Assembling Dacite in a Continental Subduction Zone: A Case Study of Tauhara Volcano

<p>Mount Tauhara is the largest dacitic volcanic complex of onshore New Zealand and comprises seven subaerial domes and associated lava and pyroclastic flows, with a total exposed volume of ca. 1 km3. The dacites have a complex petrography including quartz, plagioclase, amphibole, orthopyroxene, clinopyroxene, olivine and Fe‐Ti oxides and offer an excellent opportunity to investigate the processes and timescales involved in assembling dacitic magma bodies in a continental subduction zone with in situ and mineral specific analytical techniques. Whole rock major and trace element data and Pb isotopes ratios define linear relationships indicating that the dacites are generated by mixing of silicic and mafic magmas. Two groups of samples define separate mixing trends between four endmembers on the basis of La/Yb ratios, 87Sr/86Sr ratios and Sr contents. The older Western and Central Domes have low 87Sr/86Sr (0.7042‐0.7046) and high LREE/HREE (LaN/YbN = 8.0‐11.5) and Sr (380‐650 ppm) compared to the younger Hipaua, Trig M, Breached and Main Domes, which have higher 87Sr/86Sr (0.7047‐0.7052) and lower LREE/HREE (LaN/YbN = 6.5‐7.5) and Sr (180‐400 ppm). In situ mineral major and trace element chemistry of mineral phases, as well as Sr and Pb isotope ratios of mineral separates have been used to: (i) fingerprint the origin of each crystal phase; (ii) constrain the chemistry of the four endmembers involved in the mixing events and; (iii) estimate the timing of mixing relative to eruption and the ascent rate of the dacitic magmas. The presence of quartz and analyses of quartz‐hosted melt inclusions are used to fingerprint the chemistry of the silicic endmembers, which is a rhyolitic melt with a major element chemistry similar to that of either the Whakamaru Group Ignimbrite melts (Western, Central and Trig M Domes) or intermediate between that of the Whakamaru and the Oruanui Ignimbrite melts (Hipaua, Breached and Main Domes). Similarly, Ba‐Sr concentrations and Sr isotopic signatures of plagioclase show that this phenocryst phase also predominantly crystallized from the rhyolitic melt. Variations in the Mg# and trace element chemistry of clinopyroxenes suggest they were formed both in the mixed dacitic melts and in a mafic endmember. The chemistry of the mafic endmembers have been traced using a combination of back‐calculated Sr melt concentrations from clinopyroxene with the highest Mg# in each sample group, and the linear trends between whole rock SiO2 content and most elements. These results indicate that dacites erupted from the Western and Central Dome were generated by the mixing of a high alumina basalt and a rhyolitic melt and Trig M Dome dacites were generated by the mixing of an andesite with a rhyolitic melt. Magmas erupted from Hipaua, Breached and Main Domes were also produced by the mixing of an andesitic melt and a rhyolitic body with a composition intermediate between that of the Whakamaru and the Oruanui melt bodies. Trace element data and 87Sr/86Sr ratios of amphibole demonstrate that it crystallized from the mixed dacitic melt. Thermobarometric conditions obtained from amphibole indicate that the magma mixing event that produced the dacites occurred within a magma chamber located at ca. 9 km depth and ca. 900°C with the exception of Trig M Dome which occurred deeper at 13 km and 950°C. Diffusion profiles of Ti in quartz and Fe‐Mg in clinopyroxene indicate the magma mixing events occurred < 6 months prior to eruption. Amphibole reaction rims show the magma to have ascended over 2‐3 weeks for each dome, with the exception of Main Dome where reaction rims were not present in the amphibole, suggesting the ascent rate was faster than 0.2 m/s (< 6 hours).</p>

Assembling Dacite in a Continental Subduction Zone: A Case Study of Tauhara Volcano

