Plagioclase Hosted Melt Inclusion in Hypabyssal Rocks in Torud-Ahmad Abad Magmatic Belt

This study investigates for the first time melt inclusions (MI) that are found within fundamental minerals of subvolcanic rocks in Torud-Ahmad Abad magmatic belt. The Torud-Ahmad Abad magmatic belt is situated in south-southeast of Shahrood and belongs to the northern part of central Iran structural zone. Melt inclusions represent liquids that were trapped along growth zones (primary) or healed fractures of mineral phases, which crystallized from the silicate liquid as it cooled. Based on SEM analysis of these melt inclusions, their compositions are dacite, andesite and basaltic andesite. Thus, with the use of melt inclusions in the volcanic rocks of Torud-Ahmad Abad magmatic belt, we attempt to show the compositional evolution and origin of magma. The effective factors on magma evolution are magma mixing, fractional crystallization and crustal contamination.

Download Full-text

Relative roles of source composition, fractional crystallization and crustal contamination in the petrogenesis of Andean volcanic rocks

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1984.0014 ◽

1984 ◽

Vol 310 (1514) ◽

pp. 675-692 ◽

Cited By ~ 114

Keyword(s):

Subduction Zone ◽

Continental Crust ◽

Fractional Crystallization ◽

Magma Mixing ◽

Volcanic Rocks ◽

Crustal Contamination ◽

Oceanic Lithosphere ◽

Alkaline Basalt ◽

Complex Process ◽

Heterogeneous Mantle

There are well established differences in the chemical and isotopic characteristics of the calc-alkaline basalt—andesite-dacite-rhyolite association of the northern (n.v.z.), central (c.v.z.) and southern volcanic zones (s.v.z.) of the South American Andes. Volcanic rocks of the alkaline basalt-trachyte association occur within and to the east of these active volcanic zones. The chemical and isotopic characteristics of the n.v.z. basaltic andesites and andesites and the s.v.z. basalts, basaltic andesites and andesites are consistent with derivation by fractional crystallization of basaltic parent magmas formed by partial melting of the asthenospheric mantle wedge containing components from subducted oceanic lithosphere. Conversely, the alkaline lavas are derived from basaltic parent magmas formed from mantle of ‘within-plate’ character. Recent basaltic andesites from the Cerro Galan volcanic centre to the SE of the c.v.z. are derived from mantle containing both subduction zone and within-plate components, and have experienced assimilation and fractional crystallization (a.f.c.) during uprise through the continental crust. The c.v.z. basaltic andesites are derived from mantle containing subduction-zone components, probably accompanied by a.f.c. within the continental crust. Some c.v.z. lavas and pyroclastic rocks show petrological and geochemical evidence for magma mixing. The petrogenesis of the c.v.z. lavas is therefore a complex process in which magmas derived from heterogeneous mantle experience assimilation, fractional crystallization, and magma mixing during uprise through the continental crust.

Download Full-text

On the fO2 of slab fluids in subduction zone systems

10.5194/egusphere-egu2020-13454 ◽

2020 ◽

Author(s):

Timm John ◽

Esther Schwarzenbach ◽

Jay Ague ◽

Jilei Li

Keyword(s):

