Titanium- and water-rich metamorphic olivine in high-pressure serpentinites from the Voltri Massif (Ligurian Alps,Italy): evidence for deep subduction of high-field strength and fluid-mobile elements

Titanium- and water-rich metamorphic olivine (Fo 86–88) is reported from partially dehydrated serpentinites from the Voltri complex, Ligurian Alps. The rocks are composed of mostly antigorite and olivine in addition to magnetite, chlorite, clinopyroxene and Ti-clinohumite. In situ secondary ion mass spectrometry (SIMS) data show that metamorphic olivine has very high and strongly correlated H₂O (up to 0.7 wt%) and TiO₂ contents (up to 0.85 wt%). Ti-rich olivine shows colourless to yellow pleochroism. Olivine associated with Ti-clinohumite contains low Ti, suggesting that Ti-rich olivine is not the breakdown product of Ti-clinohumite. Fourier transform infrared spectroscopy (FTIR) absorption spectra show peaks of serpentine, Ti-clinohumite and OH-related Si vacancies. Combining FTIR and SIMS data, we suggest the presence of clustered planar defects or nanoscale exsolutions of Ti-clinohumite in olivine. These defects or exsolutions contain more H₂O (x ~ 0.1 in the formula 4Mg₂SiO₄·(1?x)Mg(OH,F)₂·xTiO₂) than Ti-clinohumite in the sample matrix (x = 0.34–0.46). In addition to TiO₂ and H₂O, secondary olivine contains significant Li (2–60 ppm), B (10–20 ppm), F (10–130 ppm) and Zr (0.9–2.1 ppm). It is enriched in ¹¹B (δ¹¹B = +17 to +23 ‰). Our data indicate that secondary olivine may play a significant role in transporting water, high-field strength and fluid-mobile elements into the deeper mantle as well as introduce significant B isotope anomalies. Release of hydrogen from H₂O-rich olivine subducted into the deep mantle may result in strongly reduced mantle domains.     

阜平杂岩中低品位磁铁矿的形成与深熔作用的关系

在中-高级变质的阜平杂岩中可以形成低品位磁铁矿.除碎屑岩中继承的磁铁矿外,新生变质磁铁矿多呈斑晶,可出现于多种岩石类型,如变基性岩、中性岩、酸性岩和变沉积岩中,表明新生磁铁矿的形成不受层位控制.磁铁矿可由物理重结晶和化学反应2种形式形成,重结晶过程主要为矿物颗粒的加粗,但没有明显的脱水反应.变质化学反应形成的磁铁矿与各单元所经受的后期变质事件改造有关,这类磁铁矿的出现与岩石中TFe的含量没有必然的联系,关键在于变质反应中是否有适量的铁组分的迁移和富集.变质反应过程中,初期黑云母变质转化形成角闪石,即变质反应不全是脱水或吸水过程,表明阜平杂岩主要的变质过程发生在含水体系中.在进一步的变质改造中,黑云母、角闪石可深熔转化形成磁铁矿.在片麻岩的含水熔融过程中,Mg、Ca优先迁移,而Fe(Ti,Al)迁移微弱,造成Fe(Ti,Al)与Mg组分的分离,残留的相对富铁组分形成磁铁矿、钛铁矿.磁铁矿结晶时没有明显的挤压或剪切,张应力可能占主导地位,相应的深熔作用主要发生在构造静应力期或体系略微抬升的过程中.     

河南舞阳地区赵案庄铁矿床成矿时代及地质意义——中国最古老的岩浆型铁矿床

河南舞阳地区赵案庄铁矿床是产于超基性岩中的隐伏矿床。超基性岩侵位于新太古界太华群赵案庄组透辉斜长角闪片麻岩中。矿体呈似层状,具有多层。矿石成分较复杂,主要矿物为磁铁矿、蛇纹石、氟磷灰石等。矿床规模为中等,其成因是与超基性岩有关的岩浆晚期磷灰石-磁铁矿矿床。在磷灰蛇纹磁铁矿石中选出的锆石样品为变质锆石,利用激光烧蚀多接收器电感耦合等离子体质谱仪(LA-MC-ICPMS)进行微区原位U_Pb同位素测年,获得不一致线的上交点年龄为(1943±5)Ma(MSWD=2.3)。同时对锆石阴极发光(CL)图像研究,表明该年龄代表锆石遭受强烈变质作用的年龄,可以限定矿体形成时代的上限,即矿体形成时代不晚于1943 Ma。超基性岩浆演化晚期,岩浆充填构造裂隙呈透辉石岩脉产出。透辉石脉与矿体同期并且穿切铁山庙组BIF型铁山铁矿。铁山庙组形成时代在2300~2500 Ma之间。透辉石岩脉形成时代不早于该组年龄,可作为赵案庄铁矿床形成时代的下限。赵案庄铁矿床的形成时代为1943~2300 Ma,为古元古代,这是中国目前发现的最古老的岩浆型铁矿床。该成矿地质时代反映了在华北陆块东南缘曾发生过一次地壳扩张和裂陷活动及华北克拉通裂解事件。     

Exsolution Lamellae in Olivine Grains of Dunite Units from Different Types of Mafic‐Ultramafic Complexes

