Melting the hydrous, subarc mantle: the origin of primitive andesites
This experimental study is the first comprehensive investigation of the melting behavior of an olivine + orthopyroxene ± spinel—bearing fertile mantle (FM) composition as a function of variable pressure and water content. The fertile composition was enriched with a metasomatic slab component of ≤0.5...
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| Format: | Article |
| Language: | English |
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Springer Berlin Heidelberg
2017
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| Online Access: | http://hdl.handle.net/1721.1/107271 https://orcid.org/0000-0002-3224-9244 |
| _version_ | 1826215929906724864 |
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| author | Mitchell, Alexandra L Grove, Timothy L. |
| author2 | Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences |
| author_facet | Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences Mitchell, Alexandra L Grove, Timothy L. |
| author_sort | Mitchell, Alexandra L |
| collection | MIT |
| description | This experimental study is the first comprehensive investigation of the melting behavior of an olivine + orthopyroxene ± spinel—bearing fertile mantle (FM) composition as a function of variable pressure and water content. The fertile composition was enriched with a metasomatic slab component of ≤0.5 % alkalis and investigated from 1135 to 1470 °C at 1.0–2.0 GPa. A depleted lherzolite with 0.4 % alkali addition was also studied from 1225 to 1240 °C at 1.2 GPa. Melts of both compositions were water-undersaturated: fertile lherzolite melts contained 0–6.4 wt% H[subscript 2]O, and depleted lherzolite melts contained ~2.5 wt% H[subscript 2]O. H[subscript 2]O contents of experimental glasses are measured using electron microprobe, secondary ion mass spectrometry, and synchrotron-source reflection Fourier transform infrared spectroscopy, a novel technique for analyzing H[subscript 2]O in petrologic experiments. Using this new dataset in conjunction with results from previous hydrous experimental studies, a thermobarometer and a hygrometer–thermometer are presented to determine the conditions under which primitive lavas were last in equilibration with the mantle. These predictive models are functions of H[subscript 2]O content and pressure, respectively. A predictive melting model is also presented that calculates melt compositions in equilibrium with an olivine + orthopyroxene ± spinel residual assemblage (harzburgite). This model quantitatively predicts the following influences of H[subscript 2]O on mantle lherzolite melting: (1) As melting pressure increases, melt compositions become more olivine-normative, (2) as melting extent increases, melt compositions become depleted in the normative plagioclase component, and (3) as melt H[subscript 2]O content increases, melts become more quartz-normative. Natural high-Mg# [molar Mg/(Mg + Fe[superscript 2+])], high-MgO basaltic andesite and andesite lavas—or primitive andesites (PAs)—contain high SiO2 contents at mantle-equilibrated Mg#s. Their compositional characteristics cannot be readily explained by melting of mantle lherzolite under anhydrous conditions. This study shows that experimental melts of a FM peridotite plus the addition of alkalis reproduce the compositions of natural PAs in SiO[subscript 2], Al[subscript 2]O[subscript 3], TiO[subscript 2], Cr[subscript 2]O[subscript 3], MgO, and Na[subscript 2]O at 1.0–1.2 GPa and H[subscript 2]O contents of 0–7 wt%. Our results also suggest that PAs form under a maximum range of extents of melting from F = 0.2–0.3. The CaO contents of the melts produced are 1–5 wt% higher than the natural samples. This is not a result of a depleted source composition or of extremely high extents of melt but is potentially caused by a very low CaO content contribution from deeper in the mantle wedge. |
| first_indexed | 2024-09-23T16:39:23Z |
| format | Article |
| id | mit-1721.1/107271 |
| institution | Massachusetts Institute of Technology |
| language | English |
| last_indexed | 2024-09-23T16:39:23Z |
| publishDate | 2017 |
| publisher | Springer Berlin Heidelberg |
| record_format | dspace |
