Three-stage modification of lithospheric mantle: Evidence from petrology, in-situ trace elements, and Sr isotopes of mantle xenoliths in the Cenozoic basalts, northeastern North China Craton

We present mineralogical and geochemical compositions of mantle xenoliths from two Cenozoic basalt localities of the northeastern North China Craton. These xenoliths include lherzolite, harzburgite, and websterite. They are generally fertile in major elements and different from the typical cratonic lithosphere, which is consistent with previous hypotheses regarding craton destruction. The ratios of 87Sr/86Sr and (La/Yb)N of clinopyroxenes (Cpx) in one lherzolite are relatively low in the core but high in the rim. The center of the Cpx grain has a high U concentration. Changes in trace elements and Sr isotopes indicate that later stage high 87Sr/86Sr melt metasomatism superimposed on the early hydrous melt/fluid. The Cpxs in some xenoliths are low in Ti/Eu but high in Ca/Al and light rare earth elements, which indicates carbonate melt metasomatism. 87Sr/86Sr is increased in the core and decreased in the rim of most Cpx grains, which reflects the superposition of two-stage metasomatism. The early agent should be high in 87Sr/86Sr, and the recent agent should be low in 87Sr/86Sr. The Cpxs in olivine websterite are low in 87Sr/86Sr (0.70220−0.70320), which reflects the recent metasomatism of asthenosphere-derived melt. Collectively, these observations reflect a three-stage modification of the lithospheric mantle. First-stage hydrous melt/fluid could come from the dehydration of young subducted plates. Second-stage melt/fluid of high 87Sr/86Sr could derive from the partial melting of the subducted altered oceanic crust, and the recent melt/fluid of low 87Sr/86Sr should be from the asthenosphere.

Metamorphic microdiamond formation is controlled by water activity, phase transitions and temperature

Metamorphic diamonds hosted by major and accessory phases in ultrahigh-pressure (UHP) metamorphic terranes represent important indicators of deep subduction and exhumation of continental crust at convergent plate boundaries. However, their nucleation and growth mechanisms are not well understood due to their small size and diversity. The Bohemian microdiamond samples represent a unique occurrence of monocrystalline octahedral and polycrystalline cubo-octahedral microdiamonds in two different metasedimentary rock types. By combining new and published data on microdiamonds (morphology, resorption, associated phases, carbon isotope composition) with P–T constraints from their host rocks, we demonstrate that the peak P–T conditions for the diamond-bearing UHP rocks cluster along water activity-related phase transitions that determine the microdiamond features. With increasing temperature, the diamond-forming medium changes from aqueous fluid to hydrous melt, and diamond morphology evolves from cubo-octahedral to octahedral. The latter is restricted to the UHP-UHT rocks exceeding 1100 °C, which is above the incongruent melting of phengite, where microdiamonds nucleate along a prograde P–T path in silicate-carbonate hydrous melt. The observed effect of temperature on diamond morphology supports experimental data on diamond growth and can be used for examining growth conditions of cratonic diamonds from kimberlites, which are dominated by octahedra and their resorbed forms.

In Situ Observations of the Transition Between Beryl and Phenakite in Aqueous Solutions Using a Hydrothermal Diamond-Anvil Cell

Beryl and phenakite are important industrial beryllium minerals. In the hydrous melt of the BeO–Al2O3–SiO2–H2O (BASH) system, experiments using quench-type high-temperature and high-pressure equipment have revealed that the different activities of Al2O3 and SiO2 (αAl2O3 and αSiO2) are the main factors that lead to different beryllium mineral assemblages. In this study, we attempted in situ observation of the crystallization process of phenakite and beryl in an aqueous solution of the BASH system using a hydrothermal diamond-anvil cell. Experimental results indicate that phenakite and beryl can crystallize faster in this regime (i.e., 2.93–0.58 × 10−5 cm/s in length and 22.39–3.23 μm3/s in volume) than from a hydrous melt. In addition, in the phenakite and beryl crystallization, pressure–temperature conditions were greater than 467 °C and 220 MPa and 495 °C and 221 MPa, respectively, and their upper temperatures and pressures attained 845–870 °C and 500–1300 MPa. These features indicate that temperature is not the main factor that controls the stability of phenakite and beryl in the BASH system. This stability can be attributed to the diffusion of components in aqueous solution that change αSiO2 and αAl2O3 during the heating and cooling processes. During heating, αSiO2 increases while beryl is dissolving, which leads to phenakite crystallization; during cooling, αSiO2 and αAl2O3 are sufficient for the remaining beryl to recrystallize. Therefore, the transition between phenakite and beryl in the aqueous solution in the BASH system may be different during heating and cooling processes. This reasoning can explain the abundance of phenakite in miarolitic cavities and the occurrence of phenakite, rather than beryl, in hydrothermally altered pegmatites, volcanic rocks, and other beryllium-rich rocks.

