On the evolution of alkali ultrabasic to intermediate magmatism: insights from experimental petrology and geochemistry

Author(s)

Andres Fabian Salazar Naranjo

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Geociências (IG/BT)
• Defense date: 2023-04-13

Examining board members

Silvio Roberto Farias Vlach; Fernanda Gervasoni; Guilherme Mallmann

• Advisor: Silvio Roberto Farias Vlach

Abstract

Two alkali ultrabasic compositions from the Meso-Cenozoic Serra do Mar Alkaline Province dike swarm were geochemically and mineralogically characterized and used as starting materials in crystallization experiments to constrain the evolution of alkali ultrabasic to intermediate magmas and their crystalline phases close to equilibrium under a wide range of physicochemical parameters. The first sample corresponds to a basanite with sodic affinity, its composition is close to primary magmas (Mg#=63) and enriched in LILE, HFSE, and REE relative to the primitive mantle. Its mineralogy comprises olivine macrocrysts in a finedgrained groundmass of olivine, clinopyroxene, spinel, and alkali-feldspar. The second sample is a tephrite (olivine-free) with potassic affinity, which has a Mg#=48 and shows clinopyroxene macrocrysts in a fined-grained groundmass of clinopyroxene, spinel, and plagioclase. The crystallization experiments were performed at one-atmosphere pressure (100kPa) and from low- to high-pressure (0.5-2.0 GPa). Under 100 kPa, experiments were carried out over a range of O2, from 2 log units below to 2 log units above the fayalitemagnetite-quartz buffer (QFM), using a high-temperature vertical furnace with CO/CO2 gas mixture. The low- to high-pressure experiments were conducted in an end-loaded pistoncylinder using a Pt-graphite capsule (CCO buffer) and NaCl-pyrex-graphite-MgO assemblage. The melts present different compositional paths. For basanite, a sodic and strongly SiO2-undersaturated liquid line of descent (basanite-tephrite-phonotephritetephriphonolite) is derived. For tephrite, it is sodic-potassic/potassic and weakly SiO2- undersaturated (tephrite-alkali-basalt-phonotephrite) or SiO2-saturated (tephrite-trachybasalttrachyandesite), under reduced or oxidized conditions, respectively. Based on the percentages of experimental glasses, we concluded that evolved sodic magmas can be derived from the crystal fractionation of basanite. The iron-magnesium exchange coefficients between mafic silicate minerals and alkali melts (2+ ) are slightly lower than those observed in tholeiitic melts, with values of 0.29±0.02 for olivine and 0.25±0.02 for clinopyroxene. In both starting compositions, the crystallized clinopyroxenes are Ti- and Al-rich, and Si-poor, with a compositional trend of evolution from Mg-rich augite to ferroan diopside. Thermobarometric proxies such as MgO-in-melts and clinopyroxene-composition were improved for application in alkali ultrabasic to intermediate rocks, following the equations: (±5°) = 27.35 () + 984 , for one-atmosphere pressure and anhydrous conditions and, (±1.4 ) = 160.20 80.68 17.683+ + 6.36, where Na, Ti, and Fe3+ are cationic proportions relative to 6O of clinopyroxene. The trace element partitioning between titanian Ca-clinopyroxene and alkali melts was also studied, which might allow petrologists and geochemists to better investigate the genesis and evolution of alkali igneous rocks. The trace elements studied here were first-row transition metals, lithium, large ion lithophile elements, high-field strength elements, and rare earth elements. The results show that P-T-O2 strongly influences the behavior of trivalent cations, in both M1 and M2 sites. / and / vary following the compositional trend of the evolution of cpx and their substitution is in agreement with Tschermakite-type. The lattice-strain model successfully fitted the divalent and trivalent cation on the M2 site. The Wood and Blundy (1997) and Sun and Liang (2012) models that predict the REE partitioning between clinopyroxene and silicate melt do not predict the negative correlations between / and pressure. However, the Wood and Blundy (1997) model based on activities for Na0.5(REE)0.5MgSi2O6 component efficiently reproduces the partitioning of REE under 100 kPa. Finally, clinopyroxene is capable of fractionating the U/Th ratio, but Zr/Hf and Ta/Nb are not. (AU)

• FAPESP's process: 18/16755-4 - Experimental studies on trace element partitioning between mafic minerals and alkaline melts
• Grantee: Andres Fabian Salazar Naranjo
• Support Opportunities: Scholarships in Brazil - Doctorate