Contributions to Mineralogy and Petrology最新文献

Carbonate–silicate liquid immiscibility is the most widely accepted petrogenetic model for explaining the origin of carbonatites, accounting for approximately three-quarters of global carbonatite occurrences. Many, however, attribute the coexistence of carbonatites and alkaline silicate rocks to the coincidental emplacement of two independent parental magmas through a single crustal conduit. Thus, the exact cause of the carbonatite-alkaline silicate rock association remains equivocal. Here, we present the results of ⁴⁰Ar/³⁹Ar dating, geochemical, and C-O-Sr–Nd-Pb isotopic investigations of the Sarnu-Dandali carbonatite-alkaline complex, part of the Deccan Large Igneous Province (LIP), to shed light on this coexistence. Additionally, we explore the roles of the Deccan-Reunion mantle plume and the Indian continental lithosphere in generating the carbonatites of the complex. ⁴⁰Ar/³⁹Ar age data reveal that, although the complex underwent multiple cycles of alkaline magmatism, the activity at ~ 68.8 Ma was synchronous with the carbonatite intrusion. Interlaced spatial association of carbonatites and alkaline silicate rocks, including melt inclusions of the former in the latter, their complementary trace element patterns (and δ¹³C and δ¹⁸O), and overlapping initial Sr–Nd-Pb isotopic ratios indicate their co-genesis through carbonate–silicate liquid immiscibility. Trace element and isotopic ratio modeling suggests that the parental melanephelinitic magma for the complex evolved through concurrent crustal assimilation (up to 6%), fractional crystallization of silicates, and immiscible separation of carbonate melt, with the latter occurring before phonolite crystallization. The least contaminated isotopic ratios suggest that the parental carbonated-silicate magma derived from a mantle source with a mixed signature of the Reunion plume and metasomatized continental lithosphere.

Previously published thermal expansion data for almandine and spessartine garnets are sparse and inconsistent. We have therefore measured the thermal expansion of two garnets of compositions Alm90 Grs2.7 Py2.3 Ski4.5 (sample Alm90) and Sps81 Alm15 Grs3 And1 (sample Sps81) from ca. 90 K to ca. 780 K by synchrotron X-ray powder diffraction using quartz as an internal calibrant for temperature. The adiabatic elastic tensors of these two samples have been determined by single-crystal Brillouin scattering at room conditions; for the Alm90 sample c₁₁ = 302.0(5), c₁₂ = 111.3(4), c₄₄ = 93.3(1), giving K_S = 174.9(4) GPa, and for the Sps81 sample c₁₁ = 303.1(4), c₁₂ = 109.5(4), c₄₄ = 93.6(2), giving K_S = 174.0(4) GPa. These new data have been used in combination with P-V, P-T-V and P-T-Ks data from the literature to determine the equations of state (EoS) of end-member almandine and spessartine. The parameters of q-compromise Mie-Grüneisen-Debye thermal pressure EoS with a third-order Birch-Murnaghan EoS to describe the compressional properties at room temperature are:

The parameters are available in .eos files for the EosFit suite of programs in the supplementary data, and from www.rossangel.com and www.mineralogylab.com. The most significant changes from the previous EoS are the lower Debye temperatures because our data show higher thermal expansion above room temperature. The biggest consequence for host-inclusion piezobarometry is that these new EoS lead to lower entrapment temperatures for zircon inclusions in garnet. We also show that there is no significant difference in the P-V/V₀ behaviour of almandine and spessartine and that the available experimental data for intermediate compositions does not indicate any significant non-ideality in the compressional or thermal expansion behaviour of almandine-spessartine garnets.

Sheared peridotite xenoliths entrained by kimberlites provide snapshots of metasomatism and deformation within the cratonic mantle. Here, we present H₂O concentrations of nominally anhydrous minerals (olivine, orthopyroxene and clinopyroxene) in 18 low-, moderate- and high-T sheared peridotites from the Kimberley and northern Lesotho kimberlite cluster (Kaapvaal craton). H₂O contents range from < 1–80 µg/g (olivine), 70–310 µg/g (orthopyroxene) and 120–300 µg/g (clinopyroxene), implying hydration through metasomatism. Core to rim heterogeneities in H₂O concentrations indicate that ortho- and clinopyroxene cores preserve an older metasomatic/deformation event in the lithospheric mantle while olivine cores record the youngest metasomatic event. The latter must have happened immediately prior to xenolith entrainment and transport to the surface. Calculated H₂O contents for the metasomatic agents range from 1 to 3 wt% H₂O. We speculate that highly-reactive hydrous (proto-)kimberlitic melts caused oxidizing metasomatism that triggered deformation prior potentially enhanced craton destabilization as well as the local resorption of diamonds.