Exsolution microstructures in olivine grains from dunite units in a few selected tectonic environments are reported here. They include lamellae of clinopyroxene and clinopyroxene-magnetite intergrowth in the Gaositai and Yellow Hill Alaskan-type complexes, clinopyroxene-magnetite intergrowth in the K?z?lda? ophiolite, and chromite lamellae in the Hongshishan mafic-ultramafic intrusive complex. These lamellae commonly occur as needle-or rod-like features and are oriented in olivine grains. The host olivine grains have Fo contents of 92.5–92.6 in the Gaositai complex, 86.5–90.1 in the Yellow Hill complex, 93.2–93.4 in the K?z?lda? ophiolite and 86.9–88.3 in the Hongshishan complex. Clinopyroxene in the rod-like intergrowth exsolved in olivine grains in the Gaositai and Yellow Hill is diopside with similar major element compositions of Ca O(23.6–24.3 wt%), SiO_2(52.2–54.0 wt%), Al_2O_3(0.67–2.15 wt%), Cr_2O_3(0.10–0.42 wt%) and Na_2O(0.14–0.26 wt%). It falls into the compositional field of hydrothermal clinopyroxene and its origin is thus probably related to reaction between dunite and fluids. The enrichment of the fluids in Ca~(2+), Fe~(3+), Cr~(3+) and Na+, resulted in elevated concentrations of these cations in olivine solid solutions via the reaction. With decreasing temperature, the olivine solid solutions altered to an intergrowth of magnetite and clinopyroxene. The Fe~(3+) and Cr~(3+) preferentially partitioned into magnetite, while Ca~(2+) and Na+ entered clinopyroxene. Since the studied Alaskan-type complexes and ophiolite formed in a subduction environment, the fluids were probably released from the subducted slab. In contrast, the exsolved chromite in olivine grains from the Hongshishan complex that formed in post-orogenic extension setting can be related to olivine equilibrated with Cr-bearing liquid. Similarly, these lamellae have all been observed in serpentine surrounding olivine grains, indicating genetic relations with serpentinization.     

Implication of Apatite and Anhydrite for Formation of an Iron‐Oxide‐Apatite (IOA) Rare Earth Element Prospect,Benjamin River,Canada

The Benjamin River apatite prospect in northern New Brunswick, Canada, is hosted by the Late Silurian Dickie Brook plutonic complex, which is made up of intrusive units represented by monzogranite, diorite and gabbro. The IOA ores, composed mainly of apatite, augite, and magnetite at Benjamin River form pegmatitic pods and lenses in the host igneous rocks, the largest of which is 100 m long and 10–20 m wide in the diorite and gabbro units. In this study, 28 IOA ore and rock samples were collected from the diorite and gabbro units. Mineralogical observations show that the apatite–augite–magnetite ores are variable in the amounts of apatite, augite, and magnetite and are associated with minor amounts of epidote‐group minerals (allanite, REE‐rich epidote and epidte) and trace amounts of albite, titanite, ilmenite, titanomagnetite, pyrite, chlorite, calcite, and quartz. Apatite and augite grains contain small anhydrite inclusions. This suggests that the magma that crystallized apatite and augite had high oxygen fugacity. In back scattered electron (BSE) images, apatite grains in the ores have two zones of different appearance: (i) primary REE‐rich zone; and (ii) porous REE‐poor zone. The porous REE‐poor zones mainly appear in rims and/or inside of the apatite grains, in addition to the presence of apatite grains which totally consist of a porous REE‐poor apatite. This porous REE‐poor apatite is characterized by low REE (<0.84 wt%), Si (<0.28 wt%), and Cl (<0.17 wt%) contents. Epidote‐group minerals mainly occur in grain boundary between the porous REE‐poor apatite and augite. These indicate that REE leached from primary REE‐rich apatite crystallized as allanite and REE‐rich epidote. Magnetite in the ores often occurs as veinlets that cut apatite grains or as anhedral grains that replace a part of augite. These textures suggest that magnetite crystallized in the late stage. Pyrite veins occur in the ores, including a large amount of quartz and calcite veins. Pyrite veins mainly occur with quartz veins in augite. These textures indicate pyrite veins are the latest phase. Apatite–augite–magnetite ore, gabbro–quartz diorite and feldspar dike collected from the Benjamin River prospect contain dirty pure albite (Ab₉₈Or₂–Ab₁₀₀) under the microscope. The feldspar dikes mainly consist of dirty pure albite. Occurrences of the dirty pure albite suggest remarkable albitization (sodic alteration) of original plagioclase (An_25.3–An₆₀ in Pilote et al., 2012) associating with intrusion of monzogranite into gabbro and diorite. SO₄^2? bearing magma crystallized primary REE‐rich apatite, augite and anhydrite reacted with Fe in the sodic fluids, which result in oxidation of Fe²⁺ and release of S^2? into the sodic fluids. REE, Ca and Fe from primary REE‐rich apatite, augite and plagioclase altered by the sodic fluids were released into the fluids. Then Fe³⁺ in the sodic fluids precipitated as Fe oxides and epidote‐group minerals in apatite–augite–magnetite ores. Finally, residual S^2? in sodic fluids crystallized as latest pyrite veins. In conclusion, mineralization in Benjamin River IOA prospect are divided into four stages: (1) oxidized magmatic stage that crystallized apatite, augite and anhydrite; (2) sodic metasomatic stage accompanying alteration of magmatic minerals; (3) oxidized fluid stage (magnetite–epidote group minerals mineralization); and (4) reduced fluid stage (pyrite mineralization).     

