scholarly journals The formation of giant podiform chromitite by asthenospheric melts in supra-subduction zone environments

2021 ◽  
Author(s):  
Tao Ruan ◽  
Hong Zhong ◽  
Jianming Zhu ◽  
Zhong-Jie Bai
Keyword(s):  
Mass Balance ◽  
Subduction Zone ◽  
Fractional Crystallization ◽  
The Other ◽  
Chromite Deposit ◽  
Hydrous Melting ◽  
Basaltic Melt ◽  
Mantle Harzburgite ◽  
Podiform Chromitite ◽  
Yarlung Zangbo Suture Zone

Abstract Podiform chromitite hosted in supra-subduction zone (SSZ) ophiolite accounts for a substantial proportion of the global chromium supply market. However, there is no consensus regarding the specific processes involved in the source and formation of this chromium. It seems unlikely that fractional crystallization of basaltic melt or the boninitic melt–mantle harzburgite reaction could provide such huge amounts of chromium given the constraints of Cr mass balance. Here we identify two specific melts responsible for the formation of the typical ophiolite-related Luobusa chromite deposit in the Yarlung–Zangbo Suture Zone in Tibet, China. One is Cr-rich melt derived from the deep asthenosphere, and the other is boninitic melt generated by hydrous melting of previously depleted peridotites. We propose that the Luobusa podiform chromitite was produced through mixing of these two melts, of which the primitive asthenospheric Cr-rich melt provided huge amounts of Cr, and the introduction of boninitic magma triggered the crystallization of chromite. The findings of this study are important for understanding the genesis of global podiform chromite deposits hosted in SSZ ophiolite.

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

1984 ◽  
Vol 310 (1514) ◽  
pp. 675-692 ◽  
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.

Targeting the hidden chromitite using geochemical vectoring for Bophivum area, northwestern Myanmar

2020 ◽  
Author(s):  
Chulho Heo ◽  
Ilhwan Oh ◽  
Seokjun Yang ◽  
Jaeho Lee ◽  
Sungwon Park ◽  
...  
Keyword(s):  
Chemical Composition ◽  
Chromian Spinel ◽  
Melting Points ◽  
Portable Xrf ◽  
Al Content ◽  
Genetic Process ◽  
Ore Body ◽  
Reaction Result ◽  
Mantle Harzburgite ◽  
Podiform Chromitite

<p>Harzburgite are the rocks that make up the mantle and consist of olivine, orthopyroxene, and clinopyroxene (<5 %). Clinopyroxene contain Ca, Al, and Ti, while orthopyroxene contain Al. On the other hand, olivine contains almost zero contents of Ca, Al and Ti. When the rising melt from the lower mantle passes through the mantle harzburgite, the clinopyroxene and orthopyroxene with lower melting points compared with olivine are fused into the melt, and the olivine is crystallized from the melt. In this genetic process, harzburgite gradually change into dunite consisting of only olivine, and Ca, Al and Ti of pyroxene in harzburgite will escape into the melt. And, as the melting point of clinopyroxene is lower than that of orhopyroxene, the Ca, Al, and Ti in clinopyroxene are escaped into the melt earlier than those in orthopyroxene. The melt with changed composition formed by melting the pyroxene are mixed with the newly rising melt with pyroxene, so that the chromian spinel in the melt becomes saturated and the chromitite are formed. By the above-mentioned mechanism, chromitite occurs with dunite and pyroxene-deficient harzburgite formed by the reaction result between melt and harzburgite. In other words, in the genetic process of high Cr chromitite, the presence of melt that fused the pyroxene within harzburgite is essential. And, in order to make high Cr chromitite, the melt must have been fused more pyroxene in harzburgite. As a result, the Ti, Ca, and Al content of harzburgite will be decreased. Therefore, considering the representative chemical composition of podiform chromitite(Robinson et al., 1997), we assumed that as we approached into harzburgite bearing high Cr chromitite(probably hidden ore body), the Ti, Ca and Al content within harzburgite will be likely to converge toward the specific contents(Ti<180ppm, Ca<0.9%, Al<0.7%). In case of Bophivum chromitite in northwestern Myanmar, it corresponds well with the representative chemical composition of high Cr chromitite in terms of the above-mentioned data. Therefore, we monitored to see whether Ti, Ca, and Al contents systematically change by the distance from the center with chromitite outcrop or high Cr anomaly zone confirmed through soil and rock geochemical exploration toward the surrounding harzburgite outcrop or not and tried to select the target element for geochemical vectoring using portable XRF. Conclusively, Ca is considered to be a more meaningful geochemical vectoring indicator than Al in terms of portable XRF measurements in the survey area.</p>

