Differentiation Of Arc Magmas Research Articles

Ascent-driven differentiation: a mechanism to keep arc magmas metaluminous?

Arc magmatism is fundamental to the generation of new continental or island arc crust. However, the mechanisms that add to the chemical complexity of natural calc-alkaline magmas ranging from basaltic to rhyolitic compositions are debated. Differentiation mechanisms currently discussed include magma mixing, assimilation, crustal melting, or (fractional) crystallisation. In this contribution, the differentiation of arc magmas by decompression-driven crystallisation is investigated. We performed a set of equilibrium crystallisation experiments at variable crustal pressures (200–800 MPa) on a hydrous high-Al basalt (3.5 wt.% of H2O in the starting material) with run temperatures varying from near-liquidus conditions (1110 °C) to 900 °C. Oxygen fugacity was buffered at moderately oxidising conditions close to the NNO equilibrium. Combining these novel experiments with previous polybaric fractional crystallisation experiments (Marxer et al., Contrib Mineral Petrol 177:3, 2022) we demonstrate the effects of pressure on the crystallisation behaviour of calc-alkaline magmas with respect to liquid and cumulate lines of descent, mineral chemistry, and phase proportions. Decompression shifts the olivine-clinopyroxene cotectic curve towards melt compositions with higher normative clinopyroxene and enlarges the stability field of plagioclase. This exerts a key control on the alumina saturation index of residual liquids. We argue that near-adiabatic (or near-isothermal) decompression accompanied by dissolution of clinopyroxene entrained during residual melt extraction in the lower crust keeps arc magmas metaluminous during crystallisation-driven differentiation thereby closely reproducing the compositional spread observed for natural arc rocks.

Nb-Ta fractionation by amphibole and biotite during magmatic evolution: Implications for the low Nb/Ta ratios of continental crust

How did the low niobium/tantalum (Nb/Ta) ratios of continental crust form is a hotly debated issue? Here, we report the Nb Ta features of a well-characterized magmatic rock series, mainly composed of amphibole and biotite cumulates, from the Northern Dabie orogenic belt in China. The Nb/Ta ratios of whole-rock (19.5–26.2), amphibole (16.8–41.1), and biotite (21.1–49.6) are negatively correlated with their Mg # values, indicating that the separation of amphibole and biotite could not only cause Nb Ta fractionation but also reduce the Nb/Ta ratios of residual magmas during magmatic evolution. Furthermore, we compile the Nb/Ta ratios of global arc magmas ( n = 15,696) to examine the role of amphibole and biotite in the differentiation of arc magmas. The Nb/Ta ratios of global arc magmas display a negative correlation with SiO 2 contents and positive correlations with Dy/Yb and Ba/Rb ratios. Additionally, the compositions of the upper continental crust (UCC) exactly fall on the evolution line of Nb/Ta vs. SiO 2 and Nb/Ta vs. Ba/Rb, yet significantly falls below the evolution line of Nb/Ta vs. Dy/Yb. These observations show that amphibole and biotite are two important magmatic phases necessary for processes of arc magma differentiation and that they play a role together in the formation of low Nb/Ta ratios of the continental crust. As a whole, we suggest that amphibole- and biotite-rich rocks stored in the deep crust are one of the potential hidden reservoirs with higher Nb/Ta ratios, which may contribute to the depleted Nb during the formation process of continental crust. • Nb Ta fractionation of arc magmas could be induced by amphibole and biotite. • Amphibole- and biotite-rich rocks in deep crust are potential high Nb/Ta reservoirs.

Petrogenesis of amphibole-rich ultramafic rocks in the Hida metamorphic complex, Japan: Its role in arc crust differentiation

Amphibole controls major and trace element chemistry of arc magma via fractionation, therefore its formation process is the key to understand arc crustal evolution. Amphibole-rich cumulates, occasionally found associated with felsic plutons or metamorphic rocks, provide complementary insights into the arc magma differentiation. In this study, we conducted a petrological examination of amphibole-rich ultramafic rocks from the Hida metamorphic complex in Japan to better understand its role in arc crust evolution. The studied samples are characterized by large poikilitic amphibole and orthopyroxene enclosing olivine grains with corroded margins. Amphibole showed high Cr2O3 and TiO2 contents (up to 0.9 and 1.2 wt%) and light rare earth element enrichment relative to heavy rare earth elements. Microscopic and geochemical features of amphibole and olivine suggest that the amphibole formed at the expense of early crystalized olivine (± clinopyroxene) from hydrous juvenile magma at a deep crustal level, which is consistent with the estimated crystallization pressure of 0.7–1.0 GPa based on an empirical Al-in-hornblende geobarometer. Considering the arc affinity of parental magma and the amphibole-fractionation trend in bulk chemistry of the Mesozoic plutonic rocks from the Hida Belt, it is implied the formation of amphibole-rich plutonic rocks contributed to arc crustal evolution as amphibole-rich residue complementary to differentiated magma.

