Extensive middle to late Jurassic felsic and mafic magmatism occurred in the southern Hunan province, southeast China. SHRIMP zircon U–Pb dating, mineral chemical, element geochemical and Sr–Nd–Hf ...isotopic data have been determined for these rocks. The results indicate that the middle Jurassic (178–170 Ma) Changchengling and Ningyuan basaltic rocks belong to tholeiitic series and alkaline series, respectively, and were formed by fractional crystallization of the parent magmas that were derived from partial melting of asthenospheric mantle triggered by the slab-released fluids of the subducted Paleo-Pacific plate. The late Jurassic (152–146 Ma) Daoxian basalts and Guiyang lamprophyre dikes are low-Ti, high-Mg potassic rocks. They were derived directly by partial melting of shallower (60–100 km) lithospheric mantle composed of amphibole- and phlogopite-bearing lherzolite in response to thermal perturbations associated with the rifting of continental arc. The middle Jurassic (164 Ma) Tongshanling granitic rocks belong to calc-alkaline series and are strongly peraluminous. They were formed by partial melting of Palaeoproterozoic metasedimentary basement in the lower-crust plus additional input from coeval basaltic magma. The late Jurassic (156 Ma) Jinjiling and Xishan granitic rocks show A
2 subtype affinity. They were generated by partial melting of granulitized Palaeoproterozoic metamorphic basement in the lower crust in response to injection of coeval low-Ti, high-Mg potassic magmas. A moderate degree (56–58%) of fractionation of these pure crustal melts could account for more felsic end-member of granitic rocks. Detailed petrologic and geochemical data for the middle to late Jurassic felsic and mafic rocks from the southern Hunan imply that during the middle Jurassic time SE China on the southeast of the Shi-Hang zone was a continental arc coupled with the subduction of the Palaeo-Pacific plate and since the beginning of late Jurassic time an intra-arc rift has been formed along the Shi-Hang zone as a consequence of slab roll-back.
To investigate how large volumes of silicic melts segregate to form granitic plutons, we conducted a case study of a zoned pluton, in which SiO2 increases from intermediate (69 wt%) to highly silicic ...compositions (74 wt%) toward the contact with metasedimentary wallrock in the outer 25 m of the pluton. All other major, minor, and trace elements vary systematically with SiO2 and indicate that outward increasing SiO2 is due to a decrease in mafic elements and minerals. Whole-rock oxygen isotopes and elemental variation diagrams do not support mixing with wallrock as an explanation for the Si-rich boundary layer. Instead, mafic enclaves, which are common in the pluton, also decrease in abundance in the outer 25 m of the pluton, suggesting a mechanical origin for the Si-rich boundary layer. The coupling of mechanical and geochemical boundary layers, combined with geochemical modeling, indicate that the silica-rich, enclave-poor boundary layer formed by hindered settling or compaction of a crystal-rich (crystal fractions >60%) magmatic mush. Segregation of melts at high crystal fraction is known to be a slow process. However, petrography and Zr-based thermometry indicate that the residual Si-rich liquids were water-saturated. Water decreases melt viscosity, which helps expulsion, but equally importantly, water also delays much of the latent heat release to late in the thermal and crystallization history of a cooling magma. We show that the higher the water content, the longer the time interval over which a magma chamber resides at the stage when water-saturated, high-silica liquids form, allowing sufficient time for exfiltration of silicic liquids before the magma body freezes.
Synthesized rare earth ion-adsorption clays (La, Ce, Nd, Dy and Y, ~ 400 ppm each) were prepared from kaolinite at various basic pH in open-air conditions. X-ray diffraction (XRD), X-ray ...photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry (ICP-MS) were used to characterize the effect of pH on the adsorption mechanism of rare earth elements (REEs) on kaolinite. The crystal structure of kaolinite by XRD analysis is not altered after REE adsorption, suggesting that REEs are bound to the surface of kaolinite. Elemental analysis by XPS and ICP-MS show that the surface concentration of REEs on kaolinite is pH dependent, and a local maximum of the surface concentration is achieved at pH 10. Three characteristics of the La 3d5/2 XPS photoemission feature were measured and compared to those of standard La compounds to probe the speciation of REEs, and electrostatically bound hydroxides such as REE(OH)2+ are identified as the primary species on the surface of kaolinite. Compared to the typical ion-adsorption clays with intrinsic acidic pH, clays synthesized at basic pH exhibit similarly high rare earth ion-exchange efficiency with ammonium sulfate (AS), suggesting that rare earth ion-adsorption clays can exist in both acidic and basic environments.