Interaction of deformation and metamorphism during subduction and exhumation of hydrated oceanic mantle: Insights from the Western Alps

Petrological investigations supported by multi‐scale structural analysis of eclogitized serpentinite in the Zermatt–Saas Zone of the Western Alps allows for the determination of mineral assemblages related to successive fabrics, upon which the P–T–d–t path of these hydrated mantle rocks can be inferred. Serpentinites of the upper Valtournanche, with lenses and dykes of metagabbro and meta‐rodingite, display an Alpine polyphase metamorphic evolution from eclogite to epidote‐amphibolite facies conditions associated with three successive foliations having different parageneses in these rocks. Serpentinite mainly consists of serpentine with minor magnetite; however, where S1 and S2 foliations are pervasive, metamorphic olivine, together with Ti‐clinohumite and clinopyroxene, are also found. The mineral assemblage associated with D1 includes serpentine1, clinopyroxene1, opaque minerals, titanite ± olivine1, Ti‐clinohumite1 and ilmenite; the D2 assemblage is the same (±chlorite) but minerals have different compositions. The assemblage associated with D3 comprises serpentine3, opaque minerals, ±chlorite3, ilmenite and amphibole3. Ti‐clinohumite is associated with veins that are older than D2 and pre‐date D3. Veins that post‐date D3 are characterized by amphibole + chlorite or by serpentine. P–T conditions for S2 parageneses evaluated using two pseudosections for different bulk compositions suggest that these rocks experienced pressures >2.5 ± 0.3 GPa at temperatures slightly higher than 600 °C. The late epidote–amphibolite facies re‐equilibration associated with D3 and D4 developed during late syn‐exhumation deformation related to folding and testifies to a small temperature decrease. These results, which were integrated in the regional framework, suggest that different portions of the Zermatt–Saas Zone registered different P–T peak conditions and underwent different exhumation paths. In addition, the inferred P–T–d–t path suggests that the Valtournanche serpentinites re‐equilibrated close to the UHP conditions registered by the Cignana meta‐cherts. These results imply that tectonic slices exhumed after UHP metamorphism might be wider than previously reported or that small‐size UHP units, tectonically sampled during the Alpine convergence, are more abundant than those that have been detected to date.     

Major and Trace Elements of Magnetite from the Qimantag Metallogenic Belt: Insights into Evolution of Ore–forming Fluids

Magnetite, as a genetic indicator of ores, has been studied in various deposits in the world. In this paper, we present textural and compositional data of magnetite from the Qimantag metallogenic belt of the Kunlun Orogenic Belt in China, to provide a better understanding of the formation mechanism and genesis of the metallogenic belt and to shed light on analytical protocols for the in situ chemical analysis of magnetite. Magnetite samples from various occurrences, including the ore–related granitoid pluton, mineralised endoskarn and vein–type iron ores hosted in marine carbonate intruded by the pluton, were examined using scanning electron microscopy and analysed for major and trace elements using electron microprobe and laser ablation–inductively coupled plasma–mass spectrometry. The field and microscope observation reveals that early–stage magnetite from the Hutouya and Kendekeke deposits occurs as massive or banded assemblages, whereas late–stage magnetite is disseminated or scattered in the ores. Early–stage magnetite contains high contents of Ti, V, Ga, Al and low in Mg and Mn. In contrast, late–stage magnetite is high in Mg, Mn and low in Ti, V, Ga, Al. Most magnetite grains from the Qimantag metallogenic belt deposits except the Kendekeke deposit plot in the " Skarn " field in the Ca+Al+Mn vs Ti+V diagram, far from typical magmatic Fe deposits such as the Damiao and Panzhihua deposits. According to the(Mg O+Mn O)–Ti O2–Al2O3 diagram, magnetite grains from the Kaerqueka and Galingge deposits and the No.7 ore body of the Hutouya deposit show typical characteristics of skarn magnetite, whereas magnetite grains from the Kendekeke deposit and the No.2 ore body of the Hutouya deposit show continuous elemental variation from magmatic type to skarn type. This compositional contrast indicates that chemical composition of magnetite is largely controlled by the compositions of magmatic fluids and host rocks of the ores that have reacted with the fluids. Moreover, a combination of petrography and magnetite geochemistry indicates that the formation of those ore deposits in the Qimantag metallogenic belt involved a magmatic–hydrothermal process.     

The stoichiometric effects of ferric iron substitutions in serpentine from microprobe data

ABSTRACT

Given that secondary magnetite is common in serpentinites, it is clear that serpentinites are oxidized rocks. Questions remain, however, concerning the distribution of ferric iron among magnetite and serpentine minerals and the role of ferric iron-rich serpentine in the formation of secondary magnetite. Direct determination of ferric iron in serpentine is not possible using an electron microprobe. We show, however, that the stoichiometic effects of ferric iron substitutions are detectable, although not quantifiable, by microprobe. First, we demonstrate that for studies that provide both microprobe analyses of major elements of serpentine and Mössbauer analysis of ferric iron, substitution effects are obvious. Next, it is equally clear that the early veins forming at the onset of olivine hydration (type 1 veins) show no indication of the presence of ferric serpentine, although a small amount of ferric ‘brucite’ may occur. Finally, we show that secondary (type 2) veins, which form as the system becomes open to fluids in equilibrium with plagioclase or pyroxene, contain, in addition to significant alumina, stoichiometric indications of ferric iron substitution. The serpentine in these veins is magnesian, usually with Mg#s around 96–98. Thus, even if a significant proportion of this iron is ferric, it comprises only a small fraction of the total ferric iron budget of the rock. Given that reduced iron is known to be abundant in early-formed brucite and early-formed serpentine and given that brucite, in particular, is absent from evolved serpentine veins, we propose that most magnetite in serpentinites forms as a tertiary product via oxidation of brucite.     