Nb-Ta systematics of Kohistan and Gangdese arc lower crust: Implications for continental crust formation

• Kohistan and Gangdese (garnet) hornblendites and/or garnet gabbros are of cumulate origins. • Amphiboles in these lower crust cumulates have high Nb/Ta but low Nb and Ta concentrations. • Amphibole fractionation will not change Nb/Ta ratio of arc magmas significantly. • Rutile likely plays an important role in Nb/Ta fractionation in continental crust formation. Earth’s continental crust has a lower Nb/Ta ratio than that of its building blocks –basaltic melts from the mantle. This Nb/Ta mass imbalance implies the existence of a complementary high Nb/Ta reservoir in deep crust. Amphibole-rich arc cumulates have been proposed to be one such candidate due to the preferential partitioning of Nb over Ta into amphibole. To test this proposal, we carried out in-situ analyses of amphibole and rutile in the lower crustal rocks from Kohistan arc and Gangdese arc. Petrological observations and geochemical results suggest that these rocks in both Kohistan arc and Gangdese arc are of magmatic origins and formed by crystal accumulation, i.e. of cumulate. The Kohistan garnet-bearing hornblendite cumulates were formed at 952–1009 ℃; the garnet-free hornblendite cumulates in Kohistan arc and Gangdese arc were formed at 941–992 ℃ and 955–1027 ℃, respectively. We find that, although the amphibole of the Kohistan and Gangdese cumulates have high Nb/Ta ratios (average 18.2 and 17.0, respectively), their Nb and Ta concentrations are far lower than mantle-derived volcanics (Nb < 1.1 ppm, Ta < 0.11 ppm). We show that, with such low concentrations of Nb and Ta, amphibole fractionation will not change arc magma Nb/Ta ratios significantly. Instead, the rutile in Kohistan cumulates has high Nb/Ta ratios and high Nb-Ta concentrations. The calculation shows that rutile may play a more important role in Nb/Ta fractionation during arc magma differentiation in thickened arc crust. Our study provides a mineralogical perspective to understand the formation of continental crust, and improves our understanding how Nb or Ta becomes enriched in arc setting.

Redox processes in subduction zones: Progress and prospect

Oxygen fugacity (fO2) is an intensive variable that describes the redox state of a system. By controlling the valence state of multivalent elements, fO2 affects the stability of iron-bearing minerals, dominants the species of volatile elements (e.g., carbon and sulfur), and controls the partitioning behaviors of multivalent elements (e.g., iron, vanadium, cerium, europium). Thus, fO2 plays a key role in understanding the generation and differentiation of arc magmas, the formation of magmatic-hydrothermal deposits, and the nature of magmatic volatiles. Subduction zones are an important site for arc magmatism and fluid action, and the study of redox processes is indispensable in subduction zone geochemistry. In this paper, we first introduce the concept, expression, and estimation methods of fO2. Then we retrospect the history and progress about the oxidation state of the metasomatized mantle wedge, summarize the redox property of slab-derived fluids, and review the latest progress on redox evolution of arc magmas during magma generation and differentiation. The main conclusions include: (1) despite its wide variation range, fO2 of the mantle wedge is generally higher than that of the oceanic mantle; (2) the redox property of the subducting slab-derived fluids is still controversial and the mechanism for the oxidization of the mantle wedge remains unclear; (3) how the fO2 varies during the generation and differentiation of the arc magmas is debated. We propose that the crux in deciphering the oxidization mechanism of the mantle wedge is to determine the mobility of iron, carbon and sulfur in subducting slab-derived fluids (especially solute-rich fluid or supercritical fluid); the key in understanding the redox evolution during arc magma generation and differentiation is to determine the partition coefficients of Fe3+ and Fe2+ between ferromagnesian minerals and silicate melts.

