Explosive activity on Kīlauea’s Lower East Rift Zone fuelled by a volatile-rich, dacitic melt

Magmas with matrix glass compositions ranging from basalt to dacite erupted from a series of 24 fissures in the first two weeks of the 2018 Lower East Rift Zone (LERZ) eruption of Kīlauea Volcano. Eruption styles ranged from low spattering and fountaining to strombolian activity. Major element trajectories in matrix glasses and melt inclusions hosted by olivine, pyroxene and plagioclase are consistent with variable amounts of fractional crystallization, with incompatible elements (e.g., Cl, F, H2O) becoming enriched by 4-5 times as melt MgO contents evolve from 6 to 0.5 wt%. The high viscosity and high H2O contents (~2 wt%) of the dacitic melts erupting at Fissure 17 account for the explosive Strombolian behavior exhibited by this fissure, in contrast to the low fountaining and spattering observed at fissures erupting basaltic to basaltic-andesite melts. Saturation pressures calculated from melt inclusions CO2-H2O contents indicate that the magma reservoir(s) supplying these fissures was located at ~2-3 km depth, which is in agreement with the depth of a dacitic magma body intercepted during drilling in 2005 (~2.5 km) and a seismically-imaged low Vp/Vs anomaly (~2 km depth). Nb/Y ratios in erupted products are similar to lavas erupted between 1955-1960, indicating that melts were stored and underwent variable amounts of crystallization in the LERZ for >60 years before being remobilized by a dike intrusion in 2018. We demonstrate that extensive fractional crystallization generates viscous and volatile-rich magma with potential for hazardous explosive eruptions, which may be lurking undetected at many ocean island volcanoes.

High Sulfur in Primitive Arc Magmas, Its Origin and Implications

Sulfur contents in 98.5% of melt inclusions (MI) from calc-alkaline subduction basalts do not exceed 4000 ppm, whereas experimentally established limits of sulfur solubility in basaltic melts with high fO2 (characteristic of subduction zones, e.g., QFM + 2) surpass 14,000 ppm. Here we show that primitive (Mg# 62-64) subduction melts may contain high sulfur, approaching the experimental limit of sulfur solubility. Up to 11,700 ppm S was measured in olivine-hosted MI from primitive arc basalt from the 1941 eruption of the Tolbachik volcano, Kamchatka. These MI often contain magmatic sulfide globules (occasionally enriched in Cu, Ni, and platinum-group elements) and anhydrite enclosed within a brown, oxidized glass. We conclude that the ubiquitous low sulfur contents in MI may originate either from insufficient availability of sulfur in the magma generation zone or early magma degassing prior to inclusion entrapment. Our findings extend the measured range of sulfur concentrations in primitive calc-alkaline basaltic melts and demonstrate that no fundamental limit of 4000 ppm S exists for relatively oxidized subduction basalts, where the maximum sulfur content may approach the solubility limit determined by crystallization of magmatic anhydrite.

Mineral formation conditions in acid magmatic systems related to the formation of massive sulfide deposits of the Urals and Altai-Sayany area

Studies of melt inclusions in quartz indicate the similarity of acid magmatic systems of massive sulfde deposits in the Urals and Altai-Sayany region. The melts of normal alkalinity corresponding to rhyodacite and rhyolite compositions and related to the tholeiitic series are dominant in all the deposits considered. The magmas are characterized by the same type evolution with a decreasing content of main oxides (TiO2, Al2O3, FeO, MgO, CaO, Na2O, K2O) and an increasing SiO2 content. Our results show the accumulation of Cu in relatively low-H2O acidic melts of ancient (Cambrian) deposits of the Altai-Sayany region and low metal contents in the intermediate (Silurian–Devonian) and H2O-saturated magmas of the Urals. The youngest (Devonian) magmas of Siberia evolve simultaneously along these two directions. The analysis of melt inclusions in quartz suggests that the minimum contents of trace and rare earth elements are characteristic of the Silurian-Devonian acid melts of the Urals, with their maximum contents in the youngest (Devonian) magmas and the intermediate contents of ancient (Cambrian) magmatic systems of the Altai-Sayany region. The features of rare and rare earth element patterns in melt inclusions in quartz indicate the similarity of acid magmatic systems of massive sulfde deposits in the Urals and Altai-Sayany region with present-day suprasubduction melts in the ocean-continent transition zones. Computational modeling using data on melt inclusions in quartz confrms our previous conclusions (Simonov, Maslennikov, 2020) that the occurrence of contrasting (basic and felsic) volcanic complexes with massive sulfde deposits in the Urals and Altai-Sayany region is a result of evolution of basaltoid magmas. Keywords: conditions of mineral crystallization, basaltic-rhyolitic complexes, massive sulfde deposits, melt inclusions, quartz, acidic melts.

