliest volcanic activity of many major hot spots, which are thought to result from deep mantle plumes. The relative volumes of melt and eruption rates of flood basalts and hot spots as well as their temporal and spatial relations can be explained by a model of mantle plume initiation: Flood basalts represent plume "heads" and hot spots represent continuing magmatism associated with the remaining plume conduit or "tail." Continental rifting is not required, although it commonly follows flood basalt volcanism, and flood basalt provinces may occur as a natural consequence of the initiation of hot-spot activity in ocean basins as well as on continents.

Melting in an Archaean mantle plume: heads it's basalts, tails it's komatiites

Abstract: THE lower part of most Archaean greenstone sequences is dominated by interlayered basaltic and komatiitic (ultrabasic) flows. These two magma types are petrologically and geochemically distinct, yet they display a close spatial and temporal association. The origin of the anomalously high-temperature komatiitic liquids has been much debated because of the implications for the thermal structure and composition of the Archaean mantle. Here we argue that both the basalts and komatiites are produced by a starting thermal plume rising in a warmer Archaean mantle. Fluid-dynamics studies show that a starting plume consists of a hot axial jet, capped by a large head into which cooler surrounding mantle is entrained. Our calculations for such a flow indicate that komatiites could form by melting in the high-temperature axis of the plume and basalts by melting in the cooler head. The sudden onset and limited duration of the basalt/komatiite sequences, seen in the greenstone belts of Western Australia and elsewhere, are explained by this model.

Isotopic and geochemical provinces of the western Indian Ocean Spreading Centers

Abstract: Basalt glasses from the Central Indian Ridge are distinct isotopically from mid-ocean ridge basalts (MORB) of the Indian Ocean triple junction and western few hundred kilometers of the Southeast Indian Ridge. In particular, very low 206Pb/204Pb and high 87Sr/86Sr signatures, which characterize the latter region, are absent over most of the Central Indian Ridge. In turn, lavas from the unusually deep eastern 1100–1500 km of the Southwest Indian Ridge are different chemically and isotopically from those of the above areas. A rather abrupt eastern boundary to Southwest Indian Ridge-type compositions occurs at or very near the geographic triple junction. This provinciality in western Indian Ocean ridge basalts partly mirrors fundamental regional differences in the underlying mantle but, at least between the eastern Southwest Indian Ridge and the western Southeast Indian Ridge and triple junction, also may reflect variations in extent and depth of melting in a vertically zoned upper mantle. A pronounced low eNd, high 206Pb/204Pb, high 87Sr/86Sr anomaly exists on the Central Indian Ridge at the Marie Celeste Fracture Zone and on the adjacent ridge segment to the south. Despite the great distance (>1100 km) of Reunion Island from the ridge, this zone appears to demark a region of mantle containing substantial Reunion hotspotlike material. Several old (35–60 m.y.) Deep Sea Drilling Project basalts which erupted on the ancestral Central Indian Ridge also record a significant Reunion hotspotlike influence, whereas a 46-m.y.-old sample that formed farther from the presumed locus of the hotspot possesses isotopic values identical to many present (non-Marie Celeste area) Central Indian Ridge MORB. The variably expressed and/or heterogeneous low 206Pb/204Pb material partly responsible for the isotopic distinctiveness of Indian Ocean ridge basalts may have entered into the Indian MORB mantle as a result of continental lithospheric remobilization preceding the breakup of Gondwana, particularly from the portion that would eventually become Greater India.

