Thermobarometry Research Papers - Academia.edu (original) (raw)

Exsolution lamellae in pyroxene and garnet porphyroblasts in pyroxenite xenoliths from the Mir, Udachnaya, and Obnazhennaya kimberlites (Siberian Craton) reveal a diverse suite of exsolved phases, including oxides (spinels, ilmenite,... more

Exsolution lamellae in pyroxene and garnet porphyroblasts in pyroxenite xenoliths from the Mir, Udachnaya, and Obnazhennaya kimberlites (Siberian Craton) reveal a diverse suite of exsolved phases, including oxides (spinels, ilmenite, rutile, and chromite), pyroxene, and garnet. Textural characteristics suggest that exsolved phases progressively increased in volumetric proportions, and in some cases, the bulk xenoliths transformed from a lithology dominated by coarse grains (i.e. > 2 cm; megacrystalline) to a significantly finer-grained texture (i.e. < 1 cm).These exsolved lamellae are the result of a complex and protracted sub solidus history following magmatic crystallization. Equilibrium pressure–temperature estimates place these xenoliths at low-to-moderate pressure–temperature conditions (690–910°C and 2.0–4.5 GPa) in the lithospheric mantle at the time of entrainment in the kimberlite. However, reconstructed compositions of initial pyroxene and garnet crystals suggest that this suite of pyroxenites formed at considerably higher temperatures and pressures that, in some instances, may have approached the majorite stability field. Pyroxenites that do not contain primary garnet may have been derived from shallower depths.Progressive exsolution in these pyroxenites is of importance inasmuch as such processes can permit localized changes in rheological properties and may also accommodate strain within portions of lithospheric mantle. Because most xenolith studies focus on peridotites and eclogites, the pyroxenite sample suite studied in this work represents an important contribution towards a greater understanding of the Siberian lithospheric mantle.

In this study, we show how veined lithospheric mantle is involved in the genesis of ultrapotassic magmatism in cratonic settings. We conducted high pressure experiments to simulate vein + wall rock melting within the Earth's lithospheric... more

In this study, we show how veined lithospheric mantle is involved in the genesis of ultrapotassic magmatism in cratonic settings. We conducted high pressure experiments to simulate vein + wall rock melting within the Earth's lithospheric mantle by reacting assemblages of harzburgite and phlogopite-rich hydrous mantle xeno-liths. These comprised a mica-, amphibole-, rutile-, ilmenite-, diopside (MARID) assemblage at 3–5 GPa and 1325–1450 °C. Melting of the MARID assemblages results in infiltration of melt through the harzburgite, leading to its chemical alteration. At 3 and 4 GPa, melts are high in K 2 O (> 9 wt%) with K 2 O/Na 2 O > > 2 comparable to anorogenic lamproites. Higher pressures and temperatures (5 GPa/1450 °C) lead to increasing MgO contents of the melt and to some extent lower K 2 O contents (5–7 wt%) at equally high K 2 O/Na 2 O ratios. Our experiments provide insights into the role of alkalis in nickel-partitioning (D Ni) between olivine and ultrapotassic melt. We observe that the high contents of Na, K, and Al are indicative of high D Ni values, implying that the melt polymerization is the dominant factor influencing the olivine/melt nickel partitioning. The change of D Ni as a function of melt composition results in a pressure independent, empirical geothermometer: = − ⎛ ⎝ ⎜ ⎜ + ⎡ ⎣ + ⎤ ⎦ + ⎞ ⎠ ⎟ ⎟ + ° () liq liq liq T (C) 88.14·ln [((Na O K O)]· 1 ·[liq(Al O)] [ (MgO FeO)] ·D 1906.2 SiO TiO 2 2 2 2 2 3 Ni Element oxides represent the composition of the glass (in wt%), and D Ni is the liquid/olivine Ni-partitioning coefficient. We propose that this geothermometer is applicable to all natural silicate melts that crystallized olivine in a temperature interval between 1000 and 1600 °C. Application to glass-olivine pairs from calc-alkaline settings (Mexico), MORB (East Pacific Rise), and OIB (Hawaii) yielded reasonable values of 996–1199 °C, 1265 °C, and 1330 °C, respectively.

