The cooling history of granulite is crucial to understanding tectonic scenarios of the continental crust. Ti-in-quartz, a useful indicator of temperature, can decipher the thermal evolution of crustal rocks. Here we apply the Ti-in-quartz (TitaniQ) thermometer to ancient ultrahigh-temperature (UHT) granulites from the Khondalite Belt (KB) in the North China Craton (NCC) and young UHT granulites from the Mogok Metamorphic Belt (MMB), Myanmar. Ti content in quartz was analyzed using a highly precise method constructed in a CAMECA SXFive electron probe microanalyzer (EPMA). The granulites from the two localities show different quartz Ti contents with a constant deforced beam of 10 μm. Matrix quartz and quartz inclusions from the NCC granulites have 57–241 ppm and 65–229 ppm, respectively, corresponding to the TitaniQ temperatures of 653–810 °C and 666–807 °C. The calculated temperatures are significantly lower than the peak temperatures (850–1096 °C) obtained by other methods, due to the formation of abundant rutile exsolution rods in quartz during cooling. Thus, the low calculated temperatures for the NCC granulites reflect a cooling state near or after the exsolution of rutile from quartz, most likely caused by a slow cooling process. However, the matrix quartz from the MMB granulites is exsolution-free and records higher Ti contents of 207–260 ppm and higher metamorphic temperatures of 894–926 °C, close to the peak UHT conditions. This feature indicates that the MMB granulites underwent rapid cooling to overcome Ti loss from quartz. Therefore, determining the amount of Ti loss from quartz by diffusion can provide new insight into the cooling behavior of UHT granulites. When a large deforced beam of 50 μm was used to cover the rutile rods, the matrix quartz in the KB granulites could also yield the TitaniQ temperatures above 900 °C. Thus, our new data suggest that the TitaniQ thermometer could be useful for revealing UHT conditions.
Abstract Subduction zone fluids are critical for transporting materials from subducted slabs to the mantle wedge. Jadeitites from Myanmar record fluid compositions and reactions in the forearc subduction channel. Here we present high‐precision Mg isotope data of the Myanmar jadeitites and associated rocks to understand the Mg isotope composition of subduction zone fluids at forearc depths. Two types of jadeitites (white and green) exhibit distinct Mg isotope compositions. The white jadeitites precipitated from Na‐Al‐Si‐rich fluids and have low δ 26 Mg values, varying from −1.55‰ to −0.92‰, whereas the green jadeitites have higher δ 26 Mg values (−0.91‰ to −0.74‰) due to metasomatic reactions between fluids and Cr spinel. The amphibole‐rich blackwall in the contact boundaries between jadeitites and serpentinites also exhibits low δ 26 Mg values (−1.17‰ to −0.72‰). Therefore, the jadeite‐forming fluids have not only high concentrations of Na‐Al‐Si but also low δ 26 Mg values. The low δ 26 Mg signature of the fluids is explained by the dissolution of Ca‐rich carbonate in subducted sediments or altered oceanic crust, which is supported by the negative correlation of δ 26 Mg with CaO/TiO 2 , CaO/Al 2 O 3 , and Sr in the white jadeitites. Given the common occurrence of Ca‐rich carbonates in the subduction channel, the Mg isotope composition of low‐Mg aqueous fluids would be significantly modified by dissolved carbonates. Metasomatism by such fluids along conduits has the potential to generate centimeter‐scale Mg isotope heterogeneity in the forearc mantle wedge. Therefore, Mg isotopes could be a powerful tracer for recycled carbonates not only in the deep mantle but also in the shallow regions of subduction zones.
Abstract The origin of cratonic lithospheric mantle has been attributed to either high‐pressure (5–7 GPa) melting in hot mantle plumes or low‐pressure (<5 GPa) melting in mid‐ocean ridges or suprasubduction zones. To resolve this long‐standing debate, it is necessary to confirm under what depths the incipient cratonic mantle melted. Compared with most cratonic mantle xenoliths and xenocrysts, which commonly experienced metasomatic modification after melt extraction, diamond inclusions with predominantly harzburgitic compositions can better track the compositional signature of pristine cratonic mantle. This paper presents thermodynamic calculations performed with THERMOCALC software to establish a quantitative relationship between garnet and cratonic peridotite compositions. Along the normal cratonic geotherm (40 mW/m 2 ), the X Ca [atomic Ca/(Ca + Mg + Fe 2+ )] and Cr# [atomic Cr/(Cr + Al) × 100] values in garnet depend mainly on the bulk CaO/Al 2 O 3 and Cr# values, respectively. Furthermore, mantle melting modeling shows that the Cr# of the residue displays a negative correlation with melting pressure at melt fractions of greater than ~15%. Therefore, the high Cr# values (mostly 12–36) of global garnet inclusions in diamond support a low‐pressure (<4 GPa) origin for cratonic lithospheric mantle. More importantly, the incipient cratonic mantle calculated from the garnet inclusions exhibits similar bulk CaO/Al 2 O 3 and Cr# compositions to oceanic lithosphere but different values from arc lithosphere with higher CaO/Al 2 O 3 and Cr# values. These results imply that cratonic mantle was initially formed by the extensive melting of hot ambient mantle in the Archean ocean ridge environments. The shallow oceanic lithosphere was subsequently stacked to generate the thick and stable cratonic lithospheric mantle.
The Chang’E-5 (CE5) mission has demonstrated that lunar volcanism was still active until two billion years ago, much younger than the previous isotopically dated lunar basalts. How the small Moon retained enough heat to drive such late volcanism is unknown, particularly as the CE5 mantle source was anhydrous and depleted in heat-producing elements. We conduct fractional crystallization and mantle melting simulations that show that mantle melting point depression by the presence of fusible, easily melted components could trigger young volcanism. Enriched in calcium oxide and titanium dioxide compared to older Apollo magmas, the young CE5 magma was, thus, sourced from the overturn of the late-stage fusible cumulates of the lunar magma ocean. Mantle melting point depression is the first mechanism to account for young volcanism on the Moon that is consistent with the newly returned CE5 basalts.
Abstract Orogenically thickened lower crust is the key site of crustal differentiation, crustal deformation, and Moho modification. However, the composition of thickened lower crust is still highly debated. Here, we calculate a set of pseudosections with mafic lower crust compositions in the Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –O 2 (NCKFMASHTO) system. Our modelling results show that the maximum thickness of the mafic lower crust increases with the Moho temperature ( T Moho ). In addition, the lithologies of stable mafic crust are characterized by medium‐pressure (MP) to high‐pressure (HP) granulites at 40–50 km, HP granulites and garnet‐omphacite granulites at 50–60 km, and garnet‐omphacite granulites at 60–70 km. Under the Pamir geothermal conditions, mafic rocks with high SiO 2 (>50.2 wt%), X Mg (>0.70), X Ca (>0.49), or low X Al (<0.11) could be stable at 70 km; however, only ~10% of global mafic granulite xenoliths lie within this compositional range. Further modelling indicates that if T Moho reaches 900–1000°C, neither the lower crust nor the upper mantle has significant strength relative to the upper crust and that only ~5–37% of mafic materials are gravitationally stable at 70 km. This implies that the base of doubly thickened (70 km) crust is dominated by intermediate‐felsic rocks, consistent with the low V p and V p / V s values seismically observed in young orogenic crustal roots. Thus, most mafic materials at >70 km could delaminate into the deep mantle. Our results provide insights on the formation of extremely thick crust with a predominantly intermediate‐felsic base and the crustal thickness variation in continental collision zones.
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