The importance of H2O in arc magmas for the formation of porphyry Cu deposits

Porphyry copper deposits (PCDs) hosted in subvolcanic intrusions at convergent margins are the primary world’s copper resources. However, the set of magmatic processes that lead to the generation of ore-bearing magmatic provinces remains unclear. In this paper we review the systematic of Cu evolution during arc magma differentiation using new and existing global compilations of whole rock geochemistry data. We trace the Cu evolution from primitive arc magmas through lower crustal plutonic to volcanic rocks. We focus on the well-known tholeiitic and calc-alkaline fractionation sequences, where arc tholeiitic series represents damp primitive melts (<2 wt% H2O) evolving with iron enrichment, and calc-alkaline series are wet primitive melts (>2 wt% H2O) that differentiate with iron depletion.Our study shows that the Cu concentration in primitive arc basalts (~80 ppm) is indistinguishable from that of primitive melts formed at mid-ocean ridges (MORBs) implying that Cu is mainly sourced from the mantle wedge in arcs with a limited contribution from the subducted oceanic lithosphere. A global compilation of plutonic rocks whole rock geochemistry (lower crustal cumulates and derivative melts) indicate no systematic difference in Cu concentrations between cumulates associated with tholeiitic or calc-alkaline series. Yet a complementary global compilation of arc volcanic whole rock geochemistry highlights the contrasting behavior of Cu in tholeiitic and calc-alkaline series during magmatic differentiation. In tholeiitic arc series, Cu shows an incompatible or compatible behavior during magma differentiation influenced by the crustal thickness. In calk-alkaline arc series, Cu is compatible during magma differentiation independently to the crustal thickness. This relates to the timing of sulfide saturation, which is controlled by the liquid lines of descent (LLD) and/or crustal thickness at redox conditions relevant for arc magmas.We demonstrate that the initial melt H2O content in primitive arc melts controls the LLD and the volume of remaining melt mass at fluid saturation. We show that the remaining H2O-saturated melt mass positively correlates with the total mass of Cu transferred into degassing fluids. The mass of extractable Cu ranges from ~3 to ~10 Mt (i.e., large PCD) for calc-alkaline series, and ranges from ~0.3 to ~2.5 Mt for tholeiitic series. The ore-forming potential of calc-alkaline arc magmas is at least ~4 to ~10 times higher relative to tholeiitic arc magmas. Despite the compatible behavior of Cu during magmatic differentiation, we propose that a single stage model for the formation of large economic PCDs (as opposed to multi-stage model for Cu-sulfides storage and remobilization) is most applicable for the calc-alkaline melts. The importance of the initial melt H2O content ultimately reflects the key role of flux melting associated with wet calc-alkaline series and high ore-forming potential, in opposition to decompression melting associated with damp tholeiitic series in arc.

The partitioning behavior of trace elements in subduction zones: Advances and prospects

The partitioning behavior of trace elements is of key importance for understanding the geochemical process and material cycle mechanism in subduction zones. This paper focuses on the advances and prospects on the studies of trace element partitioning in subduction zones from the following four aspects. (1) The properties of fluids derived from subducting slabs and their ability in element transport. How slab-derived solute-rich fluids and supercritical fluids are formed and what the roles and key control factors of these fluids are in transferring of elements (especially the high field strength elements) from slab to wedge are discussed. We point out that the detailed investigations of supercritical fluids may provide a new perspective for the element migration mechanism, material cycle process, arc magma genesis and so on. (2) The behavior of transition elements during mantle wedge melting. The behavior of the first row transition elements (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) in the mantle partial melting process is compatible or incompatible, depending on residual mineral assemblage and physicochemical conditions. The partitioning behavior of the elements such as Sc, Ti, Co, Ni and Zn whose valence states do not change in the melting process mainly depends on the residual mineral assemblage and temperature, whereas the partitioning behavior of the multivalent elements such as V and Fe is also the function of oxygen fugacity (fO2) in addition to mineral assemblage and temperature. Therefore, the partitioning behavior of transition elements has important applications in tracing lithologic inhomogeneity and fO2 of the mantle wedges. (3) The specificity of element partitioning behavior during arc magma evolution. Garnet has Dy/Yb partitioning behavior different from amphibole, and rutile has Nb/Ta partitioning behavior different from amphibole. Dy/Yb and Nb/Ta partitioning differences for these minerals enable to distinguish the specific evolution process of arc magmas. The Dy/Yb and Nb/Ta generally decrease with the increase of SiO2 in arc magmas, indicating that amphibole fractionation should be the most important during arc magma differentiation. (4) The behavior of sulfur and chalcophile elements and porphyry metallogeny. In subduction zones, the behavior of chalcophile elements such as Cu and Au is controlled by sulfide and fluid. Therefore, the stability of sulfide, the time at which the fluid exsolves from the melt relative to sulfide saturation, the fluid/sulfide mass ratio and fluid/melt Cu and Au partition coefficients in intermediate-felsic magma- H2O systems are especially important in understanding Cu and Au enrichment in magma-hydrothermal processes. Intermediate-felsic magmas mainly originate from the differentiation of arc magmas at lower crustal reservoirs, and thus the fluid exsolution from the lower crustal reservoirs and the fluid/melt and fluid/sulfide partition coefficients of Cu and Au should be the keys to understanding quantitatively how Cu and Au are migrated from the deep crust to the shallow site of mineralization.