The volcanic and magmatic evolution of Tongariro volcano, New Zealand

<p>Detailed mapping studies of Quaternary stratovolcanoes provide critical frameworks for examining the long-term evolution of magmatic systems and volcanic behaviour. For stratovolcanoes that have experienced glaciation, edifice-forming products also act as climate-proxies from which ice thicknesses can be inferred at specific points in time. One such volcano is Tongariro, which is located in the southern Taupō Volcanic Zone of New Zealand’s North Island. This study presents the results of new detailed mapping, geochronological and geochemical investigations on edifice-forming materials to reconstruct Tongariro’s volcanic and magmatic history which address the following questions: (1) Does ice coverage on stratovolcanoes influence eruptive rates and behaviour (or record completeness)? (2) What is the relationship between magmatism, its expression (i.e. volcanism) and external but related processes such as tectonics? (3) How are intermediate-composition magmas assembled and what controls their diversity? (4) What are the relative proportions of mantle-derived and crust-derived materials in intermediate composition arc magmas? (5) Do genetic relationships exist between andesite and rhyolite magmas in arc settings? Samples from 250 new field localities in under-examined areas of Tongariro were analysed for major oxide, trace element and Sr-Nd-Pb isotope compositions. Analyses were performed on whole-rock, groundmass and xenolith samples. The stratigraphic framework for these geochemical data was established from field observations and 29 new 40Ar/39Ar age determinations, which were synthesised with volume estimates and petrographic observations for all Tongariro map units. Mapping results divide Tongariro into 36 distinct map units (at their greatest level of subdivision) which were organised into formations and constituent members. New 40Ar/39Ar age determinations reveal continuous eruptive activity at Tongariro from at least 230 ka to present, including during glacial periods. This adds to the discovery of an inlier of old basaltic-andesite (512 ± 59 ka) on Tongariro’s NW sector that has an unclear source vent. Hornblende-phyric andesite boulders, mapped into the Tupuna Formation (new), yield the oldest 40Ar/39Ar age determination (304 ± 11 ka) for materials confidently attributed to Tongariro. Tupuna Formation andesites are correlated with Turakina Formation debris flows that were deposited between 349 to 309 ka in the Wanganui Basin, ~100 km south of Tongariro, which indicates that Ruapehu did not exist at this time, at least not in its current form. Tongariro has a total edifice volume of ~90 km3, 19 km3 of which is represented by exposed mapped units. The total ringplain volume immediately adjacent to Tongariro contains ~60 km3 of material. The volume of exposed glacial deposits are no more than 1 km3. During periods of major ice coverage, edifice-building rates on Tongariro were only 17-21 % of edifice-building rates during warmer climatic periods. Because shifts in edifice-building rates do not coincide with changes in erupted compositions, differences in edifice-building rates reflect a preservation bias. Materials erupted during glacial periods were emplaced onto ice masses and conveyed to the ringplain as debris, which explains reduced preservation rates at these times. MgO concentrations in Tongariro stratigraphic units with ages between 230 and 0 ka display successive and irregular cyclicity that occurs over ~10-70 kyr intervals, which reflect episodes of enhanced mafic magma replenishment. During these cycles, more rapid (≤10 kyr) increases in MgO concentrations to ≥5-9 wt% are followed by gradual declines to ~2-5 wt%, with maxima at ~230, ~160, ~117, ~88, ~56, ~35, ~17.5 ka and during the Holocene. Contemporaneous variations in Tongariro and Ruapehu magma compositions (e.g. MgO, Rb/Sr, Sr-Nd-Pb isotope ratios) for the 200-0 ka period coincide with reported zircon growth model-ages in Taupō magmas. This contemporaneity reflects regional tectonic processes that have externally regulated and synchronised the timings of elevated mafic replenishment episodes versus periods of prolonged crustal residence at each of these volcanoes. Isotopic Sr-Nd-Pb data from metasedimentary xenoliths, groundmass separates and whole-rock samples indicate that two or three separate metasedimentary terranes (in the upper 15 km of the crust) were assimilated into Tongariro magmas. These are the Kaweka terrane and the Waipapa or Pahau terranes (or both). Subhorizontal juxtapositioning of these terranes is indicated by the coexistence of multiple terranes in the same eruptive units. Paired whole-rock and groundmass (interstitial melt) samples have effectively equal Sr-Nd-Pb isotope ratios for the complete range of Tongariro compositions. Despite intra-crystal isotopic heterogeneities that are likely widespread, the new data show that crystal fractionation and assimilation occur in approximately equal balance for essentially all Tongariro eruptives. Assimilated country rock accounts for 22-31 wt% of the average Tongariro magma. Initial evolution from a Kakuki basalt-type to a Tongariro Te Rongo Member basaltic-andesite reflects the addition of 17 % assimilated metasedimentary basement with a mass assimilation rate/mass crystal fractionation rate ratio—a.k.a. ‘r value’ of 1.8-3.5. Subsequent evolution from a Te Rongo Member basaltic-andesite to other Tongariro eruptive compositions represents 5-14 % more assimilated crust (r values of ~0.1-1.0). Magma evolution from high (>1) to lower (0.1-1.0) r values can explain the dearth of andesitic melt inclusions in (bulk) andesite magmas observed globally. High relative assimilation rates characterise rapid evolution from basalt to basaltic-andesite bulk compositions which contain andesitic interstitial melts. Thus, andesitic melt inclusions have a reduced chance of being preserved in crystals which can explain their low representation in global datasets.</p>

