Predicting uniaxial compressive strength of serpentinites through physical,dynamic and mechanical properties using neural networks

The uniaxial compressive strength(UCS)of intact rock is one of the most important parameters required and determined for rock mechanics studies in engineering projects.The limitations and difficulty of conducting tests on rocks,specifically on thinly bedded,highly fractured,highly porous and weak rocks,as well as the fact that these tests are destructive,expensive and time-consuming,lead to development of soft computing-based techniques.Application of artificial neural networks(ANNs)for predicting UCS has become an attractive alternative for geotechnical engineering scientists.In this study,an ANN was designed with the aim of indirectly predicting UCS through the serpentinization percentage,and physical,dynamic and mechanical characteristics of serpentinites.For this purpose,data obtained in earlier experimental work from central Greece were used.The ANN-based results were compared with the experimental ones and those obtained from previous analysis.The proposed ANN-based formula was found to be very efficient in predicting UCS values and the samples could be classified with simple physical,dynamic and mechanical tests,thus the expensive,difficult,time-consuming and destructive mechanical tests could be avoided.

SERPENTINIZATION OF THE MANTLE WEDGE BY FLUIDS DERIVED FROM CONTINENTAL CRUSTAL MATERIAL: EVIDENCE FROM Nd ISOTOPIC SIGNATURES OF SERPENTINITES (TSO MORARI DOME,ELADAKH)

The well preserved eclogitic rocks of the Tso Morari dome in eastern Ladakh, northwest Himalaya, provide information relevant to the exhumation of high pressure/low temperature rocks, and the early stage of the Himalayan orogeny. The Tso Morari unit outcrops south of the Indus suture zone (Fig.1). The eclogitic dome is underlined on its eastern part by the Zildat normal fault where serpentinite lenses and partially hydrated peridotites are abundant. The close association of the high pressure rocks and serpentinites suggests a possible role of serpentinites in the exhumation of ultrahigh\|pressure rocks. To evaluate this possibility, geochemical analyses were carried out on the serpentinites closely associated with the Tso Morari eclogites.

Relic Oceanic Crust at Sub-Arc Depth: An Example from UHP Eclogites Enclosed in Serpentinites from the Southwestern Tianshan, China

Ultra-high pressure（UHP）eclogites that derive from subducted oceanic crust are rarely found at the Earth’s surface because they need to be enclosed in a buoyant host rock such as serpentinites that facilitate exhumation（Hermann et al.,2000;Guillot et al.,2001）.Under normal subduction geotherms,serpentinites break down just before UHP conditions are reached and therefore most of the exhumed eclogites representing subducted oceanic crust formed under fore-arc conditions.We investigated eclogite blocks enclosed into serpentinites that occur in the southwestern Tianshan oceanic subduction,China.A previous study proved that the serpentinites derive from altered oceanic crust and experienced UHP metamorphism at low temperatures of 510-530°C（Shen et al.,2015）.Three relatively fresh eclogite samples were studied in detail.Sample 129-7 shows the retrograde mineral assemblage of amphibole+biotite+albite+chlorite+minor titanite and peak metamorphic relics of omphacite+garnet±chlorite.Sample C107-23 is mainly composed of amphibole+albite+chlorite+zoisite+muscovite+minor titanite as a retrograde assemblage and garnet+phengite as the peak metamorphic relics with omphacite only found as inclusions in garnet.Similar to sample C107-23,sample C11066 preserves large-grained euhedral to subhedral garnet relics with omphacite inclusions,and epidote,diopside,amphibole,muscovite,chlorite,albite and biotite are in the matrix belong to the retrograde assemblage.These three retrograde eclogite samples were modelled using thermodynamic calculations in the Mn NCKFMSHO（Mn O-Na;O-Ca O-K;O-FeO-Mg O-Al;O;-SiO;-H;O-Fe;O;）system.Based on the peak assemblage of omphacite+garnet and the crossing of the grossular and pyrope isopleths in garnet,peak P-T conditions of;60-470oC,28-29 kbar（129-7）,450-500oC,28-35 kbar（C107-23）,;75-505oC,26-29 kbar（C11066）were calculated.The retrograde assemblages indicate near isothermal decompression resulting in a clockwise P-T evolution of these eclogites.The peak metamorphic pressures at 500°C are well within UHP conditions（coesite stability field）and are within error the same as peak conditions of the host serpentinites（Shen et al.,2015）.This provides evidence that eclogites and serpentinites shared the same evolution.We infer that the subducted low-density serpentinites were assembled with the high-density eclogites during subdution and helped the latter to exhume back to the surface.The studied eclogites thus represent rare examples of relics of oceanic crust that was subducted to sub-arc depth.

