1
|
Hotspots: The First 25 Years. ACTA ACUST UNITED AC 2013. [DOI: 10.1029/gm043p0001] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register]
|
2
|
Nd and Sr isotopic study of the Bay of Islands Ophiolite Complex and the evolution of the source of midocean ridge basalts. ACTA ACUST UNITED AC 2012. [DOI: 10.1029/jb084ib13p07429] [Citation(s) in RCA: 187] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
|
3
|
|
4
|
|
5
|
|
6
|
|
7
|
Sm-Nd, Rb-Sr, and18O/16O isotopic systematics in an oceanic crustal section: Evidence from the Samail Ophiolite. ACTA ACUST UNITED AC 2012. [DOI: 10.1029/jb086ib04p02721] [Citation(s) in RCA: 242] [Impact Index Per Article: 20.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
|
8
|
|
9
|
|
10
|
|
11
|
|
12
|
Sm-Nd and Rb-Sr systematics in volcanics and ultramafic xenoliths from Malaita, Solomon Islands, and the nature of the Ontong Java Plateau. ACTA ACUST UNITED AC 2012. [DOI: 10.1029/jb089ib04p02415] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
|
13
|
Atlantic ocean floor: Geochemistry and petrology of basalts from legs 2 and 3 of the Deep-Sea Drilling Project. ACTA ACUST UNITED AC 2012. [DOI: 10.1029/jb079i035p05507] [Citation(s) in RCA: 408] [Impact Index Per Article: 34.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
|
14
|
|
15
|
Lead and strontium isotopes and related trace elements as genetic tracers in the Upper Cenozoic rhyolite-basalt association of the Yellowstone Plateau Volcanic Field. ACTA ACUST UNITED AC 2012. [DOI: 10.1029/jb087ib06p04785] [Citation(s) in RCA: 166] [Impact Index Per Article: 13.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
|
16
|
|
17
|
Neodymium isotopes in flood basalts from the Siberian Platform and inferences about their mantle sources. Proc Natl Acad Sci U S A 2010; 76:3056-60. [PMID: 16592671 PMCID: PMC383761 DOI: 10.1073/pnas.76.7.3056] [Citation(s) in RCA: 56] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The initial isotopic compositions of Nd and Sr in basalts from the Central Siberian Plateau and other major continental flood basalts are reported. The continental flood basalts appear to be the product of partial melting of mantle sources that consist of relatively primitive undifferentiated material and are clearly distinct from midocean ridge basalts, which sample mantle reservoirs that have been modified by extraction of continental crust earlier in earth history. These observations provide fundamental constraints on models of mantle structure and dynamics. Isotopic effects of crustal contamination are clearly recognizable in some continental flood basalts, but these effects can be distinguished from isotopic patterns inherited from the mantle magma sources.
Collapse
|
18
|
Estimates of mantle thorium/uranium ratios from Th, U and Pb isotope abundances in basaltic melts. ACTA ACUST UNITED AC 1997. [DOI: 10.1098/rsta.1993.0005] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
The relationship between the abundances of Th, U and Pb isotopes in basalt melts and the [Th/U] ratio of their source is assessed. A simple melting model is used to show that whereas the activity ratio (
230
Th/
232
Th) in the initial melt before extraction is equal to the bulk source ratio, that in the extracted melt may be higher. The difference depends upon the rate of melting relative to the half life of
230
Th (73 ka). Only when the rate is fast compared to this half life will (
230
Th/
232
Th) in the extracted melt provide a correct estimate of [
232
Th/
238
U] in the source and therefore of its [Th/U ] ratio. This is normally not the case for MORB, and a better estimate of source [Th/U ] ratio is derived from [
232
Th/238U] ratio in the basalt, which does not depend upon the rate of melting. Available d a ta for MORB glasses give a best estimate for their source [Th/U] = 2.58 ± 0.06. This value is less than both that of the bulk Earth of 3.9 ± 0.1, and of the source of plume basalts from Iceland and Hawaii, which are 3.3 and 3.2 respectively. These estimates contrast with the [
232
Th/
238
U] ratio required to produce the radiogenic {
208
Pb/
206
Pb} atomic ratio of MORB over 4.55 Ga. This averages 3.8 and is little different from the average derived from Pb-isotopes in plume basalts. These observations are most easily reconciled if Th, U and Pb are efficiently stripped from the mantle by melting and have a residence time there of ≤1 Ga. The [Th/U ] ratio of 2.6 for the upper mantle requires melt fractions of ≤1 % to be involved in transferring U and Th from this region into the continents. Such melt fractions are present in subduction zones and in the source regions of continental alkali basalts.