Magnesium Isotopic Compositions of International Geological Reference Materials

Magnesium isotopic compositions are reported for twenty‐four international geological reference materials including igneous, metamorphic and sedimentary rocks, as well as phlogopite and serpentine minerals. The long‐term reproducibility of Mg isotopic determination, based on 4‐year analyses of olivine and seawater samples, was ≤ 0.07‰ (2s) for δ²⁶Mg and ≤ 0.05‰ (2s) for δ²⁵Mg. Accuracy was tested by analysis of synthetic reference materials down to the quoted long‐term reproducibility. This comprehensive dataset, plus seawater data produced in the same laboratory, serves as a reference for quality assurance and inter‐laboratory comparison of high‐precision Mg isotopic data.     

Generation and Evolution of Ore Fluids for Porphyry Cu-Au Mineralization of the Santo Tomas II (Philex) Deposit, Philippines

Abstract: The Santo Tomas II (Philex) deposit is a porphyry Cu‐Au deposit, located in the southern part of the Baguio mineral district, Benguet Province, northern Luzon, Philippines. The Santo Tomas II deposit is associated with an intrusive complex consisting of four rock types that are distinguished based on petrography. They are 1) post‐ore clinopyroxene‐bearing hornblende andesite porphyry, 2) ore‐generating hornblende andesite porphyry, 3) hornblende quartz diorite porphyry and 4) porphyritic hornblende quartz diorite. K‐Ar age of hydrothermal biotitization was estimated to be 1.5±0.4 Ma. A number of intrusive bodies having broadly similar petrography and K‐Ar age occur in the vicinity of the Santo Tomas II deposit, such as at Clifton, Ligay (Binang), Bumolo (Waterhole) and Philex Main Camp areas. The intrusions at the Santo Tomas II deposit and in the vicinity are characterized by high X_Mg (Mg/[Mg+Fe] atomic ratio, about 0.7 or higher) of mafic silicate phenocrysts such as hornblende, and high sulfur contents (> 0.2 wt% as SO₃) in accessory microphenocrystic apatite, suggesting a highly oxidizing condition. Sulfur is accommodated dominantly as oxidized species since the crystallization of phe‐nocrysts. Sub‐dendritic rim of tremolitic amphibole on hornblende phenocryst in the ore‐generating andesite porphyry at the Santo Tomas II deposit suggests interaction of magma and aqueous fluid(s) exsolved due to decompression during intrusion. Dissemination of magnetite is associated with hydrothermal biotitization and is followed by sheeted and stockwork quartz veinlets having silician magnetite and rare titanohematite instead of Cu‐Fe sulfides. The silician magnetite‐rich quartz veinlet was formed at f_O2 near the hematite‐magnetite buffer at nearly magmatic temperature, where sulfur dominantly existed as oxidized species such as SO₂. Chalcopyrite and bornite, which commonly exhibit micrographic texture often accompanying Pd telluride and native gold/Au‐rich electrum, are associated with subsequent anhydrite (‐quartz) veinlets and stringers. Both intermediate solid solution (iss) and bornite solid solution (bnss) are thought to have coprecipitated primarily at above 500°C based on fluid inclusion microthermometry and sulfur isotope thermometry applied for anhydrite and associated chal‐copyrite and bornite. The initial iss is considered to have converted to chalcopyrite partly replacing bnss during cooling. The hypersaline polyphase fluid inclusions abundantly found in the sheeted and stockwork quartz as well as anhydrite veinlets with scarce gaseous inclusions suggest that they have been trapped in the two aqueous fluid immiscible region. The western Luzon arc associated with porphyry Cu mineralization is characterized by oxidized hydrous magmatism and shallow emplacement, and by the source of sulfur enriched in ³⁴S.     

Geochemical Constraints on the Origin of Banded Iron Formation‐Hosted Iron Ore from the Archaean Ntem Complex (Congo Craton) in the Meyomessi Area,Southern Cameroon

Banded iron formations (BIFs) are the most significant source of iron in the world. In this study, we report petrographic and geochemical data of the BIF from the Meyomessi area in the Ntem Complex, southern Cameroon, and discuss their genesis and the iron enrichment process. Field investigations and petrography have revealed that the studied BIF samples are hard; compact; weakly weathered; and composed of magnetite, subordinate quartz, and geothite. The geochemical composition of the whole rock reveals that iron and silica represent more than 98 wt% of the average composition, whereas Al₂O₃, TiO₂, and high‐field strength elements (HFSE) contents are very low, similar to detritus‐free marine chemical precipitates. The total iron (TFe) contents range from 48.71 to 65.32 wt % (average of 53.29 wt %) and, together with the low concentrations of deleterious elements (0.19 wt % P on average), are consistent with medium‐grade iron ores by global standards. This interpretation is confirmed by the SiO₂/Fe₂O_3total versus (MgO + CaO + MnO)/Fe₂O_3total discrimination plot in which most of the Meyomessi BIF samples fall in the field of medium‐grade siliceous ore. Only one sample (MGT94) plots in the high‐grade magnetite–geothite ore domain. The high Fe/Ti (376.36), Fe/Al (99.90), and Si/Al (29.26) ratios of the sample are consistent with significant hydrothermal components. The rare earth elements (REE) contents of the studied BIF samples are very low (∑REE: 0.81–1.47 ppm), and the Post‐Archaean Australian Shale (PAAS)‐normalized patterns display weak positive Eu anomalies (Eu/Eu*: 1.15–1.33), suggesting a syngenetic low‐temperature hydrothermal solutions, similar to other BIF worldwide. However, the Meyomessi BIFs show high Fe contents when compared to the other BIFs. This indicates an epigenetic mineralization process affected the Meyomessi BIF. From the above results and based on the field and analytical data, we propose that the genetic model of iron ores at the Meyomessi area involves two stages of the enrichment process, hypogene enrichment of BIF protore by metamorphic and magmatic fluids followed by supergene alteration as indicated by the presence of goethite in the rocks.     

Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite‐serpentinite dehydration (Nevado‐Filábride Complex,Spain)

At sub‐arc depths, the release of carbon from subducting slab lithologies is mostly controlled by fluid released by devolatilization reactions such as dehydration of antigorite (Atg‐) serpentinite to prograde peridotite. Here we investigate carbonate–silicate rocks hosted in Atg‐serpentinite and prograde chlorite (Chl‐) harzburgite in the Milagrosa and Almirez ultramafic massifs of the palaeo‐subducted Nevado‐Filábride Complex (NFC, Betic Cordillera, S. Spain). These massifs provide a unique opportunity to study the stability of carbonate during subduction metamorphism at P–T conditions before and after the dehydration of Atg‐serpentinite in a warm subduction setting. In the Milagrosa massif, carbonate–silicate rocks occur as lenses of Ti‐clinohumite–diopside–calcite marbles, diopside–dolomite marbles and antigorite–diopside–dolomite rocks hosted in clinopyroxene‐bearing Atg‐serpentinite. In Almirez, carbonate–silicate rocks are hosted in Chl‐harzburgite and show a high‐grade assemblage composed of olivine, Ti‐clinohumite, diopside, chlorite, dolomite, calcite, Cr‐bearing magnetite, pentlandite and rare aragonite inclusions. These NFC carbonate–silicate rocks have variable CaO and CO₂ contents at nearly constant Mg/Si ratio and high Ni and Cr contents, indicating that their protoliths were variable mixtures of serpentine and Ca‐carbonate (i.e., ophicarbonates). Thermodynamic modelling shows that the carbonate–silicate rocks attained peak metamorphic conditions similar to those of their host serpentinite (Milagrosa massif; 550–600°C and 1.0–1.4 GPa) and Chl‐harzburgite (Almirez massif; 1.7–1.9 GPa and 680°C). Microstructures, mineral chemistry and phase relations indicate that the hybrid carbonate–silicate bulk rock compositions formed before prograde metamorphism, likely during seawater hydrothermal alteration, and subsequently underwent subduction metamorphism. In the CaO–MgO–SiO₂ ternary, these processes resulted in a compositional variability of NFC serpentinite‐hosted carbonate–silicate rocks along the serpentine‐calcite mixing trend, similar to that observed in serpentinite‐hosted carbonate‐rocks in other palaeo‐subducted metamorphic terranes. Thermodynamic modelling using classical models of binary H₂O–CO₂ fluids shows that the compositional variability along this binary determines the temperature of the main devolatilization reactions, the fluid composition and the mineral assemblages of reaction products during prograde subduction metamorphism. Thermodynamic modelling considering electrolytic fluids reveals that H₂O and molecular CO₂ are the main fluid species and charged carbon‐bearing species occur only in minor amounts in equilibrium with carbonate–silicate rocks in warm subduction settings. Consequently, accounting for electrolytic fluids at these conditions slightly increases the solubility of carbon in the fluids compared with predictions by classical binary H₂O–CO₂ fluids, but does not affect the topology of phase relations in serpentinite‐hosted carbonate‐rocks. Phase relations, mineral composition and assemblages of Milagrosa and Almirez (meta)‐serpentinite‐hosted carbonate–silicate rocks are consistent with local equilibrium between an infiltrating fluid and the bulk rock composition and indicate a limited role of infiltration‐driven decarbonation. Our study shows natural evidence for the preservation of carbonates in serpentinite‐hosted carbonate–silicate rocks beyond the Atg‐serpentinite breakdown at sub‐arc depths, demonstrating that carbon can be recycled into the deep mantle.     

Two occurrences of ultramafic hornfels in the Biggenden Beds,southeastern Queensland

Ultramafic hornfelses containing the assemblages hornblende + olivine + spinel + magnetite, and clinopyroxene + olivine + spinel + magnetite, are reported from two localities in the Biggenden Beds in southeastern Queensland. They are associated with mafic hornfelses in the contact metamorphic aureoles of the Mungore Adamellite and the Wateranga Gabbro. Chemical composition and minerology of the olivine + amphibole + spinel + magnetite hornfelses suggest that they represent metamorphosed picritic rocks, or possibly, altered serpentinites (blackwall rocks), whereas the clinopyroxene + olivine + spinel + magnetite hornfelses are interpreted as metamorphosed altered clinopyroxene‐rich picritic rocks. Cr‐Fe spinel relations in the hornfelses indicate partial homogenisation of primary chromian spinel with secondary magnetite ± ferrichromite during contact metamorphism.     