The hafnium paradox and the role of garnet in the source of mid-ocean-ridge basalts

Abstract: MID-OCEAN-RIDGE basalts (MORBs) are thought to result from melting in the mantle at depths of less than 60 km, in the spinel stability field1–3 MORBs have 176Hf/177Hf ratios indicating derivation from a mantle reservoir with a long-term Lu/Hf ratio greater than that of Cl chondrite meteorites, yet the measured Lu/Hf ratios in MORB are lower than in Cl chondrites: this is the 'hafnium paradox'4 Here we show that the Lu–Hf and Sm–Nd systematics of MORBs require garnet to be a residual phase in MORB melt genesis This places the onset of melting beneath a mid-ocean ridge at depths greater than 80 km A sequential melting model, in which melting starts in the garnet stability field and then continues at shallower levels, best explains the combined Nd and Hf isotope systematics, and is compatible with our present geophysical and geochemical knowledge of mid-ocean-ridge magmatism

Basaltic volcanism in Ethiopia: Constraints on continental rifting and mantle interactions

Abstract: Middle to late Cenozoic mafic lavas from the Ethiopian volcanic province exhibit considerable chemical and isotopic diversity that is linked to eruption age and eruption location. These variations provide a geochemical framework in which continental rifting can be examined. Trace element and Sr, Nd, and Pb isotopic data are interpreted to indicate involvement of up to two depleted and two enriched mantle reservoirs throughout Cenozoic rift development in Ethiopia. Superimposed on the characteristics imparted by varying degrees of melting of these distinct reservoirs are the effects of crystal fractionation and, in some instances, crustal contamination. Initial stages of Oligocene rifting and volcanism, as manifested by the rift-bounding plateau flood basalts, are attributed to asthenospheric upwelling and melting of a heterogeneous, enriched subcontinental lithospheric mantle. Mildly alkaline lavas were produced from an enriched source with characteristics similar to those of the inferred source of other mantle-derived lavas and xenoliths from east Africa (LoNd array, EMI to HIMU). Contemporaneous tholeiitic lavas were derived from a source similar to that producing oceanic basalts from Samoa and the Society Islands (EMII). As lithospheric thinning and rifting continued into the Miocene, upwelling depleted asthenosphere (depleted OIB reservoir, PREMA) interacted with the lithospheric sources producing lavas with hybrid elemental and isotopic characteristics (11–6 Ma plateau and rift margin basalts). Crustal contamination is most evident in the Oligocene to Miocene plateau basalts and is suggested to have taken place primarily at middle to lower crustal levels during initial stages of continental rifting. By 4–5 Ma b.p. continental breakup had begun in Afar, with basalts during this period being derived almost entirely from a depleted PREMA-type reservoir. In the Main Ethiopian Rift, where continental breakup is less advanced, young rift basalts retain a geochemical signature consistent with enriched (LoNd)-depleted (PREMA) mantle hybridization. During the Holocene, proto-oceanic crust and oceanic crust characterize the Afar and Red Sea/Gulf of Aden, respectively, and input from a depleted MORB source first becomes apparent. Chronologic and tectonic control on mantle melting, mantle reservoir interactions, and crust-mantle interactions is a theme common to many extensional regions. Another common feature is the apparent role of a depleted PREMA-type reservoir in these regions, supporting the idea that this reservoir is located at depth within the convecting asthenosphere. Involvement of enriched mantle during continental extension-related magmatism is also prevalent, but the geochemical signature of this component varies from region to region, suggesting a strong link to local crust formation history and local enrichment events such as subduction-driven lithospheric recycling.

The Arabian continental alkali basalt province: Part I. Evolution of Harrat Rahat, Kingdom of Saudi Arabia

