Conventionally the so-called “HIMU” (High U/Pb Mantle) ocean islands have been considered as representing subducted basaltic ocean crust that has cycled through the mantle to become part of mantle plumes. This paper by Weiss et al. presents a radically different interpretation, that the HIMU ocean island mantle source represents subcontinental lithospheric mantle (SCLM) that was metasomatized by carbonatitic fluids. In order to become a plume, the metasomatized SCLM had to delaminate from the lithosphere into the convecting mantle, and be transported to a mantle boundary layer, probably the core–mantle boundary, where it later became part of upwelling mantle plumes.
Reflecting on the use of PetDB data for this study, the authors state: “We are writing to you about it because the xenolith data in PetDB was critical for us to be able to publish it. A reviewer brought up some important issues that questioned whether the SCLM can be the HIMU mantle source. We were able to address these issues because PetDB allowed us to easily access essentially the entire body of published xenolith data, which would have been impractical for us to compile by ourselves. This was a key factor for the publication of this study because it gave us a broad overview of the composition of the SCLM, including the information we needed to confirm that our conclusions are viable.”
Figure 4. Conceptual model for the evolution of the HIMU mantle source.
The complete data set created in this study was submitted to the EarthChem Library for long term archiving and re-use (doi:10.1594/IEDA/100601), while new analytical data will be added to PetDB by PetDB staff.
Weiss, Y. et al., (2016) Key new pieces of the HIMU puzzle from olivines and diamond inclusions. Nature. doi:10.1038/nature19113 (LINK)
Iwamori et al., (2015) Isotopic heterogeneity of oceanic, arc and continental basalts and its implications for mantle dynamics
By mining geochemical data of basalts from databases, such as PetDB, Iwamori and Nakamura created a large data set of isotopic values from Mid-ocean Ridge Basalts, Ocean Island Basalts and Continental Basalts. Independent Component Analysis was found to be applicable in capturing the overall structure of the data, including mantle geochemical end-members. They propose top-down hemispherical dynamics involving both the mantle and the core, with focused subduction towards the supercontinents forming a fluid component-rich hemispheric domain anchored to the asthenosphere during continental dispersal in the past several hundred million years. This process may affect the temperature and growth rate of the inner core, resulting in synchronized hemispherical structures in the mantle and the core.
Iwamori and Nakamura, (2015). Isotopic heterogeneity of oceanic, arc and continental basalts and its implications for mantle dynamics. Gondwana Research Vol 27, 1131-1152. DOI: 10.1016/j.gr.2014.09.003
Carbotte et al., (2015) Tectonic and magmatic segmentation of the Global Ocean Ridge System: a synthesis of observation.
Do ridge axis geometry and geochemical properties reflect source composition variations of the underlying magmatic plumbing system? Carbotte et al. (2015) explore this question by examining the relationship between magmatic and tectonic segmentation at mid-ocean ridges using a synthesis of geophysical and geochemical data from a large number of sources. Combining higher resolution seafloor mapping from the GMRT Synthesis, which can identify tectonic segments including small discontinuities, with more detailed geophysical imaging of below-seafloor melt distribution and comprehensive geochemical data from PetDB, the authors confirm that global mid ocean ridges are composed of a number of magmatic spreading segments, each with its own magma plumbing system extending into the asthenosphere below.
Fig. 13. Sketch showing along-axis section of idealized magmatic spreading segment at fast and slow ridges based onstudies summarized in this article. (a) At fast-spreading ridges, rising mantle melts accumulate at the base of the crust (orange–grey) beneath each principal magmatic segment. The crustal magmatic system is composed of a more or less steady state magma lens or sill (red) that is partitioned into finer-scale segments coincident with the finest-scale seafloor segmentation. This shallow magma lens resides above a lower crustal zone of crystal mush and possible lower crustal sills (red lozenges embedded in red –grey). Red arrows in the dyke section indicate trajectories of magma transport during dyking (primarily vertical with minor lateral transport in places). (b) At slow-spreading ridges, strong focusing of mantle melts leads to thick crust/thin axial lithosphere at centre and thin crust/thick axial lithosphere at ends of each principal magmatic segment. Crustal magma bodies are more localized and ephemeral; both vertical and lateral magma transport during dyking may occur. Normal faulting is highly localized in detachments at segment ends and more distributed between several axial valley faults at segment centres. See text for further discussion.
Carbotte et al., (2015) Tectonic and magmatic segmentation of the Global Ocean Ridge System: a synthesis of observation. Geological Society of London, Special Publications 420. DOI:10.1144/SP420.5.
Eason D.E., and Dunn, R.A., (2015) Petrogenesis and structure of oceanic crust in the Lau back-arc basin
Back-arc basins such as the Lau at the Australian-Pacific plate boundary are of particular interest as productive centers of crustal formation. They host a diversity of crustal structure, compositions, and processes not found at mid-ocean ridges.
Using geochemical data culled from PetDB and other sources from the Eastern Lau Spreading Center and the Valu Fa Ridge, Eason and Dunn propose that slab-derived water carried in the near-arc ridge system enhances mantle melting, as well as plays a role in magmatic differentiation and crustal accretion processes. Their model successfully predicts both major element compositional trends of erupted lavas, and shows that slab-derived water close to the arc suppresses plagioclase crystallization leading to the formation of an ultramafic crust with high seismic velocities.
Fig 3. (a) Multibeam bathymetry of the ELSC and VFR showing sample locations (black dots). (b) Axial depth profile from Martinez et al. (2006). (c)–(e) Along-axis compositional variation of MgO and representative trace element ratios, color-coded by ridge segment.
Eason D.E., and Dubb, R.A., (2015) Petrogenesis and structure of oceanic crust in the Lau back-arc basin. EPSL 429: 128-138. DOI:10.1016/j.epsl.2015.07.065
During the past 40 years trace element indiscrimination diagrams have been widely used as a reliable method to determine the tectonic settings of mafic-ultramafic rocks. Li et al. argue that these diagrams, while widely utilized and cited, have often been based on small data sets, containing limited numbers of trace elements and using outdated analytical techniques. Although they are still widely cited, such plots are not representative of the new global data sets.
Using data from constantly updated online resources such as PetDB, Li et al. show that in both binary and ternary diagrams of trace element compositions there is significant overlap of all types of basalts. The authors conclude that, although trace element data from basalts provide useful information on source compositions and melt generation processes, all indiscrimination diagrams tested using the new data show significant overlap and fail to discriminate between any types of basalt other than mid-ocean ridge basalt (MORB) and ocean island basalt (OIB).
Fig. 4. Th-Hf-Ta ternary diagrams comparing the old (grey) and
new (this study, colour and in black) discrimination fields for basalts.
Li, C., Arndt, N.T., Tang, Q., Ripley, E.M. (2015) Trace element indiscrimination diagrams. Lithos 232: 76-83. DOI:10.1016/j.lithos.2015.06.022.