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Experimental constraints on truly conjugate alkaline silicate – carbonatite melt pairs

Weidendorfer, Daniel and Asimow, Paul D. (2022) Experimental constraints on truly conjugate alkaline silicate – carbonatite melt pairs. Earth and Planetary Science Letters, 584 . Art. No. 117500. ISSN 0012-821X. doi:10.1016/j.epsl.2022.117500.

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Physical separation of immiscible carbonatites from CO₂-bearing alkaline silicate melts is frequently invoked to explain the common presence of carbonatites in alkaline rock suites. We present new isobaric 1.0 GPa crystallization and liquid immiscibility experiments between 1250 and 925 °C on chemically complex CO₂-bearing subvolcanic basanitic, nephelinitic and nepheline-syenitic compositions from the intrusive complex on Brava Island, Cape Verdes. In the experiments, carbonatites exsolve from silicate melt along the basanite–nephelinite melt fractionation pathway throughout an unmixing temperature window from 1100 to 950 °C. Liquid immiscibility is suppressed by 950 °C by the onset of alkali-feldspathoid crystallization, which deflects the residual silicate melt away from the two-liquid field. While evolving along the two-liquid field boundary, silicate melt increases in SiO₂ from 46 to 51 wt% and decreases in CaO / (Na₂O + K₂O) (by weight) from 0.83 to 0.12, reproducing the observed compositional array of alkaline silicate subvolcanic rocks from Brava. The conjugate carbonatite melts are moderately alkaline Ca-carbonatites with 5 to 9 wt% SiO₂ and CaO / (Na₂O + K₂O) that decreases from 5.64 at 1100 °C to 1.60 at 950 °C. These coexisting silicate and carbonatite melt compositions show that previous high-pressure experimental studies have not yielded truly conjugate melt pairs, mainly due to the use of modified starting materials resulting in melt compositions too enriched in alkalis to match the natural alkaline silicate rock record.

Item Type:Article
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URLURL TypeDescription
Weidendorfer, Daniel0000-0002-6118-9287
Asimow, Paul D.0000-0001-6025-8925
Additional Information:© 2022 Elsevier. Received 27 August 2021, Revised 3 February 2022, Accepted 7 March 2022, Available online 31 March 2022, Version of Record 31 March 2022. This study was financed by SNSF grants P2EZP2_162274 and P300P2_177798. Technical support from Dr. Chi Ma during SEM and EPMA analyses is greatly acknowledged. Dr. Mike Baker is thanked for technical support in the high-pressure lab and for numerous insightful discussions. Prof. George Rossman and Prof. Ed Stolper are acknowledged for granting access to their laboratories and for numerous discussions. We acknowledge thoughtful and constructive reviews by Dr. Alexandre Corgne and one anonymous reviewer. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Funding AgencyGrant Number
Swiss National Science Foundation (SNSF)P2EZP2_162274
Swiss National Science Foundation (SNSF)P300P2_177798
Subject Keywords:carbonatite; alkaline silicate melt; liquid immiscibility; conjugate melt pairs; element partitioning
Record Number:CaltechAUTHORS:20220412-15508000
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Official Citation:Daniel Weidendorfer, Paul D. Asimow, Experimental constraints on truly conjugate alkaline silicate – carbonatite melt pairs, Earth and Planetary Science Letters, Volume 584, 2022, 117500, ISSN 0012-821X,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:114259
Deposited By: George Porter
Deposited On:13 Apr 2022 16:10
Last Modified:25 Jul 2022 23:14

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