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«DISS. ETH NO. 21411 EVOLUTION OF HYDROUS MANTLE-DERIVED CALC-ALKALINE LIQUIDS BY FRACTIONAL CRYSTALLIZATION AT 0.7 AND 0.4 GPa – AN EXPERIMENTAL ...»

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DISS. ETH NO. 21411

EVOLUTION OF HYDROUS MANTLE-DERIVED

CALC-ALKALINE LIQUIDS BY FRACTIONAL CRYSTALLIZATION

AT 0.7 AND 0.4 GPa – AN EXPERIMENTAL STUDY

A dissertation submitted to

ETH Zurich

for the degree of

Doctor of Sciences

presented by

ROHIT H. NANDEDKAR

Master of Science in Earth Sciences

ETH Zurich

born 1st of November 1980

citizen of Wohlen b. Bern (BE)

accepted on the recommendation of

supervisor Prof. Dr. P. Ulmer ETH Zurich co-supervisor Prof. Dr. O. Müntener University of Lausanne examiner Dr. T. W. Sisson US Geological Survey examiner Prof. Dr. O. Bachmann ETH Zurich

Front cover picture:

These are the Cuernos del Paine peaks (Norte, 2400 m.a.s.l.; Principal, 2600 m.a.s.l.; Este, 2200 m.a.s.l.) and the Cerro Almirante Nieto (2640 m.a.s.l.) of the Torres del Paine batholith,Chile (peaks from left to right). The shallow granitic batholith (white) is exemplarly visible against the host rock (in black). The picture was taken from the Sendero Mirador Cuernos at Lago Nordenskjöld.

Back cover picture:

All three boundaries (upper, lower and on the side) of the Torres del Paine batholith are sharbly visible. The picture was taken in the Valle del Frances (peaks from left to right: Co. Catillo, 1421 m.a.s.l.; Co. Catedral, 2168 m.a.s.l.; Co. Aguja de los Quirquinchos, 1703 m.a.s.l.; Co. los Gemelos, 1998 m.a.s.l.; Co. Trono Blanco, 2197 m.a.s.l.; Co. Aleta de Tiburón, 1717 m.a.s.l.).

“One needs to be slow to form convictions, but once formed they must be defended against the heaviest odds.” Mahatma Gandhi (1869-1948) …dedicated to my beloved Bettina Summary Summary This PhD thesis investigates the evolution of mantle-derived, hydrous basaltic magmas at intermediate to upper crustal levels in subduction-related arc systems. Near perfect fractional crystallization has been simulated in a set of experiments conducted at 0.4 and 0.7 GPa using an end loaded piston cylinder apparatus to constrain the liquid evolution of a cooling magma (liquid line of descent), the responsible phase equilibria, compositions and modes of all phases. Experiments have been carriedout in steps of 30-50 °C employing the chemical equivalent of the liquid composition of the previous experiment as starting composition in the subsequent, lower temperature experiment. The initial starting composition corresponds to a near-primary olivine tholeiitic dike from the Adamello batholith (Northern Italy), that is potentially parental to the large volume plutonic rocks forming the bulk of the batholith. The starting compositions were mixed from oxides, hydroxides, silicates, phosphates and contained, in addition, 2 wt% of a diopside glass comprising 28 elements (REE, HFSE, LILE, LLE, transition metals and actinides) providing 40 µg/g trace elements to facilitate the determination of trace element partitioning between solid and liquid phases. The oxygen fugacity was constrained close to the Ni-NiO buffer equilibrium (NNO), conditions typically inferred for subduction-related magmas crystallizing in crustal magma reservoirs, by adjusting the ferric to ferrous iron ratio in the starting material. Phases have been analyzed using electron probe micro-analyzer (EPMA) and laserablation induced coupled plasma mass spectrometry (LA-ICP-MS). Micro-Raman spectroscopy was employed to confirm anticipated H2O contents in the liquid phase after the experiment. The results of this study provide a coherent data set that is further used to interpret crystallization-driven differentiation of sub-crustal magmas at intermediate to upper crustal levels.

The liquid line of descent at 0.7 GPa follows a calc-alkaline differentiation trend (strong silica enrichment combined with low iron concentrations) that is primarily controlled by the extensive crystallization of clinopyroxene, amphibole, anorthite-rich plagioclase and Fe-Ti-oxide differentiating to peraluminous derivative liquid compositions (aluminum saturation index ASI1). The generation of peraluminous granitoids has often been associated with extensive crustal assimilation of pelitic country rocks and/or partial melting of amphibolitic to pelitic crustal lithologies. The findings of this thesis, however, provide an alternative explanation via direct fractional crystallization from precursor basaltic, sub-crustal magmas for the origin of peraluminous granitoid plutonic (and volcanic) rocks, which often constitute a significant volume of arc-related batholithic complexes (e.g. Adamello batholith (Italy) or Chelan Complex, Washington (U.S.A.)).