Display omitted
•REE ion-adsorption clays were synthesized at basic pH in open-air conditions.•Electrostatically bound hydroxides such as REE(OH)2+ are identified as the primary species on the clay surface.•Synthesized clays show high REE ion-exchange efficiency•REE ion-adsorption clays can exist in basic environments with hydroxylated REEs.
► The difference in clay mineral type is one key element controlling CH4 adsorption in clay-rich rocks. ► BET surface area is a primary proxy for CH4 adsorption capacity in clay-rich rocks. ► Clay ...type affects gas-sorption capacity and the presence of moisture can greatly reduce gas-sorption capacity. ► Quantitative model prediction is developed to estimate the methane sorption capacity of organic-lean shales.
In this study a series of CH4 adsorption experiments on clay-rich rocks were conducted at 35°C, 50°C and 65°C and at CH4 pressure up to 15MPa under dry conditions. The clay-dominated rock samples used are fresh samples from quarries and mines. Samples are individually dominated by montmorillonite, kaolinite, illite, chlorite, and interstratified illite/smectite. The experimental results show that clay mineral type greatly affects CH4 sorption capacity under the experimental conditions. In terms of relative CH4 sorption capacity: montmorillonite≫illite/smectite mixed layer>kaolinite>chlorite>illite. Physisorption is the dominant process for CH4 absorption on clay minerals, as a result, there is a linear correlation between CH4 sorption capacity and BET surface area in these clay-mineral dominated rocks. The abundance of micro-mesopores in the size range of a few to a few 10s of nanometers in montmorillonite clay and illite–smectite interstratified clay results in large BET surface area values for these mineral species.
A good linear relationship between the natural logarithm of Langmuir constant and the reciprocal of temperature exists for clay-mineral dominated rocks, which provides a way to quantify the impact of clay mineral type on gas adsorption capacity. Thermodynamic parameters, the heat of CH4 adsorption and the standard entropy, are calculated based on this linear correlations. The heat of adsorption (q) and the standard entropy (Δso) range from 9.4 to 16.6kJ/mol and from −64.8 to −79.5J/mol/K, respectively, values considerably smaller than those for CH4 adsorption on kerogens. Thus, it is expected that CH4 molecules may preferentially occupy surface sites on organic matter, in addition, the clay minerals are easily blocked by water. As a consequence, organic-rich mudrocks possess a larger CH4 sorption capacity than clay-dominated rocks lacking organic matter.
The global systematics of primitive arc melts Schmidt, M. W.; Jagoutz, O.
Geochemistry, geophysics, geosystems : G3,
August 2017, 2017-08-00, 20170801, Volume:
18, Issue:
8
Journal Article
Peer reviewed
We extracted all volcanic arc rock analyses calculated to be in equilibrium with mantle olivine from the global georoc database. This results in 938 primitive melt compositions from 30 arcs. Based on ...geochemical criteria six principal types of primitive arc melts can be distinguished: calc‐alkaline basalts and andesites, tholeiitic basalts, highly depleted tholeiitic andesites, shoshonites and low‐Si basalts. Their major element systematics indicates that last mantle equilibration occurred mostly at 1.0–2.5 GPa, 1220–1350°C for tholeiitic and calc‐alkaline basalts, at 0.5–1.2 GPa and ∼1200°C for depleted tholeiitic andesites, and at 0.7–1.2 GPa, 1050–1150°C for calc‐alkaline andesites. Quantitative treatment of major and trace elements suggests that the different melt types can be explained by a combination of variable mantle wedge preconditioning (degree of depletion prior to slab component addition, metasomatism in the lithosphere), variation in the amount and nature of the slab component added, and ‐ for primitive calc‐alkaline andesites ‐ reactive fractionation in the lithospheric top of the mantle wedge. The different slab components are best characterized by high Na2O, TiO2, Zr and Th for slab melts; high K2O/Na2O and more pronounced Nb, Sr, and Pb anomalies for fluids; and high K2O at high K2O/Na2O for supercritical liquids. A slab component that is dominantly a slab melt is common in continental but rare in intra‐oceanic arcs, consistent with comparatively cooler slabs in intra‐oceanic subduction zones. A majority of the arcs has more than one melt type, testifying for heterogeneity in the mantle wedge and added slab component.
