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Role of volatiles (S, Cl, H (sub 2) O) and silica activity on the crystallization of haueyne and nosean in phonolitic magmas (Eifel, Germany and Saghro, Morocco)

Celine Baudouin and Fleurice Parat
Role of volatiles (S, Cl, H (sub 2) O) and silica activity on the crystallization of haueyne and nosean in phonolitic magmas (Eifel, Germany and Saghro, Morocco)
American Mineralogist (October 2015) 100 (10): 2308-2322

Abstract

To constrain the crystallization of alkaline and volatile-rich lavas present in intraplate settings, we studied the petrological features and the geochemical composition of major, trace, and volatile elements of mineral and bulk-rock of two sodalite-bearing phonolites: (1) hauyne-plagioclase-bearing Si-K-rich phonolite from Laacher See (Germany) and (2) nosean-nepheline-bearing Si-poor phonolite from Saghro (Morocco). In hauyne-bearing phonolites (55-59 wt% SiO (sub 2) , K> Na, Na+K/Al=0.96-1.08), we found that the low silica and low sodium activity promoted the early crystallization of S-rich hauyne (13.7-13.9 wt% SO (sub 3) , 0.4 wt% Cl) + S-rich apatite (0.7-0.9 wt% SO (sub 3) ), titanite, and rare pyrrhotite followed by clinopyroxene-plagioclase-sanidine at relatively low pressure and temperature (P=250 MPa and T=850 degrees C) and oxidized condition (Delta NNO-NNO+1, where NNO is nickel-nickel oxide buffer). The crystallization of hauyne occurred at fluid-undersaturated conditions from a silicate melt with 6 wt% H (sub 2) O, 0.17-0.23 wt% Cl, 0.11-0.4 wt% S, and 0.07-0.14 wt% F. Nosean-bearing phonolites from Saghro are silica-poor and peralkaline (52-54 wt% SiO (sub 2) , Na>K, Na+K/Al=1.2) and crystallized at higher P and T (300 MPa and 950 degrees C) and more reduced conditions (NNO) compared to hauyne-bearing phonolites. The incongruent reaction to form nosean requires high silica and Na (sub 2) O activity. The mineral assemblage and composition suggest early crystallization of nepheline followed by nosean (7.8-8.8 wt% SO (sub 3) ; 1-1.1 wt% Cl). The sequence of crystallization is: clinopyroxene + nepheline + S-poor apatite (<0.04 wt% SO (sub 3) ) + pyrrhotite followed by nosean and titanite. Nosean-bearing magmas are fluid-undersaturated with relatively low volatile content (4 wt% H (sub 2) O, <0.25 wt% Cl, <0.056 wt% S, 0.08-0.1 wt% F), although Cl may have exsolved during ascent and formed a fluid phase (NaCl-bearing). Both hauyne- and nosean-bearing phonolites are last equilibrated at relatively low pressure and high temperature. Hauyne and nosean crystallized at oxidized and volatile-rich pre-eruptive conditions. They record the volatile concentrations at depth and may be used as oxybarometer. The incongruent reactions involved to form hauyne and nosean suggest that phonolitic magmas became more oxidized during crystallization. The initial volatile concentrations in basanite/nephelinite magmas, from partial melting of volatile-bearing K (sub 2) O-rich mantle rock, should have been one important factor influencing the crystallization of hauyne-bearing Si-K-rich phonolite and nosean-bearing Si-poor phonolite in intracontinental setting.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 100
Serial Issue: 10
Title: Role of volatiles (S, Cl, H (sub 2) O) and silica activity on the crystallization of haueyne and nosean in phonolitic magmas (Eifel, Germany and Saghro, Morocco)
Affiliation: Universite Montpellier, Geosciences Montpellier, Montpellier, France
Pages: 2308-2322
Published: 201510
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 83
Accession Number: 2016-007314
Categories: Igneous and metamorphic petrologyMineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
N27°40'00" - N36°00'00", W13°15'00" - W01°00'00"
N50°00'00" - N50°30'00", E06°19'60" - E07°19'60"
N50°23'60" - N50°23'60", E07°17'60" - E07°17'60"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201604
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