Decompression-induced Melting Research Articles

Internal igneous growth, doming and rapid erosion of a mature ocean island: the Miocene evolution of Maio (Cabo Verde)

Maio Island (Cabo Verde Archipelago) is composed of uplifted Early Mesozoic MORB-type pillow lavas and deep-sea sediments, unconformably overlain and intruded by Miocene igneous rocks. Combined structural analyses and 40Ar–39Ar dating were used to constrain the Miocene evolution of Maio. Structures and ages of uplifted Mesozoic sequences and crosscutting Miocene dykes showed that numerous intrusive events were associated with the intense growth of an igneous core complex in the middle to upper crust, causing semi-circular doming and partial disruption of the Mesozoic strata. Two nosean nephelinite dykes cut the Valanginian Batalha Formation and yielded phlogopite 40Ar–39Ar ages of 10.405 ± 0.033 Ma and 10.570 ± 0.053 Ma (2σ errors). A nosean nephelinite dyke that cuts the overlying Valanginian to Early Aptian Morro Formation yielded an age of 9.273 ± 0.020 Ma. Combined with existing K–Ar and 40Ar–39Ar ages, this confirmed a main period of island growth between ~ 16 and 8.7 Ma. We re-interpreted extensive polymict conglomerates, which occur below the Late Miocene Monte Penoso Formation, as landslide deposits. A nephelinite lava clast yielded a phlogopite 40Ar–39Ar age of 8.666 ± 0.0274 Ma, which represents a maximum age for these landslides and thus confined a period of large-scale flank collapses and erosion to between 8.7 and 6.7 Ma. Flank collapses and further mass wasting during this period may have rejuvenated the igneous activity, i.e., resulting in the formation of the Tortonian/Messinian Monte Penoso and Malhada Pedra Formations, due to decompression-induced melting at upper mantle depths. Such interaction between flank collapses and rejuvenated volcanism may be a key to better understand ocean island evolution worldwide.

Constraints on the Origin and Evolution of Magmas in the Payún Matrú Volcanic Field, Quaternary Andean Back-arc of Western Argentina

The Payún Matrú Volcanic Field (Pleistocene–Holocene) is located in the Andean back-arc of the Southern Volcanic Zone, western Argentina, and is contemporaneous with the Andean volcanic arc at the same latitude. It includes two polygenetic, mostly trachytic volcanoes: Payún Matrú (with a summit caldera 8 km wide) and Payún Liso (a smaller stratovolcano). The volcanic field includes about 200 scoria cones and alkali basaltic and trachybasaltic lava flows, forming two basaltic fields around Payún Matrú. New 40Ar–39Ar ages extend the activity of Payún Matrú up to 700 ka. The major and trace element and Sr–Nd isotopic compositions of the basaltic lavas and Payún Matrú rocks indicate that the trachytes of Payún Matrú are the result of fractional crystallization of basaltic parent magmas without significant upper crustal contamination, and that the basalts have a geochemical similarity to ocean island basalt (La/Nb = 0·8–1·5, La/Ba = 0·05–0·08). The Sr–Nd isotopic compositions of the basaltic to trachytic rocks range between 0·703813 and 0·703841 (87Sr/86Sr) and 0·512743 and 0·512834 (143Nd/144Nd). Mass-balance and Rayleigh fractionation models support the proposed origin of the trachytes, and an assimilation–fractional crystallization model indicates a low degree of upper crustal contamination in the youngest trachytes. Magnesium numbers (45–55) and contents of Ni (<20–90 ppm) and Cr (30–180 ppm) in the lavas in the basaltic fields indicate that these are not primary magmas. The data also suggest that the basaltic lavas originated in the asthenospheric mantle, probably within the spinel stability field and beneath an attenuated continental lithosphere in the back-arc area. The lack of a slab-fluid signature in the Payún Matrú Volcanic Field rocks, along with unpublished and published geophysical results (mantle tomography and electrical conductivity anomalies) suggest that magmas were generated by decompression-induced melting of upwelling mantle.

DUALITY OF CORDIERITE GRANITES RELATED TO MELT RESTITE SEGREGATION IN THE PENA NEGRA ANATECTIC COMPLEX, CENTRAL SPAIN

A large anatectic complex of Hercynian age, the Pena Negra complex, crops out in the central-north area of the Sierra de Gredos, Avila batholith, central Spain. Migmatites and anatectic granitic rocks were generated during the Upper Carboniferous from a Neoproterozoic ‐ Lower Cambrian schist‐greywacke complex with minor interlayered orthogneisses. Granitic rocks appear as subhorizontal sheets of megacrystic granodiorite and as lenticular bodies of leucogranite. These formed by in situ anatexis and correspond to melt‐restite segregations under two different conditions, namely high melt-fraction in the case of t he granodiorite, and low melt-fraction in the case of the leucogranites. The leucogranites display large enclaves of sillimanite-r ich material, representing the restitic fraction. Granodiorite was generated as consequence of crustal thickening under conditions of T ≈ 750 to 800°C and P ≈ 4 kbar, coinciding with the second phase of the Hercynian orogeny. After this event, an extensional episode involved smaller degrees of decompression-induced melting, and generated small batches of cordierite leucogranites from an already depleted source. Anatexis was enhanced, in both cases, by shear structures and by the existence of a fertile protolith.