| spelling | mit-1721.1/1072712024-05-15T05:10:00Z Melting the hydrous, subarc mantle: the origin of primitive andesites Mitchell, Alexandra L Grove, Timothy L. Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences Grove, Timothy L Mitchell, Alexandra L This experimental study is the first comprehensive investigation of the melting behavior of an olivine + orthopyroxene ± spinel—bearing fertile mantle (FM) composition as a function of variable pressure and water content. The fertile composition was enriched with a metasomatic slab component of ≤0.5 % alkalis and investigated from 1135 to 1470 °C at 1.0–2.0 GPa. A depleted lherzolite with 0.4 % alkali addition was also studied from 1225 to 1240 °C at 1.2 GPa. Melts of both compositions were water-undersaturated: fertile lherzolite melts contained 0–6.4 wt% H[subscript 2]O, and depleted lherzolite melts contained ~2.5 wt% H[subscript 2]O. H[subscript 2]O contents of experimental glasses are measured using electron microprobe, secondary ion mass spectrometry, and synchrotron-source reflection Fourier transform infrared spectroscopy, a novel technique for analyzing H[subscript 2]O in petrologic experiments. Using this new dataset in conjunction with results from previous hydrous experimental studies, a thermobarometer and a hygrometer–thermometer are presented to determine the conditions under which primitive lavas were last in equilibration with the mantle. These predictive models are functions of H[subscript 2]O content and pressure, respectively. A predictive melting model is also presented that calculates melt compositions in equilibrium with an olivine + orthopyroxene ± spinel residual assemblage (harzburgite). This model quantitatively predicts the following influences of H[subscript 2]O on mantle lherzolite melting: (1) As melting pressure increases, melt compositions become more olivine-normative, (2) as melting extent increases, melt compositions become depleted in the normative plagioclase component, and (3) as melt H[subscript 2]O content increases, melts become more quartz-normative. Natural high-Mg# [molar Mg/(Mg + Fe[superscript 2+])], high-MgO basaltic andesite and andesite lavas—or primitive andesites (PAs)—contain high SiO2 contents at mantle-equilibrated Mg#s. Their compositional characteristics cannot be readily explained by melting of mantle lherzolite under anhydrous conditions. This study shows that experimental melts of a FM peridotite plus the addition of alkalis reproduce the compositions of natural PAs in SiO[subscript 2], Al[subscript 2]O[subscript 3], TiO[subscript 2], Cr[subscript 2]O[subscript 3], MgO, and Na[subscript 2]O at 1.0–1.2 GPa and H[subscript 2]O contents of 0–7 wt%. Our results also suggest that PAs form under a maximum range of extents of melting from F = 0.2–0.3. The CaO contents of the melts produced are 1–5 wt% higher than the natural samples. This is not a result of a depleted source composition or of extremely high extents of melt but is potentially caused by a very low CaO content contribution from deeper in the mantle wedge. National Science Foundation (U.S.) (EAR-1118598) United States. Dept. of Energy. Office of Basic Energy Sciences (Contract No. DE-AC02-98CH10886) 2017-03-09T21:38:56Z 2017-03-09T21:38:56Z 2015-07 2014-12 2016-05-23T12:10:30Z Article http://purl.org/eprint/type/JournalArticle 0010-7999 1432-0967 http://hdl.handle.net/1721.1/107271 Mitchell, Alexandra L., and Timothy L. Grove. “Melting the Hydrous, Subarc Mantle: The Origin of Primitive Andesites.” Contributions to Mineralogy and Petrology 170.2 (2015): n. pag. https://orcid.org/0000-0002-3224-9244 en http://dx.doi.org/10.1007/s00410-015-1161-4 Contributions to Mineralogy and Petrology Creative Commons Attribution-Noncommercial-Share Alike http://creativecommons.org/licenses/by-nc-sa/4.0/ Springer-Verlag Berlin Heidelberg application/pdf Springer Berlin Heidelberg Springer Berlin Heidelberg |
| spellingShingle | Mitchell, Alexandra L Grove, Timothy L. Melting the hydrous, subarc mantle: the origin of primitive andesites |
| title | Melting the hydrous, subarc mantle: the origin of primitive andesites |
| title_full | Melting the hydrous, subarc mantle: the origin of primitive andesites |
| title_fullStr | Melting the hydrous, subarc mantle: the origin of primitive andesites |
| title_full_unstemmed | Melting the hydrous, subarc mantle: the origin of primitive andesites |
| title_short | Melting the hydrous, subarc mantle: the origin of primitive andesites |
| title_sort | melting the hydrous subarc mantle the origin of primitive andesites |
| url | http://hdl.handle.net/1721.1/107271 https://orcid.org/0000-0002-3224-9244 |
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