A large-scale isotope anomaly in the Southern Hemisphere mantle
TL;DR: The authors showed that the isotopic mantle anomaly is globe-encircling in extent, centred on latitude 30° S. They also showed that this mantle anomaly has been in existence for billions of years and placed severe constraints on mantle convection models.
Abstract: Basalts from many Southern Hemisphere regions have anomalous Sr and Pb isotopic characteristics. This article shows that the isotopic mantle anomaly is globe-encircling in extent, centred on latitude 30° S. Arguments suggesting that this mantle anomaly has been in existence for billions of years place severe constraints on mantle convection models.
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01 Jan 1989
TL;DR: In this article, trace-element data for mid-ocean ridge basalts and ocean island basalts are used to formulate chemical systematics for oceanic basalts, 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.
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.
19,221 citations
Cites background or methods from "A large-scale isotope anomaly in th..."
...1982; Palacz & Saunders 1986), mantle enrichment through melt migration from the LVZ into oceanic and continental lithosphere (eg Sun 1980; Hart et al. 1986) or derivation of OIBs from the lower mantle which has lost lead by continuous core formation (eg Vidal & Dosso 1978). However, Newsom et al. (1986) showed that St Helena basalts have the same molybdenum (highly chalcophile) to praseodymium (highly lithophile) abundance ratios as MORBs and other OIBs, effectively arguing against continuous core formation. The EM-type character is generally considered to be related to processes associated with convergent margins, such as the recycling of sediments with oceanic crust (eg Hawkesworth et al. 1979; Cohen & O'Nions 1982; White 1985; Weaver et al. 1986). The formation of these OIB sources from remobilization or delamination of metasomatized enriched continental lithosphere has also been proposed (eg Richardson et al. 1982; McKenzie & O'Nions 1983; Hawkesworth et al. 1986). Multiple origins for EM-type mantle is likely, including superposition of different processes. Menzies (1983) suggested that EM1 could be related to enrichment by the introduction of CO2-rich silicate melt (eg kimberlite, nephelinite) whereas the high Rb/Sr, 878r/86Sr character of EM2 is similar to some modern island arc basalts (eg White 1985)....
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...More recently, Hart et al. (1986) pointed out that HIMU and EM1...
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...Schematic diagram of the ENd versus 87Sr/S6Sr plot for OIB data to show the fields of different types of oceanic basalts and the end-member assigned by Zindler & Hart (1986). Modified from Hart et al. (1986)....
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...Zindler & Hart (1986) used different terminology to define the characteristics of mantle end-members: DMM (depleted MORB mantle), PREMA (prevalent mantle, including Iceland, Galapagos, the Easter Islands of White's MORB group and the Hawaiian islands), HIMU (long-term high U/Pb in the source, including St Helena, Tubuai, Mangaia), EM1 (enriched mantle type 1 for the Kerguelen type) and EM2 (enriched mantle type 2 for the Society Island type)....
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...Schematic diagram of the ENd versus 87Sr/S6Sr plot for OIB data to show the fields of different types of oceanic basalts and the end-member assigned by Zindler & Hart (1986). Modified from Hart et al....
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TL;DR: Basaltic volcanism'samples' the Earth's mantle to great depths, because solid-state convection transports deep material into the (shallow) melting region as mentioned in this paper.
Abstract: Basaltic volcanism 'samples' the Earth's mantle to great depths, because solid-state convection transports deep material into the (shallow) melting region. The isotopic and trace-element chemistry of these basalts shows that the mantle contains several isotopically and chemically distinct components, which reflect its global evolution. This evolution is characterized by upper-mantle depletion of many trace elements, possible replenishment from the deeper, less depleted mantle, and the recycling of oceanic crust and lithosphere, but of only small amounts of continental material.
2,397 citations
TL;DR: In this article, 15 andesite-dacite stratovolcanoes on the volcanic front of a single segment of the Andean arc show along-arc changes in isotopic and elemental ratios that demonstrate large crustal contributions to magma genesis.