Collapse
|
19
|
Abstract
Geochemical data help to constrain the sizes of identifiable reservoirs within the framework of models of layered or whole-mantle circulation, and they identify the sources of the circulating heterogeneities as mainly crustal and/or lithospheric, but they do not decisively distinguish between different types of circulation. The mass balance between crust, depleted mantle and undepleted mantle based on
143
Nd/
144
Nd, Nb/U and Ce/Pb, and the concentrations of very highly incompatible elements Ba, Rb, Th, U, and K, shows that
ca.
25- 70% (by mass) of depleted mantle balances the trace element and isotopic abundances of the continental crust. This mass balance reflects the actual proportions of mantle reservoirs only if there are no additional unidentified reservoirs. Evidence on the nature and ages of different source reservoirs comes from the geochemical fingerprints of basalts extruded at mid-ocean ridges and oceanic islands. Consideration of Nd and He isotopes
alone
indicates that ocean island basalts (oibs) may be derived from a relatively undepleted portion of the mantle. This has in the past provided a geochemical rationale for a two-layer model consisting of an upper depleted and a lower undepleted (‘primitive’) mantle layer. However, Pb-isotopic ratios, and Nb/U and Ce/Pb concentration ratios demonstrate that most or all oib source reservoirs are definitely not primitive. Models consistent with this evidence postulate recycling of oceanic crust and lithosphere or subcontinental lithosphere. Recycling is a natural consequence of mantle convection. This cannot be said for some other models such as those requiring large-scale vertical metasomatism beneath oib source regions. Unlike other trace elements, Nb, Ta, and Pb discriminate sharply between continental and oceanic crust-forming processes. Because of this, the primitive mantle value of Nb/U = 30 (Ce/Pb = 9) has been fractionated into a continental crustal Nb/U = 12 (Ce/Pb = 4) and a residual-mantle (morb (mid-ocean ridge basalt) plus oib source) Nb/U = 47 (Ce/Pb = 25). These residual mantle values are uniform within about 20% and are not fractionated during formation of oceanic crust. By using these concentrations ratios as tracers, it can be shown that the possible contribution of recycled continental crust to oib sources is limited to a few percent. Therefore, recycling must be dominated by oceanic crust and lithosphere, or by subcontinental lithosphere. Oceanic crust normally bears a thin layer of pelagic sediment at the time of subduction, and this is consistent with oib sources that are dominated by subducted oceanic crust with variable but always small additions of continental material. Primordial
3
He,
36
Ar, and excess
129
Xe, in oceanic basalts demonstrate that the mantle has been neither completely outgassed nor homogenized, but they do not constrain the degree of mixing or the size of reservoirs. Also, helium does not correlate well with other isotopic data and may have migrated into the basalt source from other regions. The high
3
He/
4
He ratios found in some oibs suggest that, even though the basalts are not derived from primordial mantle, their sources may be located close to a reservoir rich in primordial gases. This leads to models in which the oib sources are in a boundary layer within the mantle. The primordial helium migrates into this layer from below. The interpretation of the rare-gas data is still quite controversial. It is often argued that the upper mantle is a well-homogenized reservoir, but the data indicate heterogeneities on scales ranging from 10° to 10
6
m. The
206
Pb/
204
Pb ratios in the oceanic m antle range from 17 to 21, which is similar to the range in most continental rocks. The degree of mixing cannot be directly inferred from these data unless the size and composition of the heterogeneities and the time of their introduction into the system are known. The relative uniformity of Nb/U and Ce/Pb ratios in the otherwise heterogeneous morb and oib sources indicates that this reservoir was indeed homogenized after the separation of the continental crust, and that the observed isotopic and chem ical heterogeneities were introduced subsequently. Overall, the results are consistent with, but do not prove, a layered mantle where the upper layer contains both morb and oib sources, and the lower, primitive mantle is not sampled by present-day volcanism. Alternative models such as those involving a chemically graded mantle have not been sufficiently explored.
