scholarly journals K/U of the MORB Source and Silicate Earth

2020 ◽  
Vol 125 (12) ◽  
Author(s):  
B. Farcy ◽  
R. Arevalo ◽  
W. F. McDonough
Keyword(s):  
Morb Source

U-depleted versus U-enriched signatures in complementary crust-mantle sources of volcanic rocks from Asia

2021 ◽  
Author(s):  
Sergei Rasskazov ◽  
Irina Chuvashova ◽  
Tatiana Yasnygina ◽  
Elena Saranina
Keyword(s):  
Lower Mantle ◽  
Partition Coefficients ◽  
Volcanic Rocks ◽  
Anomalous Behavior ◽  
Water Soluble ◽  
Tien Shan ◽  
Mantle Sources ◽  
Continental Lower Crust ◽  
Morb Source ◽  
Transitional Processes

<p>The Nb/U~47 and Th/U~4 ratios are considered as indicative for the OIB source referred by some authors to lower mantle plumes that in fact have no specific geochemical signatures but HIMU component. The Th/U ratio may vary because of the different garnet–melt and/or clinopyroxene–melt partition coefficients of U and Th. Anomalously high or low Th/U values in rocks can also be related to the input or removal of U, the migration of which is controlled by its mobility under oxidizing conditions owing to the formation of water-soluble uranyl  compounds with hexavalent U. These variations definitely distinguish non-plume magmatic sources. The Th/U ratio decreases to 2.5 in the MORB source and increases to 6 in the continental lower crust one. We describe anomalous behavior of uranium in sources of Cenozoic basalts and basaltic andesites from Primorye, Lesser Khingan, Tunka Valley, as well as similar Cretaceous-Paleogene rocks from Tien Shan. Significant deviations of the Th/U and Nb/U ratios from the OIB values are characteristics mostly of garnet-free sources. The U-depleted and U-enriched signatures are used as sensitive indicators for deciphering crust–mantle transitional processes.</p><p>This work is supported by the RSF grant 18-77-10027.</p>

scholarly journals Identification of chondritic krypton and xenon in Yellowstone gases and the timing of terrestrial volatile accretion

2020 ◽  
Vol 117 (25) ◽  
pp. 13997-14004 ◽  
Author(s):  
Michael W. Broadley ◽  
Peter H. Barry ◽  
David V. Bekaert ◽  
David J. Byrne ◽  
Antonio Caracausi ◽  
...  
Keyword(s):  
Mantle Plume ◽  
Noble Gases ◽  
Noble Gas ◽  
Mid Ocean Ridge ◽  
Mantle Sources ◽  
Source Mantle ◽  
Morb Source ◽  
Protosolar Nebula ◽  
Ocean Ridge ◽  
Yellowstone Caldera

Identifying the origin of noble gases in Earth’s mantle can provide crucial constraints on the source and timing of volatile (C, N, H2O, noble gases, etc.) delivery to Earth. It remains unclear whether the early Earth was able to directly capture and retain volatiles throughout accretion or whether it accreted anhydrously and subsequently acquired volatiles through later additions of chondritic material. Here, we report high-precision noble gas isotopic data from volcanic gases emanating from, in and around, the Yellowstone caldera (Wyoming, United States). We show that the He and Ne isotopic and elemental signatures of the Yellowstone gas requires an input from an undegassed mantle plume. Coupled with the distinct ratio of129Xe to primordial Xe isotopes in Yellowstone compared with mid-ocean ridge basalt (MORB) samples, this confirms that the deep plume and shallow MORB mantles have remained distinct from one another for the majority of Earth’s history. Krypton and xenon isotopes in the Yellowstone mantle plume are found to be chondritic in origin, similar to the MORB source mantle. This is in contrast with the origin of neon in the mantle, which exhibits an isotopic dichotomy between solar plume and chondritic MORB mantle sources. The co-occurrence of solar and chondritic noble gases in the deep mantle is thought to reflect the heterogeneous nature of Earth’s volatile accretion during the lifetime of the protosolar nebula. It notably implies that the Earth was able to retain its chondritic volatiles since its earliest stages of accretion, and not only through late additions.

scholarly journals Sr-Nd-Pb-Hf Isotope Results from ODP Leg 187: Evidence for Mantle Dynamics of the Australian-Antarctic Discordance and Origin of the Indian MORB Source