Relic Oceanic Crust at Sub-arc Depth: an Example from UHP Eclogites Enclosed in Serpentinites from the Southwestern Tianshan Mountains, China

Ultra-high pressure(UHP)eclogites that derive from subducted oceanic crust are rarely found at the Earth’s surface because they need to be enclosed in a buoyant host rock such as serpentinites that facilitate exhumation

Petrostructural and Geochemical Characteristics of the Metamagmatites in the External Zone of the Dahomeyides Belt: Case of the KantèSerpentinites (Northern Togo)

Thanks to detailed field investigations, microstructural and geochemical analysis and relationship with enclosing rocks, microfabrics, magmatic typology and metamorphic evolution of the Kantè sepentinites have been specified for the first time. The Kantè serpentinites in northern Togo constitute a mega-lens of ultrabasic rocks tectonically intercalated in the sericite chlorite schists of the Atacora structural unit. The brecciated, schitotose or massive rock facies are strongly marked by an S1 schistocity plane superimposed by a flat C shear plane linked to a west vergence thrusting movement. The parageneses that compose the metamagmatites are essentially serpentinous, containing plagioclase, opaque minerals (magnetite, chromite, spinel) and pyroxene porphyroblasts. These microfabrics represent relics of a probable gabbroic protolith. In fact, the geochemical characteristics of the Kantè serpentinites suggest that their magmatic typology is that of komatiites or tholeiitic basalts with oceanic arc affinities. They would have been emplaced in an active margin environment. The retromorphic evolution of the protolith corresponds to the phase of involvement in a major tangential contact during the panafrican tectogenesis.

Shear strength of extremely altered serpentinites based on degree of saturation(Ankara, Turkey)

The term "mélange" has several definitions due to the origin, tectonic and petrographical features. The geotechnical engineering behaviors of mélanges are either dominated by different kinds of intact rock blocks or matrix material. Landslides were encountered within the serpentinite matrix of Ankara Mélange, which is a typical bimrock mass. The residual sections of the extremely altered serpentinites are sandy soils. Such soils undergo rapid change of saturation, leading to shear strength reduction. Undisturbed and disturbed samples were obtained from the outcrops for shear box tests. Geochemical and petrographical composition of the serpentinites were determined by XRF(X Ray Fluorescence), XRD(X-ray diffraction) and thin section inspections, in order to outline the alteration process. Based on field observations, physically decomposed core stones exist beneath 40-120 cm thick residual green and grey-dark grey residual sandy soil. Physical and mechanical properties of the soils were tested with particular emphasis on residual shear strength parameters. Disturbed samples were remolded by standard compaction. The field work was completed during both rainy and summer seasons. Disturbed samples were prepared using phase diagrams in order to attain varying saturation degree. Two distinct sandy soil groups were determined through classification tests. The volumetric compression and/or expansion of the loose and dense samples were also considered based on the angle of dilatancy. A series of consolidated and drained shear box tests were conducted with ascending degree of saturation. The reduction of effective apparent cohesion, internal friction angle and dilatancy angle were determined to obtain a threshold of the degree of saturation. All the sand samples had zero residual internal friction angle and/or apparent cohesion after reaching 70% degree of saturation.

A New Report of Serpentinites from Northern Central Indian Ridge(at 6°S)—An Implication for Hydrothermal Activity

Serpentinites from the inside corner high （6°38.5′S/68°19.34′E） from the Northern Central Indian Ridge （NCIR） are comprised mainly of high Mg-rich lizardite and chrysotile pseudomorphs with varying morphologies. ＇Mesh rim＇, ＇window＇, ＇hourglass＇ and ＇bastite＇ are the most common textures displayed by both chrysotile and lizardite. Numerous chrysotile veins in association with cross cutting magnetite veins indicate an advanced stage of serpentinisation. The relatively high abundance of chrysotile and lizardite suggest their close association and formation at a temperature below 250℃. Abundant ＇mesh rim＇ and ＇bastite＇ texture and variegated matrix reveal that the present serpentinites might have formed due to the interaction of harzburgite and seawater. Positive Eu anomaly （Eu/Eu^＊ up to ＋3.38）, higher La/Sm （up to 4.40） and Nb/La （up to 6.34） ratios suggest substantial hydrothermal influence during the formation of the serpentinites.