Key Points
We provide a complete data set of truly primitive arc melts
We identify six arc melt types as formed in the mantle and their major formation mechanisms
The nature of the slab component and the characteristics of the mantle wedge is deduced via major and trace elements
Volcán de Colima is a highly active stratovolcano at the western end of the Trans-Mexican Volcanic Belt. Present-day activity consists of lava dome growth and destruction cycles, lava flows, small ...explosions, and larger explosive Vulcanian eruptions. It has been postulated that an increased frequency of more mafic eruptions signals the run-up to the end of c. 100-year eruptive ‘cycles’, terminating with a Plinian eruption such as those in 1818 and 1913. It is therefore important to understand the role played by mafic recharge during interplinian activity. We present new petrological and geochemical data for lava and ash from the 2013–17 phase of eruption. The uniform paragenesis and geochemical homogeneity of bulk rocks indicate efficient long-term homogenisation of magmas within the plumbing system, similar to the previous 1998–2005 eruptive products. Mineral chemistry however preserves complex patterns of magma recharge and mixing. Chemical and textural information support the interpretation of two magmatic end-members – an evolved end-member, saturated with respect to Fe--Ti oxides and apatite and crystallising low-An plagioclase and pyroxenes in the Mg# 69–75 range; and a more primitive, mafic end-member, crystallising high-An plagioclase and pyroxenes in the Mg# 77–88 range. Pyroxene textures and zoning patterns suggest mixing of the mafic melts with the evolved magma and remobilisation of the crystal mush. Two-pyroxene geothermometry constrains magmatic temperatures to c. 980–1000 °C for the evolved end-member, and c. 1020–1080 °C for the mafic end-member. Pressure estimates suggest crystallisation at 4–6 kbar, or c. 12–18 km depth. We interpret this to reflect periodic injections of mafic melts and remobilised crystals into evolved reservoirs in a mushy magma storage system in the mid-crust, in agreement with geophysical data suggesting a semi-molten, partially crystallised body at this depth. An increase in reverse zoned crystals, indicative of mafic injection, from mid-2015 onwards suggests that these melts were injected into the system following the large eruption in July 2015. Our findings suggest that the intense July 2015 eruption may be linked to increased input of mafic magmas into the shallow system, indicating that mafic injections may be a key process governing the timing and style of interplinian eruption at Volcán de Colima.
•New petrological and geochemical data for the 2013–17 eruption at Volcán de Colima.•Uniform bulk paragenesis and geochemistry indicate efficient homogenisation of magmas.•Injections of mafic melts into evolved reservoirs remobilise crystal mush material.•Suggests mushy magmatic system in the mid-crust, agreeing with geophysical data.•Mafic injections may have been related to the intense July 2015 eruption.
This study describes the: (1) morphological nature of the calcites in the Toki granite, central Japan; (2) the difference in chemical compositions in terms of morphological classification; and (3) ...the identification of the stages of calcite formation and the corresponding mass transfer between minerals and fluid owing to hydrothermal alterations and groundwater-rock interactions, which reveals the sequential variations in fluid chemistry during the sub-solidus stage. Calcites in the Toki granite were classified into four types: (1) lenticular calcite in the chloritized biotite; (2) granular calcite in the altered plagioclase; (3) intergranular calcite; and (4) fracture-filling calcite. The lenticular, granular, and intergranular calcites contain greater amounts of iron, manganese, and magnesium than fracture-filling calcites. The lenticular calcite in the chloritized biotite, granular calcite in the altered plagioclase, and intergranular calcite formed due to the precipitation of calcium, iron, manganese, and magnesium released from biotite and plagioclase owing to hydrothermal alterations. The fracture-filling calcites formed at a later stage than the lenticular, granular, and intergranular forms. In the hydrothermal fluid, the concentrations of aluminum, iron, manganese, and magnesium gradually decrease, and the concentration of calcium gradually increases as the alteration proceeds. The chemical characteristics of the fluid at the late stage of hydrothermal alteration and those of the subsequent groundwater are consistent with those of fracture-filling calcites, indicating that the fracture-filling calcites precipitated from the fluid at a late stage of hydrothermal alterations and then from the groundwater. Elements released from biotite and plagioclase owing to hydrothermal alterations were incorporated into and fastened to the calcite. Therefore, the calcites influenced the sequential variations in fluid chemistry during the sub-solidus stage.