青海茫崖石棉矿区变质超基性岩的蚀变演化序列：岩石化学及矿物学证据

青海茫崖石棉矿区超基性岩体是由原岩以纯橄岩、辉橄岩和橄辉岩为主体组成的富镁质超基性岩体,经历自变质和后期多期热液的叠加变质蚀变作用,经蛇纹石化后形成蚀变完全的蛇纹岩岩体,其中部分蛇纹岩又进一步发生滑石化及碳酸盐化蚀变为滑石菱镁片岩、菱镁滑石片岩、滑石片岩和菱镁岩等。本文在野外地质调查基础上,在室内通过镜下岩矿综合鉴定、全岩化学成分分析以及电子探针成分分析等手段进行了岩石化学特征、矿物学特征及其蚀变演化过程研究。结果表明,该变质超基性岩体蛇纹岩主要特征组合矿物为蛇纹石(利蛇纹石、叶蛇纹石、纤蛇纹石)、磁铁矿、菱镁矿、滑石、水镁石、铬铁矿,变余矿物有斜方辉石、单斜辉石和铬铁矿,滑石菱镁片岩类主要组成矿物为菱镁矿、滑石、蛇纹石及磁铁矿,局部可见石英脉。该地区变质超基性岩体较完整地记录了橄榄岩水化、滑石化及碳酸盐化作用过程的各个阶段,超基性岩蚀变演化过程主要有两个作用阶段:(Ⅰ)橄榄石、辉石类矿物的蛇纹石化作用及蛇纹石绿泥石化作用;(Ⅱ)富Ca、CO2流体交代蛇纹石、滑石及水镁石的碳酸盐化作用。蛇纹石化等变质蚀变作用促进了Si、Mg及Fe元素化学活动性,使元素发生富集与迁移,对于次生矿物的形成与演化起到了一定的催化作用。多期不同组成流体热液的交代作用过程,清晰地展示了利蛇纹石、纤蛇纹石和叶蛇纹石的演化序列,以及滑石、水镁石、铬铁矿和磁铁矿的形成过程及标形特征。     

Origin of zoned spinel by coupled dissolution–precipitation and inter-crystalline diffusion: evidence from serpentinized wehrlite, Bangriposi, Eastern India

Textural and mineral–chemical characteristics in the Bangriposi wehrlites (Eastern India) provide insight into metamorphic processes that morphologically and chemically modified magmatic spinel during serpentinization of wehrlite. Aluminous chromite included in unaltered magmatic olivine is chemically homogenous. In sub-cm to 10s-of-micron-wide veins, magnetite associated with antigorite and clinochlore comprising the serpentine matrix is near-stoichiometric. But Al–Cr–Fe³⁺ spinels in the chlorite–magnetite veins are invariably zoned, e.g., chemically homogenous Al-rich chromite interior successively mantled by ferritchromite/Cr-rich magnetite zone and magnetite continuous with vein magnetite in the serpentine matrix. In aluminous chromite, ferritchromite/Cr-rich magnetite zones are symmetrically disposed adjacent to fracture-controlled magnetite veins that are physically continuous with magnetite rim. The morphology of ferritchromite–Cr-rich magnetite mimics the morphology of aluminous chromite interior but is incongruous with the exterior margin of magnetite mantle. Micropores are abundant in magnetite veins, but are fewer in and do not appear to be integral to the adjacent ferritchromite–Cr-rich magnetite zones. Sandwiched between chemically homogenous aluminous chromite interior and magnetite mantle, ferritchromite–Cr-rich magnetite zones show rim-ward decrease in Cr₂O₃, Al₂O₃ and MgO and complementary increase in Fe₂O₃ at constant FeO. In diffusion profiles, Fe₂O₃–Cr₂O₃ crossover coincides with Al₂O₃ decrease to values <0.5 wt% in ferritchromite zone, with Cr₂O₃ continuing to decrease within magnetite mantle. Following fluid-mediated (hydrous) dissolution of magmatic olivine and olivine + Al–chromite aggregates, antigorite + magnetite and chlorite + magnetite were transported in 10s-of-microns to sub-cm-wide veins and precipitated along porosity networks during serpentinization (T: 550–600 °C, f(O₂): ?19 to ?22 log units). These veins acted as conduits for precipitation of magnetite as mantles and veins apophytic in chemically/morphologically modified magmatic Al-rich chromite. Inter-crystalline diffusion induced by chemical gradient at interfaces separating aluminous chromite interiors and magnetite mantles/veins led to the growth of ferritchromite/Cr-rich magnetite zones, mimicking the morphology of chemically modified Al–Cr–Fe–Mg spinel interiors. Inter-crystalline diffusion outlasted fluid-mediated aluminous chromite dissolution, mass transfer and magnetite precipitation.     

Some petrological aspects of the Prairie Creek diamond-bearing kimberlite diatreme,Arkansas

Based on modal and chemical composition, the rocks of the Prairie Creek diatreme situated 4 km SSE of Murfreesboro, Pike County, Arkansas, are classified as micaceous kimberlite. The K-Ar isotopic analysis of phlogopite from this diatreme yielded an age of 106 ± 3 m.y. (Albian) which is in agreement with stratigraphic relations. Electron beam probe data on minerals from kimberlite breccia, one of the three textural types, are presented. The breccia is considered as the potential source of the diamonds that have been mined at the diatreme. It contains phenocrysts of olivine (Fo_90–92) and serpentine pseudomorphs after olivine embedded in a groundmass of serpentine, minor calcite, chrome-diopside, phlogopite (Mg/Mg+Fe = 84.15%), perovskite, spinels, and pentlandite. Xenoliths of shales, sandstones, and mantle-derived ultramafic material are also present. Spinels are rich in Cr, Ti, and Fe and generally low in Al. Zoned spinels show enrichments in Ti and Fe towards their rims. A positive correlation between 100(Fe³⁺+Ti)/(Cr+Al+Fe³⁺+Ti) and 100 Mg/(Mg+Fe²⁺) ratios exists in these spinels and probably reflects an oxygen fugacity increase during magma crystallization. Occluded gases in diamonds and kimberlites corroborate the hypothesis that the parent magma of the Prairie Creek kimberlite was derived by partial melting of upper-mantle garnet lherzolite under volatile-rich conditions, primarily enriched in H₂O and CO₂.     