Abstract: The Harrat Rahat lava field forms a major component of an extensive but poorly known continental alkali basalt province extending from Yemen in the south, through Saudi Arabia and Jordan, to Syria in the north. The continental intraplate volcanism which produced the Arabian lava fields (harrats) was contemporaneous with opening of the Red Sea, collision of the Arabian and Eurasian plates, and uplift of the Afro-Arabian dome. Harrat Rabat has evolved over the past 10 m.y. It contains two prominent lateritic disconformities which separate three stratigraphic units: the Shawahit (10-2.5 Ma), Hammah (2.5-1.7 Ma), and Madinah (1.7-Recent) basalts, comprising 68%, 19%, and 13% of the total harrat volume, respectively. The Shawahit basalt is composed of coarse-grained, dictytaxitic olivine transitional basalt (OTB) and minor (∼10%) alkali olivine basalt (AOB) which erupted through scoria cones and shield volcanoes. In contrast, the Hammah basalt is dominated by equal volumes of fine-grained, intergranular-to-intersertal AOB and hawaiite which erupted mainly through scoria cones and relatively few shield volcanoes. Sparse domes and flows of mugearite and benmoreite first appear in the Hammah basalt. The Madinah basalt spans the full compositional range of Harrat Rahat; 103 analyzed samples comprise OTB (8%), AOB (47%), hawaiite (32%), mugearite (4%), benmoreite (8%), and trachyte (2%). Here scoria cones dominate, whereas shield volcanoes are sparse and occur only in the lower Madinah basalt; domes and associated pyroclastic deposits are common. The final events of harrat volcanism include 11 "post-Neolithic" eruptions (<4,500 yr old) and two historical eruptions (in 641 and 1256 A.D.). Arabian harrat magmatism may have been initiated by partial melting of garnet perido-tite in the asthenosphere at a depth > 100 km. Accumulation of this partial melt at the crust-mantle boundary (37-44 km) may have resulted in Miocene uplift of the Afro-Arabian dome. Harrat Rahat volcanism began shortly after significant initial uplift with the voluminous extrusion of Shawahit OTB lava. Chemical and petrographic data suggest that these OTB magmas ascended rapidly, with little crustal residence time. The data support a model of open-system fractionation for AOB and hawaiite lavas of the younger Hammah and Madinah basalts. In this model, some of the potential Shawahit OTB was trapped in crustal chambers during magma ascent. Crystal fractionation in these chambers was accompanied by their periodic replenishment by rising OTB magma. This resulted in the mixing of magma types and the development of AOB and hawaiite magmas which are anomalously enriched in highly incompatible elements. The mugearite, benmoreite, and trachyte lavas are the products of advanced fractionation in these crustal chambers with little or no magma replenishment.

Coexisting calcalkaline and high‐niobium basalts from Turrialba Volcano, Costa Rica: Implications for residual titanates in arc magma sources

Abstract: Turrialba volcano, the southeasternmost volcano in the Central American arc, is constructed of medium to high-K calcalkaline basalts, andesites, and dacites, plus rare basalts with unusually high Nb concentrations. The compositions of these high-Nb basalts are more similar to those of intraplate basalts than they are to typical calcalkaline or arc-tholeiitic basalts. The association of calcalkaline and high-Nb basalts is rare in arc front volcanoes, seemingly being restricted to volcanoes that overlie Oligocene or younger subducting crust or that overlie the edges of subducting plates. The calcalkaline and high-Nb basalts at Turrialba have generally similar major element, trace element, and isotopic compositions but differ significantly in their Ba/La and La/Nb ratios. The geochemical similarities imply that they were derived from similar ocean island basalt sources. Their geochemical differences suggest that residual rutile stabilized by a large ion lithophile element bearing slab-derived fluid was present during calcalkaline basalt genesis but not during high-Nb basalt genesis. To explain the stability of rutile in a calcalkaline melt with a relatively low TiO2 concentration, we use a model that involves two stages of melting for both basalt types. Silica saturated high degree melts with mid-ocean ridge basalt like incompatible element concentrations generated by upwelling mantle are used as mixing end-members for both the calcalkaline and the high-Nb basalts. The calcalkaline basalts represent mixtures of the high-degree melts and oxidized small-degree melts generated by amphibole breakdown in mantle overlying the subducting slab. This small-degree melt has high incompatible element concentrations and is saturated in rutile. Arc-related lamprophyric rocks have compositions that are appropriate for these small-degree melts. High-Nb basalts are mixtures of the high-degree melts and more reduced small-degree melts that are undersaturated in rutile. These reduced melts may migrate around or through the subducting slab into the wedge to become involved in arc magma genesis.