The role of clinopyroxene in amphibole fractionation of arc magmas: Evidence from mafic intrusive rocks within the Gangdese arc, southern Tibet

Amphibole is thought to be a key control on the differentiation of arc magmas but the fractionation of this mineral remains enigmatic because it is rarely present in erupted lavas. This study presents detailed petrological, mineralogical, and geochronological data for newly identified Late Triassic (217–215 Ma) mafic intrusive rocks from the Gangdese arc of southern Tibet. These gabbros host an amphibole-rich cumulate enclave and clinopyroxene crystal clots, including two mineral assemblages in chemical disequilibrium: (1) amphibole and Type-1 clinopyroxene within the enclave and clots; and (2) small crystals of Type-2 clinopyroxene, plagioclase, and biotite within the host. Textural and geochemical evidence indicate that the enclave and clots represent captured fragments of amphibole- and clinopyroxene-dominated lower crustal crystal mushes, respectively, whereas small host crystals formed by the shallow crystallization of evolved melts extracted from these crystal mushes. The compositional variations between the two types of clinopyroxene also reveal this melt segregation process. Amphibole with similar rare earth element patterns to associated Type-1 clinopyroxene inclusions suggest that the former formed through peritectic reactions consuming clinopyroxene. Enhanced amphibole fractionation may also have occurred as a result of the presence of high-density clinopyroxene inclusions. This indicates that the fractionation of amphibole together with precursory clinopyroxene inclusions within the deep crust controls the early chemical evolution of mafic arc magmas, a process that is consistent with the geochemical variations in lavas from three other active arc volcanoes. The associated cumulates can efficiently delaminate back into the mantle, ultimately leading to the generation of SiO2-rich continental crustal material.

Nb/Ta systematics in arc magma differentiation and the role of arclogites in continent formation

The surfaces of rocky planets are mostly covered by basaltic crust, but Earth is unique in that it also has extensive regions of felsic crust, manifested in the form of continents. Exactly how felsic crust forms when basaltic magmas are the dominant products of melting the mantles of rocky planets is unclear. A fundamental part of the debate is centered on the low Nb/Ta of Earth’s continental crust (11–13) compared to basalts (15–16). Here, we show that during arc magma differentiation, the extent of Nb/Ta fractionation varies with crustal thickness with the lowest Nb/Ta seen in continental arc magmas. Deep arc cumulates (arclogites) are found to have high Nb/Ta (average ~19) due to the presence of high Nb/Ta magmatic rutiles. We show that the crustal thickness control of Nb/Ta can be explained by rutile saturation being favored at higher pressures. Deep-seated magmatic differentiation, such as in continental arcs and other magmatic orogens, is thus necessary for making continents.

Lower crustal H2O controls on the formation of adakitic melts

At volcanic arcs, fluids released from the subducting slab lower the solidus of the mantle wedge and cause melting. Furthermore, slab melts may infiltrate the mantle wedge, and have been suggested to generate adakitic (residual garnet) signatures at some arc volcanoes. However, experimental work indicates that the garnet stability field will expand in the lower overriding crust in the presence of somewhat less hydrous melts, suggesting that such signatures may also develop at crustal levels. Here we use geothermometry and plagioclase hygrometry of mafic eruptives from southwest Japan to demonstrate that the adakitic compositions of associated intermediate magmas are of lower crustal origin due to a decrease in the water content of parental melts, and are not generated by partial melting of the eclogitic subducting slab at elevated temperatures. Lower crustal melt evolution at reduced water contents may represent an important process for generating adakitic signatures in all tectonic settings that have previously been considered to enhance slab melting. Our results demonstrate that magmatic water plays a key role in the differentiation of arc magmas in modern and ancient subduction settings.