Contact metamorphosed dunite-harzburgite complex in the Chugoku district,western Japan

The Tari-Misaka ultramafic complex, which is emplaced into the Paleozoic sediments and thermally metamorphosed by two younger granitic masses, is divided into four zones on the basis of the mineral assemblage. They are, in order of increasing metamorphic grade: Zone I antigorite-olivine-orthopyroxene-clinopyroxene. Zone II olivine-talc. Zone III olivine-anthophyllite. Zone IV olivine-orthopyroxene. Strongly serpentinized clinopyroxene-bearing harzburgite in Zone I is similar to ordinary Alpine-type harzburgite. In Zonne II, two kinds of olivine are recognized. One is Mg-rich olivine (Fo₉₃ to Fo₉₇) with opaque inclusions and is probably a recrystallization product of serpentine with talc. The other is Fe-rich olivine (Fo₈₈ to Fo₉₃) free of opaque inclusions and is probably a relic of the primary peridotite. Olivine in Zone III and Zone IV is also relatively Mg-rich (Fo₉₁ to Fo₉₅). Chromitite in Zone IV commonly has an assemblage, olivine+cordierite+Mg-Al spinel (Mg/Mg+Fe²⁺, more than 0.9). Enstatite is rare and coexists with less magnesian Mg-Al spinel (Mg/Mg+Fe²⁺, less than 0.9). Petrological and mineralogical characters of the ultramafic rocks can be well explained by thermal metamorphism of strongly serpentinized peridotite by granitic intrusion. Metamorphic zones are consistent with the experimental results in the system MgO-SiO₂-H₂O. The assemblage olivine+cordierite indicates that the metamorphism occurred at relatively low pressures (<3kb).     

Differentiation of nelsonitic magmas in the formation of the ~1.74 Ga Damiao Fe–Ti–P ore deposit, North China

The ~1.74 Ga Damiao anorthosite complex, North China, is composed of anorthosite and leuconorite with subordinate melanorite, mangerite, oxide-apatite gabbronorite, perthite noritic (i.e., jotunitic) and ferrodioritic dykes. The complex hosts abundant vein-, pod- and lens-like Fe–Ti–P ores containing variable amounts of apatite (10–60 modal%) and Fe–Ti oxides. In addition to Fe–Ti–P ores, there are also abundant Fe–Ti ores which are closely associated with Fe–Ti–P ores in the deposit. Most of Fe–Ti–P ores are dominated by Fe–Ti oxides and apatite, devoid of silicate minerals, mineralogically similar to the common nelsonites elsewhere. In contrast, Fe–Ti ores are dominated by Fe–Ti oxides with minor apatite (<5 modal %). The parental magma of these ores, estimated from olivine and apatite compositions using mineral-melt partition coefficients, has composition similar to the ferrodioritic dykes. Fe–Ti–P ores have variable Fe–Ti oxides and apatite proportions, indicating that they are cumulates. Their simple assemblage of Fe–Ti oxides and apatite and local net-texture suggest that the Fe–Ti–P ores in Damiao have formed from nelsonitic melts immiscibly separated from the ferrodioritic magma during late-stage differentiation. Fe–Ti ores are also cumulates and have mineral compositions similar to Fe–Ti–P ores. The close association between Fe–Ti and Fe–Ti–P ores indicates that the Fe–Ti ores may have also formed from the nelsonitic melts. We proposed that differentiation of nelsonitic melts accompanied by gravity settling is responsible for the formation of Fe–Ti and Fe–Ti–P ores. Such a differentiation process in nelsonitic melts is well supported by variations of Sr, Y, Th, U, REE and Eu/Eu* of apatite in Fe–Ti–P ores. Using oxides/apatite ratio of 2:1 and compositions of apatite and calculated primary oxides, we estimate the composition of the nelsonitic melt as ~52.0 wt% Fe₂O₃t, ~18.5 wt% CaO, ~14.2 wt% P₂O₅, ~8.7 wt% TiO₂, ~4.0 wt% Al₂O₃ and ~1.1 wt% MgO with minor SiO₂, K₂O, Na₂O and F. Such a nelsonitic melt is suggested to be possibly conjugated with Si-rich melts compositionally similar to the Damiao jotunitic dykes (~50 wt% SiO₂ and ~15 wt% Fe₂O₃t) which may subsequently evolve to mangeritic rocks in Damiao. Our modeling also indicates that the onset of immiscibility occurs at a time when the evolved melt has ~44 wt% SiO₂, ~21 wt% Fe₂O₃t, ~3.0 wt% TiO₂ and ~2.6 wt% P₂O₅. High oxygen fugacity and phosphorous content in magmas may play important roles in the immiscibility of nelsonitic magmas, including promoting iron enrichments and widening the two-liquid field.     

