(. Ingalls, P. Savage, E. Pringle, and J. Foriel, WU) for assistance with MC-ICP-MS data collection and Don Swanson (HVO-USGS) for samples and discussion

, Grit Steinhoefel, two anonymous reviewers, and Associate Editor Clark Johnson for input that

L. Y. Alleman, D. Cardinal, C. Cocquyt, P. Plisnier, J. P. Descy et al., Silicon isotopic fractionation in Lake Tanganyika and its main tributaries, J. Great Lakes Res, vol.31, pp.509-519, 2005.
URL : https://hal.archives-ouvertes.fr/hal-02552900

M. Azaroual, C. Fouillac, and J. M. Matray, Solubility of silica polymorphs in electrolyte solutions.1. Activity coefficient of aqueous silica from 25 degrees to 250 degrees C, Pitzer's parameterisation, Chem. Geol, vol.140, pp.155-165, 1997.

S. Q. Bai, S. Urabe, Y. Okaue, and T. Yokoyama, Acceleration effect of sulfate ion on the dissolution of amorphous silica, J. Colloid Inter. Sci, vol.331, pp.551-554, 2009.

I. Basile-doelsch, J. D. Meunier, and C. Parron, Another continental pool in the terrestrial silicon cycle, Nature, vol.433, pp.399-402, 2005.
URL : https://hal.archives-ouvertes.fr/hal-01695941

C. Bern, M. Brzezinski, C. Beucher, K. Ziegler, C. et al., Weathering, dust, and biocycling effects on soil silicon isotope ratios, Geochim. Cosmochim. Acta, vol.74, pp.876-889, 2010.

R. H. Busey, E. Schwartz, R. E. Mesmer, D. Cardinal, J. Gaillardet et al., Contrasting silicon isotope signatures in rivers from the Congo Basin and the specific behaviour of organic-rich waters, Geophys. Res. Lett, vol.19, p.12403, 1980.

R. Chakrabarti, A. H. Knoll, S. B. Jacobsen, and W. W. Fischer, Si isotope variability in Proterozoic cherts, Geochim. Cosmochim. Acta, vol.91, pp.187-201, 2012.

S. M. Chemtob, B. L. Jolliff, G. R. Rossman, J. M. Eiler, and R. E. Arvidson, Silica coatings in the Ka'u Desert, Hawaii, a Mars analog terrain: A micromorphological, spectral, chemical, and isotopic study, J. Geophys. Res. Planets, 2010.

S. M. Chemtob and G. R. Rossman, Timescales and mechanisms of formation of amorphous silica coatings on fresh basalts at Kilauea Volcano, Hawai'i, J. Volcanol. Geotherm. Res, vol.286, pp.41-54, 2014.

S. M. Chemtob, G. R. Rossman, and J. F. Stebbins, Natural hydrous amorphous silica: quantitation of network speciation and hydroxyl content by 29 Si MAS NMR and vibrational spectroscopy, Am. Mineral, vol.97, pp.203-211, 2012.

C. T. Chen and W. L. Marshall, Amorphous silica solubilities. 4. Behavior in pure water and aqueous sodium chloride, sodium sulfate, magnesium chloride, and magnesium sulfate solutions up to 350° C, Geochim. Cosmochim. Acta, vol.46, pp.279-287, 1982.

J. Chmeleff, I. Horn, G. Steinhoefel, and F. Von-blanckenburg, In situ determination of precise stable Si isotope ratios by UV-femtosecond laser ablation high-resolution multi-collector ICP-MS, Chem. Geol, vol.249, pp.155-166, 2008.

A. Das, S. Krishnaswami, M. M. Sarin, and K. Pande, Chemical weathering in the Krishna Basin and Western Ghats of the Deccan Traps, India: Rates of basalt weathering and their controls, Geochim. Cosmochim. Acta, vol.69, pp.2067-2084, 2005.