Fluid Flow ◽

Subduction Zone ◽

Melt Inclusions ◽

Melt Inclusion ◽

Volcanic Rocks ◽

Ore Deposits ◽

Molybdenum Isotope ◽

Element Cycling ◽

Plate Interface ◽

Rock Interaction

One of the most pressing riddles of the subduction cycle to be solved is linked to the fO2 of the slab-released fluids. It is now well accepted that the fluids liberated during slab dehydration play a crucial role in subduction zone seismicity, element cycling, and arc magmatism. However, whether these slab fluids are oxidizing or reducing transport agents is poorly understood and thus, there is still a lot we need to understand. This is of particular importance for gaining a mechanistic view on the formation processes of economically important arc related ore deposits, which certainly require understanding of the behavior of redox sensitive mobilities of the relevant elements. In brief, while some field-based studies from the slab perspective are suggesting rather reduced conditions (e.g., based on sulfides and sulfur isotope work, ref. 1) others, mainly related to higher temperature systems (e.g., based on bulk-rock &#8211; rutile systems and molybdenum isotope work, ref. 2), are indicative of more oxidizing slab fluids. Especially for m&#233;lange-like structures developed at the plate interface, studies on sulfur-bearing minerals result in contrasting fO2 of the related slab fluids (ref. 3 vs ref. 4). It appears that at least during retrogression along the plate interface the reactively flowing fluids tend to have a more oxidizing potential (ref. 5). Interestingly, the prime fluid source of subducting slabs, i.e. dehydrating slab mantle serpentinites, is thought to release reduced fluids (ref. 6) but melt inclusions in arc volcanic rocks are often oxidized. Recent studies suggest that this is likely linked to fluid-rock interaction at local scales (ref. 7) and/or possibly within the magma reservoirs that comprise rather low-melt-fraction mush (ref. 8). This in turn would suggest that the slab fluids might change their fO2 during reactive intra-slab fluid flow, or would not need to be oxidized prior to melt inclusion entrapment and that the oxidizing potential of the fluids may be the result of magmatic processes during melt ascent in the arc. In this contribution we review the current state of knowledge, provide new ideas and models regarding channelized though reactive intra-slab fluid flow, and illustrate the next steps to unravel this exiting and thus far poorly understood topic of subduction zone element cycling.&#160;1]&#160;&#160;&#160;&#160;&#160;&#160;&#160; Li, J.-L., et al. (2020). Nature Communications. https://doi.org/10.1038/s41467-019-14110-42]&#160;&#160;&#160;&#160;&#160;&#160;&#160; Chen, S., et al. (2019). Nature Communications. http://doi.org/10.1038/s41467-019-12696-33]&#160;&#160;&#160;&#160;&#160;&#160;&#160; Schwarzenbach, E.M., et al. (2018). Scientific Reports 8, 15517.4] &#160;&#160;&#160;&#160;&#160;&#160; Walters, J. B., et al. (2019). Geochemistry Geophysics Geosystems, 286, 185&#8211;28. http://doi.org/10.1029/2019GC0083745] &#160;&#160;&#160;&#160;&#160;&#160; Li, J.-L., et al. (2016). Contributions to Mineralogy and Petrology, 171:72. http://doi.org/10.1007/s00410-016-1284-26]&#160;&#160;&#160;&#160;&#160;&#160;&#160; Piccoli, F., et al. (2019). Scientific Reports, 1&#8211;7. http://doi.org/10.1038/s41598-019-55944-87]&#160;&#160;&#160;&#160;&#160;&#160;&#160; Tollan, P. & Hermann, J. (2019). Nature Geoscience 12, 667&#8211;671.8]&#160;&#160;&#160;&#160;&#160;&#160;&#160; Jackson, M. D., et al. (2018). Nature, 564, 405&#8211;409. http://doi.org/10.1038/s41586-018-0746-2&#160;

Download Full-text

LA-ICP-MS analysis of crystallized melt inclusions in olivine, plagioclase, apatite and pyroxene: quantification strategies and effects of post-entrapment modifications

Journal of Petrology ◽

10.1093/petrology/egaa085 ◽

2020 ◽

Author(s):

Jia Chang ◽

Andreas Audétat

Keyword(s):