Geochemical variations in Andean basaltic and silicic lavas from the Villarrica-Lanin volcanic chain (39.5° S): an evaluation of source heterogeneity, fractional crystallization and crustal assimilation

Abstract: At 39.5° S in the southern volcanic zone of the Andes three Pleistocene-recent stratovolcanoes, Villarrica, Quetrupillan and Lanin, form a trend perpendicular to the strike of the Andes, 275 to 325 km from the Peru-Chile trench. Basalts from Villarrica and Lanin are geochemically distinct; the latter have higher incompatible element abundances and La/Sm but lower Ba/La and alkali metal/La ratios. These differences are consistent with our previously proposed models involving: a) a west to east decrease in an alkali metal-rich, high Ba/La slab-derived component which causes an across strike decrease in degree of melting; or b) a west to east increase in the contamination of subduction-related magma by enriched subcontinental lithospheric mantle. Silicic and mafic lavas from the stratovolcanoes have overlapping Sr, Nd and O isotopic ratios. Silicic lavas also have geochemical differences that parallel those of their associated basalts, e.g., rhyolite from Villarrica has lower La/Sm and incompatible element contents than high-SiO2 andesite from Lanin. At each volcano the most silicic lavas can be modelled by closed system fractional crystallization while andesites are best explained by magma mixing. Apparently crustal contamination was not an important process in deriving the evolved lavas. Basaltic flows from small scoria cones, 20–35 km from Villarrica volcano have high incompatible element contents and low Ba/La, like Lanin basalts, but trend to higher K/Rb (356–855) and lower 87Sr/ 86Sr (0.70361–0.70400) than basalts from either stratovolcano. However all basalts have similar Nd, Pb and O isotope ratios. The best explanation for the unique features of the cones is that the sources of SVZ magmas, e.g., slab-derived fluids or melts of the subcontinental lithospheric mantle, have varying alkali metal and radiogenic Sr contents. These heterogeneities are not manifested in stratovolcano basalts because of extensive subcrustal pooling and mixing. This model is preferable to one involving crustal contamination because it can account for variable Sr isotope ratios and uniform Nd and Pb isotope ratios among the basalts, and the divergence of the cones from across-strike geochemical trends defined by the stratovolcanoes.

The volcanism of southern Italy: Role of subduction and the relationship between potassic and sodic alkaline magmatism

Abstract: The ultrapotassic magmatism of southern Italy (the Roman province) is well known, and recently these highly unusual lavas have been explained in terms of subduction-related processes. Less well studied are the coeval calc-alkaline to potassic rocks of the nearby Aeolian Islands, which are situated above a Benioff zone and are therefore demonstrably related to recently active subduction. On a number of geochemical diagrams the Roman and Aeolian provinces define continuous trends, which may be accommodated in a single petrogenetic model involving mixing of three isotopically and elementally distinct components. Two of these are subduction-related: first, a high Sr/Nd, high Th/Ta component derived largely from basaltic ocean crust and, second, a component with extremely high Th/Ta, but relatively low Sr/Nd derived largely from subducted sediments. These are mixed with mantle wedge material which, prior to subduction, was characterised by highly radiogenic Pb isotope ratios, and is therefore comparable to the mantle source of Mount Etna volcanism. Thus it would appear that midplate tholeiitic to Na-alkalic magmatism and continental margin calc-alkaline to ultrapotassic magmas were derived from mantle sources which, prior to subduction, had similar isotopic signatures. This observation has important implications for the potential involvement of trace element and isotope enriched (OIB-like) mantle in the genesis of subduction-related volcanism.