The volcanic and magmatic evolution of Tongariro volcano, New Zealand

<p>Detailed mapping studies of Quaternary stratovolcanoes provide critical frameworks for examining the long-term evolution of magmatic systems and volcanic behaviour. For stratovolcanoes that have experienced glaciation, edifice-forming products also act as climate-proxies from which ice thicknesses can be inferred at specific points in time. One such volcano is Tongariro, which is located in the southern Taupō Volcanic Zone of New Zealand’s North Island. This study presents the results of new detailed mapping, geochronological and geochemical investigations on edifice-forming materials to reconstruct Tongariro’s volcanic and magmatic history which address the following questions: (1) Does ice coverage on stratovolcanoes influence eruptive rates and behaviour (or record completeness)? (2) What is the relationship between magmatism, its expression (i.e. volcanism) and external but related processes such as tectonics? (3) How are intermediate-composition magmas assembled and what controls their diversity? (4) What are the relative proportions of mantle-derived and crust-derived materials in intermediate composition arc magmas? (5) Do genetic relationships exist between andesite and rhyolite magmas in arc settings? Samples from 250 new field localities in under-examined areas of Tongariro were analysed for major oxide, trace element and Sr-Nd-Pb isotope compositions. Analyses were performed on whole-rock, groundmass and xenolith samples. The stratigraphic framework for these geochemical data was established from field observations and 29 new 40Ar/39Ar age determinations, which were synthesised with volume estimates and petrographic observations for all Tongariro map units. Mapping results divide Tongariro into 36 distinct map units (at their greatest level of subdivision) which were organised into formations and constituent members. New 40Ar/39Ar age determinations reveal continuous eruptive activity at Tongariro from at least 230 ka to present, including during glacial periods. This adds to the discovery of an inlier of old basaltic-andesite (512 ± 59 ka) on Tongariro’s NW sector that has an unclear source vent. Hornblende-phyric andesite boulders, mapped into the Tupuna Formation (new), yield the oldest 40Ar/39Ar age determination (304 ± 11 ka) for materials confidently attributed to Tongariro. Tupuna Formation andesites are correlated with Turakina Formation debris flows that were deposited between 349 to 309 ka in the Wanganui Basin, ~100 km south of Tongariro, which indicates that Ruapehu did not exist at this time, at least not in its current form. Tongariro has a total edifice volume of ~90 km3, 19 km3 of which is represented by exposed mapped units. The total ringplain volume immediately adjacent to Tongariro contains ~60 km3 of material. The volume of exposed glacial deposits are no more than 1 km3. During periods of major ice coverage, edifice-building rates on Tongariro were only 17-21 % of edifice-building rates during warmer climatic periods. Because shifts in edifice-building