Preprints
https://doi.org/10.5194/egusphere-2023-1499
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/egusphere-2023-1499
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
29 Aug 2023
Status: this preprint is open for discussion.
Carbonatite-induced petit-spot melts squeezed upward from the asthenosphere beneath the Jurassic Pacific Plate
AIST, Geological Survey of Japan, Research Institute of Geology and Geoinformation, Central 7, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8567, Japan
Graduate School of Science, Tohoku University, 6-3 Aramaki-Aoba, Aoba-ku, Sendai 980–8578, Japan
Naoto Hirano
Graduate School of Science, Tohoku University, 6-3 Aramaki-Aoba, Aoba-ku, Sendai 980–8578, Japan
Center for Northeast Asian Studies, Tohoku University, 41 Kawauchi, Aoba-ku, Sendai 980–8576, Japan
Shiki Machida
Ocean Resources Research Center for Next Generation, Chiba Institution of Technology, 2-17-1 Tsudanuma, Narashino 275-0016, Japan
Hirochika Sumino
Research Center for Advanced Science and Technology, the University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan
Norikatsu Akizawa
Atmosphere and Ocean Research Institute, the University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa 277-8564, Japan
Akihiro Tamura
Earth Science Course, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan
Tomoaki Morishita
Earth Science Course, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan
Yasuhiro Kato
Ocean Resources Research Center for Next Generation, Chiba Institution of Technology, 2-17-1 Tsudanuma, Narashino 275-0016, Japan
Department of Systems Innovation, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Submarine Resources Research Center, Research Institute for Marine Resources Utilization, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, Kanagawa, 237-0061, Japan
Abstract. The lithosphere–asthenosphere boundary (LAB), which can be seismically detected, stabilizes plate tectonics. Several conflicting hypotheses have been proposed as the causes of LAB discontinuity, such as the contribution of hydrated minerals, mineral anisotropy, and partial melts. The petit-spot melts ascending from the asthenosphere, owing to subducting plate flexures, support the partial melting at the LAB. Here, we observed the lava outcrops of six monogenetic volcanoes formed by petit-spot volcanism in the western Pacific. Thereafter, we determined the 40Ar/39Ar ages, major and trace element compositions, and Sr, Nd, and Pb isotopic ratios of the petit-spot basalts. The 40Ar/39Ar ages of two monogenetic volcanoes were ca. 2.6 Ma (million years ago) and ca. 0 Ma, respectively. The isotopic compositions of the western Pacific petit-spot basalts suggest their geochemically similar melting sources. They were likely derived from a mixture of high-μ (HIMU) mantle-like and enriched mantle (EM) -1-like components related to carbonatitic/carbonated materials and recycled crustal components. A mass balance-based melting model implied that the characteristic trace element composition (i.e., Zr, Hf, and Ti depletions) of the western Pacific petit-spot magmas could be explained by the partial melting of garnet lherzolite with a small degree of carbonatite melt flux with crustal components. This result confirms the involvement of carbonatite melt and recycled crust in the source of petit-spot melts and provides an implication for the genesis of tectonic-induced volcanism with similar geochemical signatures to those of petit-spots.
How to cite. Mikuni, K., Hirano, N., Machida, S., Sumino, H., Akizawa, N., Tamura, A., Morishita, T., and Kato, Y.: Carbonatite-induced petit-spot melts squeezed upward from the asthenosphere beneath the Jurassic Pacific Plate, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-1499, 2023.
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Kazuto Mikuni et al.
Status: open (until 10 Oct 2023)
Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor
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RC1: 'Comment on egusphere-2023-1499', Anonymous Referee #1, 29 Sep 2023
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Review of the manuscript “Carbonatite-induced petit-spot melts squeezed upward from the asthenosphere beneath the Jurassic Pacific Plate” by Mikuni, K. et al.
General comments
The manuscript presents new geochemical and geochronological (40Ar/39Ar) data on lavas from petit-spot volcanism in western Pacific Ocean. By using trace element and Sr-Nd-Pb isotopes, the authors model the mantle source from which basaltic melts were generated as a garnet lherzolite fluxed by small amounts of carbonatite melts, and speculate about recycling of crustal components into the mantle. According to their model, carbonatite melt and recycled oceanic crust components induced partial melting at the lithosphere-asthenosphere boundary (LAB).
The case study is extremely interesting, making the data worth of being published. On the other hand, a poor data filtering, coupled to a general lack of rigour and structure through the modelling and discussion, make the manuscript hard to follow, thus making a major revision necessary.
The authors should focus more in detail on the presented model, maybe cutting some side information (e.g., mineral chemistry and treatment of mantle xenoliths in the supplementary data, or all other informations in the main text that were already published elsewhere) to leave space to the main message of their study. Some results (e.g., negative Ar/Ar ages) must be also better supported to be included in the manuscript.
English language should be also improved.
Specific comments
- Title: the title is totally un-representative of the content of the paper. A better title could be “Crustal recycled and carbonate components required for the genesis of petit-spot alkaline basalts”, or something similar.
- Abstract: the data in the manuscript do not contribute to define the origin nor the structure of the LAB. The model provided is only petrogenetic. The opening of the abstract must be modified.