Drilling deep into young oceanic crust, Hole 504B, Costa Rica Rift

Abstract: Hole 504B is by far the deepest hole yet drilled into the oceanic crust in situ, and it therefore provides the most complete “ground truth” now available to test our models of the structure and evolution of the upper oceanic crust. Cored in the eastern equatorial Pacific Ocean in 5.9-m.y.-old crust that formed at the Costa Rica Rift, hole 504B now extends to a total depth of 1562.3 m below seafloor, penetrating 274.5 m of sediments and 1287.8 m of basalts. The site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust. As a result, the conductive heat flow approaches the value of about 200 mW/m² predicted by plate tectonic theory, and the in situ temperature at the total depth of the hole is about 165°C. The igneous section was continuously cored, but recovery was poor, averaging about 20%. The recovered core indicates that this section includes about 575 m of extrusive lavas, underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes; the latter have been sampled in situ only in hole 504B. The igneous section is composed predominantly of magnesium-rich olivine tholeiites with marked depletions in incompatible trace elements. Nearly all of the basalts have been altered to some degree, but the geochemistry of the freshest basalts is remarkably uniform throughout the hole. Successive stages of on-axis and off-axis alteration have produced three depth zones characterized by different assemblages of secondary minerals: (1) the upper 310 m of extrusives, characterized by oxidative “seafloor weathering“; (2) the lower extrusive section, characterized by smectite and pyrite; and (3) the combined transition zone and sheeted dikes, characterized by greenschist-facies minerals. A comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology, alteration, and porosity into three zones that are analogous to layers 2A, 2B, and 2C described by marine seismologists on the basis of characteristic seismic velocities. Many of the logs and experiments suggest the presence of a 100- to 200-m-thick layer 2A comprising the uppermost, rubbly pillow lavas, which is the only significantly permeable interval in the entire cored section. Layer 2B apparently corresponds to the lower section of extrusive lavas, in which original porosity is partially sealed as a result of alteration. Nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900–1000 m below seafloor, within the transition between extrusives and sheeted dikes, indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2B and 2C and confirming that layer 2C consists of intrusive sheeted dikes. A vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends—that between the sheeted dikes of seismic layer 2C and the gabbros of seismic layer 3, which has never been sampled in situ—may be within reach of the next drilling expedition to hole 504B. Therefore despite recent drilling problems deep in the hole, current plans now include revisiting hole 504B for further drilling and experiments when the Ocean Drilling Program returns to the eastern Pacific in 1991.

Melt inclusion investigation of the volatile behaviour in historic alkali basaltic magmas of Etna

Abstract: Crystallization paths of basaltic (1763 eruption) and hawaiitic (1865 and 1329 eruptions) scoria from Etna were deduced from mineralogy and melt inclusion chemistry. The volatile behaviour was investigated through the study of melt inclusions trapped in the phenocrysts and those of the whole rocks and the matrix glasses. The results from the 1763 eruption point to the early crystallization of olivine Fo 81.7 from a water-rich alkaline basalt, with high Cl (1750–2000 ppm) and S (2100–2400 ppm) concentrations. The hawaiitic melt inclusions trapped in olivine Fo 74, salite and plagioclase are characterized by a decrease in Cl/K2O and S/K2O ratios. In each investigated system there is good correlation between K2O and P2O5. In the whole rocks, Cl ranges from 980 to 1680 ppm, from basaltic to hawaiitic lavas, whereas S (110–136 ppm) remains low. Cl and S behaviour in the 1763 magma suggests an early degassing stage of Cl and S, with CO2 and a water-rich gaseous phase for a pressure close to 100 MPa, consistent with a permanent outgassing at the summit craters of Etna. During the eruption, the sulphur remaining in the hawaiitic liquid is lost, and the degassing of chlorine is limited. Such a degassing model can be extended to the 1865 and 1329a.d. eruptions.

Compositional constraints on the continental lithospheric mantle from trace elements in spinel peridotite xenoliths

Abstract: DATA for niobium in mid-ocean-ridge basalts (MORBs) and ocean-island basalts (OIBs) have provided new constraints on global geochemical models of continental growth and mantle differentiation. Until now, there were no such data in the concentration range of parts per 109 (p.p.b.) for peridotite samples. Here we report new measurements of Nb and other trace elements in fertile peridotite xenoliths. Our data show that the Nb/Ta ratio is chondritic, whereas Nb/Th (and presumably Nb/U) ratios are higher than chondritic. High Nb/Th ratios are characteristic of sources and/or residues of divergent-margin (for example, MORB) and intraplate (for example, OIB) magmatism. Compared with primitive mantle, these peridotites show a systematic and uniform depletion of the highly inompatible elements. The depleted patterns are apparently inconsistent with these peridotites being sources or residua from convergent-margin (that is, island-arc) magmatism. These data may indicate that the continental lithospheric mantle does not possess a Nb enrichment to complement the depletion found in the continental crust and island-arc basalts.