Unsolved mysteries of water in its liquid and glassy phases

Although H2O has been the focus of a considerable amount of research since the beginning of the century, its peculiar physical properties are still not well understood. First we discuss some of the anomalies of this `complex fluid'. Then we describe a qualitative interpretation in terms of percolation concepts. Finally, we discuss recent experiments and simulations relating to the liquid-liquid phase transition hypothesis that, in addition to the known critical point in water, there exists a `second' critical point at low temperatures. In particular, we discuss very recent measurements at Tsukuba of the compression-induced melting and decompression-induced melting lines of high-pressure forms of ice. We show how knowledge of these lines enables one to obtain an approximation for the Gibbs potential G(P,T ) and the equation of state V(P,T ) for water, both of which are consistent with the possible continuity of liquid water and the amorphous forms of solid water.

Liquid water: A very complex fluid

Although H2O has been the topic of considerable research since the beginning of the century, the peculiar physical properties are still not well understood. First we discuss some of the anomalies of this ‘complex fluid’. Then we describe a qualitative interpretation in terms of percolation concepts. Finally, we discuss recent experiments and simulations relating to the hyothesis that, in addition to the known critical point in water, there exists a ‘second’ critical point at low temperatures. In particular, we discuss very recent measurements of the compression-induced melting and decompression-induced melting lines of high-pressure forms of ice. We show how knowledge of these lines enables one to obtain an approximation for the Gibbs potential G(P, T) and the equation of state V(P,T) for water, both of which are consistent with the possible continuity of liquid water and the amorphous forms of solid water.

The puzzling statistical physics of liquid water

Although H 2O has been the topic of considerable research since the beginning of the century, the peculiar physical properties are still not well understood. We discuss recent experiments and simulations relating to the hypothesis that, in addition to the known critical point in water, there exists a “second” critical point at low temperatures. In particular, we discuss very recent measurements of the compression-induced melting and decompression-induced melting lines of high-pressure forms of ice. We show how knowledge of these lines enables one to obtain an approximation for the Gibbs potential G( P, T) and the equation of state V( P, T) for water, both of which are consistent with the possible continuity of liquid water and the amorphous forms of solid water. We also comment on some of the evidence that is equally consistent with other scenarios for the behavior of liquid water.

Decompression-induced melting of ice IV and the liquid–liquid transition in water

Although liquid water has been the focus of intensive research for over 100 years, a coherent physical picture that unifies all of the known anomalies of this liquid1,2,3, is still lacking. Some of these anomalies occur in the supercooled region, and have been rationalized on the grounds of a possible retracing of the liquid–gas spinodal (metastability limit) line into the supercooled liquid region4,5,6,7, or alternatively the presence of a line of first-order liquid–liquid phase transitions in this region which ends in a critical point8,9,10,11,12,13,14,. But these ideas remain untested experimentally, in part because supercooled water can be probed only above the homogeneous nucleation temperature TH at which water spontaneously crystallizes. Here we report an experimental approach that is not restricted by the barrier imposed by TH, involving measurement of the decompression-induced melting curves of several high-pressure phases of ice in small emulsified droplets. We find that the melting curve for ice IV seems to undergo a discontinuity at precisely the location proposed for the line of liquid–liquid phase transitions8. This is consistent with, but does not prove, the coexistence of two different phases of (supercooled) liquid water. From the experimental data we calculate a possible Gibbs potential surface and a corresponding equation of state for water, from the forms of which we estimate the coordinates of the liquid–liquid critical point to be at pressure Pc ≈ 0.1 GPa and temperature Tc ≈ 220 K.

Metastable Melting Lines of Ice Phases at Low Temperature.

When ice Ih in an emulsion is compressed below 250 K, it melts to supercooled liquid water, avoiding the formation of other crystal phases [1]. Here, we create emulsified high-pressure ices tinder high pressure and low temperature, and measure their temperature while these ices are decompressed at a constant rate at different temperatures. We detect metastable melting points of high-pressure ices, and identify their melting lines. We find what could be possibly two new ice phases, and discuss the relationship between decompression-induced melting and decompressioninduced amorphization.