J. C. De-hoog, M. J. Van-bergen, and M. H. Jacobs, Vapour-phase crystallisation of silica from SiF4-bearing volcanic gases, Annals Geophys, vol.48, pp.775-785, 2005.

C. L. De-la-rocha, M. A. Brzezinski, and M. J. Deniro, Purification, recovery, and laserdriven fluorination of silicon from dissolved and particulate silica for the measurement of natural stable isotope abundances, Anal. Chem, vol.68, pp.3746-3750, 1996.
URL : https://hal.archives-ouvertes.fr/hal-00660855

C. L. De-la-rocha, M. A. Brzezinski, and M. J. Deniro, A first look at the distribution of the stable isotopes of silicon in natural waters, Geochim. Cosmochim. Acta, vol.64, pp.2467-2477, 2000.
URL : https://hal.archives-ouvertes.fr/hal-00660843

C. L. De-la-rocha, M. A. Brzezinski, M. J. Deniro, and A. Shemesh, Silicon-isotope composition of diatoms as an indicator of past oceanic change, Nature, vol.395, pp.680-683, 1998.
URL : https://hal.archives-ouvertes.fr/hal-00660848

S. Delstanche, S. Opfergelt, D. Cardinal, F. Elsass, L. Andre et al., Silicon isotopic fractionation during adsorption of aqueous monosilicic acid onto iron oxide, Geochim. Cosmochim. Acta, vol.73, pp.923-934, 2009.
URL : https://hal.archives-ouvertes.fr/hal-02665841

T. Ding, D. Wan, C. Wang, and F. Zhang, Silicon isotope compositions of dissolved silicon and suspended matter in the Yangtze River, China. Geochim. Cosmochim. Acta, vol.68, pp.205-216, 2004.

C. B. Douthitt, The geochemistry of the stable isotopes of silicon, Geochim. Cosmochim. Acta, vol.46, pp.1449-1458, 1982.

W. M. Edmunds, P. L. Smedley, O. Selinus, B. Alloway, J. A. Centeno et al., Fluoride in natural waters, Essentials of Medical Geology -Impacts of the Natural Environment on Public Health, pp.301-315, 2005.

K. W. Flach, W. D. Nettleton, L. H. Gile, and J. G. Cady, Pedocementation: induration by silica, carbonates, and sesquioxides in the quaternary, Soil Science, vol.107, pp.442-453, 1969.

S. Geilert, P. Z. Vroon, D. L. Roerdink, P. Van-cappellen, and M. J. Van-bergen, Silicon isotopic fractionation during abiotic silica precipitation at low temperatures: inferences from flow-through experiments, Geochim. Cosmochim. Acta, vol.142, pp.95-114, 2014.

S. Geilert, P. Z. Vroon, N. S. Keller, S. Gudbrandsson, A. Stefansson et al., Silicon isotope fractionation during silica precipitation from hot-spring waters: evidence from the Geysir geothermal field, Geochim. Cosmochim. Acta, vol.164, pp.403-427, 2015.

R. B. Georg, B. C. Reynolds, M. Frank, and A. N. Halliday, Mechanisms controlling the silicon isotopic compositions of river waters, Earth Planet. Sci. Lett, vol.249, pp.290-306, 2006.

R. B. Georg, B. C. Reynolds, A. J. West, K. W. Burton, and A. N. Halliday, Silicon isotope variations accompanying basalt weathering in Iceland, Earth Planet. Sci. Lett, vol.261, pp.476-490, 2007.
URL : https://hal.archives-ouvertes.fr/hal-00320081

R. B. Georg, C. Zhu, B. C. Reynolds, and A. N. Halliday, Stable silicon isotopes of groundwater, feldspars, and clay coatings in the Navajo Sandstone aquifer, Geochimica Et Cosmochimica Acta, vol.73, pp.2229-2241, 2009.

T. M. Gerlach, Oxygen buffering of Kilauea volcanic gases and the oxygen fugacity of Kilauea basalt, Geochim. Cosmochim. Acta, vol.57, pp.795-814, 1993.