The compositional variation of plagioclase and the partitioning of major elements between plagioclase and melt have been experimentally measured as a function of the cooling rate. Crystals were grown from a basaltic melt at a pressure of... more

The compositional variation of plagioclase and the partitioning of major elements between plagioclase and melt have been experimentally measured as a function of the cooling rate. Crystals were grown from a basaltic melt at a pressure of 500 MPa under (i) variable cooling rates of 0.5, 2.1, 3, 9.4, and 15 °C/min from 1250 °C down to 1000 °C, (ii) quenching temperatures of 1025, 1050, 1075, 1090, and 1100 °C at the fixed cooling rate of 0.5 °C/min, and (iii) isothermal temperatures of 1000, 1025, 1050, 1075, 1090, and 1100 °C. Our results show that euhedral, faceted plagioclases form during isothermal and slower cooling experiments exhibiting idiomorphic tabular shapes. In contrast, dendritic shapes are observed from faster cooled charges. As the cooling rate is increased, concentrations of Al + Ca + Fe + Mg increase and Si + Na + K decrease in plagioclase favoring higher An and lower Ab + Or contents. Significant variations of pl–liqKd are also observed by the comparison between iso...

New mineral-melt thermobarometry and mineral chemistry data are presented for basaltic scoriae erupted from the Mbwelesu crater of Ambrym volcano, Vanuatu, during persistent lava lake activity in 2005 and 2007. These data reveal... more

New mineral-melt thermobarometry and mineral chemistry data are presented for basaltic scoriae erupted from the Mbwelesu crater of Ambrym volcano, Vanuatu, during persistent lava lake activity in 2005 and 2007. These data reveal crystallisation conditions and enable the first detailed attempt at reconstruction of the central magma plumbing system of Ambrym volcano. Pressures and temperatures of magma crystallisation at Ambrym are poorly constrained. This study focuses on characterising the magma conditions underlying the quasi-permanent lava lakes at the basaltic central vents, and examines petrological evidence for magma circulation. Mineral-melt equilibria for clinopyroxene, olivine and plagioclase allow estimation of pressures and temperatures of crystallisation, and reveal two major regions of crystallisation, at 24–29 km and 11–18 km depth, in agreement with indications from earthquake data of crustal storage levels at c. 25–29 km and 12–21 km depth. Temperature estimates are ~1150–1170 °C for the deeper region, and ~1110–1140 °C in the mid-crustal region, with lower temperatures of ~ 1090–1100 °C for late-stage crystallisation. More primitive plagioclase antecrysts are thought to sample a slightly more mafic melt at sub-Moho depths. Resorption textures combined with effectively constant mafic mineral compositions suggest phenocryst convec-tion in a storage region of consistent magma composition. In addition, basalt erupted at Ambrym has predominantly maintained a constant composition throughout the volcanic succession. This, coupled with recurrent periods of elevated central vent activity on the scale of months, suggest frequent magmatic recharge via steady-state melt generation at Ambrym.

During and aft er the closure of the Neo-Tethyan Ocean and progressive collision of the Tauride-Anatolide Platform with the Sakarya Continent, widespread magmatism occurred in NW Anatolia. Th is magmatism is manifested in a NW-trending... more

During and aft er the closure of the Neo-Tethyan Ocean and progressive collision of the Tauride-Anatolide Platform with the Sakarya Continent, widespread magmatism occurred in NW Anatolia. Th is magmatism is manifested in a NW-trending belt along the northern border of the Menderes Massif. Due to the complex geodynamic setting of this region, the exact emplacement depth of the granitoids is still a matter of debate. Here we present Al-in-hornblende barometrical data and Sr-Nd-Pb isotope compositions of the Early Miocene Alaçam granite. Th e results imply a shallow emplacement depth of this granite (4.7±1.6 km) in contrast to previous studies which suggested emplacement along the brittle-ductile boundary of the crust. Furthermore, an evaluation of literature data let us reconsider the general emplacement mechanism of the Alaçam and other Early Miocene granitoids in the region. Initial isotopic signatures of the Alaçam granite are 87 Sr/ 86 Sr(I)= 0.70865–0.70915, e Nd (I)= –5.8 to –6.4, δ 18 O= 9.5–10.5, 206 Pb/ 204 Pb isotope ratios vary between 18.87 and 18.90. Th ese features indicate an assimilation-dominated crustal crystallization and melt derivation from an older middle crustal protolith. Özet: Neo-Tetis okyanusunun kapanmasını izleyen Anatolid-Torid Platformu'nun Sakarya Kıtası ile progresif çarpışması sırasında ve sonrasında, KB Anadolu' da yaygın bir magmatizma meydana gelmiştir. Bu magmatizma, Batı Anadolu' da yer alan kuzey Menderes Masifi boyunca KB doğrultulu bir magmatik kuşak ortaya çıkarmaktadır. Bölgenin karmaşık jeodinamik evriminden dolayı, granitoidlerin yerleşim derinliği, halen tartışmalıdır. Bu çalışmada, Erken Miyosen yaşlı Alaçam granitinin Al-hornblend barometresi sonuçları ve Sr-Nd-Pb-O izotop bileşimleri sunulmaktadır. Elde edilen sonuçlar, Alaçam granitinin, önceki çalışmaların aksine, kabuğun sığ kesimlerinde (4.7±1.6 km) yerleştiğini ve önerilen kabuğun derin, elastik-plastik deformasyon sınırında gerçekleşen bir sokulum olmadığını belirtmektedir. Bununla beraber, literatürün yeniden değerlendirilmesiyle, Alaçam granitinin ve bölgedeki diğer Erken Miyosen granitoidlerinin yerleşim mekanizmasının yeniden incelenmesine neden olmuştur. Alaçam Graniti'nin birincil izotopik değerleri; 87 Sr/ 86 Sr(I)= 0.70865–0.70915, e Nd (I)= –5.8 to –6.4, δ 18 O= 9.5–10.5, 206 Pb/ 204 Pb= 18.87–18.90' dır. Bu izotop verileri asimilasyonun baskın olduğu bir kabuksal kristallenmeyi ve granitin daha yaşlı bir orta kabuk köken kayasından türediğini göstermektedir.

