Sr ISOTOPE, MAJOR AND TRACE ELEMENTAL MOBILITY OF CENTRAL GANGA ALLUVIAL PLAIN

Abstract

In northern part of Indian sub-continent, Indo-Ganga-Brahmaputra Plain forms the largest alluvial plain in the world which has been formed due to Himalayan orogeny. The sediments deposited in the foreland basin are from the Himalayan as well as from the craton regions. The center part of the Indo-Ganga-Brahmaputra Plain is widely known as the Ganga Alluvial Plain (GAP) which covers an area of about 250,000 km2 and located between longitudes 77ºE to 88ºE and latitudes 30ºN to 24ºN. The Gomati River (a tributary of the Ganga River) drains the Ganga Alluvial Plain and derives its sediments from weathering of alluvial deposits under monsoon controlled sub-tropical climate. The present study mainly aims at major, trace and rare earth element geochemistry along with Sr isotopic ratio study of the Central Ganga Alluvial Plain within Gomati River Basin. For this purpose four sampling sites were chosen and samples were collected for further processing and geochemical analysis. One set of mica-rich bedload and normal bedload sediments was collected from each site. The normal and mica-rich bedload sediments along with separated minerals (muscovite, biotite and feldspar) were analyzed for element geochemistry to obtain following objectives: 1. To look at the mobility of major and trace element during chemical weathering processes in Ganga Alluvial Plain (GAP). 2. To constrain the source of 87Sr/86Sr isotopic ratios in Ganga Alluvial Plain (GAP). Chapter 1 contains a brief introduction of weathering of sediments and minerals in fluvial environment, mica minerals, rare earth elements and Strontium isotope. It also reports previous studies related to weathering processes and elemental mobility phenomenon. This introductory chapter provides a framework for the study. Study area begins with Chapter 2, which presents background information of Ganga Alluvial Plain. The Gomati River originates within the Ganga Alluvial Plain (GAP) and its basin experiences a humid sub-tropical climate. Weathering products in the Ganga Alluvial Plain are transported by the Gomati River as dissolved, bedload and suspended load. The Gomati River sediments act as weathering products of the Ganga Alluvial Plain having its ultimate sediment source in the Himalayan region. The mineralogy of bedload sediments is mainly composed of quartz, followed by plagioclase, alkali feldspar and mica. Average mineral composition of the river sand consists of quartz (55%), rock fragments (19%), muscovite (15%), K-feldspar (8%), biotite (2%) and plagioclase (1%). ii Chapter 3 addresses analytical techniques and methods applied to accomplish this study. Sediment samples of mica-rich bedload and normal bedload were collected from four sampling sites- Naimeserayan Haidergarh, Sultanpur and Chandwak. Minerals - muscovite, biotite and feldspar were separated from bedload sediments with the help of Isodynamic separator. After dissolution, samples were analyzed by ICP-MS. Sr isotopic analysis was done using Thermal Ionisation Mass Spectrometer (TIMS) TRITON T1. The analysis of mica grains has been done with the help of Scanning Electron Microscope with attached Energy Dispersive X-Ray (EDX) Spectrometer. Chapter 4 report the results generated from the geochemical analysis of bedload sediments along with associated silicate minerals-biotite, muscovite and feldspar. Biotite has higher percentage of MgO & FeO(t) than muscovite and feldspar. Muscovite has higher percentage of Al2O3 & K2O and feldspar has higher percentage of CaO & Na2O. Biotite has higher concentration of trace elements- As, Co, Cr, Cu, Li, Ni, Rb, V, Zn than muscovite and feldspar. Muscovite has higher concentration of Ba, Sr than biotite and feldspar. In the normal bedload sediments, total-REE concentration (ƩREE), light-REE concentration (LREE) and heavy-REE concentration (HREE) are 92.1–323.1 ppm, 83.7–302.1 ppm and 7.8–21.0 ppm, respectively. Ce is the most abundant of the ƩREE compositions. The decreasing order of ƩREE concentration in common silicate minerals is biotite > muscovite > feldspar. Chondrite-normalised REE patterns of the river sediments show a strong LREE enrichment, relatively