Metastable Melting Lines for H2O and the Liquid-Liquid Phase Transition Hypothesis

ABSTRACTWhen ice Ih in an emulsion is compressed below 250 K, it melts to supercooled liquid water, avoiding the formation of other crystal phases. Here, we create emulsified high-pressure ices under high pressure and low temperature, and measure their temperature while these ices are decompressed at a constant rate at different temperatures. We detect metastable melting points of high-pressure ices, and identify their melting lines. We find what could be possibly two new ice phases, and discuss the relationship between decompression-induced melting and decompression-induced amorphization. Finally, we discuss briefly the analysis of experimental data and simulation results that are consistent with the hypothesized “second critical point” with temperature and pressure coordinates of approximately 200 K and 100 Mpa.

MORBCAL: a program for calculating the compositions of primary basaltic melts produced by decompression-induced melting below mid-ocean ridges

A widely accepted model proposes that mid-ocean ridge basalts (MORB) are produced by decompression-induced partial melting of oceanic upper mantle which adiabatically upwells in response to plate separation (McKenzie, 1984; Klein and Langmuir, 1987; McKenzie and Bickle, 1988). In this model, rising mantle begins to melt as it intersects the solidus at a depth determined by its temperature and melting characteristics. Continued upwelling involves continuous melting, matrix compaction, and melt segregation. To quantitatively evaluate this type of melting, Niu and Batiza (1991) developed an empirical scheme based on the peridotite melting experiments of Jaques and Green (1980), Falloon and Green (1987, 1988), and Falloon and others (1988). We used the experimental data to calculate the bulk partition coefficients (Kd) for the major elements. In addition, we determined pressure and temperature (extent of partial melting) dependance of these Kds and used these to calculate the compositions of pooled polybaric melts created by incremental melting. Finally, we developed a method for calculating the melting parameters (initial and final melting pressures and temperatures, plus total extent of melting) for actual MORB melt compositions. This method assumes that primary polybaric pooled melts fractionate only at low pressure and employs the chemical back-tracking method of Klein and Langmuir (1987). The purpose of this Short Note is to present the MORBCAL program. This program is an easy-to-use tool for readers interested in mid-ocean ridge processes and igneous petrogenesis. For details of the melting model, readers are referred to Niu and Batiza (1991). In the rest of the note, we briefly discuss MORBCAL and its availability.

In Situ Densities of Morb Melts and Residual Mantle: Implications for Buoyancy Forces beneath Mid-Ocean Ridges

Recent theoretical studies of mid-ocean ridge (MOR) accretion have shown that the density differences between silicate melt and solid phases and between fertile and residual mantle may be important controls on the upward flow of upper mantle and separation of melt from the upwelling matrix. Both compositional and thermal buoyancy may play an important role. To assess quantitatively the magnitude of these effects, we have compiled the available data for calculating densities of silicate melts and solid phases as a function of composition, temperature (1000-1500°C), and pressure (1 atm to any pressure where the solid phases of interest remain stable ). We include measured and calculated values of thermal expansion and compressibility for both solids and liquids. Our calculations for mantle material undergoing decompression-induced melting show that density differences between instantaneous melt and residual solids increase toward the surface, implying an increasing melt migration rate with continuing upwelling and melting. However, the density difference is significantly less than the value of commonly assumed in theoretical models. In agreement with previous studies, we also find that residual mantle is less dense than unmelted mantle; however, using realistic pressures for MORB generation, the difference is small.

Late Precambrian alkaline plutons in southwest India: Geochronologic and rare-earth element constraints on Pan-African magmatism

The Precambrian granulite facies terrain of southwestern India is intruded by a suite of alkali granite and syenite plutons. RbSr whole-rock isotope data for the Angadimogar syenite (AM) and the Peralimala alkali granite (PM), belonging to this suite, define isochron ages of 638 ± 28 and 750 ± 40 Ma, respectively, with initial 87 Sr 86 Sr ratios of 0.7032 ± 0.0008 and 0.7031 ± 0.0008, respectively. These age data, together with data from previous studies, demonstrate long-lived magmatic activity in the time span from the late Proterozoic to the early Palaeozoic, broadly contemporaneous with Pan-African events in other fragments of the Gondwana supercontinent. REE patterns are reported for four plutons of this Pan-African alkali granite-syenite suite: Chengannoor (CR), Vellingiri (VL) and the two dated intrusions (AM and PM). CR and AM are characterised by high total REE, strongly LREE-enriched patterns with no Eu anomaly, associated with low Sr, Rb, U and Th. K 2O, K 2O Na 2O , K 2O MgO and the agpaitic index are lower for these plutons as compared to the other two. The PM and VL intrusions have lower total REE and less strongly fractionated REE patterns, associated with high K 2O, K 2O Na 2O and K 2O MgO ratios, high Sr and Rb levels, but low U and Th. The geochemical patterns in these rocks compare them well with A-type granites and their tectonic relations assign affinities to magmatism of within-plate type. The alkaline magmatism manifests an extensional phase associated with the pre-rift tectonics of the Indian continent within the Gondwana assemblage. A petrogenetic model is development for these plutons, involving decompression-induced melting of deep crustal source materials characterised by low initial 87 Sr 86 Sr and high K Rb ratios.