Crystal origins and magmatic system beneath Ngauruhoe volcano (New Zealand) revealed by plagioclase textures and compositions (original) (raw)
Related papers
Journal of Petrology, 2012
Ruapehu, New Zealand' s largest active andesite volcano, is located at the southern tip of the Taupo Volcanic Zone (TVZ), the main locus of subduction-related volcanism in the North Island. Geophysical data indicate that crustal thickness increases from 525 km within theTVZ to 40 km beneath Ruapehu.The volcano is built on a basement of Mesozoic meta-greywacke, and geophysical evidence together with xenoliths contained in lavas indicates that this is underlain by oceanic, meta-igneous lower crust. The present-day Ruapehu edifice has been constructed by a series of eruptive events that produced a succession of lava flow-dominated stratigraphic units. In order from oldest to youngest, these are the Te Herenga (250^180 ka), Wahianoa (160^115 ka), Mangawhero (55^45 ka and 20^30 ka), and Whakapapa (15^2 ka) Formations. The dominant rock types are plagioclase-and pyroxene-phyric basaltic andesite and andesite. Dacite also occurs but only one basalt flow has been identified. There have been progressive changes in the minor and trace element chemistry and isotopic composition of Ruapehu eruptive rocks over time. In comparison with rocks from younger formations, Te Herenga eruptive rocks have lower K 2 O abundances and a relatively restricted range in major and trace element and Nd^Sr isotopic composition. Post-Te Herenga andesites and dacites define a Sr^Nd isotopic array that overlaps with the field forTVZ rhyolites and basalts, but Te Herenga Formation lavas and the Ruapehu basalt have higher 143 Nd/ 144 Nd ratios.The isotopic, and major and trace element composition of Te Herenga andesite can be replicated by models involving mixing of an intra-oceanic andesite with a crustal component derived from a meta-igneous composition. Post-Te Herenga andesites show considerable variation in major and trace element and Sr and Nd isotopic compositions ( 87 Sr/ 86 Sr ranges from 0·7049 to 0·7060 and 143 Nd/ 144 Nd from 0·51264 to 0·51282). The range of compositions can be modeled by assimilation^fractional crystallization (AFC) involving meta-greywacke as the assimilant, closed-system fractionation, or by mixing of intra-oceanic andesite or basalt and a meta-greywacke crustal composition. Plagioclase and pyroxene compositions vary over wide ranges within single rocks and few of these have compositions consistent with equilibration with a melt having the composition of either the host-rock or groundmass. The 87 Sr/ 86 Sr compositions of plagioclase also vary significantly within single whole-rock samples. Glass inclusions and groundmasses of andesitic rocks all have dacitic or rhyolitic major and trace element compositions. The application of various mineral geothermometers and geobarometers indicates pre-eruption temperatures between 950 and
Magma mingling in an andesite pyroclastic flow of the Pourahu Member, Ruapehu volcano, New Zealand
Journal of Volcanology and Geothermal Research, 1995
We describe a magma mingling episode from Ruapehu volcano between two andesite magmas, one very much minor in volume relative to the other. The event acted to trigger eruption of the andesitic Pourahu pyroclastic flow which is preserved in a thick sequence of tephras and laharic deposits in the southeastern ring plain of the volcano. The predominant andesite is pale brown coloured and porphyritic containing phenocrysts of plagioclase-clinopyroxene-orthopyroxene-Fe-Ti oxides. Rare clasts of a darker andesite are different texturally, less vesicular, and contain distinctive microphenocrysts of plagioclase and quench olivine. Equally rare clasts, of streaky pumice consisting of interbanded 'dark' and 'light' andesite attest to mingling between these two andesite components. Chemical analyses of discrete clasts demonstrate that the Pourahu pyroclastic flow andesites span much of the compositional spectrum of Ruapehu andesites. This observation demonstrates heterogeneity in the products of a relatively small eruption. The darker clast analyses and those from associated distal fall deposits lie within the fields defined by the dominant light coloured clasts. Phenocryst and microphenocryst geothermometry suggest slightly higher temperatures in the dark component. However, glasses from groundmass and phenocryst inclusions in the same specimen may differ considerably, leading us to conclude that many phenocrysts are in fact xenocrystic and were incorporated in the melts as they migrated towards the surface. We prefer a model in which a small volume of hot andesite magma injects a vent-feeding magma chamber, triggering vesiculation and eruption. We infer that the process of magma withdrawal extended downward into the magma body causing the dark component to intermingle with the lighter (dominant) component, 'sucking' more dark magma into the chamber. Our observations are entirely consistent with the existence of a plexus of small, possibly interlinked magma chambers beneath Ruapehu.