Partition coefficients between amphiboles and coexisting melts have been determined and subsequently used to conduct trace element modeling to reconcile trace element patterns obtained from natural rocks representing either cumulate or derivative liquid compositions. Amphibole/liquid partition coefficients reveal a strong increase with decreasing temperature except for potassium, which behaves oppositely. The increase of the partition coefficients of amphiboles is attributed to (1) the increasing polymerization of the coexisting melt and (2) an increase in the cummingtonite component with increasing differentiation.

Experimentally determined solid and liquid fractions revealed a strongly non-linear behavior along the liquid line of descent with moderate crystallization of olivine and increasing amount of clinopyroxene close to the liquidus (1160-1070 °C, 20 % of the original mass), followed by a narrow temper

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iSummary





ature interval (1070-1010 °C) where 40 % of the original magma mass crystallizes to clinopyroxene, An-rich plagioclase, Al-rich spinel and amphibole (starting at 1020 °C) driving the remaining liquid composition from basaltic to andesitic. This is followed by a large temperature interval (1010-720 °C) where amphibole-plagioclase-magnetite crystallizes (solidifying 50 % of the remaining magma, which is equal to 20 % of the original magma mass). This strongly non-linear crystallization behavior has considerable consequences for magma rheology. The large solid fraction produced during the temperature interval characterized by massive crystallization potentially results in a crystal mush containing less than 50 % liquid that, in turn, increases the probability that the liquid could be separated from its solid fraction, leaving dense clinopyroxene-dominated cumulates behind. This might actually represent the prime reason that andesite/tonalite compositions dominate shallow level igneous as well as volcanic complexes at convergent plate margins. If correct, the often inferred compositional gap between basaltic and andesitic composition would be rheologically and not phase equilibria controlled; the experiments generate a continuous spectrum of liquid compositions. At lower temperature, the crystallization from an andesitic to a high-silica rhyolite composition is very gentle and smooth; the amphibole-plagioclase-magnetite assemblage coexists over the entire andesite – rhyolite evolution and magmas will only achieve 50 % crystallinity (if solids are not immediately extracted) close to the granite minimum. Thus, the magmas exist as dilute to concentrated suspensions, allowing them to ascent and/ or form long-living, dynamic, convecting magma reservoirs. Such conditions may promote open system processes such as assimilation, replenishment and mixing/mingling in high-level magma reservoirs.

The liquid line of descent determined at 0.4 GPa follows a “mainly” calc-alkaline differentiation trend and crystallizes olivine-clinopyroxene-plagioclase (-ilmenites) over the entire temperature range (1100–920 °C) investigated in this study. The liquid composition evolves from basaltic to dacitic compositions, but remains metaluminous, even after 80 % solidification of the original magma mass.

The crystallization – temperature relationship is more linear at 0.4 GPa than at 0.7 GPa over the investigated temperature range.

In these experimental studies, the H2O-content of the initial magma were fixed to 3.0 wt%; thus the effect of varying initial H2O-contents on phase equilibria, compositions and modes was not evaluated. However, the effect of water on the course of the magmatic differentiation is well known and is clearly inferred by comparison with similar experiments conducted under anhydrous conditions: a calc-alkaline differentiation trend is not achieved at low water contents and the derivative liquids are characterized by iron-enrichment at low silica concentration resulting in a tholeiitic trend. H2O suppresses plagioclase saturation relative to less polymerized Fe-Mg silicates and stabilizes Fe-Ti oxides under comparable fO2 conditions. In the present experimental study, continuous crystallization increases the H2O-content in the derivative liquids until volatile saturation occurs. H2O is additionally responsible for the stabilization of hydrous minerals, most importantly amphibole that significantly contributes to increasing silica-contents of derivative liquids characterizing the calc-alkaline differentiation trend.

The effect of pressure on the liquid line of descent is most evidently expressed by different mineral assemblages stabilizing in the differentiating magmas: At deepest crustal levels (1.0 GPa), clinopyroxene and garnet fractionation control the differentiation of the magma, whereas at intermediate crustal levels (0.7–1.0 GPa) clinopyroxene, amphibole and, with decreasing pressure more prominently, An-rich plagioclase fractionation dominate the differentiation towards peraluminous high

–  –  –

silica liquids. At upper crustal levels (0.4 GPa), the stability of olivine is considerably increased and An-rich plagioclase dominates the differentiation.