T. W. Giambelluca, Q. Chen, A. G. Frazier, J. P. Price, Y. Chen et al., Online rainfall atlas of Hawai'i, Bull. Amer. Meteor. Soc, vol.94, pp.313-316, 2013.

N. N. Golovnev, V. B. Nogteva, and I. I. Golovneva, Formation of Si(IV) and Ge(IV) fluoride complexes in dilute aqueous solutions, Russian J. Gen. Chem, vol.73, pp.1388-1394, 2003.

L. P. Greenland, W. I. Rose, and J. B. Stokes, An estimate of gas emissions and magmatic gas content from Kilauea volcano, Geochim. Cosmochim. Acta, vol.49, pp.125-129, 1985.

A. P. Gysi and A. Stefánsson, Mineralogical aspects of CO 2 sequestration during hydrothermal basalt alteration -an experimental study at 75 to 250 °C and elevated pCO 2, Chem. Geol, pp.146-159, 2012.

P. R. Heck, J. M. Huberty, N. T. Kita, T. Ushikubo, R. Kozdon et al., SIMS analyses of silicon and oxygen isotope ratios for quartz from Archean and Paleoproterozoic banded iron formations, Geochim. Cosmochim. Acta, vol.75, pp.5879-5891, 2011.

J. Heicklen and V. Knight, The infrared spectrum of SiF4, Spectrochim. Acta, vol.20, pp.295-298, 1974.

R. Hellmann, D. Daval, and R. Wirth, Formation of amorphous silica surface layers by dissolution-reprecipitation during chemical weathering: implications for CO 2 uptake, Proc. Earth Planet. Sci, vol.7, pp.346-349, 2013.

P. S. Hill, E. A. Schauble, and E. D. Young, Effects of changing solution chemistry on Fe 3+ /Fe 2+ isotope fractionation in aqueous Fe-Cl solutions, Geochim. Cosmochim. Acta, vol.74, pp.6669-6689, 2010.

C. J. Horwell, B. J. Williamson, E. W. Llewellin, D. E. Damby, L. Blond et al., The nature and formation of cristobalite at the Soufrière Hills volcano, Montserrat: implications for the petrology and stability of silicic lava domes, Bull. Volcanol, vol.75, 2013.

H. J. Hughes, C. Delvigne, M. Korntheuer, J. De-jong, L. André et al., Controlling the mass bias introduced by anionic and organic matrices in silicon isotopic measurements by MC-ICP-MS, J. Anal. At. Spectrom, vol.26, pp.1892-1896, 2011.

J. A. Hurowitz, S. M. Mclennan, D. H. Lindsley, and M. A. Schoonen, Experimental epithermal alteration of synthetic Los Angeles meteorite: Implications for the origin of Martian soils and identification of hydrothermal sites on Mars, J. Geophys. Res. Planets, vol.110, p.7002, 2005.

J. A. Hurowitz, S. M. Mclennan, N. J. Tosca, R. E. Arvidson, J. R. Michalski et al., In situ and experimental evidence for acidic weathering of rocks and soils on Mars, J. Geophys. Res, vol.111, pp.2-19, 2006.

S. Hurwitz, F. Goff, C. J. Janik, W. C. Evans, D. A. Counce et al., Mixing of magmatic volatiles with groundwater and interaction with basalt on the summit of Kilauea Volcano, Hawaii, J. Geophys. Res, vol.108, 2003.

B. F. Jones, H. P. Eugster, and S. L. Reitig, Hydrochemistry of the Lake Magadi basin, Kenya. Geochim. Cosmochim. Acta, vol.41, pp.53-72, 1977.

K. G. Knauss, W. L. Bourcier, K. D. Mckeegan, C. I. Merzbacher, S. N. Nguyen et al., Dissolution kinetics of a simple analogue nuclear waste glass as a function of pH, time and temperature, MRS Proc, vol.176, 1989.