Cryptic variation in the Rhum layered intrusion

1978 ◽  
Vol 42 (323) ◽  
pp. 347-356 ◽  
Author(s):  
A. C. Dunham ◽  
W. J. Wadsworth
Keyword(s):  
Magma Chamber ◽  
Fractional Crystallization ◽  
Electron Microprobe ◽  
Magma Mixing ◽  
Layered Intrusion ◽  
The Other ◽  
Layered Series ◽  
Cryptic Variation ◽  
High Calcium

SummaryElectron-microprobe analyses of cumulus olivine, chromite, pyroxene, and plagioclase from layered peridotites and allivalites of the Eastern and Western Layered Series of Rhum demonstrate the presence of cryptic variation. Olivine varies from Fo88-78 within individual units, and there are corresponding changes in the Mg/(Mg+Fe2+) ratios in the pyroxenes and chromites. Plagioclase changes are not so dramatic, but the An-content broadly follows the Mg/(Mg+Fe2+) ratio in the other minerals. The most Fe-(and Na-) rich phases do not occur at the top of lithological units, but some way below. The composition trend above them is reversed. The data are interpreted as the result of periodic infilling of a magma chamber, the new magma mixing with the remains of the previous pulse. Each pulse was followed by a period when fractional crystallization produced the layered rocks. New data on Ni in the olivines suggests that the ratio of the volume of initial magma to volume of layered rocks was about four to one, the initial magma being allied to the high-calcium low-alkali tholeiitic basalts of Skye.

scholarly journals Dokriani glacier(Dingarh glacier) and its geological analysis

2018 ◽  
Vol 6 (1) ◽  
Author(s):  
Tanuj Shukla ◽  
Sanjay Shukla
Keyword(s):  
Mass Balance ◽  
Garhwal Himalaya ◽  
The Other ◽  
Negative Mass ◽  
Environmental System ◽  
Terminal Moraine ◽  
Environmental Processes ◽  
Global Environmental ◽  
Five Phases ◽  
Geomorphological Analysis

The study of geomorphological system not only gives information about environmental processes operating there but also relate them whith global environmental system. The geomorphological analysis of Dokriani glacier, Garhwal Himalaya shows five phases of glacial advancement and retreat in the form of well preserved lateral and terminal moraines. The observed retreat rate of glacier in last two decades is about 17.2 m/yr which represents its negative mass balance followed by change in snout position, area and surface height. The farthest glacier expansion of the valley represented as terminal moraine is situated at 8.3 km from present day snout. Whereas, the other sucsessive glacial stages has followed the similar fashion of glacial advancement due to climatic sensitiveness.

Trondhjemitic and granitic melts formed by fractional crystallization of an olivine tholeiite from Reykjanes Peninsula, Iceland

2005 ◽  
Vol 142 (6) ◽  
pp. 651-658 ◽  
Author(s):  
E. MARTIN ◽  
O. SIGMARSSON
Keyword(s):  
Continental Crust ◽  
Fractional Crystallization ◽  
Significant Role ◽  
Major Element ◽  
Low Pressure ◽  
The Other ◽  
Olivine Tholeiite ◽  
Host Lava ◽  
Reykjanes Peninsula ◽  
Silicic Magmas

