Alteration of Feldspathoids Changes pH of Late-Magmatic Fluids: A Case Study from the Lovozero Peralkaline Massif, Russia
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Abstract:
The 360-370-Ma-old Lovozero peralkaline massif (NW Russia) is a layered nepheline syenitic–foidolitic pluton. In the rocks of the massif, late-stage (auto)metasomatic alterations of rock-forming minerals are quite intense. We studied the products of the alteration of nepheline and sodalite via microtextural, microprobe, and spectroscopic methods. We found that these minerals are extensively replaced by the association between natrolite + nordstrandite ± böhmite ± paranatrolite in accordance with the following reactions: 3Nph + 4H2O → Ntr + Nsd + NaOH; 6Nph + 9H2O → Ntr + Pntr + 2Nsd + 2NaOH; Sdl + 4H2O → Ntr + Nsd + NaOH + NaCl, where Nph is nepheline, Ntr is natrolite, Nsd is nordstrandite, Pntr is paranatrolite, and Sdl is sodalite. As a result, about one-third of the sodium from nepheline (and sodalite) is set free and passes into the fluid. This leads to an increase in the Na/Cl ratio and, hence, the pH of the fluid. An increase in pH stabilizes hyperagpaitic minerals (e.g., ussingite, villiaumite, thermonatrite, and trona), which can crystallize in close proximity to pseudomorphized nepheline and sodalite. Thus, the alteration of feldspathoids increases the pH of late-magmatic fluids, which in turn can lead to the crystallization of hyperagpaitic minerals.Keywords:
Nepheline
Peralkaline rock
Sodalite
Massif
Nepheline syenite
Aegirine
Metasomatism
Baddeleyite
Nepheline
Peralkaline rock
Pyroxene
Nepheline syenite
Aegirine
Mineral redox buffer
Grossular
Fractional crystallization (geology)
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Peralkaline rock
Aegirine
Nepheline
Nepheline syenite
Carbonatite
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Peralkaline rock
Nepheline syenite
Nepheline
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Hyperagpaitic rocks are highly peralkaline nepheline syenites in which minerals such as ussingite [Na2AlSi2O8(OH)] and naujakasite (Na6FeAl4Si8O26) crystallize instead of, or in addition to, feldspars and feldspathoids; eudialyte is succeeded by steenstrupine-(Ce) and members of the lovozerite and lomonosovite groups; highly water-soluble minerals such as villiaumite (NaF) and natrosilite (Na2Si2O5) form part of magmatic mineral assemblages. Hyperagpaitic magmatic rocks in the Ilímaussaq alkaline complex in South Greenland include intrusive bodies of villiaumite- and naujakasite-bearing lujavrite (i.e. melanocratic, silica-undersaturated syenite), and late veins and pegmatites. The transition from agpaitic to hyperagpaitic magmatic conditions is controlled by increasing peralkalinity (or aNa2Si2O5) and changes in activities of water, halogens and phosphorus. The relative importance of these parameters can be evaluated by chemographic analysis based on observed mineral assemblages and mineral compositions. Highly peralkaline melts can crystallize agpaitic albite + microcline + nepheline + arfvedsonite + eudialyte ± sodalite magmatic assemblages over a wide range of alkali, water and halogen activities. In most cases, a net increase in HF activity is needed to induce villiaumite crystallization, suggesting hyperagpaitic conditions, but because of the orientation of the villiaumite-saturation surface in log aNa2Si2O5–log aH2O–log aHF space, loss of an aqueous fluid may stabilize villiaumite without increasing peralkalinity. Naujakasite indicates elevated aNa2Si2O5, ussingite elevated aH2O, and steenstrupine-(Ce) and vuonnemite high aNa2Si2O5 and aP2O5. Closed-system fractionation of agpaitic magma may lead to hyperagpaitic residual liquids if aegirine, eudialyte or sodalite do not crystallize early, which is not applicable to the main line of magma evolution in Ilímaussaq, with abundantly eudialyte-bearing kakortokite and sodalite-bearing naujaite cumulates. Hyperagpaitic residual liquids can have developed if fractionation of these minerals were suppressed, or by open-system processes such as intraplutonic assimilation of sodalite-bearing cumulates in lujavritic magma at low aHCl.
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The 360-370-Ma-old Lovozero peralkaline massif (NW Russia) is a layered nepheline syenitic–foidolitic pluton. In the rocks of the massif, late-stage (auto)metasomatic alterations of rock-forming minerals are quite intense. We studied the products of the alteration of nepheline and sodalite via microtextural, microprobe, and spectroscopic methods. We found that these minerals are extensively replaced by the association between natrolite + nordstrandite ± böhmite ± paranatrolite in accordance with the following reactions: 3Nph + 4H2O → Ntr + Nsd + NaOH; 6Nph + 9H2O → Ntr + Pntr + 2Nsd + 2NaOH; Sdl + 4H2O → Ntr + Nsd + NaOH + NaCl, where Nph is nepheline, Ntr is natrolite, Nsd is nordstrandite, Pntr is paranatrolite, and Sdl is sodalite. As a result, about one-third of the sodium from nepheline (and sodalite) is set free and passes into the fluid. This leads to an increase in the Na/Cl ratio and, hence, the pH of the fluid. An increase in pH stabilizes hyperagpaitic minerals (e.g., ussingite, villiaumite, thermonatrite, and trona), which can crystallize in close proximity to pseudomorphized nepheline and sodalite. Thus, the alteration of feldspathoids increases the pH of late-magmatic fluids, which in turn can lead to the crystallization of hyperagpaitic minerals.
Nepheline
Peralkaline rock
Sodalite
Massif
Nepheline syenite
Aegirine
Metasomatism
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Nepheline
Nepheline syenite
Aegirine
Peralkaline rock
Ilmenite
Carbonatite
Alkali feldspar
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