M. J. Leng, G. E. Swann, M. J. Hodson, J. J. Tyler, S. V. Patwardhan et al., The potential use of silicon isotope composition of biogenic silica as a proxy for environmental change, Silicon, vol.1, pp.65-77, 2009.

Y. H. Li, T. P. Ding, and D. F. Wan, Experimental study of silicon isotope dynamic fractionation and its geological application, Mineral Deposits, vol.13, pp.282-288, 1994.

M. C. Malin, D. Dzurisin, and R. P. Sharp, Stripping of Keanakakoi tephra on Kilauea Volcano, Hawaii. GSA Bull, vol.94, pp.1148-1158, 1983.

J. Marin-carbonne, M. Chaussidon, M. C. Boiron, and F. Robert, A combined in situ oxygen, silicon isotopic and fluid inclusion study of a chert sample from Onverwacht Group, 2011.

S. Ga and . Africa, New constraints on fluid circulation, vol.286, pp.59-71

W. L. Marshall, C. , and C. A. , Amorphous silica solubilities -VI. Postulated sulfate-silicic acid solution complex, Geochim. Cosmochim. Acta, vol.46, pp.367-370, 1982.

M. Meheut, M. Lazzeri, E. Balan, and F. Mauri, Structural control over equilibrium silicon and oxygen isotopic fractionation: A first-principles density-functional theory study, Chemical Geol, vol.258, pp.28-37, 2009.

A. J. Milligan, D. E. Varela, M. A. Brzezinski, and F. O. Morel, Dynamics of silicon metabolism and silicon isotopic discrimination in a marine diatom as a function of pCO 2, Limnol. Oceanogr, vol.49, pp.322-329, 2004.

M. E. Minitti, C. M. Weitz, M. D. Lane, and J. L. Bishop, Morphology, chemistry, and spectral properties of Hawaiian rock coatings and implications for Mars, J. Geophys. Res. Planets, vol.112, p.5015, 2007.

P. A. Munter, O. T. Aepli, and R. A. Kossatz, Hydrofluoric acid -water and hydrofluoric acid -hydrofluosilicic acid -water, Industrial Engineering Chem, vol.39, pp.427-431, 1947.

S. I. Naboko, A case of gaseous fluorine metasomatism at an active volcano, Geochemistry 1957, pp.452-455, 1957.

J. T. Nanyaro, U. Aswathanarayana, and J. S. Mungure, A geochemical model for the abnormal fluoride concentration in waters in parts of northern Tanzania, J. Afr. Earth Sci, vol.2, pp.129-140, 1984.

L. O. Ohman, A. Nordin, I. F. Sedeh, and S. Sjoberg, Equilibrium and structural studies of silicon(IV) and aluminum(III) in aqueous solution. 28. Formation of soluble silicic acid-ligand complexes as studied by potentiometric and solubility measurements, Acta Chemica Scandinavica, vol.45, pp.335-341, 1991.

S. Opfergelt, D. Cardinal, L. Andre, C. Delvigne, L. Bremond et al., Variations of ? 30 Si and Ge/Si with weathering and biogenic input in tropical basaltic ash soils under monoculture, Geochim. Cosmochim. Acta, vol.74, pp.225-240, 2010.
URL : https://hal.archives-ouvertes.fr/hal-01982866

S. Opfergelt, D. Cardinal, C. Henriet, X. Draye, L. Andre et al., Silicon isotopic fractionation by banana (Musa spp.) grown in a continuous nutrient flow device, Plant Soil, vol.285, pp.333-345, 2006.

S. Opfergelt and P. Delmelle, Silicon isotopes and continental weathering processes: assessing controls on Si transfer to the ocean, Comptes Rendus Geosci, vol.344, pp.723-738, 2012.

E. A. Pringle, P. S. Savage, J. Badro, J. Barrat, and F. Moynier, Redox state during core formation on asteroid 4-Vesta, Earth Planet. Sci. Lett, vol.373, pp.75-82, 2013.
URL : https://hal.archives-ouvertes.fr/insu-00933993

M. E. Raymo, W. F. Ruddiman, and P. N. Froelich, Influence of late Cenozoic mountain building on ocean geochemical cycles, Geology, vol.16, pp.649-653, 1988.