Inductively Coupled Plasma ◽

Melt Inclusions ◽

Melt Inclusion ◽

Volcanic Rocks ◽

Internal Standard ◽

Spot Size ◽

Slow Cooling ◽

Inductively Coupled ◽

Ms Analysis ◽

Icp Ms

Abstract Melt inclusions represent a unique tool to reconstruct the composition and chemical evolution of silicate melts in magmatic systems. Laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS) is the most commonly used microanalytical technique to analyze crystallized melt inclusions without prior re-homogenization. Well-preserved melt inclusions can be quantified by subtracting the contribution of co-ablated host with a carefully selected internal standard. However, post-entrapment compositional re-equilibration commonly renders this task difficult, to the same degree as it would affect any quantification after prior re-homogenization. In this study, we first examine well-preserved, crystallized melt inclusions hosted in olivine, plagioclase, apatite, clinopyroxene and orthopyroxene from porphyry dikes and volcanic rocks to test various quantification strategies and evaluate the associated uncertainties, and then we use these strategies to quantify coarsely crystallized melt inclusions from gabbroic rocks at Marble Canyon (USA) and Laiyuan (China) that experienced severe post-entrapment modifications due to relatively slow cooling rates. The results demonstrate that even for well-preserved melt inclusions hosted in chemically complex minerals the uncertainty related to inclusion–host deconvolution can be rather high (up to 30‒50% for host-incompatible trace elements significantly above their limits of detection), though other uncertainties inherent to LA-ICP-MS analysis are relatively small (typically ≤5‒10%). The deconvolution-related uncertainty can be minimized to ca. 10% by (i) choosing whole rocks that are fresh and representative of magmatic liquids, (ii) choosing the smallest possible spot size to ablate the melt inclusions, and (iii) choosing a host endmember that is compositionally as similar as possible to the one ablated together with the inclusion. Results of coarsely crystallized melt inclusions from gabbroic rocks suggest that the range of elements affected by post-entrapment re-equilibration varies from intrusion to intrusion. Olivine-hosted melt inclusions from Marble Canyon appear to have diffusively lost Fe, Ti and Ca, whereas those from Laiyuan lost Fe, Na, Al, Ca, Ti and Y and gained V. However, the relative abundances of K, P, Rb, Sr, Zr, Nb, Mo, Cs, Ba, Ce, Ta, Pb, Th, U and ±Cu appear unchanged. Plagioclase-hosted melt inclusions from Marble Canyon are relatively well-preserved, whereas those from Laiyuan lost significant amounts of Fe, K, Mg, Mn, Rb and Co. Apatite-hosted melt inclusions seem well preserved with regard to most elements except for Cu. These results suggest that despite the post-entrapment modification of certain element concentrations and the associated difficulties in melt inclusion quantification, information on the approximate abundances of other elements that are invaluable for petrogenetic and metallogenic studies can still be retrieved from melt inclusions in gabbroic rocks using the LA-ICP-MS technique.

Download Full-text

Geochemical Insights from Clinopyroxene Phenocrysts into the Magma Evolution of an Alkaline Magmatic System from the Sanshui Basin, South China

Minerals ◽

10.3390/min11111295 ◽

2021 ◽

Vol 11 (11) ◽

pp. 1295

Author(s):

Peijia Chen ◽

Nianqiao Fang ◽

Xiaobo Yuan

Keyword(s):

Fractional Crystallization ◽

South China ◽

Magma Mixing ◽

Volcanic Rocks ◽

Magma Ascent ◽

The South ◽

Magma Evolution ◽

Magma Plumbing System ◽

Magma System ◽

Structural Levels

The Sanshui Basin is located at the northern continental margin of the South China Sea and characterized by a continental rift basin. The bimodal volcanic rocks in Sanshui Basin record the early Cenozoic magmatic activity in the South China Block, but the magmatic evolution that produced the bimodal volcanic rocks is poorly understood. Clinopyroxenes in bimodal volcanic rocks in the Sanshui Basin provide an opportunity to investigate magma during magma ascent. In this work, we classified nine types of clinopyroxene phenocrysts according to composition and texture in cogenetic basalt-trachyandesite-comenditic trachyte, while the composition of unzoned clinopyroxene have an evolution sequence of diopside-hedenbergite-aegirine along with an increase in trace element contents with a decrease of Mg#, indicating that the genesis of clinopyroxene was dominated by fractional crystallization in a closed magma system. However, the clinopyroxenes with reversed zoning and multiple zoning record the process of magma mixing and recharge indicating an open magma system. While fractional crystallization is the dominant process, magma mixing, recharge, and crystal settling were also found to influence magma evolution. Thermobarometric calculations showed that clinopyroxene crystallized a several structural levels in the crust during magma ascent. In this study, we established a magma plumbing system that provides new constraints for the magma evolution in the Sanshui Basin.