Abstract: Fifteen andesite-dacite stratovolcanoes on the volcanic front of a single segment of the Andean arc show along-arc changes in isotopic and elemental ratios that demonstrate large crustal contributions to magma genesis. All 15 centers lie 90 km above the Benioff zone and 280±20 km from the trench axis. Rate and geometry of subduction and composition and age of subducted sediments and seafloor are nearly constant along the segment. Nonetheless, from S to N along the volcanic front (at 57.5% SiO2) K2O rises from 1.1 to 2.4 wt %, Ba from 300 to 600 ppm, and Ce from 25 to 50 ppm, whereas FeO*/MgO declines from >2.5 to 1.4. Ce/Yb and Hf/Lu triple northward, in part reflecting suppression of HREE enrichment by deep-crustal garnet. Rb, Cs, Th, and U contents all rise markedly from S to N, but Rb/Cs values double northward — opposite to prediction were the regional alkali enrichment controlled by sediment subduction. K/Rb drops steeply and scatters greatly within many (biotite-free) andesitic suites. Wide diversity in Zr/Hf, Zr/Rb, Ba/Ta, and Ba/La within and among neighboring suites (which lack zircon and alkali feldspar) largely reflects local variability of intracrustal (not slab or mantle) contributions. Pb-isotope data define a limited range that straddles the Stacey-Kramers line, is bracketed by values of local basement rocks, in part plots above the field of Nazca plate sediment, and shows no indication of a steep (mantle+sedimentary) Pb mixing trend. 87Sr/86Sr values rise northward from 0.7036 to 0.7057, and 143Nd/144Nd values drop from 0.5129 to 0.5125. A northward climb in basal elevation of volcanic-front edifices from 1350 m to 4500 m elevation coincides with a Bougueranomaly gradient from −95 to −295 mgal, interpreted to indicate thickening of the crust from 30–35 km to 50–60 km. Complementary to the thickening crust, the mantle wedge beneath the front thins northward from about 60 km to 30–40 km (as slab depth is constant). The thick northern crust contains an abundance of Paleozoic and Triassic rocks, whereas the proportion of younger arc-intrusive basement increases southward. Primitive basalts are unknown anywhere along the arc. Base-level isotopic and chemical values for each volcano are established by blending of subcrustal and deep-crustal magmas in zones of melting, assimilation, storage and homogenization (MASH) at the mantle-crust transition. Scavenging of mid-to upper-crustal silicic-alkalic melts and intracrustal AFC (prominent at the largest center) can subsequently modify ascending magmas, but the base-level geochemical signature at each center reflects the depth of its MASH zone and the age, composition, and proportional contribution of the lowermost crust.
2,013 citations
Cites background from "A large-scale isotope anomaly in th..."
...For comparison, oceanic regression line (ORL) is that of Tilton (1983); northern hemisphere Pb reference line (NHRL) is that of Hart (1984) ; S-K is the average crustal Pb-evolution curve of Stacey and Kramers (1975); field of Nazca plate basalts is from Unruh and Tatsumoto (1976); and field of Nazca plate sediments is constructed from Unruh and Tatsumoto (1976), Dasch (1981), and Tilton (1983)...
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TL;DR: In this article, a combination of approaches is required to estimate the major and trace element abundances in the depleted mantle (DM), the source for mid-ocean ridge basalts (MORBs).
Abstract: [1] We present an estimate for the composition of the depleted mantle (DM), the source for mid-ocean ridge basalts (MORBs). A combination of approaches is required to estimate the major and trace element abundances in DM. Absolute concentrations of few elements can be estimated directly, and the bulk of the estimates is derived using elemental ratios. The isotopic composition of MORB allows calculation of parent-daughter ratios. These estimates form the “backbone” of the abundances of the trace elements that make up the Coryell-Masuda diagram (spider diagram). The remaining elements of the Coryell-Masuda diagram are estimated through the composition of MORB. A third group of estimates is derived from the elemental and isotopic composition of peridotites. The major element composition is obtained by subtraction of a low-degree melt from a bulk silicate Earth (BSE) composition. The continental crust (CC) is thought to be complementary to the DM, and ratios that are chondritic in the CC are expected to also be chondritic in the DM. Thus some of the remaining elements are estimated using the composition of CC and chondrites. Volatile element and noble gas concentrations are estimated using constraints from the composition of MORBs and ocean island basalts (OIBs). Mass balance with BSE, CC, and DM indicates that CC and this estimate of the DM are not complementary reservoirs.
1,432 citations
References
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TL;DR: In this article, the authors propose a model for the origin of hot-spot volcanism, where oceanic crust is returned to the mantle during subduction and sinks into the deeper mantle and accumulates at some level of density compensation, possibly the core-mantle boundary.
1,397 citations
TL;DR: Lead isotopic compositions of young volcanic rocks from different tectonic environments have distinctive characteristics their differences are evaluated within the framework of global tectonics and mantle differentiation Ocean island leads are in general more radiogenic than mid-ocean ridge basalt (morb) leads as discussed by the authors.
