skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


Title: Lithological Heterogeneities in the Mantle: Origins and Contributions to Magma Genesis (Keynote 3a)
The quantification of the mantle heterogeneity and its contribution to magma genesis are crucial for our understanding of the nature of Earth’s geochemical reservoirs and recycling processes. Today, the source of basalts is often envisioned as a heterogeneous mantle that comprises a range of lithological heterogeneities, especially pyroxenites, introduced into the mantle by various geodynamic and magmatic processes. Different chemical parameters have been proposed to trace evidence for the contribution of lithological heterogeneities during magma genesis (e.g., major elements1-2, major element ratios1,3 or logratios4, first row transition element concentrations5-6, iron isotopes7) and several empirical models for partial melting of a heterogeneous mantle have been developed in an attempt to quantify the proportion of pyroxenites in the mantle source of magmas8-11. However, the large range of compositions covered by the potential lithologies present in the mantle makes it challenging to determine a unique proxy for their contributions in the magmas2,12. Additionally, these contributions are controlled by various factors, such as the abundances of the different lithologies in the mantle, their respective melting behaviors (solidus temperatures and melt productivities) and the thermal and melting regimes of the mantle. Finally, interpretations from the magma compositions are complicated by the potential presence of volatiles in the mantle source and/or by modifications experienced by the magmas during their journey through the mantle8,12. This keynote presentation will present examples of challenges that can rise when we try to quantify the nature and abundance of the mantle heterogeneity, the efforts that have been published by various authors, and a few potential research directions that could bring prospects for success. 1- Hauri (1996), Nature 382, 2- Lambart et al. (2013), Lithos 160-161, 3- Hirschmann et al. (2003), Geology 31, 4- Yang et al. (2019), JGR-Solid Earth 124, 5- Sobolev et al (2007), Science 316, 6- Sobolev et al. (2008), Science 321, 7- Williams and Bizimis, EPSL 404, 8- Mallik and Dasgupta (2014), GGG 15, 9- Kimura and Kawabata (2015), GGG 16, 10- Lambart et al. (2016), JGR-Solid Earth 121, 11- Brown and Lesher (2016), GGG 17, 12- Mallik et al. (in press), in: Konter J., Ballmer M, Cottaar S, & Marquardt H. (Eds. ), AGU monograph.  more » « less
Award ID(s):
1946346
PAR ID:
10218536
Author(s) / Creator(s):
;
Date Published:
Journal Name:
Goldschmidt 2020
Page Range / eLocation ID:
1405 to 1405
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; (, American Geophysical Union - Fall meeting 2020)
    null (Ed.)
    Mantle heterogeneity has a first-order control on the petrological and geochemical differences of erupted mafic lavas across the globe. It is debated whether this heterogeneity reflects only chemical variability or also lithological differences in source regions. Because of their various partitioning behaviors between mantle minerals, First Row Transition Elements (FRTEs) have been identified as potential lithological tracers. Here, we investigate the various parameters that control FRTE partitioning between common mantle phases through a comparison of partition coefficients calculated from natural pyroxenites obtained from the Earthchem database with previous partitioning experiments and new electron microprobe analyses. Using naturally occurring pyroxenites from alpine massifs and xenoliths provides the opportunity to explore the behavior of FRTEs on a much larger range of compositions and temperatures than covered by experimental studies. Our preliminary results show that natural partition coefficients for Fe and Mn depend on temperature and vary distinctly between lithologies. The effect of composition, however, is difficult to resolve and will require further inspection. Natural exchange coefficients, or Kd’s (mineral/mineral) for Mn/Fe, largely match previous experimental data across peridotite and pyroxenite compositions for garnet/clinopyroxene(cpx), orthopyroxene/cpx, and olivine/cpx. However, natural samples often present evidence of chemical disequilibrium and/or secondary alteration which can significantly increase the scatter in analyses. Importantly, despite the larger uncertainty on the natural Kd’s than on experimental ones, natural exchange coefficients show distinct values between the various pairs of minerals. These distinctions, and the fact that Kd’s do not seem to be influenced by temperature, make the bulk Mn/Fe ratio in lavas a good lithological tracer, supporting previous claims. Hence, we show that natural compositions can be used to expand trends in FRTE distribution behavior across a wider range of temperatures (500-1500°C) and compositions than determined previously by experiments alone. 
    more » « less
  2. ; (, Goldschmidt 2022)
    Mantle heterogeneity has a first-order control of the petrological and geochemical differences of erupted mafic lavas worldwide. Whether this heterogeneity reflects only chemical variations or also lithological heterogeneity in source regions is debated. Because of their contrasted partitioning behaviors between mantle phases, First Row Transition Elements (FRTEs) are considered as potential lithological tracers. Using a combination of published data on natural and experimental samples and new high current microprobe analyses on a variety of pyroxenite samples, we investigated the parameters that control FRTE exchange coefficients (Kd) between common mantle minerals and performed inverse modeling to test if FRTE ratios from oceanic basalt compositions can be used to solve for modal proportions in their mantle source. We applied the Kd determined from mantle lithologies in this study, along with experimental melt-mineral partitioning coefficients and a simplified melting model, on two basalt suites selected for their contrasted Mn/Fe and Zn/Fe ratios. Our results show that a same FRTE ratio can be explained by a range of modal proportions in the source. However, when combined, FRTE ratios become a powerful tool to constrain the nature of the source. 