rates do not coincide with changes in erupted compositions, differences in edifice-building rates reflect a preservation bias. Materials erupted during glacial periods were emplaced onto ice masses and conveyed to the ringplain as debris, which explains reduced preservation rates at these times. MgO concentrations in Tongariro stratigraphic units with ages between 230 and 0 ka display successive and irregular cyclicity that occurs over ~10-70 kyr intervals, which reflect episodes of enhanced mafic magma replenishment. During these cycles, more rapid (≤10 kyr) increases in MgO concentrations to ≥5-9 wt% are followed by gradual declines to ~2-5 wt%, with maxima at ~230, ~160, ~117, ~88, ~56, ~35, ~17.5 ka and during the Holocene. Contemporaneous variations in Tongariro and Ruapehu magma compositions (e.g. MgO, Rb/Sr, Sr-Nd-Pb isotope ratios) for the 200-0 ka period coincide with reported zircon growth model-ages in Taupō magmas. This contemporaneity reflects regional tectonic processes that have externally regulated and synchronised the timings of elevated mafic replenishment episodes versus periods of prolonged crustal residence at each of these volcanoes. Isotopic Sr-Nd-Pb data from metasedimentary xenoliths, groundmass separates and whole-rock samples indicate that two or three separate metasedimentary terranes (in the upper 15 km of the crust) were assimilated into Tongariro magmas. These are the Kaweka terrane and the Waipapa or Pahau terranes (or both). Subhorizontal juxtapositioning of these terranes is indicated by the coexistence of multiple terranes in the same eruptive units. Paired whole-rock and groundmass (interstitial melt) samples have effectively equal Sr-Nd-Pb isotope ratios for the complete range of Tongariro compositions. Despite intra-crystal isotopic heterogeneities that are likely widespread, the new data show that crystal fractionation and assimilation occur in approximately equal balance for essentially all Tongariro eruptives. Assimilated country rock accounts for 22-31 wt% of the average Tongariro magma. Initial evolution from a Kakuki basalt-type to a Tongariro Te Rongo Member basaltic-andesite reflects the addition of 17 % assimilated metasedimentary basement with a mass assimilation rate/mass crystal fractionation rate ratio—a.k.a. ‘r value’ of 1.8-3.5. Subsequent evolution from a Te Rongo Member basaltic-andesite to other Tongariro eruptive compositions represents 5-14 % more assimilated crust (r values of ~0.1-1.0). Magma evolution from high (>1) to lower (0.1-1.0) r values can explain the dearth of andesitic melt inclusions in (bulk) andesite magmas observed globally. High relative assimilation rates characterise rapid evolution from basalt to basaltic-andesite bulk compositions which contain andesitic interstitial melts. Thus, andesitic melt inclusions have a reduced chance of being preserved in crystals which can explain their low representation in global datasets.</p>

Metals in subduction related magmatism: Insights from melt inclusions and associated glassy groundmass from the Southern Kermadec Arc, New Zealand