Origin of the serpentinites in the Lichi melange,eastem Taiwan,China:implication from petrology and geochronology

Lichi melange,located in the southern coastal range,eastern Taiwan,China,is a typical tectonic melange of the plate's boundary zone between the Eurasian Plate and the Philippine Sea Plate.It formed during the collision of the Luzon arc with the Eurasian Continent (arc-continent collision).It is composed of sandstone and/or mudstone matrix and many kinds and sizes of rock fragments,including some sedimentary rocks,volcanic rocks and a few metamorphic rocks.The serpentinite is one of the common fragments in the Lichi melange.By the petrographic characteristics and the zircon U-Pb chronology analyses,protolith of the serpentinite is peridotite,the age is 17.7 ±0.5 Ma.Taking the tectonic background into account,it is inferred that the serpentinite (serpentinised peridotite)come from the forearc basin (the North Luzon Trough)and was taken into the melange by a second thrust westwards.The origin of the serpentimte in Lichi melange is helpful to understand the formation of the Lichi melange and can provide reliable detailed information for the study of the arc-continent collision orogenic activity in and offshore Taiwan.

Geochemical constraints on the origin and tectonic setting of the serpentinized peridotites from the Paleoproterozoic Nyong series,Eseka area,SW Cameroon

Serpentinized rocks closely associated with Paleoproterozoic eclogitic metabasites were recently discovered at Eseka area in the northwestern edge of the Congo craton in southern Cameroon.Here,we present new field data,petrography,and first comprehensible wholerock geochemistry data and discuss the protolith and tectonic significance of these serpentinites in the region.The studied rock samples are characterized by pseudomorphic textures,including mesh microstructure formed by serpentine intergrowths with cores of olivine,bastites after pyroxene.Antigorite constitutes almost the whole bulk of the rocks and is associated(to the less amount)with tremolite,talc,spinel,and magnetite.Whole-rock chemistry of the Eseka serpentinites led to the distinction of two types.Type 1 has high MgO(>40 wt%)content and high Mg#values(88.80)whereas Type 2 serpentinite samples display relatively low MgO concentration and Mg#values(<40 and 82.88 wt%,respectively).Both types have low Al/Si and high Mg/Si ratios than the primitive mantle,reflecting a refractory abyssal mantle peridotite protolith.Partial melting modeling indicates that these rocks were derived from melting of spinel peridotite before serpentinization.Bulk rock high-Ti content is similar to the values of subducted serpentinites(>50 ppm).This similarity,associated with the high Cr contents,spinel-peridotite protolith compositions and Mg/Si and Al/Si ratios imply that the studied serpentinites were formed in a subductionrelated environment.The U-shaped chondrite normalizedREE patterns of serpentinized peridotites,coupled with similar enrichments in LREE and HFSE,suggest the refertilized nature due to melt/rock interaction prior to serpentinization.Based on the results,we suggest that the Eseka serpentinized peridotites are mantle residues that suffered a high degree of partial melting in a subductionrelated environment,especially in Supra Subduction Zone setting.These new findings suggest that the Nyong series in Cameroon represents an uncontested Paleoproterozoic suture zone between the Congo craton and the Sao Francisco craton in Brazil.

Serpentinized Peridotites at the North Part of the Wadi Allaqi District(Egypt): Implications for the Tectono-Magmatic Evolution of Fore-arc Crust