Download Full-text

The Magmatic to Magmatic-Hydrothermal Evolution of the El Laco Deposit (Chile) and Its Implications for the Genesis of Magnetite-Apatite Deposits

Economic Geology ◽

10.5382/econgeo.2017.4523 ◽

2017 ◽

Vol 112 (7) ◽

pp. 1595-1628 ◽

Cited By ~ 24

Author(s):

Fernando Tornos ◽

Francisco Velasco ◽

John M. Hanchar

Keyword(s):

Melt Inclusions ◽

Volcanic Rocks ◽

Significant Proportion ◽

Crustal Contamination ◽

Central Andes ◽

Coarse Grained ◽

Primitive Mantle ◽

Low Density ◽

Magnetite Ore ◽

Field Evidence

Abstract The geology and geochemistry of the El Laco iron oxide deposit (Central Andes, Chile) support a genesis related to the ascent, degassing, and subvolcanic emplacement of an unusual oxidized silica-poor but water-and iron-rich melt that took place during the growth of the host Pliocene-Holocene andesitic volcano. The model proposed in this paper for the evolution of the deposit involves the formation of a shallow telescoped magmatichydrothermal system with complex melt-fluid unmixing in a vertical column of less than 1 km. The dominant mineralization occurs as large stratabound apatite-poor magnetite bodies interfingered with an andesite host and rooted in vertical dikes of magnetite with minor apatite. The stratabound mineralization is crosscut by abundant coeval diatreme-like structures indicative of vigorous degassing. The andesite underlying the mineralization is pervasively replaced by a high-temperature alkali-calcic alteration assemblage (K feldspar-diopside-magnetite-scapolite) that includes coarse-grained diopside-magnetite-anhydrite veins and large subvertical bodies of magmatic-hydrothermal breccias. The host andesite also shows a large strata-bound steam-heated acid alteration that is devoid of any magnetite but has produced the replacement of a significant proportion of the early magnetite by hematite. The El Laco system is rich in anhydrite but poor in sulfides, suggesting that there were persistent oxidizing conditions that inhibited the formation of a sulfide-bearing mineralization. Field evidence, oxygen isotope geothermometry, and thermodynamic constraints suggest that the magnetite mineralization formed close to the surface at temperatures above 800°C. The magnetite textures, similar to those of subaerial low-viscosity basalts, and the presence of melt inclusions in the host andesite recording the presence of immiscible Fe-Mg-Ca-(Si-Ti-P-S) and Si-K-Na-Al melts, suggest that the magnetite ore formed by direct crystallization from an iron-rich melt; its chemistry inhibited the formation of most other magmatic phases except minor apatite, anhydrite, and diopside. The crystallization of the iron-rich melt at shallow depths promoted the separation of large amounts of two immiscible aqueous fluids: a dominant low-density vapor phase and a small volume of hypersaline fluid. Diopside-magnetite-anhydrite veins are interpreted as the product of the crystallization of the residual melts, whereas the interaction of the brine with the host andesite formed the deep alkali-calcic hydrothermal assemblage. The condensation and mixing of the low-density magmatic vapor with meteoric water produced the steam-heated alteration. Isotope data from the host andesite (87Sr/86Sr: 0.7066–0.7074; εNd: −5.5 to −4.1; δ18Owhole rock: 7.2–9.6‰; δ18Omagnetite: 5.1–6.2‰) and an underlying andesite porphyry (87Sr/86Sr: 0.7075–0.7082; εNd: −5.9 to −4.6) reflect the interaction of a primitive mantle melt with Andean crustal rocks. The isotope geochemistry of the magnetite ore (87Sr/86Sr: 0.7083; εNd: −5.4 to −5.1; δ18O 3.5–5.5‰) and the alkali-calcic alteration and related diopside-magnetite-anhydrite veins (87Sr/86Sr: 0.7080–0.7083; εNd: −5.1 to −4.6; δ18Odiopside: 7.2–8.2%c; δ18Omagnetite 4.4–6.3‰) show that the mineralization has a more crustal signature than the host andesite and all the volcanic rocks of the Central Andes. Therefore, ore-forming fluids/melts were not equilibrated with the host volcanic rocks and are interpreted as related to a deep yet undiscovered batch of highly contaminated igneous rocks. Crustal contamination is interpreted as due to major interaction of a juvenile melt with the underlying Late Mesozoic-Tertiary Salta Group, located 1 to 6 km beneath the volcano and which has high 87Sr/86Sr values (0.7140–0.7141).

