Development of the negative gravity anomaly of the 85°E Ridge,northeastern Indian Ocean – A process oriented modelling approach

The 85°E Ridge extends from the Mahanadi Basin, off northeastern margin of India to the Afanasy Nikitin Seamount in the Central Indian Basin. The ridge is associated with two contrasting gravity anomalies: negative anomaly over the north part (up to 5°N latitude), where the ridge structure is buried under thick Bengal Fan sediments and positive anomaly over the south part, where the structure is intermittently exposed above the seafloor. Ship-borne gravity and seismic reflection data are modelled using process oriented method and this suggest that the 85°E Ridge was emplaced on approximately 10–15 km thick elastic plate (Te) and in an off-ridge tectonic setting. We simulated gravity anomalies for different crust-sediment structural configurations of the ridge that were existing at three geological ages, such as Late Cretaceous, Early Miocene and Present. The study shows that the gravity anomaly of the ridge in the north has changed through time from its inception to present. During the Late Cretaceous the ridge was associated with a significant positive anomaly with a compensation generated by a broad flexure of the Moho boundary. By Early Miocene the ridge was approximately covered by the post-collision sediments and led to alteration of the initial gravity anomaly to a small positive anomaly. At present, the ridge is buried by approximately 3 km thick Bengal Fan sediments on its crestal region and about 8 km thick pre- and post-collision sediments on the flanks. This geological setting had changed physical properties of the sediments and led to alter the minor positive gravity anomaly of Early Miocene to the distinct negative gravity anomaly.     

Geophysical signatures over and around the northern segment of the 85°E Ridge,Mahanadi offshore,Eastern Continental Margin of India: Tectonic implications

The nature and origin of the subsurface 85°E Ridge in the Bay of Bengal has remained enigmatic till date despite several theories proposed by earlier researchers. We reinterpreted the recently acquired high quality multichannel seismic reflection data over the northern segment of the ridge that traverses through the Mahanadi offshore, Eastern Continental Margin of India and mapped the ridge boundary and its northward continuity. The ridge is characterized by complex topography, multilayer composition, intrusive bodies and discrete nature of underlying crust. The ridge is associated with large amplitude negative magnetic and gravity anomalies. The negative gravity response across the ridge is probably due to emplacement of relatively low density material as well as ∼2–3 km flexure of the Moho. The observed broad shelf margin basin gravity anomaly in the northern Mahanadi offshore is due to the amalgamation of the 85°E Ridge material with that of continental and oceanic crust. The negative magnetic anomaly signature over the ridge indicates its evolution in the southern hemisphere when the Earth’s magnetic field was normally polarized. The presence of ∼5 s TWT thick sediments over the acoustic basement west of the ridge indicates that the underlying crust is relatively old, Early Cretaceous age.The present study indicates that the probable palaeo-location of Elan Bank is not between the Krishna–Godavari and Mahanadi offshores, but north of Mahanadi. Further, the study suggests that the northern segment of the 85°E Ridge may have emplaced along a pseudo fault during the Mid Cretaceous due to Kerguelen mantle plume activity. The shallow basement east of the ridge may have formed due to the later movement of the microcontinents Elan Bank and Southern Kerguelen Plateau along with the Antarctica plate.     

Petrogenesis of anomalous K-enriched MORB from the Southwest Indian Ridge: 11°53′E to 14°38′E