The Neoproterozoic Allaqi-Heiani suture （800-700 Ma） in the south Eastern Desert of Egypt is the northernmost linear ophiolitic belt that defines an arc-arc suture in the Arabian- Nubian shield （ANS）. The Neoproterozoic serpentinized peridotites represent a distinct lithology of dismembered ophiolites along the Allaqi-Heiani suture zone. The alteration of peridotites varies, some contain relicts of primary minerals （Cr-spinel and olivine） and others are extremely altered, especially along thrusts and shear zones, with development of talc, talc-carbonate and quartz-carbonate. The fresh cores of the chromian spinels are rimmed by ferritchromite and Cr- magnetite. The fresh chromian spinels have high Cr# （0.62 to 0.79）, while Mg# shows wider variation （0.35-0.59）. High Cr# in the relict chromian spinels and Fo content in the primary olivines indicate that they are residual peridotites after extensive partial melting. The studied ophiolitic upper mantle peridotites are highly depleted and most probably underwent high degrees of partial melting at a supra-subduction zone setting. They can be produced by up to -20%-22% dynamic melting of a primitive mantle source. The mineralogical and geochemical features of the studied rocks reflect that the mantle peridotites of the north part of the Wadi Allaqi district are similar to the fore-arc peridotites of a supra-subduction zone.

The Formation and Evolution of Uvarovite in UHP Serpentinite and Rodingite and its Constraints on Chromium Mobility in the Oceanic Subduction Zone

The uvarovite-andradite and uvarovite-andradite-grossular solid-solution series are rare in nature.The discovery of uvarovite-andradite in serpentinite and rodingite from the ultra-high pressure(UHP)metamorphic belt in southwestern Tianshan provided an opportunity to investigate its behavior in the subduction zone.Uvarovite(defined as chromiumgarnet)from serpentinite is homogeneous in a single grain,covering compositions in the uvarovite-andradite solid solution series of Adr_(58-66)Uv_(33-41),with few grossular components.Uvarovite from rodingites contain various Cr_(2)O_(3) contents(1.7-17.9 wt%)and mineral compositions being in the range of Adr_(21-31)Uv_(41-50)Grs_(22-37),Adr_(52-90)Uv_(5-25)Grs_(0-21) and Adr_(19-67)Uv_(3-63)Grs_(13-42).Discontinuous chemical variation of uvarovite from core to rim indicates that uvarovite formed by consuming andradite and chromite,which could provide Ca,Cr,Al and Fe.Raman signals of water were identified for uvarovite from both serpentinite and rodingite,with high water content in uvarovite from serpentinite.The high pressure mineral assemblage,as well as the association with perovskite,indicated that the studied uvarovite from serpentinite and rodingite was formed through high pressure metamorphism,during the subduction zone serpentinization and rodingitization.High alkaline and highly reduced fluids released from serpentinization or rodingitization in the oceanic subduction zone promote the mobility of chromium and enable its long-distance migration.

In situ characterization of forearc serpentinized peridotite from the Sulu ultrahigh-pressure terrane: Behavior of fluid-mobile elements in continental subduction zone

Serpentinized peridotites in the Yangkou(YK),Suoluoshu(SLS) and Hujialin(HJL) areas in the Sulu ultrahighpressure terrane represent the relic of ancient subcontinental lithospheric mantle below the North China Craton.Their protoliths,harzburgite and dunite,were variably hydrated by aqueous fluids released from subducting Yangtze continent.The rocks are enriched in fluid-mobile elements(FME) including Sb(42–333 times the depleted mantle value) and Pb(30–476 times).The degrees of the FME enrichment are comparable to that of the Himalayan forearc serpentinites,and greater than forearc mantle serpentinites from Marianas,suggesting that the degrees of FME enrichment in the forearc serpentinites are greater in continental subduction zones than those in the oceanic subduction zones.Lizardite after olivine in the SLS serpentinite shows higher degrees of enrichment in Sb and As than those for antigorite after both olivine and orthopyroxene in the YK area.The antigorite has highly enriched in Pb,U,Cs,and LREE,but not for the lizardite.The abundance of FME in two different species of serpentine reflects the different temperature of hydration.At temperature lower than 300 ℃,formed lizardite at shallow depths of the mantle wedge incorporates elements that are fluid mobile at low temperatures,such as Sb and As.When the temperature greater than 300 ℃,formed antigorite at a relatively deep mantle wedge incorporate more FME from the subducting continental slab(or fragments),including Pb,U,Cs,LREE as well as Sb and As.The eventual breakdown of antigorite(600–700 ℃) in prograde metamorphism would discharge water as well as FME into the subducting channel and/or the overlying mantle.