A pair of samples, from host lava and an included segregation vein from the Reykjanes Peninsula, Iceland, allows the assessment of a complete fractional crystallization of an olivine tholeiite at low pressure. The final product consists of silicic glasses with bimodal composition: trondhjemitic and more rarely granitic. Compilation of data on major element compositions of Icelandic silicic rocks reveals a clear difference from those of the segregation glasses. Fractional crystallization of basalts at low pressure is therefore not the most likely mechanism for the origin of silicic magmas in Iceland. Similar conclusions have been reached in studies on O- and Th-isotope compositions. On the other hand, the trondhjemitic compositions of the glasses in the segregation vein from Reykjanes Peninsula suggest that fractional crystallization of olivine tholeiites could have played a significant role during the formation of the very early continental crust.

scholarly journals Evaluation of Seasonal and Synoptic Changes in Snow Accumulation in Antarctica between Five Reanalyses Products and In Situ Observations

Atmosphere ◽  
2018 ◽  
Vol 9 (12) ◽  
pp. 473
Author(s):  
Yihui Liu ◽  
Fei Li ◽  
Weifeng Hao
Keyword(s):  
Mass Balance ◽  
Snow Accumulation ◽  
The Other ◽  
Surface Mass Balance ◽  
Surface Mass ◽  
In Situ Observations ◽  
High Precipitation Events ◽  
High Precipitation ◽  
Precipitation Events

The performance of recent reanalysis products (i.e., ERA-Interim, NCEP2, MERRA, CFSR, and JRA-55) was evaluated based on in situ observations from nine automatic weather stations and one stake network to investigate the monthly and seasonal variability of the surface mass balance in Antarctica. Synoptic precipitation simulations were also evaluated by an investigation of high precipitation events. The seasonal variations showed large fluctuations and were inconsistent at each station, probably owing to the large interannual variability of snow accumulation based on the short temporal coverage of the data. The ERA-Interim and JRA-55 datasets revealed better simulated precision, with the other three models presenting similar simulations at monthly and seasonal timescales. The JRA-55 dataset captured a greater number of synoptic high precipitation events at four of the nine stations. Such events at the other five stations were mainly captured by ERA and CFSR. The NCEP2 dataset was more weakly correlated with each station on all timescales. These results indicate that significant monthly or seasonal correlations between in situ observations and the models had little effect on the capability of the reanalyses to capture high precipitation events. The precision of the five reanalysis datasets widely fluctuated in specific regions or at specific stations at different timescales. Great caution is needed when using a single reanalysis dataset to assess the surface mass balance over all of Antarctica.

scholarly journals Spatial Patterns of Mass-balance Fluctuations of North American Glaciers

1989 ◽  
Vol 35 (120) ◽  
pp. 163-168 ◽  
Author(s):  
Anne Letrèguilly ◽  
Louis Reynaud
Keyword(s):  
Principal Component Analysis ◽  
Mass Balance ◽  
Spatial Patterns ◽  
North American ◽  
Principal Component ◽  
The Other ◽  
Time Dependent ◽  
Climatic Variations ◽  
Common Signal

Abstract Long-term records (10–30 years of measurements) of North American glaciers are compared using Lliboutry’s simplified linear model. This model separates the mass balance into two additive terms, one dependent on the location of the glacier and the other on time. The time-dependent term provides a common signal for the variations of different glaciers. Principal-component analysis indicates that these similarities amount to between 65 and 70% of the total variance for glaciers up to about 500 km apart. Within this distance, similar variations of mass balance and, therefore the same yearly climatic variations, can be observed.

scholarly journals Mass-balance model parameter transferability on a tropical glacier

2013 ◽  
Vol 59 (217) ◽  
pp. 845-858 ◽  
Author(s):  
Wolfgang Gurgiser ◽  
Thomas Mölg ◽  
Lindsey Nicholson ◽  
Georg Kaser
Keyword(s):  
Mass Balance ◽  
The Other ◽  
Balance Model ◽  
Small Scale ◽  
Model Parameters ◽  
Glacier Mass Balance ◽  
Ablation Zone ◽  
Model Errors ◽  
Mass Balance Model ◽  
Cumulative Mass