Glassy pillow basalts with unusual geochemical characteristics for mid-ocean ridge basalt (MORB) have been dredge sampled from the Southwest Indian Ridge between 12 and 15°E during Leg ANT IV/4 of the F.S. POLARSTERN. Lavas from 4 of 6 dredges are moderately nepheline normative, highly K-enriched (0.5–1.77 wt% K₂O) alkali basalts and hawaiites. Mg-numbers indicate that many of the lavas are fairly primitive (Mg No.=63–67), yet show extreme enrichment in incompatible elements; e.g. Nb (24–60 ppm), Ba (170–470 ppm) and Sr (258–460 ppm). Incompatible-element ratios such as Zr/Nb (3–5) and Y/Nb (0.46–1.1) are extremely low even for E-type (enriched) MORB, whereas (La/Yb)_n ratios are particularly high (3.4–7.8). ⁸⁷Sr/⁸⁶Sr (0.70290–0.70368), ¹⁴³Nd/¹⁴⁴Nd (0.51302–0.51284) and ²⁰⁶Pb/²⁰⁴Pb (18.708–19.564) isotopic ratios further indicate the geochemically enriched nature of these lavas, which range from the compositional field for depleted N-type (normal) MORB towards the composition of Bouvet Island lavas. Mutually correlated incompatible-element and Sr-, Nd- and Pb-isotopic ratios allow a fairly well constrained model to be developed for the petrogenesis of these unusually alkalic mid-ocean ridge lavas. The alkalic nature and degree of enrichment in incompatible elements is ascribed to particularly low degrees of partial melting (3–5 wt%), at greater than usual depth, of a source region that has experienced prior geochemical enrichment (by veining) related to the upwelling Bouvet mantle plume. To account for the observed compositional variations, a model is proposed whereby mixing between partial melts derived from these geochemically enriched silicate veins, and an incipient to low percentage (±2%) melt from the surrounding geochemically depleted suboceanic asthenosphere occurs as a consequence of increasing degree of melting with adiabatic upwelling. Eruption of these alkalic lavas in this spreading ridge environment is attributed to a temporary hiatus in tholeiitic volcanism and associated spreading along this section of the Southwest Indian Ridge, related to readjustment of spreading direction to a more stable plate geometry.     

Widespread tectonic extension at the Central Indian Ridge between 8°S and 18°S

The middle part of the Central Indian Ridge (MCIR) between 8°S and 18°S is representative of mid-ocean ridges in the Indian Ocean but has not previously been systematically surveyed. Here we present results from the first high-resolution mapping survey over both on- and off-axis sections of the MCIR including multibeam bathymetry, magnetics, hydrocasting, and seabed sampling. The 700-km-long MCIR consists of six first-order segments that are offset by > 30 km along well-developed transform faults. Three of the first-order segments are further divided into seven second-order segments with < 30 km offset along non-transform discontinuities. We have recognized for the first time 11 prominent ocean core complexes (OCCs). These occur at nearly all segment ends, corresponding to an occurrence every 60 km of the surveyed ridge. Seafloor spreading model studies using magnetic reversals show that the MCIR is a slow-spreading ridge with average full opening rates ranging from 33.7 to 45.1 mm/yr, increasing from north to south. The highly curved and intermittent axial ridge geometry, rugged flank fabric, variation in the depth and width of the middle valley, and the frequent occurrences of ocean core complexes and non-transform discontinuities demonstrate that asymmetric accretionary processes are dominant along the ridge. The spreading rate symmetry combined with morphotectonic features, reveal that the MCIR segments developed mainly via tectonic extension with little magmatism. Segments with asymmetric accretion controlled by tectonic extension makes up ~ 96% of the MCIR, whereas symmetric accretion controlled by robust magmatism make up < 4%. Hydrothermal plumes with high methane concentrations occur frequently over the OCCs. This finding indicates that abundant OCCs exposed by detachment faults lead to extensive hydrothermal circulation at off-axis areas and that detachment faults are the primary fluid path for hydrothermal fluid circulation at off-axis regions. Serpentinization of mantle-derived rock at OCCs may be one of the major sources of heat and methane in off-axis areas.     