Journal of Petrology, 2008
Products of the 1915 Lassen Peak eruption reveal evidence for a magma recharge^magma mixing event that may have catalyzed the eruption and from which four compositional members were identified: light dacite, black dacite, andesitic inclusion, and dark andesite. Crystal size distribution, textural, and in situ chemical (major and trace element and Sr isotope) data for plagioclase from these compositional products define three crystal populations that have distinct origins: phenocrysts (long axis40Á5 mm) that typically have core An contents between 34 and 36 mol %, microphenocrysts (long axis between 0Á1 and 0Á5 mm) that have core An contents of 66^69, and microlites (long axis50Á1mm) with variable An core contents from 64 to 52. Phenocrysts are interpreted to form in an isolated dacitic magma chamber that experienced slow cooling. Based on textural, compositional, and isotopic data for the magma represented by the dacitic component, magma recharge was not an important process until just prior to the 1915 eruption. Average residence times for phenocrysts are in the range of centuries to millennia. Microphenocrysts formed in a hybrid layer that resulted from mixing between endmember reservoir dacite and recharge magma of basaltic andesite composition. High thermal contrast between the two end-member magmas led to relatively high degrees of undercooling, which resulted in faster crystal growth rates and acicular and swallowtail crystal habits. Some plagioclase phenocrysts from the dacitic chamber were incorporated into the hybrid layer and underwent dissolution^precipitation, seen in both crystal textures and rim compositions. Average microphenocryst residence times are of the order of months. Microlites may have formed in response to decompression and/or syn-eruptive degassing as magma ascended from the chamber through the volcanic conduit. Chemical distinctions in plagioclase microlite An contents reveal that melt of the dark andesite was more mafic than the melt of the other three compositions. We suggest that mixing of an intruding basaltic andesite and reservoir dacite before magma began ascending in the conduit allowed formation of a compositionally distinct microlite population. Melt in the other three products was more evolved because it had undergone differentiation during the months following initial mixing; as a consequence, melt and microlites among these three products have similar compositions. The results of this study highlight the integrated use of crystal size distribution, textural, and in situ chemical data in identifying distinct crystal populations and linking these populations to the thermal and chemical characteristics of complex magma bodies.
Tracking magma dynamics at Mount Etna from plagioclase textures and compositional zoning
A systematic study of textural and compositional zoning (An% and FeO variation) in plagioclase phenocrysts of historic (pre-1971) and recent (post-1971) lavas at Mount Etna was made through back-scattered electron (BSE) images and electron microprobe analyses (EMP). The textures considered include oscillatory zoning and several types of dissolution, resorption and growth textures at the phenocryst cores and/or rims. Two patterns of oscillation were recognized from the combined An-FeO variation: 1) Low Amplitude-High Frequency (LAHF) and 2) High Amplitude-Low Frequency (HALF). The first pattern is interpreted here as due to kinetic effects at the plagioclase/melt interface which developed during crystallization in closed reservoirs. The second, which sometimes involves thin dissolution surfaces marked by irregular edges, angular unconformities and complex dissolution-regrowth patterns, might imply crystallization in a more dynamic regime, probably driven by chemical and physical gradients of the system (e.g., convection in a steadily degassing open-conduit). Dissolution and resorption textures at the core vary from patchy (exclusive to plagioclases within pre-1971 lavas) to strongly sieved, and can be related to increasing rates of decompression under H 2 O-undersaturated conditions. Thick sieve-textured envelopes at the phenocryst rims, generally coupled with marked An-FeO increase, result from mixing with more primitive and volatilerich magmas. In the same crystals from recent activity, An and, to a lesser extent, FeO increase, consistent with the mixing of H 2 O-rich magmas similar in their mafic character to the resident magma (cryptic mixing). Two types of growth textures were also recognized at the crystal rims: 1) stripes of regularly-shaped melt inclusions and 2) swallow-tailed, skeletal crystals. In the first instance, the concordant An-FeO decrease suggests crystallization caused by fast ascent-related decompression accompanied by volatile loss. In the second, An decrease at effectively constant FeO contents may indicate crystallization at a high level of undercooling from already degassed magma, followed by rapid quenching; such a feature might be acquired during syn-or post-eruptive conditions. Although textures found in historic lavas are rather similar to those in the recent ones, some differences occur, such as lack of crystals with patchy cores in recent products and lower An contents in crystals of historic ones. The available data allowed us to obtain information on the dynamics of the feeding system, highlighting their possible modifications over time. In particular, historically erupted magmas, generally acknowledged to be volatile-poor, may have ascended through the deep portions of the plumbing system under H 2 O-undersaturated conditions at lower rates than the recent ones, recognized as more volatile-rich. Eruption triggering mechanisms from closed reservoirs in the shallow portions of the feeding system are similar for both historic and recent events, and may be generally favoured by a recharging phase of more primitive, undegassed magma or by a few episodes of important fracture opening (e.g., in response to an earthquake swarm).