Major element, volatile, and stable isotope geochemistry of Hawaiian submarine tholeiitic glasses

Abstract: Tholeiitic glasses were dredged from the submarine rift zones of the five volcanoes comprising the island of Hawaii and Loihi Seamount. The major element composition of the glasses follows a systematic trend that is related to the stage of evolution of the volcano. Glasses from Loihi Seamount (the youngest Hawaiian volcano) are enriched in Fe, Ca, Ti, Na, and K and depleted in Si and Al relative to the glasses from the other, older volcanoes. Kilauea is intermediate in age and its glasses are intermediate in composition between those from Loihi and Mauna Loa, the largest and oldest of the active Hawaiian tholeiitic volcanoes. The volatile contents (H20, CO2, S, F, Cl) of the glasses from these volcanoes follow the same trend (highest in Loihi; lowest in Mauna Loa). Glasses from Hualalai Volcano are similar in composition to those from Mauna Loa; those from Kohala Volcano are similar to Kilauea; Mauna Kea glasses range from Mauna Loa-like to Kilauea-like. The observed systematic variation in composition of Hawaiian tholeiites may be related to the progressive melting and depletion of the source of these volcanoes during their growth. Oxygen and hydrogen isotope analyses were made on many of the glasses from each volcano. The δ18O values of Hawaiian tholeiites are distinctly lower than those of mid-ocean ridge basalt (MORB) (averages: 5.1 versus 5.7). These low values are probably a distinct feature of hot spot lavas. The δD values for these glasses (−88 to −61) are typical of mantle and MORB values. Thus the H2O in the Hawaiian glasses is probably of magmatic origin. Previous isotopic and trace element data indicate that the source of Hawaiian tholeiites contains two distinct source components. Based on the results of this study, the plume component in the source for Hawaiian tholeiites is characterized by moderate 87Sr/86Sr (0.7035–0.7037) and 206Pb/204Pb ratios (18.6–18.7), a low δ18O value (∼5.0), and greater contents of volatiles, Fe, Ca, Ti, Na and K relative to the MORB source.

Primitive calc-alkaline and alkaline rock types from the Western Mexican Volcanic Belt

Abstract: Since the early Pliocene, small volumes of alkaline magmas have erupted in the western Mexican Volcanic Belt (MVB) in close association with the volumetrically dominant calc-alkaline magmas. Both suites include relatively rare “primitive” types, characterized by high Mg#, Cr, and Ni contents. Primitive hypersthene-normative basalts, parental to the calc-alkaline suite, are found along the volcanic front but are absent at greater distances from the trench. The volcanic-front calc-alkaline suites are typically hornblende bearing, indicating relatively high water contents. Associated primitive alkaline magmas at volcanic-front locations also contain hydrous minerals. These nepheline-normative suites include basanites, phlogopite-bearing minettes, and other hornblende-bearing lamprophyres. These are probably the youngest and freshest lamprophyres yet discovered on Earth, and ideal samples for addressing the origin of this exotic class of rocks. On extended MORB-normalized elemental plots, all lamprophyres show patterns similar to the calc-alkaline basalts, but have overall enrichments of 2 to 25X in Ti, K, P, Ba, Sr, light rare earth elements, and other incompatible elements. The lamprophyres show the same strong relative enrichments of Ba, K, and Sr, and depletions in Ti and Nb that characterize the calc-alkaline rocks of western Mexico and all subduction zones. These similarities in relative element abundances support a common source region for all primitive magmas of the western MVB. The primitive calc-alkaline and alkaline rocks of western Mexico show narrow, overlapping ranges in Sr, Nd, and Pb isotopic ratios, also consistent with derivation of all magmas from a common source. The presence of the relatively cold subducted Cocos and Rivera plates below the volcanic front demands that all primitive magmas originated in the depth range 30 to 75 km. These magmas are enriched in Ba, K, Sr, and other elements, probably transported by hydrous fluids rising from the subducted slab. Several lines of evidence support the presence of phlogopite in the source region of western MVB magmas, probably in the form of phlogopite-rich veins cutting asthenospheric peridotite. The first liquids formed upon partial melting of this veined source region will concentrate the vein component and generate the lamprophyres. These lamprophyric melts rarely erupt in arc settings, more commonly forming dikes in the arc crust. Eruption of lamprophyric magmas in the western MVB is favored by through-crustal extensional fracture systems related to active rifting of the Jalisco block from the N. American plate. Larger volumes of calc-alkaline basalt are generated by melting of the same source which dilutes the vein component with larger proportions of peridotitic wall rock.