Occurrence of ice-rafted erratics and the petrology of the KR1 seamount trail from the Australian–Antarctic Ridge

ABSTRACT

A multi-disciplinary study of the KR1 segment of the Australian–Antarctic Ridge has been conducted since 2011. We present geochemical and age dating results for samples dredged from three sites on the KR1 seamount trail. The majority of the samples are alkaline ocean island basalts with subdominant enriched tholeiites. The samples from the DG05 bathymetric depression include ice-rafted erratics from Antarctica, which consist of gabbro, diabase, various granitoids, volcanic rocks such as trachyte and rhyolite and deformed or undeformed sedimentary rocks. The main provenance of glacial erratics is considered to be the Ross Sea region. However, Carboniferous to Cretaceous ages of erratics indicate that some of these may originate from the western regions of West Antarctica. Based on the size and topography of the volcanic features and geochemical characteristics of the alkaline ocean island basalts (La/Sm_N = 2.62–3.88; Tb/Yb_N = 1.54–2.67) and the enriched tholeiites, the KR1 seamount trail is interpreted to be a submarine hotspot chain that is the product of alkaline volcanic eruption and seafloor spreading.     

Geochemistry and petrogenesis of mafic-ultramafic rocks from the Central Indian Ridge,latitude 8°–17° S: denudation of mantle harzburgites and gabbroic rocks and compositional variation of basalts

This study investigates the formation of lower oceanic crust and geochemical variations of basalts along the Central Indian Ridge (CIR, lat. 7°45′–17°10′ S). Harzburgites, various gabbroic cumulates, medium- to fine-grained oxide gabbros, diabases, and pillow basalts were recovered by dredging from segment ends such as ridge-transform intersections (RTIs), non-transform discontinuities (NTDs), and transform offset areas. The occurrence of both harzburgites and gabbroic rocks with minor basalts at all segments ends, and leucogabbro intrusive into harzburgite at the 12°45′ S NTD indicates that oceanic crust at segment ends exposes mantle-derived harzburgites and gabbroic intrusions with a thin basaltic cover due to sparse magmatic activity. Basalts collected along the entire ridge show wide compositional variations between N (normal)- and E (enriched)-mid-ocean ridge basalt (MORB). T (transitional)-MORBs with enriched affinities are more prominent than N-MORBs. There is no tendency of enrichment towards specific directions. (La/Sm)_N variations in MORB along the CIR (8°–21°S) fluctuates at a regional scale with local high positive anomalies reflecting compositional heterogeneity of the sub-CIR mantle domain.     

Major and trace elements of abyssal peridotites: evidence for melt refertilization beneath the ultraslow-spreading Southwest Indian Ridge (53° E segment)

This study examines the geochemistry of major and trace elements of abyssal peridotites from the Southwest Indian Ridge (SWIR) (53° E amagmatic segment), to determine the influence of mafic melts on mantle peridotites during melt extraction. The results show a great geochemical variability in the ~90 km-long ridge segment, with a degree of mantle melting ranging from 4% to 24%. An ancient melting event may explain the presence of highly depleted peridotites at the ultraslow-spreading ridge. The 53° E segment peridotites show enrichment of light rare earth elements (LREEs) (average La_N/Sm_N = 1.87) and significant positive anomaly of U and Pb normalized to primitive mantle (PM). The positive correlations between LREEs (La, Ce, Pr, Nd) and high field strength elements (HFSEs; e.g. Nb and Zr) suggest that the enrichment of LREEs is caused by melt refertilization, which is also supported by prevalent magmatic microstructures in the peridotites. The melt refertilization model shows that the addition of 0.02–2.7% basaltic melts to peridotites can be responsible for the LREE enrichment. We suggest that the positive anomaly of U is probably attributed to fluid alteration whereas the enrichment of Pb is probably attributed to both melt refertilization and fluid alteration. Melt refertilization in the 53° E segment peridotites may be a result of melt–rock reaction and crystallization of melts trapped in peridotites. These processes may be enhanced by increased melt permeability in the mantle owing to the refractory peridotites produced by ancient melting and the decreasing efficiency of melt extraction in the cold and thick lithosphere at the 53° E ridge segment. The presence of melt refertilization implies that melt extraction is incomplete in the ridge mantle, which may be one of the reasons for the extremely thin and irregular variation of the crustal thickness at ultraslow-spreading ridges.     