Journal of Petrology, 2010
Kilauea volcano is a very intensively studied, active basaltic magmatic system and thus represents an ideal location to study magma solidification processes in a natural environment. Understanding solidification is important in refining models of magma chamber dynamics and its detailed study may improve our knowledge of magma system evolution. In this study magma solidification processes are examined and quantified using samples from the 19691 974 Mauna Ulu (MU) rift eruption. We have collected major and trace element whole-rock data plus in situ olivine compositions, along with crystal size distribution data on 11 lava samples. The observed whole-rock chemical variation was partly produced by olivine addition within the Kilauea edifice. At least two distinct olivine populations are inferred from quantitative textural analysis: (1) a 3^40-year-old population characterized by a low crystal density, greater crystal length and flatter slopes of the crystal size distributions (CSDs); (2) a 1•5^15-year-old population marked by a high density of smaller crystals and steep CSD slopes.The range in olivine composition suggests that all these crystals grew from a range of different magmas, probably closely related by crystal fractionation. The ubiquitous presence of deformed olivine crystals shows that population 1 reflects a component that must have mostly originated by disruption of a deformed cumulate. This antecrystic olivine population represents an earlier-coarsened and aggregated, cumulate-forming magma component. In contrast, the phenocrystic population 2 represents a late magma component formed in the summit magma storage region. Our results are consistent with the hypothesis that the components of the MU magmas followed two different routes. The deformed-olivine-bearing magma moved along the deep basal de¤ collement then rose through vertical pipe-like conduits under the MU rift.The undeformed-olivine-bearing magma rose via the main conduit to the summit reservoir and then moved out along the rift zone, where the magmas mixed in small chambers. The presence of narrow, reversely zoned olivine rims suggests that the mixing occurred just prior to eruption.
Lithos, 2010
A systematic study of textural and compositional zoning (An% and FeO variation) in plagioclase phenocrysts of historic (pre-1971) and recent (post-1971) lavas at Mount Etna was made through back-scattered electron (BSE) images and electron microprobe analyses (EMP). The textures considered include oscillatory zoning and several types of dissolution, resorption and growth textures at the phenocryst cores and/or rims. Two patterns of oscillation were recognized from the combined An–FeO variation: 1) Low Amplitude–High Frequency (LAHF) and 2) High Amplitude–Low Frequency (HALF). The first pattern is interpreted here as due to kinetic effects at the plagioclase/melt interface which developed during crystallization in closed reservoirs. The second, which sometimes involves thin dissolution surfaces marked by irregular edges, angular unconformities and complex dissolution–regrowth patterns, might imply crystallization in a more dynamic regime, probably driven by chemical and physical gradients of the system (e.g., convection in a steadily degassing open-conduit). Dissolution and resorption textures at the core vary from patchy (exclusive to plagioclases within pre-1971 lavas) to strongly sieved, and can be related to increasing rates of decompression under H2O-undersaturated conditions. Thick sieve-textured envelopes at the phenocryst rims, generally coupled with marked An–FeO increase, result from mixing with more primitive and volatile-rich magmas. In the same crystals from recent activity, An and, to a lesser extent, FeO increase, consistent with the mixing of H2O-rich magmas similar in their mafic character to the resident magma (cryptic mixing). Two types of growth textures were also recognized at the crystal rims: 1) stripes of regularly-shaped melt inclusions and 2) swallow-tailed, skeletal crystals. In the first instance, the concordant An–FeO decrease suggests crystallization caused by fast ascent-related decompression accompanied by volatile loss. In the second, An decrease at effectively constant FeO contents may indicate crystallization at a high level of undercooling from already degassed magma, followed by rapid quenching; such a feature might be acquired during syn- or post-eruptive conditions.Although textures found in historic lavas are rather similar to those in the recent ones, some differences occur, such as lack of crystals with patchy cores in recent products and lower An contents in crystals of historic ones. The available data allowed us to obtain information on the dynamics of the feeding system, highlighting their possible modifications over time. In particular, historically erupted magmas, generally acknowledged to be volatile-poor, may have ascended through the deep portions of the plumbing system under H2O-undersaturated conditions at lower rates than the recent ones, recognized as more volatile-rich. Eruption triggering mechanisms from closed reservoirs in the shallow portions of the feeding system are similar for both historic and recent events, and may be generally favoured by a recharging phase of more primitive, undegassed magma or by a few episodes of important fracture opening (e.g., in response to an earthquake swarm).