<p>The southern Kermadec Arc – Havre Trough (SKAHT) is an intra-oceanic arc – back-arc system where the Pacific plate is subducting beneath the Australian plate. The Kermadec volcanic arc front consists of 33 volcanic centres, four of which host hydrothermal mineralization (Brothers, Haungaroa, Rumble II West, and Clark) such as volcanogenic massive sulfide (VMS) deposits, which are characterised by high concentrations of base and precious metals (e.g., Au, Cu, Zn, Pb). The sources of these metals are strongly tied to the metal contents within underlying magmatic rocks and associated magmatic systems with which the hydrothermal fluids interact. Understanding the sources, movements, and accumulation of metals associated with porphyry copper and exhalative base metal deposits within a subduction – arc setting remains limited. This study reports major, trace, and volatile element contents in basaltic groundmass glasses and olivine-hosted melt inclusions from lavas from four locations within the arc – back-arc setting of the SKAHT. The focus is on understanding the controls on base metal (Pb, Cu, Zn, Mo, V) contents in the magmas. The sample locations, Rumble III and Rumble II West volcanoes, and back-arc Basins D and I, form an arc-perpendicular transect extending from arc front into the back-arc. The analysed melt inclusion and groundmass glasses are all basalt to basaltic andesite in composition, with back-arc basin samples more mafic than arc front volcano samples. The magmatic evolution of the melts is primarily controlled by crystal fractionation of olivine + pyroxene + plagioclase. All glasses have undergone variable degassing, indicated by an absence of detectable CO₂ and curvilinear decreases in S contents with increasing SiO₂. Of the volatile phases analysed, only Cl appears unaffected by degassing. Distinct compositional differences are apparent between arc front and back-arc melts. The arc front magmas formed from higher degrees of melting of a less fertile mantle source and are more enriched in trace elements then the back-arc magmas due to greater additions of slab-derived aqueous fluids to their source. Magmas from a single arc front volcano (Rumble II West) incorporate melts that have tapped variably enriched sources, indicating heterogeneity of the mantle at small scales. Significant variation in mantle composition, however, is also apparent laterally along strike of the arc. Rumble III volcano and Basin I lie on an arc-perpendicular transect south of Rumble II West volcano and Basin D. Their greater enrichment in trace elements and higher concentrations of base metals than Rumble II West and Basin D lavas can be attributed to higher fluxes of subduction derived components. Base metals (Cu, Zn, Pb, Mo, and V) are variably enriched in the SKAHT melts compared with typical mid-ocean ridge basalts with relative enrichments in the order Pb >> Cu > Mo, V > Zn. All metals appear to be affected by mantle metasomatism related to slab-derived fluids, either directly from slab components introduced to the mantle source (e.g., Pb) or through mobilisation of metals within the ambient mantle wedge. The apparently compatible behaviour of Zn, Cu, and V in the mantle means that these elements may be enriched in arc front magmas relative to back-arc magmas by higher degrees of partial melting and/or melting of more depleted sources. All base metals behave incompatibly in the magma during crystal fractionation between 48 – 56 wt.% SiO₂. Lead and Cu concentrations, however, begin to level out from ~ 52 wt.% SiO₂ suggesting some subsequent loss to fractionating volatile phases as metal sulfide complexes. Rumble III samples show a decrease in metal concentration (Pb, Cu, V), from melt inclusions to groundmass glasses, suggestive of more significant loss associated with sulfur degassing. Although other factors such as heat generation, hydrothermal flow, fault systems, and magma venting are key in the development of VMS deposits, this study shows that variations in subduction parameters can significantly affect metal concentrations in arc magmas that may host hydrothermal systems, and hence the amount of metals available to be scavenged into the deposits.</p>

Metals in subduction related magmatism: Insights from melt inclusions and associated glassy groundmass from the Southern Kermadec Arc, New Zealand

