Experimental modal metasomatism of a spinel lherzolite and the production of amphibole-bearing peridotite
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Amphibole
Metasomatism
Peridotite
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Peridotite
Carbonatite
Ultramafic rock
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Distinct equilibration temperatures, deformation and trace element characteristics are observed in amphibole-bearing and amphibole-free peridotite xenoliths from Nushan, Sino-Korean Craton, eastern China. Amphibole-free peridotites are predominantly deformed, fine-grained (∼1 mm) and equilibrated at 990–1110°C. Their cpx are characterized by either light rare earth element (LREE)-depleted or relatively flat REE patterns with only a slight depletion in high field strength elements (HFSE). LREE enrichment is generally associated with Fe-rich samples, consistent with 'wall-rock' metasomatism adjacent to basaltic veins. In contrast, amphibole-bearing peridotites are less deformed, coarse-grained (>3 mm) and display chemical zonation in the pyroxenes suggesting cooling from 1050 to 850°C. Their cpx show a large variation in LREE (Cen = 1·7–68) and almost constant heavy rare earth element (HREE) contents (Ybn = 9·8–11·6). The highest LREE contents occur in cpx from amphibole-rich samples, coupled with Fe enrichment, strong enrichment in Th and U, and marked depletion in the HFSE. These characteristics may be accounted for by combined 'wall-rock' and 'diffuse' metasomatism involving an agent rich in volatiles and incompatible elements. As such the Nushan xenoliths could represent samples from two spatially separate metasomatic aureoles. Conversely, the cryptic and modal metasomatism could be genetically related, because the amphibole-peridotites and Fe-rich amphibole-free samples show similar Sr–Nd isotopic ranges that are indistinguishable from those of the Cenozoic basalts from eastern China. The different metasomatic assemblage and the trace element composition can be accounted for in terms of P–T control on amphibole stability and progressive chemical evolution of asthenosphere-derived melts during upward migration. Trace element signatures and metasomatic assemblages, together with the fertile composition of the Nushan peridotites, suggest an origin as newly accreted lithosphere rather than as relic cratonic mantle. Metasomatism may have occurred after late Mesozoic lithospheric thinning, which marked a dramatic change in lithospheric architecture beneath the Sino-Korean Craton.
Metasomatism
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Peridotite
Xenolith
Trace element
Rare-earth element
Primitive mantle
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Metasomatism
Amphibole
Trace element
Peridotite
Xenolith
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Amphibole
Metasomatism
Peridotite
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Amphibole
Metasomatism
Peridotite
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Peridotite
Metasomatism
Chromite
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Peridotite
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Amphibole is a common hydrous mineral in mantle rocks. To better understand the processes leading to the formation of amphibole-bearing peridotites and pyroxenites in mantle rocks, we have undertaken an experimental study reacting lherzolite with hydrous basaltic melts in Au-Pd capsules using the reaction couple method. Two melts were examined, a basaltic andesite and a basalt, each containing 4 wt% of water. The experiments were run at 1200°C and 1 GPa for 3 or 12 h, and then cooled to 880°C and 0.8 GPa over 49 h. The reaction at 1200°C and 1 GPa produced a melt-bearing orthopyroxenite-dunite sequence. The cooling stimulates crystallization of orthopyroxene, clinopyroxene, amphibole, and plagioclase, leading to the formation of an amphibole-bearing gabbronorite–orthopyroxenite–peridotite sequence. Compositional variations of minerals in the experiments are controlled by temperature, pressure, and reacting melt composition. Texture, mineralogy, and mineral compositional variation trends obtained from the experiments are similar to those from mantle xenoliths and peridotite massif from the field including amphibole-bearing peridotites and amphibole-bearing pyroxenite and amphibolite that are spatially associated with peridotites, underscoring the importance of hydrous melt-peridotite reaction in the formation of these amphibole-bearing rocks in the upper mantle. Amphiboles in some field samples have distinct textual and mineralogical features and their compositional variation trends are different from that defined by the melt-peridotite reaction experiments. These amphiboles are either crystallized from the host magma that entrained the xenoliths or product of hydrothermal alterations at shallow depths.
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Subduction of oceanic crust buries an average thickness of 300–500 m of sediment that eventually dehydrates or partially melts. Progressive release of fluid/melt metasomatizes the fore-arc mantle, forming serpentinite at low temperatures and phlogopite-bearing pyroxenite where slab surface reaches 700–900 °C. This is sufficiently high to partially melt subducted sediments before they approach the depths where arc magmas are formed. Here, we present experiments on reactions between melts of subducted sediments and peridotite at 2–6 GPa/750–1100 °C, which correspond to the surface of a subducting slab. The reaction of volatile-bearing partial melts derived from sediments with depleted peridotite leads to separation of elements and a layered arrangement of metasomatic phases, with layers consisting of orthopyroxene, mica-pyroxenite, and clinopyroxenite. The selective incorporation of elements in these metasomatic layers closely resembles chemical patterns found in K-rich magmas. Trace elements were imaged using LA-ICP-TOFMS, which is applied here to investigate the distribution of trace elements within the metasomatic layers. Experiments of different duration enabled estimates of the growth of the metasomatic front, which ranges from 1–5 m/ky. These experiments explain the low contents of high-field strength elements in arc magmas as being due to their loss during melting of sedimentary materials in the fore-arc.
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Phlogopite
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