flat HREE, weak positive Gd anomaly and absence of prominent negative Eu-anomaly. In associated minerals, positive Eu-anomaly was shown by feldspar and muscovite, whereas negative Eu-anomaly by biotite. Biotite differs from feldspar and muscovite in REE enrichment and positive Gd-anomaly. The most characteristic sign of chemical alteration of biotite in the normal bedload sediments is its consistent downstream REE depletion. Microanalysis of grain of biotite mineral shows depletion of element from core to marginal edge. Chapter 5 deals with the discussion of generated dataset and focuses on the distribution of bedload sediments along with associated silicate minerals- biotite, muscovite and feldspar to understand the mobility of major, trace and rare earth elements in Ganga plain. Biotite show enrichment in Al2O3, P2O5, FeO, MgO, MnO,TiO2 and depletion in Na2O, K2O, CaO relative to UCC. Muscovite show enrichment in Al2O3, P2O5, and depletion in Na2O, K2O, FeO, MnO, CaO. Except for CaO, and TiO2, all other major elements are depleted in Feldspar relative to UCC. Bedload Sediments are enriched in CaO and TiO2 and depleted in Al2O3, FeO(t), MgO ,MnO, Na2O, P2O5 relative to UCC. Biotite shows enrichment in As, Co, Cs, Cr, Cu, Ga, Li, Ni, Pb, Rb, V and Zn and Muscovite show enrichment in As, Ba, Co, Cs, Cr, Cu, Ga, Li, Ni, iii Pb, Rb, V and Zn relative to UCC. Feldspar show enrichment in As and other trace elements are depleted. Bedload sediments are enriched in As, Cs, Ga, Li, Rb, and Se relative to UCC. Bivariant plots were plotted to find out association and source of elements. In sediments, K2O shows positive correlation with Al2O3, indicating the common mineral sources in mica minerals and Mg shows strong positive correlation with Al2O3, K2O, CaO, and FeO(t), indicating the common mineral sources in plagioclase and mica minerals. Significant positive correlation of Ba with K2O and Al2O3 favours its association with K-aluminosilicates. Weathering influences the spatial variability in ƩREE content in biotite of the bedload sediments. The ƩREE concentrations in biotite of the normal bedload sediments decrease from 471 ppm to 142 ppm with downstream suggesting the release of REE through the intensive chemical weathering of easily weatherable biotite. On the other hand, total REE concentration in biotite of the mica-rich bedload sediments increased markedly with downstream from 375 ppm to 1445 ppm due to secondary deposition on the mineral’s surface. Biotite has lower 87Sr/86Sr and lower Sr content in sediments of Gomati River Basin. The Sr isotopic ratio is increasing from Biotite to Muscovite to sediments in Gomati River Basin. Lower values of Sr may be due to relative increase in carbonate weathering or due to dilution of Sr concentration due to increase in water discharge. Elevated 87Sr/86Sr ratios of the Gomati River throughout post-monsoon season demonstrate that the interaction among the GAP sediments increased the Sr-influx after monsoon season. Chapter 6 introduces the conclusion derived from the results and discussion of present study. This study provides major, trace and rare earth element geochemistry of bedload sediments of Gomati river along with associated silicate minerals-biotite, muscovite and feldspar to evaluate their usefulness in weathering studies. Major and trace element distribution in the weathering products is controlled by mineral sorting during transportation and deposition. REE geochemistry has utility for understanding the impacts of biotite weathering process and its significance in REE mobilization in the Gomati River Basin under its humid sub-tropical climatic condition. Gomati river water and biotite have lower values of Sr isotopic ratio while muscovite and sediments have higher Sr isotopic ratios. At Ramnagar cliff, a marked correlation between climate and Sr isotopic ratios is noted indicating the influence of climate over Sr isotopic concentration. At Behta nadi cliff section, Sr isotopic values are increasing from bottom to top of the facies revealing that Sr ratio is increasing within the Ganga Alluvial Plain. Higher Sr isotopic ratios of the Gomati River are due to chemical weathering of alluvial material present within the Ganga Alluvial Plain.