Download Full-text

STUDYING THE GENESIS OF IGNEOUS ROCKS IN ZARIN-KAMAR REGION (SHAHROOD, NORTHEASTERN IRAN) BY RARE EARTH ELEMENTS

GÃªnero & Direito ◽

10.22478/ufpb.2179-7137.2019v8n4.48442 ◽

2019 ◽

Vol 8 (4) ◽

Author(s):

Ebrahim Nazemi ◽

Mohammad-Ali Arian ◽

Abdolreza Jafarian ◽

Mohsen Pourkermani ◽

Abdollah Yazdi

Keyword(s):

Rare Earth ◽

Total Concentration ◽

Volcanic Rocks ◽

Crustal Contamination ◽

Oceanic Island ◽

Igneous Rocks ◽

Magma Evolution ◽

North East ◽

Enriched Mantle ◽

Island Basalt

Zarin-Kamar region is located north east of Shahrood (36o37’-36o42’N, 55007’-55012’E). Plutonic rocks in this area belongs to syenite group and their texture is intergranular hypidiomorphic. Volcanic rocks in the area have porphyritic, amygdale intersertal texture. which quartz is also seen among their cavities and porosities. Total concentration of REEs (ΣREE) in the study igneous rocks varies between 450 and 683 ppm. Diagram of Eu/Eu* versus Sr and Eu/Eu* versus Ba show negative anomalies of Eu. This phenomenon as well as Ba and Sr trends show that plagioclase removal has happened during the magma evolution. These rocks have rock has originated from an enriched mantle source. The rate of Dy/Yb in the igneous rocks of the region varies between 1.32 and 2.62. it shows that it stemmed from a garnet lherzolitic source. The rate of (Tb /Yb )N was between 0.97 to 2.25 showing a garnet source. Also other related figures showed that the samples belonged to OIB (Oceanic Island Basalt). The rate of La/Ta was between 6.6 to 14.01. It also showed that they had a source from asthenosphere. The rate of La/Nb was 0.5 to 0.91. It also shows a less crustal contamination among these samples.

Download Full-text

Magmatic Processes Associated with Oceanic Crustal Accretion at Slow-spreading Ridges: Evidence from Plagioclase in Mid-ocean Ridge Basalts from the South China Sea

Journal of Petrology ◽

10.1093/petrology/egz027 ◽

2019 ◽

Vol 60 (6) ◽

pp. 1135-1162 ◽

Cited By ~ 3

Author(s):

Fan Yang ◽

Xiao-Long Huang ◽

Yi-Gang Xu ◽

Peng-Li He

Keyword(s):