The experimentally constrained compositions of cumulates at 0.4 and 0.7 GPa favorably compare with ultramafic to mafic cumulate rocks of the Adamello batholith and the Chelan Complex. More differentiated intermediate to acidic rock compositions, however, are compositionally closer to the experimentally-derived liquids. Trace element modeling using experimentally constrained partition coefficients and modal proportions of crystallizing phases further supports the importance of crystallization-driven differentiation for the generation of intermediate to acidic supra-subduction magmas in many cases. Derivations from the experimentally-derived liquid line of descent and/or mismatch between calculated and observed trace element patterns potentially provide evidence and assist quantification of open-system processes such as FARM (Fractionation-Assimilation-ReplenishmentMixing; Baker et al., 1991) and MASH (Melting-Assimilation-Storage-Homogenization; Hildreth and Moorbath, 1988) clearly operating in addition to crystallization-driven differentiation in igneous systems at convergent-plate margins.

References Baker, M.B., Grove, T.L., Kinzler, R.J., Donnellynolan, J.M., and Wandless, G.A., 1991, Origin of compositional zonation (high-alumina basalt to basaltic andesite) in the Giant Crater Lava Field, Medicine Lake Volcano, northern California: Journal of Geophysical Research-Solid Earth, v. 96, p. 21819-21842.

Hildreth, W., and Moorbath, S., 1988, Crustal contributions to arc magmatism in the Andes of Central Chile: Contributions to Mineralogy and Petrology, v. 98, p. 455-489.

–  –  –

Zusammenfassung Diese Dissertation untersucht die Entwicklung von wasserhaltigen basaltischen Magmen aus dem Erdmantel in mittleren und flachen Tiefen der Erdkruste oberhalb von Subduktionszonen. Die Entwicklung der Schmelze mit sinkender Temperatur (liquid line of descent) und der dazugehörigen Mineralphasen wurde untersucht, indem fraktionierte Kristallisationsexperimente bei 0.4 und

0.7 GPa unter Zuhilfenahme einer Druckstempelpresse durchgeführt wurden. Fraktionierte Kristallisation wurde in den Experimenten simuliert, indem die Schmelzzusammensetzung des jeweils vorangegangenen Experiments bei 30-50 °C höherer Temperatur als Startzusammensetzung des folgenden Experiments diente. Die ursprünglichste Ausgangszusammensetzung der Experimentreihe stammt von einem Olivin-tholeiitischen Ganggestein, welches sich wahrscheinlich parental zu den benachbarten plutonischen Gesteinen der grossen Adamello Intrusion (in Norditalien) verhält. Die jeweiligen Startzusammensetzungen wurden durch Oxide, Hydroxide, Silikate, Phosphate und 2 Gew.-% eines diopsidischen Spurenelementträgerglases gemischt. Dieses besteht aus 28 Elementen, welche Seltene Erden, verschiedene Spurenelemente, Übergangsmetalle sowie Aktinide beinhalten. Die Spurenelemente wurden benötigt, um das Verhalten der Elemente – bevorzugter Einbau in eine Mineralphase oder Anreicherung in der Schmelze - zu bestimmen. Daher wurde ihre Konzentration im Ausgangsmaterial auf konstant 40 µg/g festgesetzt. Die Redoxbedingungen der Experimente wurden, analog der angenommenen Redoxverhältnisse für diese geotektonische Situation, entsprechend der Gleichgewichtspufferung von Ni zu NiO bestimmt und durch das Verhältnis von Eisen(III) zu Eisen(II) im Experiment festgelegt. Die chemische Zusammensetzung aller Phasen wurden mit Hilfe der Elektrodenstrahlmikrosonde (electron probe micro-analyzer (EPMA)) und durch Laserablation gekoppelt an einen Massenspektrometer (laser-ablation induced coupled plasma mass spectrometry (LA-ICP-MS)) untersucht. Wassergehalte wurden an der abgeschreckten Schmelze (Glas) nach den durchgeführten Experimenten durch Ramanspektroskopie bestimmt. Die Resultate dieser Doktorarbeit liefern einen kohärenten Datensatz, der die kristallisationsbedingte Differentiation von subkrustalen Magmen in intermediären bis flachen Tiefen der Erdkruste beschreibt.



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