Sediment thickness map of the Indian region 79°E–86°E, 2°S–8°S based on satellite gravity interpretation

Satellite free air gravity anomalies over the Indian ocean region 79°E–86°E, 2°S–8°S were obtained from the website http://topex.ucsd.edu and a contour map was compiled. Five profiles of the anomaly have been interpreted in terms of two-dimensional structures in the ocean. Thickness of sediments lying on the oceanic crust determined from the interpretation of gravity profiles were used to compile an isopach map of the region 79°E–86°E, 2°S–8°S. This map in combination with one of the isopach maps compiled by previous workers, provides information regarding the thickness of sediments up to 6° S. According to this map sediment thickness varies from ～600 m over the middle part of the region to ～800 m further south, indicating that thinning of sediments in the middle part of the region is only localized. Information provided by this gravity study may be useful in planning detailed seismological studies to delimit the outer edge of the continental margin of Sri Lanka, defined according to the United Nations Convention of the Law of the Sea (UNCLOS).     

A Petrogenetic Model of Basalts from the Northern Central Indian Ridge:3-11°S

1 Introduction The Northern Central Indian Ridge (NCIR) between 3° and 11°S latitudes is joined to the north with the slow spreading Carlsberg Ridge (CR; ~24–26 mm/a, full spreading rate) and to the south with the intermediate spreading Southern Central Indian Ridge (SCIR; ~50 mm/ a) (Fig. 1). Earlier petrological investigations of the Central Indian Ridge were concentrated either on or along the CR to the north and at the southern end of the CIR up to the Rodriguez Triple Junc…     

A Petrogenetic Model of Basalts from the Northern Central Indian Ridge: 3-11°S

Mid-Ocean Ridge Basalts (MORB) from the Northern Central Indian Ridge (NCIR) were recovered between latitudes 3° and 11°S and are olivine tholeiite with higher abundances of K and Rb.They are of typical transitional MORB (T-MORB) variety and appear to have been generated from an enriched-mantle peridotite source. The primitive NCIR MORBs having Mg# ＞ 0.68 are the product of partial melting at an estimated pressure of ～ 1 GPa. It is inferred that the magma was subsequently modified at a pressure ＞ 1 GPa by crystal fractionation and spinel was the first mineral to crystallize followed by separation of relatively Fe-rich olivine with subsequent decrease in pressure. During progressive fractionation at lower pressure (between 1-0.5 GPa), the bulk composition of the magma became systematically depleted in MgO, and enriched in ∑FeO, TiO2, P2O5 and Na2O. There was,however, limited gradual depletion in Al2O3 and CaO and concomitant enrichment in K2O. With the progressive fractionation these basalts became gradually enriched in V, Co, Y, Zr and to some extent in Sr, and depleted in Ni and Cr. In addition, the ∑REE of the magma also increased with fractionation,without any change in (La/Yb)n value.     

Crustal structure and nature of emplacement of the 85°E Ridge in the Mahanadi offshore based on constrained potential field modeling: Implications for intraplate plume emplaced volcanism

An integrated interpretation of multi-channel seismic reflection, gravity and magnetic datasets belonging to northern most part of the 85°E Ridge in the Mahanadi offshore is carried out to study the crustal structure and mode of its emplacement. The basement structure map of the ridge reveals that it is 130–150 km wide and is composed of an eastern high which appears as a continuous, broad and smooth topographyand the western high characterized by several steep isolated highs. The seismic velocities reported for the first time over the ridge indicate several sedimentary sequences ranging in velocities between 1.6 and 4.0 km/s above the acoustic basement top. The salient aspects of the sedimentary velocities are; a low velocity layer (2.6–3.2 km/s) within the Cretaceous sequence in the intervening depressions encompassing the flank region, and a regionally widespread higher velocity layer (3.5–3.8 km/s) belonging to the Eocene–Oligocene section overlying the ridge. A layer having a velocity of 4.2–4.7 km/s probably made of volcanoclastic rocks is observed immediately below the acoustic basement. The sediment isopach maps presented here for three major horizons are used to compute the 3-D sediment gravity effect to obtain a crustal Bouguer anomaly map of the region. Detailed analysis of the gravity and magnetic anomaly maps clearly demonstrates the continuity of ridge up to the Mahanadi coast at Chilka Lake. Seismically constrained gravity and magnetic models indicate that the ridge is composed of volcanic material that was emplaced on continental crust in the shelf-slope areas and over the oceanic crust in the deep offshore areas. The modeled crustal structure below the ridge further indicates volcanic emplacement of the ridge on a relatively younger lithosphere. We propose two alternative models for the emplacement of the ridge.     