2002 ◽  
Vol 3 (12) ◽  
pp. 1-35 ◽  
Author(s):  
Pamela D. Kempton ◽  
Julian A. Pearce ◽  
Tiffany L. Barry ◽  
J. Godfrey Fitton ◽  
Charles Langmuir ◽  
...  
Keyword(s):  
Hf Isotope ◽  
Mantle Dynamics ◽  
Morb Source

STORAGE OF C IN OLIVINE AND CARBONATED MELTING IN THE MORB SOURCE REGION

2017 ◽  
Author(s):  
Marc M. Hirschmann ◽  
◽  
Lora Amstrong ◽  
Erik H. Hauri
Keyword(s):  
Source Region ◽  
Morb Source

Xenon isotopes in the MORB source, not distinctive of early global degassing

2015 ◽  
Vol 42 (11) ◽  
pp. 4367-4374 ◽  
Author(s):  
P. Boehnke ◽  
M. W. Caffee ◽  
T. M. Harrison
Keyword(s):  
Morb Source ◽  
Xenon Isotopes

Large local-scale isotopic heterogeneities of the MORB source mantle: A case study on the SWIR

2006 ◽  
Vol 70 (18) ◽  
pp. A585 ◽  
Author(s):  
N. Shimizu ◽  
J.M. Warren ◽  
C. Sakaguchi ◽  
E. Nakamura ◽  
H.J.B. Dick
Keyword(s):  
Local Scale ◽  
Source Mantle ◽  
Morb Source

Low recycling efficiency of boron into the deep mantle

2020 ◽  
Author(s):  
Horst Marschall ◽  
Matthew Jackson
Keyword(s):  
Melt Inclusions ◽  
Mobile Elements ◽  
Primitive Mantle ◽  
Arc Magmas ◽  
Ocean Island Basalts ◽  
Deep Mantle ◽  
Mantle Sources ◽  
Source Mantle ◽  
Geochemical Tracer ◽  
Morb Source

<p>Boron is a distinctly crustal element in that it is strongly enriched in the surface reservoirs, such as continental crust, seawater, sediments, serpentinites and altered oceanic crust, relative to the mantle. These B-enriched reservoirs are also isotopically very distinct from the regular depleted upper mantle (d<sup>11</sup>B = -7.1 ±0.9 ‰ [10.1016/j.gca.2017.03.028]). This has encouraged the idea that boron could be an ideal tracer for subducted surface materials in the deep mantle in the form of isotopically anomalous recycled components in ocean island basalts (OIB) and enriched MORB. Yet, the potential of a geochemical tracer of this type is weakened by its extraction from the slab at the onset of subduction by dewatering and metamorphic dehydration, because this process depletes the recycled components in fluid-mobile elements. As such, this “subduction barrier” diminishes the deep recycling efficiency of incompatible, fluid-mobile tracers like B.</p><p>This study focuses on the B abundances and B isotopic compositions of glasses and melt inclusions that show low Cl/K ratios and are thought to represent the uncontaminated mantle signal from the HIMU (Tuvalu and Mangaia), EM1 (Pitcairn) and EM2 (Samoa) sources. Strikingly, all samples are depleted in boron by a factor of approximately 1.5 to 4 relative to non-fluid-mobile elements of similar incompatibility (e.g. LREE, P, Be). This negative boron anomaly is ubiquitous in OIB and is consistent with the results of previous studies [10.1016/0016-7037(95)00402-5; 10.1016/j.epsl.2018.12.005]. It also mirrors their characteristic negative Pb anomaly. These anomalies show that the mantle sources of OIB are depleted in B (and Pb) relative to non-fluid-mobile elements of similar incompatibility and relative to the MORB-source mantle. This is best explained by the presence in the OIB sources of recycled components that are enriched in all incompatible elements except for the fluid-mobile B (and Pb). The fluid mobile elements must have been preferentially extracted in the subduction barrier and returned to the surface on the short path via arc magmas. Arc magmas consistently show a general enrichment in isotopically heavy boron [10.1007/978-3-319-64666-4_9] with positive B anomalies.</p><p>Despite of the low recycling efficiency of boron into the convecting mantle, OIB still have B isotope signatures that are distinct from those of MORB. Previous studies have reported OIB signatures slightly lighter than MORB and the primitive mantle [10.1016/j.epsl.2018.12.005]. However, our study exclusively finds isotopically heavy B with a range in d<sup>11</sup>B from MORB-like values (-8.6 ±2.0 ‰) up to -2.5 ±1.5‰ for EM1 and HIMU lavas. The total OIB range is small but significant, and is consistent with the deep recycling of material that is strongly depleted in boron, but isotopically distinct (with isotopically heavy B in the case of our EM1 and HIMU samples). The B depletion combined with the B isotopic anomaly in OIB shows that B is efficiently (but not quantitatively) removed from the slab during subduction, and that isotopically distinct mantle domains are thus produced. The subduction barrier for boron increases its strength as a tracer in arcs, but it diminishes its potential as a tracer of deep mantle recycling.</p>