Geochemical and Petrological Studies on the Early Carboniferous Sidingheishan Mafic–Ultramafic Iintrusion in the Southern Margin of the Central Asian Orogenic Belt, NW China

The Sidingheishan mafic-ultramafic intrusion is located in the eastern part of the Northern Tianshan Mountain, along the southern margin of the Central Asian Orogenic Belt in northern Xinjiang autonomous region of China. The Sidingheishan intrusion is mainly composed of wehrlite, olivine websterite, olivine gabbro, gabbro and hornblende gabbro. At least two pulses of magma were involved in the formation of the intrusion. The first pulse of magma produced an olivine-free unit and the second pulse produced an olivine-bearing unit. The magmas intruded the Devonian granites and granodiorites.An age of 351.4±5.8 Ma(Early Carboniferous) for the Sidingheishan intrusion has been determined by U-Pb SHRIMP analysis of zircon grains separated from the olivine gabbro unit. A U-Pb age of 359.2±6.4 Ma from the gabbro unit has been obtained by LA-ICP-MS. Olivine of the Sidingheishan intrusion reaches 82.52 mole% Fo and 1414 ppm Ni. On the basis of olivine-liquid equilibria, it has been calculated that the MgO and FeO included in the parental magma of a wehrlite sample were approximately10.43 wt% and 13.14 wt%, respectively. The Sidingheishan intrusive rocks are characterized by moderate enrichments in Th and Sm, slight enrichments in light REE, and depletions in Nb, Ta, Zr and Hf. The ε_(Nd)(t) values in the rock units vary from +6.70 to +9.64, and initial ~(87)Sr/~(86)Sr ratios range between 0.7035 and0.7042. Initial ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb and ~(208)Pb/~(204)Pb values fall in the ranges of 17.23-17.91,15.45-15.54 and 37.54-38.09 respectively. These characteristics are collectively similar to the Heishan intrusion and the Early Carboniferous subduction related volcanic rocks in the Santanghu Basin, North Tianshan and Beishan area. The low(La/Gd)_(PM) values between 0.26 and 1.77 indicate that the magma of the Sidingheishan intrusion was most likely derived from a depleted spinel-peridotite mantle.(Th/Nb)_(PM)ratios from 0.59 to 20.25 indicate contamination of the parental magma in the upper crust.Crystallization modeling methods suggest that the parental magma of the Sidingheishan intrusion was generated by flush melting of the asthenosphere and subsequently there was about 10 vol%contamination from a granitic melt. This was followed by about 5 vol% assimilation of upper crustal rocks. Thus, the high-Mg basaltic parental magma of Sidingheishan intrusion is interpreted to have formed from partial melting of the asthenosphere during the break-off of a subducted slab.     

Metasomatism and ore formation at contacts of dolerite with saliferous rocks in the sedimentary cover of the southern Siberian platform

The data on the mineral composition and crystallization conditions of magnesian skarn and magnetite ore at contacts of dolerite with rock salt and dolomite in ore-bearing volcanic—tectonic structures of the Angara—Ilim type have been integrated and systematized. Optical microscopy, scanning and transmission electron microscopy, electron microprobe analysis, electron paramagnetic resonance, Raman and IR spectroscopy, and methods of mineralogical thermometry were used for studying minerals and inclusions contained therein. The most diverse products of metasomatic reactions are found in the vicinity of a shallow-seated magma chamber that was formed in Lower Cambrian carbonate and saliferous rocks under a screen of terrigenous sequences. Conformable lodes of spinel-forsterite skarn and calciphyre impregnated with magnesian magnetite replaced dolomite near the central magma conduit and apical portions of igneous bodies. At the postmagmatic stage, the following mineral assemblages were formed at contacts of dolerite with dolomite: (1) spinel + fassaite + forsterite + magnetite (T = 820?740°C), (2) phlogopite + titanite + pargasite + magnetite (T = 600–500°C), And (3) clinochlore + serpentine + pyrrhotite (T = 450°C and lower). Rock salt is transformed at the contact into halitite as an analogue of calciphyre. The specific features of sedimentary, contact-metasomatic, and hydrothermal generations of halite have been established. The primary sedimentary halite contains solid inclusions of sylvite, carnallite, anhydrite, polyhalite, quartz, astrakhanite, and antarcticite; nitrogen, methane, and complex hydrocarbons have been detected in gas inclusions; and the liquid inclusions are largely aqueous, with local hydrocarbon films. The contact-metasomatic halite is distinguished by a fine-grained structure and the occurrence of anhydrous salt phases (CaCl₂ · KCl, CaCl₂, nMgCl₂ · mCaCl₂) and high-density gases (CO₂, H₂S, N₂, CH₄, etc.) as inclusions. The low-temperature hydrothermal halite, which occurs in skarnified and unaltered silicate rocks and in ore, is characterized by a low salinity of aqueous inclusions and the absence of solid inclusions. The composition and aggregative state of inclusions in halite and forsterite indicate that salt melt-solution as a product of melting and dissolution of salt was the main agent of high-temperature metasomatism. Its total salinity was not lower than 60%. The composition and microstructure of magnetite systematically change in different mineral assemblages. Magnetite is formed as a result of extraction of iron together with silicon and phosphorus from dolerite. The first generation of magnetite is represented by mixed crystals, products of exsolution in the Fe-Mg-Al-Ti-Mn-O system. The Ti content is higher at the contact of dolerite with rock salt, whereas, at the contact with dolomite, magnetite is enriched in Mg. The second generation of magnetite does not contain structural admixtures. The distribution of boron minerals and complex crystal hydrates shows that connate water of sedimentary rocks could have participated in hydrothermal metasomatic processes.