In situ determination of water-saturated solidus by electrical discontinuity

<p><span>Water plays an important role in lowering melting temperature of rocks. The water-saturated solidus of rock is critical for understanding the magma generation and the dynamics of the Earth. There have been a lot of water-saturated solidi of rocks constrained by traditional quench method in literature. However, since both of the hydrous silicate melt and aqueous fluid can be quenched to glasses at high pressure, it is difficult to discriminate whether the quenched glasses were from melt or not. As a result, the water-saturated solidi of rocks from different studies may show significant discrepancy. One way to solve this problem is to detect the characteristics change of the rock system in situ, and electrical conductivity measurement is one of the good options. It is known that hydrous melt has much higher electrical conductivity than solid rock, and temperature is much more effective in enhancing melt electrical conductivity than that for aqueous fluid. Once the partial melting is triggered, the electrical conductivity of the water-saturated rock system may have remarkable increase if the hydrous melt is interconnected in the system. Accordingly, the abrupt change of electrical conductivity may mark the solidus temperature. In this study, we performed electrical conductivity measurement for the determination of water-saturated solidus of albite. We adopted albite as the starting material because its water-saturated solidus is well known, which can help to verify the accuracy our method, and its quenched products are not so controversial. The electrical conductivity measurements were carried out at four different pressures ranging from 0.35 GPa to 1.7 GPa in a 3/4″ piston cylinder apparatus with impedance spectroscopy. The obvious change of electrical conductivity was observed at solidus temperature within error, with increase of 1.8-0.18 log unit at 0.35-1.7 GPa. The results showed a stronger increase of conductivity at lower pressures, and fitted well with the water-saturated solidus of albite in literature. One defect of this method is the loss of water during experiment. The final water content in the system is about 1-2 wt%, comparing to the initial 10-15 wt% H<sub>2</sub>O. Nevertheless, the whole system is still water saturated, since water solubility in albite is fairly low. Therefore, if such a method can be improved to keep more water, it may be applied to other rocks to better constrain the water-saturated solidi in the future.</span></p><p> </p>

Subduction sediment-lherzolite interaction at 2.9 GPa: effects of metasomatism and partial melting

We present the results of analogue experiments carried out in a piston–cylinder apparatus at 750–900°C and 2.9 GPa aimed to simulate metasomatic transformation of the fertile mantle caused by fluids and melts released from the subducting sediment. A synthetic H2O- and CO2-bearing mixture that corresponds to the average subducting sediment (GLOSS, Plank, Langmuir, 1998) and mineral fractions of natural lherzolite (analogue of a mantle wedge) were used as starting materials. Experiments demonstrate that the mineral growth in capsules is controlled by ascending fluid and hydrous melt (from 850°C) flows. Migration of the liquids and dissolved components develops three horizontal zones in the sedimentary layer with different mineral parageneses that slightly changed from run to run. In the general case, however, the contents of omphacite and garnet increase towards the upper boundary of the layer. Magnesite and omphacite (± garnet ± melt ± kyanite ± phengite) are widespread in the central zone of the sedimentary layer, whereas SiO2 polymorph (± kyanite ± phengite ± biotite ± omphacite ± melt) occurs in the lower zone. Clinopyroxene disappears at the base of lherzolite layer and the initial olivine is partially replaced by orthopyroxene (± magnesite) in all experiments. In addition, talc is formed in this zone at 750°C, whereas melt appears at 850°C. In the remaining volume of the lherzolite layer, metasomatic transformations affect only grain boundaries where orthopyroxene (± melt ± carbonate) is developed. The described transformations are mainly related to a pervasive flow of liquids. Mineral growth in the narrow wall sides of the capsules is probably caused by a focused flow: omphacite grows up in the sedimentary layer, and talc or omphacite with the melt grow up in the lherzolite layer. Experiments show that metasomatism of peridotite related to a subducting sediment, unlike the metasomatism related to metabasites, does not lead to the formation of garnet-bearing paragenesis. In addition, uprising liquid flows (fluid, melt) do not remove significant amounts of carbon from the metasedimentary layer to the peridotite layer. It is assumed that either more powerful fluxes of aqueous fluid or migration of carbonate-bearing rocks in subduction melanges are necessary for more efficient transfer of crustal carbon from metasediments to a mantle in subduction zones.