<p>The southern Kermadec Arc – Havre Trough (SKAHT) is an intra-oceanic arc – back-arc system where the Pacific plate is subducting beneath the Australian plate. The Kermadec volcanic arc front consists of 33 volcanic centres, four of which host hydrothermal mineralization (Brothers, Haungaroa, Rumble II West, and Clark) such as volcanogenic massive sulfide (VMS) deposits, which are characterised by high concentrations of base and precious metals (e.g., Au, Cu, Zn, Pb). The sources of these metals are strongly tied to the metal contents within underlying magmatic rocks and associated magmatic systems with which the hydrothermal fluids interact. Understanding the sources, movements, and accumulation of metals associated with porphyry copper and exhalative base metal deposits within a subduction – arc setting remains limited. This study reports major, trace, and volatile element contents in basaltic groundmass glasses and olivine-hosted melt inclusions from lavas from four locations within the arc – back-arc setting of the SKAHT. The focus is on understanding the controls on base metal (Pb, Cu, Zn, Mo, V) contents in the magmas. The sample locations, Rumble III and Rumble II West volcanoes, and back-arc Basins D and I, form an arc-perpendicular transect extending from arc front into the back-arc. The analysed melt inclusion and groundmass glasses are all basalt to basaltic andesite in composition, with back-arc basin samples more mafic than arc front volcano samples. The magmatic evolution of the melts is primarily controlled by crystal fractionation of olivine + pyroxene + plagioclase. All glasses have undergone variable degassing, indicated by an absence of detectable CO₂ and curvilinear decreases in S contents with increasing SiO₂. Of the volatile phases analysed, only Cl appears unaffected by degassing. Distinct compositional differences are apparent between arc front and back-arc melts. The arc front magmas formed from higher degrees of melting of a less fertile mantle source and are more enriched in trace elements then the back-arc magmas due to greater additions of slab-derived aqueous fluids to their source. Magmas from a single arc front volcano (Rumble II West) incorporate melts that have tapped variably enriched sources, indicating heterogeneity of the mantle at small scales. Significant variation in mantle composition, however, is also apparent laterally along strike of the arc. Rumble III volcano and Basin I lie on an arc-perpendicular transect south of Rumble II West volcano and Basin D. Their greater enrichment in trace elements and higher concentrations of base metals than Rumble II West and Basin D lavas can be attributed to higher fluxes of subduction derived components. Base metals (Cu, Zn, Pb, Mo, and V) are variably enriched in the SKAHT melts compared with typical mid-ocean ridge basalts with relative enrichments in the order Pb >> Cu > Mo, V > Zn. All metals appear to be affected by mantle metasomatism related to slab-derived fluids, either directly from slab components introduced to the mantle source (e.g., Pb) or through mobilisation of metals within the ambient mantle wedge. The apparently compatible behaviour of Zn, Cu, and V in the mantle means that these elements may be enriched in arc front magmas relative to back-arc magmas by higher degrees of partial melting and/or melting of more depleted sources. All base metals behave incompatibly in the magma during crystal fractionation between 48 – 56 wt.% SiO₂. Lead and Cu concentrations, however, begin to level out from ~ 52 wt.% SiO₂ suggesting some subsequent loss to fractionating volatile phases as metal sulfide complexes. Rumble III samples show a decrease in metal concentration (Pb, Cu, V), from melt inclusions to groundmass glasses, suggestive of more significant loss associated with sulfur degassing. Although other factors such as heat generation, hydrothermal flow, fault systems, and magma venting are key in the development of VMS deposits, this study shows that variations in subduction parameters can significantly affect metal concentrations in arc magmas that may host hydrothermal systems, and hence the amount of metals available to be scavenged into the deposits.</p>

Crystal Forensics of Historical Lava Flows from Mt Ngauruhoe

<p>Mt Ngauruhoe is a 900 m high andesitic cone constructed over the last 2500 yr, and is the youngest cone of the Tongariro Massif. It was previously one of the most continuously active volcanoes in New Zealand, with ash eruptions having occurred every few years since written records for the volcano began in 1839. However, it has now been more than 30 yr since the last eruption. Eruptions in 1870, 1949, 1954 and 1974-1975 were accompanied by lava and block-and-ash flows. Detailed sampling of these historical lava and block-and-ash flows was conducted, including sampling from seven different lava flows erupted over the period June-September 1954 to investigate changes in magma geochemistry and crystal populations over short timescales, and to enable observed changes to be related back to known eruption dates. Mineral major and trace element chemistry highlights the importance of mixing between distinct basaltic and dacitic melts to generate the basaltic andesite whole rock compositions erupted. The basaltic end member can be identified from the presence of olivine crystals with Mg# 75-87, clinopyroxene cores with Mg# 82-92, and plagioclase cores of An80-90. The dacitic melt is identified by SiO2-rich clinopyroxene melt inclusions, clinopyroxene zoning with Mg# 68-76 and plagioclase rims of An60-70. Textural evidence from complex mineral zoning and large variability in the widths of reaction rims on olivine crystals suggests that mafic recharge of the more evolved system is frequent, and modelling of Fe-Mg inter-diffusion applied to the outermost rims of the clinopyroxene crystal population indicates that such recharge events have occurred weeks to months or even shorter prior to each of the historical eruptions, and thus likely trigger the eruptions.</p>