Serpentine Jade(Bowenite) in Korea

A bowenite deposit, recently discovered in Booyo County, Republic of Korea, is the first of its kind in the country. This deposit is located along the contacts of garnet veins intruding the serpentinite. The bowenite is translucent, dark green in color, greasy in luster, 5 in Mohs ’ scale of hardness, ～2.57 in SG , 2.58 in RI , and 1.56 in N D . The X ray powder diffraction analysis identified the bowenite as antigorite of the serpentine group minerals, whose major chemical compositions are composed of SiO 2 (42.49 %), MgO (39.08 %), Fe 2O 3 (3.85 %), and H 2O (11.87 %) and whose important trace elements include Cr (2 188×10 -6 ), Ni (1 110×10 -6 ), and Co (58×10 -6 ). The IR spectrum shows the absorptions at 3 670 (OH stretching), 1 190, 1 070, 980 (SiO stretching) and 610 cm -1 (OH bending). The DTA/TGA thermogram shows the peaks at 343, 755 and 830.1 ℃. The endothermic reaction at 755 ℃ denotes expulsion of structural water, and the strong exothermal reaction at 830.1 ℃ is related to the formation of olivine. Black inclusions finely dispersed in the mineral are identified, with X ray Gandolfi camera method, as magnetite.

Natural Mg silicates with different structures and morphologies: Reaction with K to produce K2MgSiO4 catalyst for biodiesel production

In this work, different magnesium silicate mineral samples based on antigorite, lizardite, chrysotile(which have the same general formula Mg3Si2O5(OH)4), and talc(Mg3Si4O10(OH)2) were reacted with KOH to prepare catalysts for biodiesel production. Simple impregnation with 20 wt% K and treatment at 700–900°C led to a solid-state reaction to mainly form the K2MgSiO4 phase in all samples. These results indicate that the K ion can diffuse into the different Mg silicate structures and textures, likely through intercalation in the interlayer space of the different mineral samples followed by dehydroxylation and K2MgSiO4 formation. All the materials showed catalytic activity for the transesterification of soybean oil(1:6 of oil : methanol molar ratio, 5 wt% of catalyst, 60°C). However, the best results were obtained for the antigorite and chrysotile precursors, which are discussed in terms of mineral structure and the more efficient formation of the active phase K2MgSiO4.

Serpentinite Slurries against Forest Fires

Forest fires are one of the commonest natural hazards. Forest fires make the largest contribution to CO2 emissions after the burning of fossil fuels. Here a new technology is proposed to extinguish forest fires not with water, but with a slurry of serpentine. Serpentinites are abundantly available in many countries on every continent. If serpentine is calcined, it weathers very fast and captures CO2. Calcination, however, requires a lot of heat, which makes it counterproductive to produce calcined serpentine for CO2 capture. In cases, however, where heat is the problem, like in forest fires, one can extinguish them to greater advantage by using serpentinite slurries instead of plain water. The calcined residue that is left as a thin cake on the burning material prevents oxygen to reach the burning material. It also prevents the escape of inflammable gases, and the calcination itself withdraws large quantities of heat from the fire. After the fire is extinguished, the calcined material in contact with the atmosphere will rapidly weather and capture CO2. This compensates part of the CO2 that is produced by the fire. In tests, where the efficacy of quenching fires with serpentine slurries was compared to the effect of water, it turned out that serpentinite slurries performed far better.

Geology and Geochemistry of the Chromiferous Mineralization in the External Zone of the Pan-African Dahomeyides Belt,Northwestern Benin(Gulf of Guinea,West Africa)

Occurrences of chromium mineral exist in Bénin within the external nappes of the Pan-African Dahomeyides belt. These chromite deposits are podiform types and hosted in the serpentinite. Blackish color, the chromite appears under microscope in transmitted light, as cracked phenocrystals with tangles of the serpentinite venules. Chemical analyses using X-ray fluorescence spectrometer showed the chromite ore consists of about 38% Cr2O3 and total (Cr2O3 + Al2O3) is 60%. This chromiferous mineralisation resulted from the magmatic cumulates formation by fractional crystallization of an ultramafic magma.