Abstract. This study provides data about the chemical composition of garnet and associated minerals from the quartz-mica schists occurring to the south of the village Oreschnik, Sakar Mountain, and coustrains the P - T conditions of... more

Abstract. This study provides data about the chemical composition of garnet and associated minerals from the quartz-mica schists occurring to the south of the village Oreschnik, Sakar Mountain, and coustrains the P - T conditions of metamorphism. The chemical composition of the following minerals have been studied by electron microprobe: garnet, biotite, staurolite, white mica, chlorite, plagioclase. The metamorphic grade is of lower amphibolite facies. The P - T estimate for a staurolite - garnet - white mica - biotite - chlorite - plagioclase schist is T = 6160 C, σ (T) = 21 and P = 7,8 kbar, σ (P) = 1.3.
Key words: garnet, amphibolite facies, metamorphism, Sakar Mountain.

The metamorphic basement (i.e. Palaeozoic micaschists, Porto Azzurro Unit) cropping out in the Mt. Calamita promontory, in the southeastern Elba Island, represents the roof of the mostly unexposed late Miocene Porto Azzurro granite. A... more

The metamorphic basement (i.e. Palaeozoic micaschists, Porto Azzurro Unit) cropping out in the Mt. Calamita promontory, in the southeastern Elba Island, represents the roof of the mostly unexposed late Miocene Porto Azzurro granite. A thermobarometric study was performed on micaschists and interlayered amphibolites to constrain P-T conditions during contact metamorphism. Adopted methods were based on P-T grids, conventional thermobarometry and analysis of uid inclusions. It was found that the basement was originally located at a depth of 6.5 ± 1.3 km and that the peak metamorphic conditions were close to the muscovite dehydration reaction in micaschist. Therefore, small differences of temperatures conditioned the presence or absence of K-feldspar in the eastern and western side of the Mt. Calamita promontory, respectively. A maximum peak temperature of c. 625 °C was estimated by amphibole-plagioclase thermometry in amphibolite. Fluid inclusions in micaschist quartz lenses document signi cant episodes of uid circulation during the cooling history of the metamorphic aureole. Finally, a conductive thermal model was built up to reproduce the thermal evolution during the contact metamorphic event and following cooling. Simulated temperatures resulted signi cantly lower than the observed peak values. This discrepancy is explained as the effect of the localized heat advection in zones affected by in ltration of magmatic uids and by the intrusion of numberless felsic dykes.

The compositional variation of clinopyroxene and the partitioning of major elements between clinopyroxene and melt are estimated as a function of the cooling rate. Clinopyroxenes were crystallized under variable cooling regimes (15, 9.4,... more

The compositional variation of clinopyroxene and the partitioning of major elements between clinopyroxene and melt are estimated as a function of the cooling rate. Clinopyroxenes were crystallized under variable cooling regimes (15, 9.4, 3, 2.1, and 0.5°C/min from 1250 down to 1000°C) and at isothermal conditions of 1000°C from a basaltic composition at a pressure of 500MPa under anhydrous and hydrous (H2O=1.3wt.%) conditions. The clinopyroxene chemistry shows that, as the cooling rate increases, crystals are progressively depleted in Ca, Mg, Fe2+ and Si and enriched in Na, Fe3+, Al (mainly AlIV), and Ti. Di and Hd versus CaTs and CaFeTs form a continuous binary solid solution characterized by higher amounts of tschermakitic components with increasing cooling rate. Two parameters (DH=Di+Hd and TE=CaTs+CaFeTs+En) are calculated to describe the effect of cooling rate on the clinopyroxene composition. The variation of DH/TE with increasing cooling rate evidences the kinetic process ind...