AbstractWe explore the small-scale spatial and temporal transferability of model parameters between two points in the ablation zone of tropical Glaciar Shallap, Cordillera Blanca, Peru (9°S, −77° W; ∼4800 m a.s.l.) in order to provide a robust assessment of the performance of a process-based glacier mass-balance model. Relative surface height change is calculated at hourly time-steps, and cumulative values are compared to surface height measurements made at irregular intervals (14–64 days) over the course of two continuous hydrological years (August 2006–August 2008). Best-performing parameter combinations were determined for each point from the outcome of 1000 model simulations for which parameters were varied randomly within a defined range. With these parameter combinations measurements for a specific location and time-span are well reproduced. Transferring the parameter combination as optimized for one location to the other location in the ablation zone increases the errors of modeled cumulative mass balance by 5–1326 mm ice eq.a−1. Transferring the parameter combinations as optimized for one year to the other year increases the modeled errors in cumulative mass balance by 18–3179 mm ice eq.a−1. Model errors generally increase during periods with frequent snowfall and snow cover. This could reflect either the inherent difficulty of modeling complex snow processes, or the inability of the model to correctly capture the pattern of albedo evolution at this site. The magnitude of errors associated with parameter transfer in space and time highlights the need for improving model performance for robust climatological and/or hydrological analyses on tropical glaciers.

Identification of Jurassic mafic arc magmatism in the eastern North China Craton: Geochemical evidence for westward subduction of the Paleo-Pacific slab

2020 ◽  
Author(s):  
Wei Fang ◽  
Li-Qun Dai ◽  
Yong-Fei Zheng ◽  
Zi-Fu Zhao ◽  
Li-Tao Ma ◽  
...  
Keyword(s):  
Subduction Zone ◽  
North China Craton ◽  
North China ◽  
The Other ◽  
Arc Magmatism ◽  
Early Mesozoic ◽  
Mantle Sources ◽  
Pacific Slab ◽  
Cratonic Mantle ◽  
T Values

Subduction of the Paleo-Pacific slab beneath the North China Craton (NCC) has exerted a strong influence on the Mesozoic destruction of the craton. However, no Andean-type arc magmatism has been reliably identified in the eastern NCC. Here we report the occurrence of Jurassic arc-like lamprophyres in the Liaodong Peninsula, providing a snapshot of the Paleo-Pacific slab subduction beneath the NCC in the early Mesozoic. Zircon U-Pb dating of the lamprophyres yields consistent ages of 158−155 Ma for magma crystallization. These lamprophyres all exhibit typical arc-like trace element distribution patterns, but show a series differences in their radiogenic isotope compositions and the other geochemical variables. Type 1 lamprophyres exhibit weakly enriched Sr-Nd-Hf isotopes with (87Sr/86Sr)i ratios of 0.7075−0.7085, εNd(t) values of −3.9 to −1.3 and εHf(t) values of −5.4 to −0.3, whereas Type 2 lamprophyres exhibit moderately enriched radiogenic isotopes with (87Sr/86Sr)i ratios of 0.7096−0.7117, εNd(t) values of −12.2 to −7.6 and εHf(t) values of −12.8 to −4.7. There are also systematic differences in zircon Hf isotopes and whole-rock Ba/Th, Ba/La, Sr/Nd, Th/Nd, Th/Yb, and La/Sm ratios for the two types of lamprophyre. Taken together, these similarities and differences can be accounted for by metasomatic reaction of the cratonic mantle wedge with two properties of liquid phase derived from subducting Paleo-Pacific slab. One is aqueous solutions from the subducting basaltic oceanic crust, and the other is hydrous melts from the subducting terrigenous. The two properties of subduction zone fluids were incorporated in different proportions into the mantle sources of these lamprophyres. Accordingly, the lamprophyres were derived from the metasomatic mantle sources. This qualitative interpretation is verified by quantitative modeling of the geochemical transfer at the slab-mantle interface in a paleo-oceanic subduction zone. Therefore, the Jurassic lamprophyres in the eastern NCC provide the geochemical evidence for the crust-mantle interaction during the Paleo-Pacific slab subduction beneath eastern Asia in the early Mesozoic, when the chemical metasomatism by the slab-derived fluids would have weakened the cratonic mantle for its thinning and destruction in the Early Cretaceous.

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