Origin and Evolution of Ultramafic Rocks along the Sagaing Fault, Myanmar

The active Sagaing fault in Myanmar defines the boundary between the Indian Plate and the Eurasian Plate and causes seismic damage in the major cities of Myanmar. Small bodies of serpentinite occur along the fault. We for the first time investigated the highly sheared serpentinite bodies in the Sheinmagar area and Yega Inn area along the Sagaing fault. Extensively sheared/brecciated serpentinites and related rocks, such as talc and/or chlorite-bearing rocks contains small rock fragments of serpentinites. Serpentine texture and mineral chemistry indicate that the protolith of these serpentinites were mainly harzburgite with minor amounts of dunite, some of which are cut by gabbroic veins. No shape-preferred orientation of the antigorite is present, indicating that the serpentinization was occurred under relatively static conditions. Protolith and serpentine minerals are similar to those of the jadeitite-bearing serpentinites in the north of the Sagaing fault(the Jade Mine belt). Chemical variations of spinels in the studied area are within the compositional range of forearc peridotites and those in the mantle section of nearby ophiolites. After the formation of antigorite serpentinite under static conditions, these serpentinites were subsequently, but locally deformed, probably due to the activity of the Sagaing fault, resulting in the formation of serpentinite schist/brecciated rock. The presence of the less-deformed antigorite serpentinite in the sheared/brecciated zone indicates the strain localization mainly along the surrounding serpentine-talc(±chlorite) schistose rocks, which is probably formed by the reaction between serpentinite and country rocks. Further studies are needed to better understand whether the distribution of serpentinized peridotites cause variations in the activity of the Sagaing fault.

A stepwise process for carbon dioxide sequestration using magnesium silicates

This work involves the production of magnesium in the form of Mg(OH)_(2)from serpentinite rock(nickel mine tailing)material followed by conversion into MgCO_(3)using a pressurised fluidised bed(PFB)reactor operating at 400℃-600℃and pressures up to 2.85 MPa.Our approach is rooted in the thermodynamic fact that the reaction between Mg(OH)_(2)and gaseous CO_(2)forming MgCO_(3)and water releases significant amounts of heat.The main problem is,however,the chemical kinetics;the reaction is slow and has to be accelerated in order to be used in an economically viable process for large-scale(~1 Mt/a)CO_(2)sequestration.We have constructed a labscale PFB reactor test-setup for optimising the carbonation reaction.At high enough temperatures and conversion levels the reaction should provide the heat for the proceeding Mg(OH)_(2)production step,making the overall process energy neutral.So far we have been able to achieve a conversion degree of 26%at 500℃and 2.85 MPa after 30 min(particle size 125-212μm).In this paper the test facility and our latest results and progress on CO_(2)mineral carbonation are summarised.Also,the possible integration of the iron as a feedstock for iron and steel production will be briefly addressed.An interesting side-effect of this carbon dioxide capture and storage(CCS)route is that significant amounts of iron are obtained from the serpentinite rock material.This is released during the Mg(OH)_(2)production and can be of great interest to the iron-and steel producing sector,which at the same time is Finland’s largest CO_(2)producer.

Geo-vegetation Mapping and Soil Geochemical Characteristics of the Indikolapelessa Serpentinite Outcrop,Southern Sri Lanka

The serpentinite blocks of Indikolapelessa, located along an identified litho-tectonic boundary between the Highland Complex （HC） and the Vijayan Complex （VC） of Sri Lanka, have undergone extensive lateralization with metal enrichment. Characteristic serpentinite vegetation with some endemic species was recognized in the soils and supergene deposits develop on serpentinite lithology. This type of geological and ecological relationship forms vegetation covers on serpentinite lithologies which are sharply demarcated from the surrounding metamorphic terrains. The aforesaid ＂geo-ecological phenomenon＂ can be used as a tool for geo-vegetation mapping in ultramafic terrains to trace the geological boundaries in landscapes where rock outcrops are virtually absent. We success- fully applied the concept of geo-vegetation mapping in order to demarcate the boundary of underlain serpentinite rocks from surrounding non-serpentinite metamorphic rocks （e.g. granitic gneiss）. The hypothesis was supported by the geochemical variations of soils/supergene deposits found at serpen- tinite and non-serpentinite sites, especially immobile elements and some trace elements. Based on whole rock chemistry and soil chemical data obtained, we suggest that the Indikolapelessa serpentinite outcrop, together with the other four serpentinite outcrops, is more likely to represent the Mg-rich mantle fragments at the time of overthrusting of the two crustal blocks of HC and VC during the Pan- African event.