- Lines 74-82: The list of alkali basalts from random worldwide localities is not very meaningful, as readers might be not aware of each specific geodynamic context. Better re-shape this section, discriminating between tectonic settings and inferred geodynamic context, i.e. plume vs. not plume-related origin, continental vs. oceanic, etc….as it is done for petit-spot volcanoes shortly below. Intracontinental alkali basalts are not occurring only in China and North America; intraoceanic plume-related alkali basalts are not occurring only in Hawaii
- Lines 87-88: references are needed here! Falloon & Green 1989 EPSL; Falloon & Green 1990 Geology; Foley et al. 2009 Lithos; Ghosh et al. 2009 ChemGeo
- Lines 124-131: In this (long) sentence, the concepts of LAB nature and regional petit-spot volcanism are put together, but I feel some explanation is missing. Petit-spot volcanism provides info about asthenospheric melting as many other types of volcanism do. Why should the reader find this type of magmatism particularly useful? This section is the key of the scientific background, as it links the “big picture” to the case study!
- Section 4.4 (geochemical modelling) should be removed from the methodology and included later in the modelling/discussion section, at line 679. At the moment is out of context, and forces the reader to go back to 4.4 section while being in the middle of the discussion about the nature of the mantle source and the partial melting processes.
- Geochronology: 40Ar/39Ar dating on these samples is for sure problematic, due to the intense state of alteration. However, at the moment the presentation of the data is not convincing enough to be acceptable for publication: no discussion about K/Ca as a function of alteration is provided; the negative age (-0.11 Ma) obtained for the sample R01 is simply meaningless, and obtained with only 49% of released 39Ar; why for sample 1466R6-001 was the inverse isochron age chosen? More details are needed. On the other hand, the age ranges obtained using Fe-Mn crust or palagonite are interesting and useful.
- Geochemistry: along the whole results and discussion sections, sample 1466-R6 and R7 are treated as “anomalous” Si-undersaturated rocks, more vesicular, ascribable to a genesis from a different magma suite and thus mantle source (lines 564). This is a crucial issue, because authors use this discrepancy to speculate about different mantle sources for the magmatism. However, from Table 2 it is clear that samples 1466-R6 and R7 have the highest LOI (up to 6 wt%), as reported by the authors themselves later on (lines 594-596; lines 604 “…alteration was not extensive…excluding sample 1466R7-001, 1466R7-003”). In Supplementary Fig. S1 we see that sample 1466-R7 contains the most Fe-rich olivine phenocrysts, but at the same time also altered peridotite xenoliths or xenocrysts are included in this sample (Fig. S4). The question is: is it possible that the whole-rock analyses are just effect of alteration, or have just been “doped” because xenoliths/xenocrysts fragments were simply mixed up with the host rock during sample preparation? This would also explain why Sr-Nd-Pb isotopes are not showing any difference with respect to the other samples (lines 575-576), not requiring any differential contribution of carbonatite flux to the source (lines 584-586). As large portions of fresh glass are analysable in these samples (Fig. 4a, BSE image), is it possible to compare the whole-rock analysis of sample 1466-R7 with the composition of its glass (as done for the other samples) and thus verify the difference between the two magmatic suites?
- Chapter 6.2, containing the petrographic description of the samples and the evaluation of their alteration state and chemical representativeness, must be moved right at the beginning of the result section.
- Lines 670-673: Carbonatites are not depleted in U, Th and Nb. A typical feature is the high Nb/Ta, together with low Zr-Hf. Include reference, and/or plot an average carbonatite trace element pattern in Fig. 7 (using the same dataset as that for isotopes).
- Why for major elements and isotopes are author using the whole carbonatites GEOROC database, but for the partial melting model (section 6.4 and Fig. 11) only the “average carbonatite” of Bizimis et al. (2003) is used?
- Lines 698-707: I agree the partial melting of garnet lherzolite with small crustal component + low carbonatite flux is the most plausible among the model performed, but still this model is not able to fully reproduce the Ta, La, Sm-Eu-Ti and Y concentrations. Is there a reason? Did authors try to change the modal composition of the source, melting contribution percentages and/or amount of recycled oceanic crustal material to better fit the petit-spot pattern?
- Lines 732-747, Isotopic modelling: as authors in the previous section model the percentage of recycled crustal components, peridotitic fertile mantle and carbonatitic flux, it would be extremely interesting to verify if such percentages are also applicable to the isotopic mixing models. Day and Hilton (2011, EPSL; 2021, Geology) and Day et al. (2022, GCA) tried to perform such model for the Canary Island magmatism; the same was done by Markse et al. (2008, JPet) and Pietruszka et al. (2013, EPSL) for Hawaii.
- Chapter 6.4 is entirely speculative, does not add value nor constraints to the model. I suggest to remove it up to line 785. The content of lines 785-792, that is the hypotheses made to reconcile the model with the geodynamic context of the area, can be incorporated at the end of section 6.3.
- Lines 811-812: remove this sentence. In this paper the origin of LAB is not discussed at all.
Technical corrections
- Many analyses in the manuscript come from already published papers by the same authors. This information must be clarified through the text and the tables, as they cannot be presented as new analyses.