    more » « less
  3. ; ; (, Journal of Petrology)
    Abstract Siliceous slab-derived partial melts infiltrate the sub-arc mantle and cause rock-melt reactions, which govern the formation of diverse primary arc magmas and lithological heterogeneities. The effect of bulk water content, composition of reactants, and nature of melt infiltration (porous versus channelized) on the rock-melt reactions at sub-arc conditions have been investigated by previous studies. However, the effect of multiple episodes of rock-melt reactions in such scenarios has not been investigated before. Here, we explore mantle wedge modifications through serial additions of hydrous-silicic slab partial melts and whether such a process may ultimately explain the origin of high-Mg# andesites found in arcs worldwide. A series of piston-cylinder experiments simulate a serial addition of silicic slab melts in up to three stages (I through III) at 3 GPa and 800–1050°C, using rock-melt proportions of 75–25 and 50–50. A synthetic KLB-1 and a natural rhyolite (JR-1) represented the mantle and the slab components, respectively. Right from the first rock-melt interaction, the peridotite mantle transforms into olivine-free mica-rich pyroxenites ± amphibole ± quartz/coesite in equilibrium with rhyolitic-hydrous melts (72–80 wt% SiO2 and 40–90 Mg#). The formation of olivine-free pyroxenite seems to be controlled by complex functions of T, P, rock-melt ratio, wedge composition, and silica activity of the slab-melt. Remarkably, the pyroxenites approach a melt-buffered state with progressive stages of rock-melt reactions, where those rhyolitic melts inherit and preserve the major (alkalis, Fe, Mg, Ca) and trace element slab-signature. Our results demonstrate that lithological heterogeneities such as pyroxenites formed as products of rock-melt reactions in the sub-arc mantle may function as melt ‘enablers,’ implying that they may act as pathways that enable the infiltrating melt to retain their slab signature without undergoing modification. Moreover, the density contrast between the products of rock-melt reaction (melts and residues) and the average mantle wedge (~150 to 400 kg/m3) may help forming instabilities and diapiric rise of the slab components into the mantle wedge. However, the fate of the primitive slab-melts seems to be associated with the length of the pathway of mantle interaction which explains the evident wide magma spectrum as well as their degree of slab garnet-signature dilution. This work and the existence of high-Mg# Mexican-trondhjemites indicates that almost pristine slab-melts can make their way up to crustal levels and contribute to the arc magma diversity. 
    more » « less
  4. (, Journal of Petrology)
    Abstract Mixing has been widely used in the interpretation of radiogenic isotope ratios and highly incompatible trace element variations in basalts produced by melting of a heterogeneous mantle. The binary mixing model is constructed by considering mass balance of endmember components, which is independent of physical state and spatial distribution of the endmembers in the mantle source. Variations of radiogenic isotope ratios and highly incompatible trace elements in basalts also depend on the size and spatial distribution of chemical and lithological heterogeneities in the mantle source. Here we present a new mixing model and a mixing scheme that take into account of the size, spatial location, and melting history of enriched mantle (EM) and depleted mantle (DM) parcels in the melting column. We show how Sr, Nd, and Hf concentrations and isotope ratios in the aggregated or pooled melt collected at the top of the melting column vary as a function of location of the EM parcel in the melting column. With changing location of the EM parcel in the upwelling melting column, compositions of the pooled melt do not follow a single mixing curve expected by the binary mixing model. Instead, they define a mixing loop that has an enriched branch and a depleted branch joined by two extreme points in composition space. The origin of the mixing loop can be traced back to four types of EM distribution or configuration in the melting column. The shape of the mixing loop depends on the relative melting rate of the EM to that of the DM and the number and spacing of EM parcels in the melting column. Probabilities of sampling the enriched and depleted branches in the pooled melt are proportional to volume fractions of the enriched and depleted materials in the mantle source. Mixing of pooled melts from a bundle of melting columns results in mixing envelopes in the isotope ratio correlation diagrams. The mixing envelope is a useful tool for studying chemical variations in mantle-derived melts. As an application, we consider scattered correlations in 87Sr/86Sr vs. 143Nd/144Nd and 143Nd/144Nd vs. 176Hf/177Hf in mid-ocean ridge basalts. We show that such correlations arise naturally from melting of a spatially heterogeneous mantle. 
    more » « less
  5. ; (, Volcanica)
    We present computational modeling outcomes for bilithologic (peridotite and pyroxenite) mantle melting in divergent environments, considering equilibrium and disequilibrium porous flow melting of 0–50 % pyroxenite in thermal equilibrium with peridotite, potential temperatures of 1300 and 1400 °C, upwelling rates from 1–50 cm yr−1, maximum porosities of 0.1–2.0 %, and four compositions that span pyroxenite melting behavior. Basalt-like pyroxenites (G2) uniquely produce low (226Ra/230Th) and (231Pa/235U) with high (230Th/238U), but quantities greater than ~10 % produce anomalously thick crust, restricting their global abundance. Silica-deficient pyroxenite (M7-16 and MIX1G) melts are more moderate, but require chemical re-equilibration during transport to resemble global basalts, while hybrid lithologies (KG1) produce melts similar to those of peridotites. Uranium-series disequilibria in partial melts can also be decoupled from trace elements by radioactive decay in two-dimensional regimes. The mantle must thus contain multiple types of pyroxenite on a global scale, with melts traveling by complex networks and experiencing heterogeneous extents of chemical re-equilibration. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email