Abstract: Lead isotopic compositions of young volcanic rocks from different tectonic environments have distinctive characteristics Their differences are evaluated within the framework of global tectonics and mantle differentiation Ocean island leads are in general more radiogenic than mid-ocean ridge basalt (morb) leads They form linear trends on lead isotopic ratio plots Many of the trends extend toward the field of morb On plots of 207 P b / 204 Pb against 206 Pb / 204 Pb, their slopes are generally close to 01 Island arc leads in general are confined between sediment and morb type leads with slopes of ca 030 on a plot of 207 P b / 204 Pb against 206 Pb / 204 Pb Pb, Sr and Nd isotopic data of Hawaiian volcanics are closely examined Data from each island support a two-component mixing model However, there is a lack of full range correlation between islands, indicating heterogeneity in the end members This mixing model could also be extended to explain data from the Iceland-Reykjanes ridge, and from 45° N on the Atlantic Ridge The observed chemical and isotopic heterogeneity in young volcanic rocks is considered to be a result of long-term as well as short-term mantle differentiation and mixing Lead isotopic data from ocean islands are interpreted in terms of mantle evolution models that involve long-term (more than 2 Ga) mantle chemical and isotopic heterogeneity Incompatible element enriched ‘plume’-type morb have Th/U ratios ca 30 too low and Rb/Sr ratios ca 004 too high to generate the observed 208 Pb and 87 Sr respectively for long periods of time Elemental fractionation in the mantle must have occurred very recently This conclusion also applies to mantle sources for ocean island alkali basalts and nephelinites Depletion of incompatible elements in morb sources is most probably due to continuous extraction of silicate melt and/or fluid phase from the low-velocity zone throughout geological time Data on Pb isotopes, Sr isotopes and trace elements on volcanic rocks from island arcs are evaluated in terms of mixing models involving three components derived from (1) sub-arc mantle wedge, (2) dehydration or partial melting of subducted ocean crust, and (3) continental crust contamination In contrast to the relation between 87 Sr/ 86 Sr and 143 Nd / 144 Nd ratios of ocean volcanics, there is a general lack of correlation between Pb and Sr isotopic ratios except that samples with very radiogenic Pb ( 206 Pb / 204 Pb > 195) have low 87 Sr/ 87 Sr ratios (07028- 07035) These samples also have inferred source Th/U ratios (30-35) not high enough to support long-term growth of 208 Pb Data suggest that their mantle sources have long-term integrated depletion in Rb, Th, U and light ree High 238 U / 204 Pb (y a)values required by the Pb isotopic data are most probably due to depletion of Pb by separation of a sulphide phase Relations between Pb, Sr and Nd isotopic ratios of young volcanic rocks could be explained by simultaneous upward migration of silicate and/or fluid phase and downward migration of a sulphide phase in a differentiating mantleration of a sulphide phase in a differentiating mantle
1,167 citations
TL;DR: In this paper, a critical re-assessment of the construction of simple ore lead isotopic development curves is followed by three fresh approximations, all designed to involve the minimum possible number of assumptions.
1,023 citations
TL;DR: Sr and Nd isotope ratios for 17 mid-ocean ridge basalts and for 11 oceanic islands and island groups are reported in this article, and the results are not explained by binary mixing of depleted and undepleted mantle reservoirs or variable magmatic depletion of a planetary reservoir, but support mantle evolution models involving re-injection of crust material into the mantle.
Abstract: Sr and Nd isotope ratios are reported for 17 mid-ocean ridge basalts and for 11 oceanic islands and island groups. Data from the Azores, Samoa and the Society Islands diverge significantly from the mantle array. These results are not explained by binary mixing of depleted and undepleted mantle reservoirs or by variable magmatic depletion of a planetary reservoir, but support mantle evolution models involving re-injection of crust material into the mantle.
688 citations
TL;DR: Pb and Sr isotopic compositions from the Indian Ocean (active ridges, old ocean floor and aseismic ridge samples) confirm the characteristic nature of the mantle record in this region as mentioned in this paper.
Abstract: Pb and Sr isotopic compositions from the Indian Ocean (active ridges, old ocean floor and aseismic ridge samples) confirm the characteristic nature of the mantle record in this region. The results emphasize the importance of mixing processes between the lower mantle (oceanic-island basalt source), and the upper mantle (ridge-basalt source). The isotopic characteristics of the Indian Ocean islands seem to be in agreement with the hypothesis of the reinjection of sediments into the mantle.
601 citations