Melt Inclusions ◽

Melt Inclusion ◽

Magma Mixing ◽

Pillow Lava ◽

Crustal Accretion ◽

Spreading Ridges ◽

Mid Ocean Ridge ◽

Magmatic Processes ◽

Slow Spreading ◽

Ocean Ridge

Abstract Magmatic processes associated with oceanic crustal accretion at slow-spreading mid-oceanic ridges are less well understood compared with those at fast-spreading ridges. Zoned plagioclase in the basalts might record these magmatic processes as a result of the very slow intra-crystal diffusion of CaAl–NaSi. Plagioclase phenocrysts in plagioclase-phyric basalt from Hole U1433B of International Ocean Discovery Program (IODP) Expedition 349 in the South China Sea show complex zoning patterns (e.g. normal, reverse, oscillatory and patchy). These samples provide a rare opportunity to determine the magma dynamics associated with oceanic crustal accretion at slow-spreading ridges through time. Igneous lithological units in Hole U1433B consist of a series of massive lava flows at the bottom and a thick succession of small pillow lava flows at the top. Most of the plagioclase phenocrysts in the massive lava show core–rim zonation with high-An cores (An ∼85%; in mole fraction; Pl-A) in equilibrium with melts that are more primitive than their host. Some high-An cores of Pl-A phenocrysts contain melt inclusions and are depleted in La, Ce, Y and Ti, but enriched in Sr and Eu; this is interpreted as resulting from dissolution–crystallization processes during reaction of hot melt with pre-existing plagioclase cumulates. In the pillow lavas, most of the plagioclase phenocrysts show normal core–mantle–rim zonation (Pl-B) with An contents decreasing gradually from the core to the mantle to the rim, suggesting extensive magma mixing and differentiation. Reversely zoned plagioclases (Pl-C) are sparsely present throughout the basalts, but mostly occur in the lower part of the drill hole. The cores of euhedral Pl-C phenocrysts are compositionally comparable with the mantles of Pl-B phenocrysts, suggesting that the evolved magma was recharged by a relatively primitive magma. Melt inclusion-bearing Pl-A phenocrysts occur mainly in the massive lava, but rarely in the pillow lava, whereas Pl-B phenocrysts are present dominantly in the pillow lava, which reflects reducing melt–rock interaction and enhanced magma mixing, recharging and differentiation from the bottom to the top of the hole. In addition, the extensive magma mixing and differentiation recorded by Pl-B phenocrysts in the pillow lava require the existence of a melt lens beneath the mid-ocean ridge. Consistently, the plagioclase phenocrysts in the pillow lava mostly lack melt inclusions, corresponding to very weak melt–rock reactions, which indicates that the magma was transported through plagioclase cumulates by channel flow and requires a higher magma supply to the magma conduit. Therefore, the textural and compositional variations of plagioclase phenocrysts in the samples reflect the changes in magma dynamics of the mid-ocean ridge basalt through time with respect to oceanic crustal accretion at slow-spreading ridges. Overall, the oceanic crustal accretion process is sensitive to the magma supply. In the period between two episodes of extension, owing to a low melt supply the primitive melt percolates through and interacts with the mush zone by porous flow, which produces melt inclusion-bearing high-An plagioclase through dissolution–crystallization processes. At the initial stage of a new episode of extension, the melt infiltrates the mush zone and entrains crystal cargoes including melt inclusion-bearing high-An plagioclase. During the major stage of extension, owing to a relatively high melt supply the melt penetrates the mush zone by channel flow and can pool as melt lenses somewhere beneath the dikes; this forms intermediate plagioclases and the reverse zoning of plagioclases by magma mixing, recharging and differentiation in the melt lens. Such magmatic processes might occur repeatedly during the episodic extension that accompanies oceanic crustal accretion at slow-spreading ridges, which enhances the lateral structural heterogeneity of the oceanic crust.

Download Full-text

Geochemical Insights From Clinopyroxene Phenocrysts Into the Magma Evolution of an Alkaline Magmatic System From the Sanshui Basin, South China

10.20944/preprints202111.0046.v1 ◽

2021 ◽

Author(s):

Peijia Chen ◽

Nianqiao Fang ◽

Xiaobo Yuan

Keyword(s):

Fractional Crystallization ◽

South China ◽

Magma Mixing ◽

Volcanic Rocks ◽

Magma Ascent ◽

Magmatic Evolution ◽

The South ◽

Magma Evolution ◽

Magma Plumbing System ◽

Magma System

The Sanshui Basin (SSB) is located at the northern continental margin of the South China Sea and characterized by a continental rift basin. The bimodal volcanic rocks in SSB record the early Cenozoic magmatic activity in the South China Block, on the magmatic evolution process of bimodal volcanic rocks are poorly understood. Clinopyroxenes in bimodal volcanic rocks in the SSB provide an opportunity to investigate the magma process during magma ascent. We classified nine types of clinopyroxene phenocrysts according to the different compositions and textures types in cogenetic basalt-trachyandesite-comenditic trachyte, the composition of unzoned clinopyroxene have an evolution sequence of diopside- hedenbergite- aegirine with the decrease of Mg#, and the trace element contents of unzoned clinopyroxenes also increase systematically during magma evolution, indicating that the genesis of clinopyroxene dominated by fractional crystallization in a closed magma system; however, the clinopyroxenes with reverse zoning and multiple zoning record the process of magma mixing and recharge indicating an open magma system. Whilst fractional crystallization is the dominated process, magma mixing, recharge, and crystal settling complicate magma evolution. Thermobarometric calculations show that clinopyroxene phenocrysts in bimodal volcanic rocks of SSB are distributed in the whole crust during magma ascent. We have established a magma plumbing system, which provides a new constrain for the complex magmatic evolution history in the SSB by detailed mineral-scale analysis.

Download Full-text