B. C. Reynolds, J. Aggarwal, L. Andre, D. Baxter, C. Beucher et al., An inter-laboratory comparison of Si isotope reference materials, J. Anal. Atom. Spectrom, vol.22, pp.561-568, 2007.

C. S. Riebe, J. W. Kirchner, and R. C. Finkel, Erosional and climatic effects on long-term chemical weathering rates in granitic landscapes spanning diverse climate regimes, Earth Planet. Sci. Lett, vol.224, pp.547-562, 2004.

F. Robert and M. Chaussidon, A palaeotemperature curve for the Precambrian oceans based on silicon isotopes in cherts, Nature, vol.443, pp.969-972, 2006.

E. Ruiz-agudo, C. V. Putnis, C. Rodriguez-navarro, and A. Putnis, Mechanism of leached layer formation during chemical weathering of silicate minerals, Geology, vol.40, pp.947-950, 2012.

P. S. Savage, R. M. Armytage, R. B. Georg, and A. N. Halliday, High temperature silicon isotope geochemistry, Lithos, vol.190, pp.500-519, 2014.

P. S. Savage, R. B. Georg, H. M. Williams, K. W. Burton, and A. N. Halliday, Silicon isotope fractionation during magmatic differentiation, Geochim. Cosmochim. Acta, vol.75, pp.6124-6139, 2011.

K. D. Seelos, R. E. Arvidson, B. L. Jolliff, S. M. Chemtob, R. V. Morris et al., Silica in a Mars analog environment: Ka'u Desert, J. Geophys. Res. Planets, vol.115, pp.0-15, 2010.

A. Shahar, K. Ziegler, E. D. Young, A. Ricolleau, E. A. Schauble et al., Experimentally determined Si isotope fractionation between silicate and Fe metal and implications for Earth's core formation, Earth Planet. Sci. Lett, vol.288, pp.228-234, 2009.

E. J. Stefurak, W. W. Fischer, and D. R. Lowe, Texture-specific Si isotope variations in Barberton Greenstone Belt cherts record low temperature fractionations in early Archean seawater, Geochim. Cosmochim. Acta, vol.150, pp.26-52, 2015.

G. Steinhöfel, I. Horn, and F. Blanckenburg, Micro-scale tracing of Fe and Si isotope signatures in banded iron formation using femtosecond laser ablation, 2009.

G. Steinhöfel, F. Von-blanckenburg, I. Horn, K. O. Konhauser, N. J. Beukes et al., Deciphering formation processes of banded iron formations from the Transvaal and the Hamersley successions by combined Si and Fe isotope analysis using UV femtosecond laser ablation, Geochim. Cosmochim. Acta, vol.74, pp.2677-2696, 2010.

N. J. Tosca, S. M. Mclennan, D. H. Lindsley, M. A. Schoonen, S. H. Van-den-boorn et al., Silicon isotope and trace element constraints on the origin of similar to 3.5 Ga cherts: Implications for early Archaean marine environments, J. Geophys. Res. Planets, vol.109, pp.1077-1103, 2004.

A. F. White and M. F. Hochella, Surface chemistry associated with the cooling and subaerial weathering of recent basalt flows, Geochim. Cosmochim. Acta, vol.56, pp.3711-3721, 1992.

K. Ziegler, O. A. Chadwick, M. A. Brzezinski, and E. F. Kelly, Natural variations of ? 30 Si ratios during progressive basalt weathering, Hawaiian islands, Geochim. Cosmochim. Acta, vol.69, pp.4597-4610, 2005.

K. Ziegler, E. D. Young, E. A. Schauble, and J. T. Wasson, Metal-silicate silicon isotope fractionation in enstatite meteorites and constraints on Earth's core formation, Earth Planet. Sci. Lett, vol.295, pp.487-496, 2010.