Although volumetrically minor, ultramafic magmatism in arc settings is globally widespread. The genesis of arc-related ultramafic magmas is hypothesized to require efficient slab dehydration that results in high water concentrations in the hottest portion of the sub-arc mantle wedge where mantle-equilibrated magmas attain picritic compositions. Using the highly variable concentrations of Ca in olivine (Fo_89.6−93.5), measured by EPMA, we show that ultramafic volcanic rocks of the Late Triassic Nicola Group in the North American Cordillera record the ascent of crystal-rich picritic slurries (~ 40–60 wt% accumulated olivine + Cr-spinel) driven by limited escape of exsolved H₂O ± other volatiles. The picrites record initial crystallization of low-Ca olivine (~ 400 ppm) from an oxidized and water-rich (≥ 8wt.%) near-primary magma containing ~ 19.3 wt% MgO, followed by rapid degassing-driven ascent, during which olivine with high Ca contents (> 2000 ppm) crystallized. The concomitant changes in Mn content and Fe/Mn ratios of olivine and the increasing Fe³⁺ contents in coexisting Cr-spinel suggest that degassing also resulted in significant oxidation of the melt, possibly through SO₂ degassing.

Constraining the cooling timescales/rates of metamorphic rocks of orogens is critical for understanding their tectonic settings and geodynamic mechanisms, especially their links to plate tectonics and the formation of the continental lithosphere. The Orosirian period (2.1–1.8 Ga) is characterized by widespread orogenesis and is potentially linked to the onset of global plate tectonics, yet the metamorphic cooling histories of Orosirian orogens remain underexplored. This study applies a dual-constraint diffusion modeling approach, integrating both major (Fe, Mn, Mg, Ca) and trace elements (HREE + Y) in garnet. This is based on multi-mineral microprobe and laser ablation mapping to two metamorphic rocks from the Wutai–Fuping crustal section of the Trans-North China Orogen (TNCO). Sample 20FP09, collected from the Wanzi Supracrustal Rocks, records a two-stage retrograde P–T path: an isothermal decompression (ITD) from the M2 peak (725 ° C/0.9 GPa) to M3 decompression (725 ° C/0.6 GPa), followed by isobaric cooling (IBC) to M4 (550 ° C/0.4 GPa). Sample L17, from the Shizui Subgroup, preserves a single cooling segment from the M2T peak (680 ° C/0.6 GPa) to M3 (550 ° C/0.4 GPa). Diffusion modeling using multiple experimental datasets of diffusion coefficients yields an ITD duration of 1–4 Myr and an IBC duration of 11–31 Myr for sample 20FP09, and a single 35–50 Myr cooling interval for sample L17. Together, these results indicate a protracted cooling duration of 13–50 Myr, corresponding to a cooling rate of 2–13 ° C/Myr for the TNCO. Such rates are significantly slower than those of Phanerozoic orogens (> 100 ° C/Myr). This implies that Orosirian orogenesis may have involved in prolonged thermal relaxation within a hot, weak Paleoproterozoic lithosphere, likely influenced by higher mantle temperatures and orogenic thermal structures. This study also demonstrates the importance of large datasets containing high spatial element maps for obtaining robust constraints on metamorphic timescales; and offers a first-order empirical estimate of Sc diffusivity in garnet, which remains experimentally unconstrained.

The Osorno and Calbuco stratovolcanoes are situated just 27 km apart along the volcanic arc front of the Central Southern Volcanic Zone (Chile), yet they exhibit significantly different characteristics. Notably, Calbuco intermediate to andesitic lavas contain amphibole, which is absent in Osorno, and has a much higher explosivity (up to VEI = 4 to 5). Their olivine-hosted melt inclusions (MI) display similar major elements differentiation trends. Incompatible trace element ratios overlap in both volcanoes and therefore do not correlate with the H₂O and Cl contents. Indeed, MIs data and petrological hygrometers indicate higher volatiles content at Calbuco (~ 3 wt% @ 51.3 wt% SiO₂, 951–3232 ppm) compared to Osorno (~ 1 wt% @ 50.5 wt% SiO₂, 521–1091 ppm). Because the Cpx-only geobarometers and geophysical data both place the Osorno and Calbuco main magma reservoirs at the same depth, their differing volatile contents are interpreted as inherited from the mantle melts. Their calculated primary magmas result from similar to identical partial melting fractions of mantle sources that differ only in their H₂O content. The mantle source beneath Calbuco partially melted at a lower temperature, possibly because of the slightly more trench ward position of the volcano, which projects vertically into a cooler part of the mantle wedge “nose”. The lack of correlation between incompatible elements and H₂O content is interpreted as resulting from evidence for a decoupling between H₂O, transported mainly in a solute-poor fluid, and incompatible elements, which are delivered by melts derived from the subducting oceanic lithosphere. The variability of the mantle wedge H₂O concentration and the uncorrelated content of slab tracers and H₂O have also been observed at a global scale.