Pb and Hf isotope variations along the Southeast Indian Ridge and the dynamic distribution of MORB source domains in the upper mantle

2013 ◽  
Vol 375 ◽  
pp. 196-208 ◽  
Author(s):  
Barry B. Hanan ◽  
Janne Blichert-Toft ◽  
Christophe Hemond ◽  
Kaan Sayit ◽  
Arnaud Agranier ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Hf Isotope ◽  
Dynamic Distribution ◽  
Southeast Indian Ridge ◽  
Morb Source ◽  
Indian Ridge

scholarly journals Dynamical geochemistry of the mantle

Solid Earth ◽  
2011 ◽  
Vol 2 (2) ◽  
pp. 159-189 ◽  
Author(s):  
G. F. Davies
Keyword(s):  
Trace Elements ◽  
Oceanic Crust ◽  
Primitive Mantle ◽  
Mantle Heterogeneity ◽  
The Past ◽  
Incompatible Elements ◽  
The Earth ◽  
Depleted Mantle ◽  
Morb Source ◽  
Dynamical Modelling

Abstract. The reconciliation of mantle chemistry with the structure of the mantle inferred from geophysics and dynamical modelling has been a long-standing problem. This paper reviews three main aspects. First, extensions and refinements of dynamical modelling and theory of mantle processing over the past decade. Second, a recent reconsideration of the implications of mantle heterogeneity for melting, melt migration, mantle differentiation and mantle segregation. Third, a recent proposed shift in the primitive chemical baseline of the mantle inferred from observations of non-chondritic 142Nd in the Earth. It seems most issues can now be resolved, except the level of heating required to maintain the mantle's thermal evolution. A reconciliation of refractory trace elements and their isotopes with the dynamical mantle, proposed and given preliminary quantification by Hofmann, White and Christensen, has been strengthened by work over the past decade. The apparent age of lead isotopes and the broad refractory-element differences among and between ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs) can now be quantitatively accounted for with some assurance. The association of the least radiogenic helium with relatively depleted sources and their location in the mantle have been enigmatic. The least radiogenic helium samples have recently been recognised as matching the proposed non-chondritic primitive mantle. It has also been proposed recently that noble gases reside in a so-called hybrid pyroxenite assemblage that is the result of melt from fusible pods reacting with surrounding refractory peridotite and refreezing. Hybrid pyroxenite that is off-axis may not remelt and erupt at MORs, so its volatile constituents would recirculate within the mantle. Hybrid pyroxenite is likely to be denser than average mantle, and thus some would tend to settle in the D" zone at the base of the mantle, along with some old subducted oceanic crust. Residence times in D" are longer, so the hybrid pyroxenite there would be less degassed. Plumes would sample both the degassed, enriched old oceanic crust and the gassy, less enriched hybrid pyroxenite and deliver them to OIBs. These findings can account quantitatively for the main He, Ne and Ar isotopic observations. It has been commonly inferred that the MORB source is strongly depleted of incompatible elements. However it has recently been argued that conventional estimates of the MORB source composition fail to take full account of mantle heterogeneity, and in particular focus on an ill-defined "depleted" mantle component while neglecting less common enriched components. Previous estimates have also been tied to the composition of peridotites, but these probably do not reflect the full complement of incompatible elements in the heterogeneous mantle. New estimates that account for enriched mantle components suggest the MORB source complement of incompatibles could be as much as 50–100 % larger than previous estimates. A major difficulty has been the inference that mass balances of incompatible trace elements could only be satisfied if there is a deep enriched layer in the mantle, but the Earth's topography precludes such a layer. The difficulty might be resolved if either the Earth is depleted relative to chondritic or the MORB source is less depleted than previous estimates. Together these factors can certainly resolve the mass balance difficulties.

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