Palaeomonsoon and palaeoproductivity records of δ18O, δ{13}C and CaCO3 variations in the northern Indian Ocean sedimentsC and CaCO3 variations in the northern Indian Ocean sediments

δ¹⁸ O and δ¹³C of G.sacculifer have been measured in five cores from the northern Indian Ocean. In addition, high resolution analysis (1 to 2 cm) was performed on one core (SK-20-185) for both δ¹⁸O and gd¹³C in five species of planktonic foraminifera. CaCO₃ variation was measured in two cores. The results, presented here, show that

Conditions of the formation of plagiogranite from the Markov trough,Mid-Atlantic Ridge, 5°52′-6°02′N

Doklady Earth Sciences -     

Structure of the Earth’s crust and tectonic evolution history of the Southern Indian Ocean (Antarctica)

The Southern Indian Ocean comprises large sedimentary basins of the Riiser-Larsen Sea (western sector); the Cosmonauts, Cooperation (Commonwealth), Davis seas (central sector); and the Mawson-d’Urville seas (eastern sector). The main tectonic provinces of the Southern Indian Ocean (Antarctica) have been outlined as a result of comprehensive interpretation of the geophysical data. Special attention is paid to determining the boundary between the rifted continental and oceanic crust. The basin of the Riiser-Larsen Sea was formed in the Early Jurassic under the action of the Karoo mantle plume. The intrusive complex, as a remote manifestation of the mantle plume, occurs along the inner boundary of the marginal rift. Opening of the ocean in the basin of Riiser-Larsen Sea started about 160 Ma ago and was characterized by rearrangement of plate motion and intense volcanic activity at the early stage. In the basin of the Cosmonauts, Cooperation, and Davis seas, the final stage of rifting was accompanied by the rise of the lithospheric mantle and by intrusive magmatism. The opening of the ocean started here 134 Ma ago. Emplacement of the Kerguelen plume resulted in jumping of ridges and detachment of continental crustal blocks from the Indian margin with the formation of the Kerguelen Plateau (microcontinent). The basin of the Mawson-d’Urville seas has evolved under conditions of long-term rifting since the Late Jurassic and is characterized by an extended zone of mantle unroofing. Breakup of the lithosphere between Australia and Antarctica developed asynchronously over a time interval of 95–65 Ma ago with propagation of MOR from the west eastward. The research was carried out using a great body of geophysical information (～140000 km of CDP seismic profiling, more than 250 stations of seismic refraction sounding, and more than 250000 km of magnetic and gravity profiles) obtained by expeditions from many countries over more than 30 years.     

Temporal analysis of air pollution and its relationship with meteorological parameters in Bahrain, 2006–2012

The objective of this paper is to analyze temporal and seasonal trends of air pollution in Bahrain between 2006 and 2012 by utilizing datasets from five air quality monitoring stations. The non-parametric and robust Theil-Sen approach is employed to study quantitatively temporal variations of particulate matter (PM₁₀ and PM_2.5), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), and ozone (O₃). The calculated annual concentrations for PM₁₀ and PM_2.5 in Bahrain were substantially higher than recommended World Health Organization (WHO) guideline standards. Results showed increasing trends for PM₁₀, PM_2.5, and SO₂ whereas O₃ and its precursor NO₂ showed decreasing behavior. The general increase in air pollution trends is in agreement with prediction of air pollution models for Middle East region due to economic growth, industrialization, and urbanization. The significances of long-term trends were examined. Additional to actual (unadjusted) trends, meteorological adjusted (deseasonalized) trends and seasonal trends were quantified. The box-plot analysis visually illustrated monthly variations of key air pollutants. It showed that only PM₁₀ and PM_2.5 exhibited seasonal pattern, and their concentrations increased during summer and decreased during winter. The effects of ambient air temperature, relative humidity, wind speed, and rainfall on particulate matter (PM) concentrations were further investigated. The Spearman correlation coefficient results demonstrated significant negative correlation between relative humidity and PM concentrations (??0.595 for PM₁₀ and ??0.526 for PM_2.5) while significant positive correlation was observed between temperature and PM concentrations (0.420 for PM₁₀ and 0.482 for PM_2.5).     