For petrological calculations, including geothermobarometry and the calculation of phase diagrams (for example, P-T petrogenetic grids and pseudosections), it is necessary to be able to express the ...activity-composition ( a- x) relations of minerals, melt and fluid in multicomponent systems. Although the symmetric formalism--a macroscopic regular model approach to a- x relations--is an easy-to-formulate, general way of doing this, the energetic relationships are a symmetric function of composition. We allow asymmetric energetics to be accommodated via a simple extension to the symmetric formalism which turns it into a macroscopic van Laar formulation. We term this the asymmetric formalism (ASF). In the symmetric formalism, the a- x relations are specified by an interaction energy for each of the constituent binaries amongst the independent set of end members used to represent the phase. In the asymmetric formalism, there is additionally a "size parameter" for each of the end members in the independent set, with size parameter differences between end members accounting for asymmetry. In the case of fluid mixtures, for example, H 2O-CO 2, the volumes of the end members as a function of pressure and temperature serve as the size parameters, providing an excellent fit to the a- x relations. In the case of minerals and silicate liquid, the size parameters are empirical parameters to be determined along with the interaction energies as part of the calibration of the a- x relations. In this way, we determine the a- x relations for feldspars in the systems KAlSi 3O 8-NaAlSi 3O 8 and KAlSi 3O 8-NaAlSi 3O 8-CaAl 2Si 2O 8, for carbonates in the system CaCO 3-MgCO 3, for melt in the melting relationships involving forsterite, protoenstatite and cristobalite in the system Mg 2SiO 4-SiO 2, as well as for fluids in the system H 2O-CO 2. In each case the a- x relations allow the corresponding, experimentally determined phase diagrams to be reproduced faithfully. The asymmetric formalism provides a powerful and flexible way of handling a- x relations of complex phases in multicomponent systems for petrological calculations. PUBLICATION ABSTRACT
The Late Archean Taihua complex, mainly composed of amphibolite and TTG and TTG-like gneisses, is volumetrically most important metamorphic rock suites scattered along the southern margin of the ...North China Craton (NCC). Zircon SHRIMP U–Pb dating shows two episodes (2765
±
13 and 2723
±
9
Ma) of Archean magmatism in the Lushan area with distinct geochemical features. The early (2765
±
13
Ma) suite (TTG-like gneisses) has low-SiO
2 (52.5–66.1
wt%), high-Mg
# (0.47–0.68), low HREE (Yb
N
=
3.0–5.4) and Y (8.07–13.9
ppm) with low to moderate (La/Yb)
N (6.7–37.1) and Sr/Y (25.9–119.3). The younger (2723
±
9
Ma) suite (TTG gneisses) has high-SiO
2 (63.5–74.3
wt%), low-Mg
# (0.13–0.52), very low REE (Yb
N
<
1.8) and Y (<4
ppm) with a wide range of (La/Yb)
N (5.2–86.6), Sr/Y (71.4–949) and showing Eu/Eu*
>
1 (1.20–2.43). Both suites show pronounced negative Nb–Ta and Ti anomalies on the primitive mantle-normalized spidergram. The TTG-like gneiss suite has similar bulk-rock Nd and zircon Hf model ages (∼3.0
Ga) with
ɛ
Nd(
t)
>
0 (0.26–1.46), and is interpreted as resulting from melt of mantle interactions with the melts derived from partial melting of subducted ocean crust with a residual assemblage of garnet
+
clinopyroxene
+
rutile
±
amphibole, which favors subducted slab model for the late Archean TTG. The TTG gneiss suite has abundant relic zircons (2.95–2.80
Ga) with
ɛ
Nd(
t)
<
0 (−1.31 to −0.23), which is best interpreted as derived from partial melting of thickened lower continental crust with a garnet-amphibolite residue (garnet
+
amphibole
±
rutile). Significant high-pressure fractional crystallization (garnet
±
amphibole) and accumulation (plagioclase) are also required in the petrogenesis. The Lushan amphibolite with nearly flat primitive mantle-normalized trace-element pattern is interpreted to represent early ocean crust metamorphism. These observations suggest a possible model of late Archean crustal accretion from ocean crust to continental terrain in the southern North China Craton.
PDF