The last known eruption at Cerro Machín Volcano (CMV) in the Central Cordillera of Colombia occurred w900 years BP and ended with the formation of a dacitic lava dome. The dome rocks contain both normally and reversely zoned plagioclase (An 24e54 ), unzoned and reversely zoned amphiboles of dominantly tschermakite and pargasite/magnesio-hastingsite composition and olivine xenocrysts (Fo ¼ 85e88) with amphibole/clinopyroxene overgrowth, all suggesting interaction with mafic magma at depth. Plagioclase additionally exhibits complex oscillatory zoning patterns reflecting repeated replenishment, fractionation and changes in intrinsic conditions in the magma reservoir. Unzoned amphiboles and cores of the reversely zoned amphiboles give identical crystallization conditions of 910 AE 30 C and 360 AE 70 MPa, corresponding to a depth of about 13 AE 2 km, at moderately oxidized conditions (f O2 ¼ þ0.5 AE 0.2 DNNO). The water content in the melt, calculated based on amphibole chemistry, is 7.1 AE 0.4 wt.%. Rims of the reversely zoned amphiboles are relatively enriched in MgO and yield higher crystallization temperatures (T ¼ 970 AE 25 C), slightly lower melt H 2 O contents (6.1 AE 0.7 wt.%) and overlapping pressures (410 AE 100 MPa). We suggest that these rims crystallized following an influx of mafic melt into a resident magma reservoir at mid-crustal depths, further supported by the occurrence of xenocrystic olivine. Crystallization of biotite, albite-rich plagioclase and quartz occurred at comparatively low temperatures (probably <800 C) during early stages of ascent or storage at shallower levels. Based on amphibole mineral chemistry, the felsic resident melt had a rhyolitic composition (71 AE 2 wt.% SiO 2 ), whereas the hybrid magma, from which the amphibole rims crystallized, was dacitic (64 AE 3 wt.% SiO 2 ). The bulk rock chemistry of the CMV lava dome dacites is homogenous. They have elevated (La/Nb) N ratios of 3.8e4.5, typical for convergent margin magmas, and display several geochemical characteristics of adakites. Both Sr and Nd isotope compositions ( 87 Sr/ 86 Sr w0.70497, 143 Nd/ 144 Nd w0.51267) are among the most radiogenic observed for the Northern Volcanic Zone of the Andes. They are distinct from oceanic crust that has been subducted in the region, pointing to a continental crustal control on the isotope composition and hence the adakitic signature, possibly in a crustal "hot zone".
Contributions To Mineralogy and Petrology, 2002
Textural and compositional zoning in plagioclase phenocrysts in a sample from Parinacota volcano (Chile) was investigated using backscattered electron images and electron microprobe analysis of major and trace elements. Large (2 mm) oscillatory zoned crystals (type I) with resorption surfaces of moderate An discontinuities (⩽10% An) and decreasing trace-element contents (Sr, Mg, Ti) towards the rim reflect melt differentiation and turbulent convection in the main magma body. Early recharge with a low-Sr mafic magma is seen in the core. Small-scale Sr variations in the core indicate limited diffusion and thus residence and differentiation times of the magma shorter than a few thousand years. Smaller crystals (type II) with low trace-element/An ratio reflect the influence of an H2O-rich melt probably from a differentiated boundary layer. Closed-system in-situ crystallisation, mafic magma recharge and the role of a water-rich differentiated boundary layer can be distinguished from the An–trace element relationships. Crystals apparently move relatively freely between different parts and regimes in the magma chamber, evidence for "convective crystal dispersion". High-Sr type II crystals indicate an earlier input of Sr-rich mafic magma. Recharge of two distinct mafic magma types is thus identified (high-Sr and low-Sr), which must have been present – at increasing recharge rates with time – in the plumbing system throughout the volcano's history.