Pillow lavas of the Sierra Leone test site,Mid-Atlantic Ridge, 5°–7°N: Sr-Nd isotope systematics,geochemistry, and petrology

Based on detailed petrological, geochemical, and isotope-geochemical study, fragments of fresh pillow lavas with chilled glass margins dredged at the Sierra-Leone test site in the axial MAR rift zone between 5° and 7°N correspond to MORB tholeiites, which are not primitive mantle melts but were differentiated in intermediate magmatic (intrusive) chambers. Small-scale geochemical and Sr-Nd isotope heterogeneities were established for the first time in the basalts and their glasses. It was shown that some samples show significant nonsystematic differences in the ⁸⁷Sr/⁸⁶Sr ratio between the basalts and their chilled glasses and less significant difference in ?_Nd; higher Sr ratios can be observed both in the glasses and basalts of the same lava fragments. No significant correlation is observed between the isotope characteristics of the samples and their geochemistry; it was also shown that seawater did not affect the Sr and Nd isotope composition of the chilled glasses of the studied pillow lavas. It is suggested that such differences in isotope ratios are related to a small-scale heterogeneity of the melts owing to incomplete homogenization during their rapid ascent to the surface. The heterogeneity of the basaltic melts is explained by their partial contamination by the older plutonic rocks (especially gabbroids) of the lower oceanic crust, through which they ascended to the ocean floor surface. The wider scatter of the Sr isotopic ratios relative to Nd is related to the presence of xenocrysts of calcic plagioclase; correspondingly, the absence of a Nd mineral carrier in the rocks results in less distinct Nd isotope variations. It was shown that all of the studied basalts define a single trend along the mantle correlation array in the Sr-Nd isotope diagram. The causes of this phenomenon remain unclear.     

Geochemistry of mafic rocks and melt inclusions and their implications for the heat source of the 14.0°S hydrothermal field,South Mid-Atlantic Ridge

Fresh rocks sampled from the 14.0°S hydrothermal field of the South Atlantic Ridge can be divided into two categories： olivine-gabbro and basalt. The olivine-gabbro is composed mainly of three types of minerals： olivine, clinopyroxene and plagioclase, while a multitude of melt inclusions occur in the plagioclase phenocrysts of the basalts. We analyzed the whole-rock, major and trace elements contents of the basaks, the mineral chemistry of phenocrysts and melt inclusions in the basalts, and the mineral chemistry of olivine-clinopyroxene-plagioclase in the olivine-gabbro, then simulated magma evolution within the crust using the COMAGMAT program. The whole-rock geochemistry shows that all the basalts exhibit typical N-MORB characteristics. In addition, the mineral chemistry characteristics of the olivine-gabbro （low-Fo olivine, low-Mg# clinopyroxene, high-TiO2 clinopyroxene, low-An plagioclase）, show that strong magma differentiation occurred within the crust. Nevertheless, significant discrepancies between those minerals and phenocrysts in the basalts （high-Fo olivine, high-An plagioclase） reflect the heterogeneity of magma differentiation. High Mg# （-~0.72） melt inclusions isobaric partial crystallization simulations suggest that the magma differentiation occurred at the depth shallower than 13.03 km below the seafloor, and both the vertical differentiation column shows distinct discrepancies from that of a steady-state magma chamber. Instead, a series of independent magma intrusions probably occurred within the crust, and their corresponding crystallized bodies, as the primary high-temperature thermal anomalies within the off-axis crust, probably act as the heat source for the development of the 14.0°S hydrothermal system.