Collapse
|
20
|
Terrestrial helium degassing fluxes and the atmospheric helium budget: Implications with respect to the degassing processes of continental crust. ACTA ACUST UNITED AC 1989. [DOI: 10.1016/0168-9622(89)90002-x] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
|
21
|
Fluid kinematics, fluid residence times, and rock degassing in oceanic crust determined from noble gas contents of deep sea drilling project pore waters. ACTA ACUST UNITED AC 1987. [DOI: 10.1029/jb092ib12p12491] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
|
22
|
|
23
|
|
24
|
The Rare Earth Element Characteristics of Igneous Rocks from the Ocean Basins. ACTA ACUST UNITED AC 1984. [DOI: 10.1016/b978-0-444-42148-7.50011-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register]
|
25
|
|
26
|
|
27
|
|
28
|
Tholeiitic basalts from spreading ocean ridges the growth of the oceanic crust. Naturwissenschaften 1981. [DOI: 10.1007/bf01047247] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
|
29
|
|
30
|
|
31
|
Isotope geochemical studies Of North Atlantic Ocean basalts and their implications for mantle evolution. ACTA ACUST UNITED AC 1979. [DOI: 10.1029/me002p0342] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register]
|
32
|
Abstract
Combined neodymium and strontium isotope studies on Tertiary volcanics from northwest Scotland indicate that their parental mantle isotopic compositions have been substantially modified in many instances by contamination with the Precambrian continental crust through which they were erupted. The occurrence of samarium-neodymium and rubidium-strontium "pseudoisochrons" of different ages in these contaminated continental volcanics indicates that they are artifacts of the contamination processes and have no temporal significance with respect to mantle fractionation events.
Collapse
|
33
|
|
34
|
Abstract
The power of a heat engine ignited by tidal energy can account for geologically reasonable rates of average magma production and sea floor spreading. These rates control similarity of heat flux over continents and oceans because of an inverse relationship between respective depth intervals for mass transfer and consequent distributions of radiogenic heat production.
Collapse
|
35
|
Abstract
The lead isotopic compositions and uranium, thorium, and lead concentrations have been measured on six samples of material from the Sea of Tranquillity. The leads are moderately to very radiogenic; the initial lead concentrations are very low; the uranium and thorium levels are 0.26 to 0.88 and 0.87 to 3.35 parts per million, respectively. The Th/U ratios cluster about a 3.6 value. Apparent ages calculated for four rocks are 4.1 to 4.2 x 10(9) years. Dust and breccia yield apparent ages of 4.60 to 4.63 x 10(9) years. The uranium-lead ages are concordant, or nearly so, in all cases. The lunar surface is an ancient region with an extended record of events in the early history of the solar system. discrepancy between the rock ages and dust ages poses a fundamental qusestion about rock genesis on the moon.
Collapse
|
36
|
Petrological, Magnetic and Chemical Properties of Basalt dredged from an Abyssal Hill in the North-east Pacific. Nature 1969. [DOI: 10.1038/2231049a0] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
|
37
|
Abstract
Geochemical studies of Wyoming Precambrian graywackes derived from continental crust older than 3.2 x 10(9) years indicate that their source area was at least as highly differentiated as most younger Precambrian crust. The composition of this early crust (approximately that of calcium-rich granite) is not unlike that of the 2.5 to 3.2 x 10(9) year old North American crust. Limited geochemical data suggest that the composition of North America may not have changed significantly during the last 3.0 to 3.5 x 10(9) years.