Metasomatic reactions affect the mechanical properties of the interface between the mantle wedge and a subducting plate. However, the controls on fluid-mediated reactions that occur at the interface, involving mass transfer, volume change, and deformation, are poorly constrained. In this study, we experimentally investigated metasomatic reactions at the boundary between mantle rocks (serpentinite and harzburgite) and sedimentary rocks (quartzite or pelitic schist) at 500 °C and 1 GPa, using an assembly with a core of metasedimentary rock sandwiched between serpentinite and harzburgite. In all experimental runs, talc formed preferentially in the mantle rocks. In experiments using quartzite, talc formed along pre-existing fractures in serpentinite and as a layer within harzburgite, while no obvious alteration occurred in the quartzite. In experiments using pelitic schist, talc formed within tree-like fractures in serpentinite, whereas in harzburgite it formed as a layer or wedge-like filling. In the pelitic schist, albite porphyroblasts were preferentially replaced by Mg-rich saponite. These observations and mass balance calculations indicate that: (1) the solid volume-decreasing reactions in serpentinite accompanying reaction-induced fracturing and solid volume-increasing reaction in harzburgite caused the contrasting textures in the two rock types; and (2) Al-bearing minerals in the sedimentary rocks absorbed Mg, which facilitated the overall progress of the Mg-releasing, talc-forming reactions in the mantle rocks. These results suggest that compositional heterogeneity in subducting sediments can produce variations in the talc distribution and rheological properties at the slab–mantle interface, potentially affecting the location of episodic tremor and slip in a mantle wedge corner.

The Assam-Meghalaya Gneissic Complex (AMGC) in northeast India represents the eastern fringe of the Precambrian Indian Shield and comprises Meso- to Neoproterozoic migmatitic quartzofeldspathic gneisses, foliated granitoids, metapelites and Ediacaran-Cambrian granites. Our work focuses on the petrogenesis of garnet-bearing and garnet-absent granites of the West Garo Hills (WGH), in the westernmost AMGC, that are intrusive into basement quartzofeldspathic gneisses (QFG) and foliated granitoids. We use phase equilibria modelling to evaluate whether partial melting of basement rocks could have produced the Ediacaran-Cambrian granites in the area. Textural and chemical characteristics of garnet support a magmatic origin. Phase equilibria modelling, carried out on garnet-bearing granite, indicates that the garnet core composition equilibrated at ∼7.5 kb/830°C, while rim composition record ∼5.3 kb/730°C. Phase equilibria modelling suggests that the QFG and metapelite basement rocks were metamorphosed at ∼5.5 kb/750°C and ~ 5.7 kb/740°C, respectively. U-Pb dating of monazite and xenotime in the granite, the host QFG, and pegmatites yielded comparable 591 − 542 Ma, 582 − 540 Ma and 562 − 460 Ma ages, respectively, which date the timing of high-grade metamorphism and granite emplacement during Pan-African orogenesis. Our melt-reintegrated phase equilibria modelling suggests that partial melting of basement QFG and associated metapelite with varying degrees of fractional crystallization could produce melts having composition similar to those of the granites in the WGH. A comparison of the modelled melt compositions with the compiled granite data from the entire AMGC suggests that basement QFG-derived melts can largely explain the composition of the Ediacaran-Cambrian granites.

Dunite channels are critical pathways for melt extraction and migration within the asthenosphere. Although their formation within mantle peridotite sequences is often attributed to melt-rock interaction, the underlying physical processes remain poorly constrained. This study examines variations in both major and trace elements across a dunite-harzburgite sequence in the Zedong ophiolite to better understand these physical processes. Geochemical profiles across the dunite-harzburgite transect reveal concentration gradients near the dunite-harzburgite contact, particularly within the harzburgite. Such compositional variations were interpreted through two-dimensional numerical simulations, coupled with diffusion and advection in porous media. The results indicate that trace element gradients in olivine can be produced by a melt flow along the dunite–harzburgite boundary with lateral advection from dunite to harzburgite, which is consistent with those observed in the field. Trace element patterns of the interstitial clinopyroxene grains within the dunite suggest their crystallization from a final pulse of percolating melt through the channel during the waning stage of the melt transport system, which probably occurred shortly before the ophiolite obduction. Integrating these results with previous studies on mafic–ultramafic rocks in the Moho Transition Zone (MTZ), we propose that the dunite–harzburgite sequence in the Zedong ophiolite was initially generated in the asthenosphere beneath an ultra-slow spreading ridge, which subsequently experienced a multi-stage history of melt migration and melt–rock interaction during progressive mantle upwelling. Such a complex history significantly influenced the lithological and geochemical architecture of the MTZ of the ophiolite. Overall, results from this study demonstrate that both dunites and their surrounding harzburgites bear variable concentration gradients, resulting from both mantle heterogeneity and variations in melt compositions within individual dunite channel.