Easter microplate evolution: Pb isotope evidence

Abstract: We report on 53 Pb isotope analyses of basalts from 48 dredge stations occupied along the spreading boundaries of the Easter microplate and adjacent East Pacific Rise (EPR). Also included in the study are seven analyses of basalts from Easter and Sala y Gomez islands. A major anomaly is observed on the East Rift, around 27°S, where this ridge is shallowest and closest to Easter and Sala y Gomez islands. Basalts from the West Rift are less radiogenic. The means for the two rift populations are distinct, but their ranges overlap significantly. On the average, there is a systematic westward decrease in radiogenic Pb content with distance from Sala y Gomez. The Pb isotope anomaly is confined to the boundaries of the Microplate and the total range exceeds that of the entire EPR, both in the most and the least end of radiogenic Pb content. Radiogenic Pb content covaries with (La/Sm)N ratios with the exception of a nepheline-normative picritic basalt glass outlier. The trends are curvilinear. There is no correlation between the Pb isotope ratios and the bulk composition of the lavas. In Pb versus Pb isotope diagrams, basalts from the East and West rifts form two tight linear trends of statistically indistinguishable slope. Basalts from Easter and Sala y Gomez islands lie on the upper end of these trends. A binary mixing process between a radiogenic source similar to that present beneath Sala y Gomez and the large ion lithophile element (LILE)-depleted mid-ocean ridge basalt (MORB) source is strongly suggested. There is no trace of the Dupal anomaly beneath the microplate nor beneath Easter or Sala y Gomez Islands. If the Dupal anomaly is indeed continuous and of semi global extent, it must lie deeper in the mantle than the depths at which basaltic melts take source beneath the microplate and these two islands. There is also no correlation between the apparent dispersion of Pb isotope ratios and the rate at which the various ridge segments of the microplate spread. Tests of the plum pudding model across fracture zones, where smaller degrees of melting might have prevailed and preferential melting of the LILE-rich veins or plums may take place, were found to be inconclusive. In contrast, the overall variation in Pb isotopes, (La/Sm)N, and tectonic and kinematic evolution of the EPR, strongly support that the hotspot source-migrating ridge model may indeed be applicable to the region. Independent evidence suggests that the tectonic and geochemical anomaly associated with the Easter microplate is the result of the influence of a lateral mantle plume flow at shallow depth in the upper-mantle, connecting the Sala y Gomez plume with the westward migrating EPR. A small discontinuity in Pb isotope variation associated with the 25°S propagating East Rift, as also found across the 95.5°W propagator on the Galapagos Spreading Center, further supports the concept that the flux of the plume may pulsate; that is, the plume is discontinuous and probably rises in the form of a chain of blobs. The repeated tectonic disturbances and propagation of new rifts which characterize the evolution of the Easter microplate may coincide and be caused by the appearance of such blobs in the upper most mantle, as we have previously suggested for the Galapagos. There is a remarkable similarity in the geochemical, petrological, and tectonic configuration of the Easter microplate-Sala y Gomez hotspot system with that of the Galapagos, which suggests that very similar processes are at work in the two regions.