Collapse
|
38
|
Abstract
We show that the rubidium in amphiboles is generally depleted with respect to potassium. The K:Rb ratios of 50 analyzed amphiboles range from 100 to 5000, averaging 1120. This fractionation effect holds for potassium concentrations ranging from 0.05 to 1.5 percent. The K:Rb ratios of abyssal tholeiites do not place unambiguous limits on the K:Rb ratio of the upper mantle, since partial melting of a mantle material such as amphibole peridotite would produce a liquid with a K:Rb ratio higher than that in the initial material. Large-scale mineralogic control of distributions of trace elements in the mantle could produce trends with depth that are the reverse of trends normally attributed to differentiation processes.
Collapse
|
39
|
Abstract
Analysis of several well-documented specimens from each of the three volcanic series on Oahu gives the following mean ratios of Sr(87) to Sr(86): the Waianae series, 0.7030 +/- 0.00010 (sigma); the Koolau series, 0.70385+/- 0.00009 (sigma); and the Honolulu series, 0.7029 ++/- 0.00006 ( sigma). The mean ratio of Sr(87) to Sr(86) of the Koolau series specimens is significantly higher than the means of the other two series. With one exception, significant differences in Sr(87)/ Sr(86) within a series were not found, even though some large compositional differences existed.
Collapse
|
40
|
Abstract
The isotopic compositions of lead and the concentrations lead, uranium, and thorium in samples of oceanic tholeiite and alkali suites are determined, and the genetic relations of the oceanic basalts are discussed. Lead of the oceanic tholeiites has a varying lead-206: lead-204 ratio between 17.8 and 18.8, while leads of the alkali basalt suites from Easter Island and Guadalupe Island are very radiogenic with lead-206: lead-204 ratios between 19.3 and 20.4 It is concluded that (i) the isotopic composition of lead in oceanic tholeiite suggests that the upper mantle source region of the tholeiite was differentiated from and original mantle material more than 1 billion years ago and that the upper mantle is not homogeneous at the present time, (ii) less than 20 million years was required for the crystal differentiation within the alkali suite from Easter Island, (iii) no crustal contamination was involved in the course of differentiation of rocks from Easter Island; however, some crustal contamination may have affected Guadalupe Island rocks, and (iv) alkali basalt may be produced from the tholeiite in the oceanic region by crystal differentiation. Alternatively the difference in the isotopic composition of lead in oceanic basalts may be produced by partial melting at different depths of a differentiated upper mantle.
Collapse
|
41
|
Abstract
The increase in K:Rb ratio with decrease in potassium content found in basaltic rocks does not seem to apply to ultramafic rocks. The ratios in a series of alpine ultramafic rocks and ultramafic inclusions in basals and kimberlite pipes are about 200 to 500-significantly lower than those in oceanic tholeiites. This characteristic of ultramafic rocks appears to be consistent with a simplified model in which early differentiation of the primitive mantle led to formation of an upper mantle region enriched in alkali elements and having a low K:Rb ratio. Alpine ultramafic rocks may be residuals from such an upper mantle region.
Collapse
|
42
|
Abstract
Four dredge hauls from near the crest and from the eastern flank of the seismically active Mid-Indian Ocean Ridge at 23 degrees to 24 degrees S, at depths of 3700 to 4300 meters, produced only low-potassium tholeiitic basalt similar in chemical and mineralogic composition to basalts characteristic of ridges and rises in the Atlantic and Pacific oceans. A fifth haul, from a depth of 4000 meters on the lower flank of a seamount on the ocean side of the Indonesian Trench, recovered tholeiitic basalt with higher concentrations of K and Ti and slightly lower amounts of Si and Ca than the typical-oceanic tholeiite of the ridge. The last sample is vesicular, suggesting depression of the area since the basalt was emplaced. Many of the rocks dredged are variously decomposed and hydrated, but there is no evidence of important chemical modification toward conversion of the lava flows to spilite during extrusion or solidification.
Collapse
|