A systematic study of textural and compositional zoning (An% and FeO variation) in plagioclase phenocrysts of historic (pre-1971) and recent (post-1971) lavas at Mount Etna was made through back-scattered electron (BSE) images and electron microprobe analyses (EMP). The textures considered include oscillatory zoning and several types of dissolution, resorption and growth textures at the phenocryst cores and/or rims. Two patterns of oscillation were recognized from the combined An-FeO variation: 1) Low Amplitude-High Frequency (LAHF) and 2) High Amplitude-Low Frequency (HALF). The first pattern is interpreted here as due to kinetic effects at the plagioclase/melt interface which developed during crystallization in closed reservoirs. The second, which sometimes involves thin dissolution surfaces marked by irregular edges, angular unconformities and complex dissolution-regrowth patterns, might imply crystallization in a more dynamic regime, probably driven by chemical and physical gradients of the system (e.g., convection in a steadily degassing open-conduit). Dissolution and resorption textures at the core vary from patchy (exclusive to plagioclases within pre-1971 lavas) to strongly sieved, and can be related to increasing rates of decompression under H 2 O-undersaturated conditions. Thick sieve-textured envelopes at the phenocryst rims, generally coupled with marked An-FeO increase, result from mixing with more primitive and volatilerich magmas. In the same crystals from recent activity, An and, to a lesser extent, FeO increase, consistent with the mixing of H 2 O-rich magmas similar in their mafic character to the resident magma (cryptic mixing). Two types of growth textures were also recognized at the crystal rims: 1) stripes of regularly-shaped melt inclusions and 2) swallow-tailed, skeletal crystals. In the first instance, the concordant An-FeO decrease suggests crystallization caused by fast ascent-related decompression accompanied by volatile loss. In the second, An decrease at effectively constant FeO contents may indicate crystallization at a high level of undercooling from already degassed magma, followed by rapid quenching; such a feature might be acquired during syn-or post-eruptive conditions. Although textures found in historic lavas are rather similar to those in the recent ones, some differences occur, such as lack of crystals with patchy cores in recent products and lower An contents in crystals of historic ones. The available data allowed us to obtain information on the dynamics of the feeding system, highlighting their possible modifications over time. In particular, historically erupted magmas, generally acknowledged to be volatile-poor, may have ascended through the deep portions of the plumbing system under H 2 O-undersaturated conditions at lower rates than the recent ones, recognized as more volatile-rich. Eruption triggering mechanisms from closed reservoirs in the shallow portions of the feeding system are similar for both historic and recent events, and may be generally favoured by a recharging phase of more primitive, undegassed magma or by a few episodes of important fracture opening (e.g., in response to an earthquake swarm).
Journal of Petrology, 2017
The residence time of crystals in magmatic systems is an essential parameter to describe the dynamics of these systems and to evaluate the temporal representativeness of the mineral data used to document the physical conditions in the magmas. Uranium-series disequilibria in mineral separates from young volcanic rocks with a known eruption date provide unique insights into these residence times. We present 238 U-230 Th-226 Ra measurements for plagioclase separates and groundmass from magmas erupted at Volc an de Colima, Mexico in 2004 and 2007. The (230 Th/ 232 Th) activity ratios in plagioclase show limited variations within the range measured in groundmasses and previously documented for whole-rocks. The (230 Th/ 232 Th) and (238 U/ 232 Th) activity ratios in plagioclase are predominantly controlled by glass present as inclusions in the crystals or adhering to their rims, even in high-purity crystal separates with less than 2% glass. Variations in these activity ratios are best explained by crustal assimilation during crystallization and do not require ageing of the crystals. One plagioclase separate with very limited contamination by glass impurities has a demonstrable zero 230 Th-238 U crystal age. Precise 226 Ra-230 Th model ages are difficult to obtain owing to the strong influences of uncertainties in the partition coefficients, the temperature of crystallization and the correction for glass impurities in crystal separates. Nevertheless, if these uncertainties are taken into account, 226 Ra-230 Th model ages in the range 0-2000 years are obtained for the plagioclase separates. More complex models elaborated on the basis of the compositional zoning observed in plagioclase phenocrysts suggest that the cores of the crystals are at most 8500 years old. Overall, these results indicate that the plagioclase phenocrysts are relatively young and are not sourced from a long-lived near-solidus crystal mush as is often suggested for arc volcanoes. A more dynamic system in which crystallization predominantly occurs in response to degassing during ascent of the melts is conceivable at Volc an de Colima.