- Line 73-74: the opening sentence is too broad, as it is having a poor meaning. Maybe correct to: “the petrogenesis of alkali basalts and the nature/evolution of their source mantle…”. Which tectonic settings? Be more specific.
- Lines 104-105: Maybe better re-phrasing to: “…to understand the nature of the underlying lithosphere-asthenosphere system and model the geodynamic evolution of the region”
- Lines 109-112: Maybe the concept can be smoothened and made more concise: “In the last 20 years, the increasing knowledge of petit-spot volcanic settings has provided useful insights on the nature of the lithosphere-asthenosphere system, especially in the NW Pacific region (Hirano et al., 2006, Hirano & Machida 2022)”
- Lines 117-121: Sentence too long, can be cut to clarify the concept
- Lines 121-122: Where? Provide some names of localities, as this might be of broad interest to the reader!
- Line 129: what does “hybrid factor” mean?
- Line 276: EPMA analyses were not made on glasses only. Please correct and distinguish in this section the analytical conditions for glasses (defocussed beam?) from those used for minerals. How about matrix correction (Pouchou and Pichoir 1991)?
- Line 417 and figure 4a: alkaline and sub-alkaline; dividing line is from Irvine and Baragar (1971)
- Line 433: “…compared to the representative ocean island basalt (OIB)”
- Data from worldwide carbonatites presented in Fig. 9 are extremely scattered and not very useful, as they cover the entire diagram. Are they needed? If so, I recommend the authors to filter the dataset at least for the geodynamic settings or sample freshness/data quality.
- Line 556: “petrography and geochemistry”.
- Line 607-608: not entirely true. Some samples are indeed altered, and are specifically those used to argue for the heterogeneity of the mantle!
- Lines 617-619: “mass balance calculation of fractional phases….could not be performed because of inadequate phenocrysts” what does it mean?
- Lines 668-670: concept repeated.
- Line 729: Herzberg
Figures and Tables
- 1 can be re-organized in a 2x2 panel, as at the moment the panels are too small to be read properly. In the tomographic image to the right, maybe it would be useful to have also a more “zoomed” view of the upper 700-800 km.
- Line 144: Replace “although” with “Notwithstanding”
- 3: mineral labels should be added to all the optical microscope photos and BSE images. Why not splitting the figure in 4 different figures, or move some material to the supplementary information? As it is, it cannot be presented as a single figure composed by 4 multi-panels…
- 4: Panel b does not look very helpful, could be replaced with a K2O vs Na2O diagram to discriminate Na- from K-affinity. Also, plotting the literature data for kimberlites causes a “squeezing” of the OIB data down to K2O/Na2O <1.5, with all the data overlapping with each other. Is this really necessary?
- 5-6: I would recommend to plot MgO wt% in reverse order on the x-axis, so that to increase the degree of differentiation rightwards. This would also mark better the differences with respect to NW Pacific rocks, clearly visible on the CaO/Al2O3 vs. MgO plot.
- 7 caption: “Primitive-mantle- (PM, Sun & McDonough, 1989) normalized trace element…..”. Delete the last sentence.
- 9: carbonatite symbols in light green are poorly readable. The details about the mixing model in the caption can be moved to the supplementary material.
- 10b: provide the reference for the age of the detrital zircon in the Fig. caption.
- S9. Is it different from Fig. 11 in the main text? If not, why duplicate it?
- Table 2: Is this dataset including both glass analyses performed by EPMA and LA-ICP-MS and whole-rock analyses performed by XRF? If so, it must be clearly defined, and possibly the 2 samples set must be distinguishable from each other, not mixed. Values of 0.00 have no sense. Are they not analysed (n.a.) or below detection limit (b.d.l.)?
- Supplementary Table S1: mineral chemistry analyses must be filtered for data quality, at the moment they are totally un-presentable. Some olivine analyses sum up to <98 or >101 (sometimes >102!!!!), must be discarded. Some olivine analyses have Al2O3 >0.2 wt% (sometimes >1 wt%): must be deleted, are just mixed analyses. Filters must also be applied according to the quality of apfu calculations. Values 0.00 are not presentable: are they be not analysed (n.a.) or below detection limit (b.d.l.)? The same applies for cpx and plag, as many grains sum up to <98. Many spinel analyses have >0.8 wt% SiO2, must be deleted. One spinel analysis has 15 wt% SiO2, it is just a mixed analysis, delete.
Citation: https://doi.org/10.5194/egusphere-2023-1499-RC1
Kazuto Mikuni et al.
Kazuto Mikuni et al.
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Short summary
Plate tectonics theory is understood as the moving of rocky plate (lithosphere) on ductile zone (asthenosphere). The causes of lithosphere–asthenosphere boundary (LAB) is controversial, but petit-spot volcanism supports the presence of melt at the LAB. We analyzed chemical composition and eruption age of petit-spot volcanoes on the western Pacific Plate, and the results suggested that carbonatite melt and recycled oceanic crust have induced the partial melting at the LAB.
Plate tectonics theory is understood as the moving of rocky plate (lithosphere) on ductile zone...