<p>Mount Tauhara is the largest dacitic volcanic complex of onshore New Zealand and comprises seven subaerial domes and associated lava and pyroclastic flows, with a total exposed volume of ca. 1 km3. The dacites have a complex petrography including quartz, plagioclase, amphibole, orthopyroxene, clinopyroxene, olivine and Fe‐Ti oxides and offer an excellent opportunity to investigate the processes and timescales involved in assembling dacitic magma bodies in a continental subduction zone with in situ and mineral specific analytical techniques. Whole rock major and trace element data and Pb isotopes ratios define linear relationships indicating that the dacites are generated by mixing of silicic and mafic magmas. Two groups of samples define separate mixing trends between four endmembers on the basis of La/Yb ratios, 87Sr/86Sr ratios and Sr contents. The older Western and Central Domes have low 87Sr/86Sr (0.7042‐0.7046) and high LREE/HREE (LaN/YbN = 8.0‐11.5) and Sr (380‐650 ppm) compared to the younger Hipaua, Trig M, Breached and Main Domes, which have higher 87Sr/86Sr (0.7047‐0.7052) and lower LREE/HREE (LaN/YbN = 6.5‐7.5) and Sr (180‐400 ppm). In situ mineral major and trace element chemistry of mineral phases, as well as Sr and Pb isotope ratios of mineral separates have been used to: (i) fingerprint the origin of each crystal phase; (ii) constrain the chemistry of the four endmembers involved in the mixing events and; (iii) estimate the timing of mixing relative to eruption and the ascent rate of the dacitic magmas. The presence of quartz and analyses of quartz‐hosted melt inclusions are used to fingerprint the chemistry of the silicic endmembers, which is a rhyolitic melt with a major element chemistry similar to that of either the Whakamaru Group Ignimbrite melts (Western, Central and Trig M Domes) or intermediate between that of the Whakamaru and the Oruanui Ignimbrite melts (Hipaua, Breached and Main Domes). Similarly, Ba‐Sr concentrations and Sr isotopic signatures of plagioclase show that this phenocryst phase also predominantly crystallized from the rhyolitic melt. Variations in the Mg# and trace element chemistry of clinopyroxenes suggest they were formed both in the mixed dacitic melts and in a mafic endmember. The chemistry of the mafic endmembers have been traced using a combination of back‐calculated Sr melt concentrations from clinopyroxene with the highest Mg# in each sample group, and the linear trends between whole rock SiO2 content and most elements. These results indicate that dacites erupted from the Western and Central Dome were generated by the mixing of a high alumina basalt and a rhyolitic melt and Trig M Dome dacites were generated by the mixing of an andesite with a rhyolitic melt. Magmas erupted from Hipaua, Breached and Main Domes were also produced by the mixing of an andesitic melt and a rhyolitic body with a composition intermediate between that of the Whakamaru and the Oruanui melt bodies. Trace element data and 87Sr/86Sr ratios of amphibole demonstrate that it crystallized from the mixed dacitic melt. Thermobarometric conditions obtained from amphibole indicate that the magma mixing event that produced the dacites occurred within a magma chamber located at ca. 9 km depth and ca. 900°C with the exception of Trig M Dome which occurred deeper at 13 km and 950°C. Diffusion profiles of Ti in quartz and Fe‐Mg in clinopyroxene indicate the magma mixing events occurred < 6 months prior to eruption. Amphibole reaction rims show the magma to have ascended over 2‐3 weeks for each dome, with the exception of Main Dome where reaction rims were not present in the amphibole, suggesting the ascent rate was faster than 0.2 m/s (< 6 hours).</p>

Thermodynamic model for a pilot balloon

In the early part of the 20th century, tracking a pilot balloon from the ground with an optical theodolite was one of the few methods that was able to provide information from the upper air. One of the most significant sources of error with this method, however, was involved in calculating the balloon height as a function of time, a calculation dependent on the ascent rate which was traditionally taken to be constant. This study presents a new thermodynamic model which allows us to compute the thermal jump between the surrounding environment and the lifting gas as a function of different parameters such as the atmospheric temperature lapse rate or the physical characteristics of the balloon. The size of the thermal jump and its effect on the ascent rate is discussed for a 30 g pilot balloon, which was the type used at the Ebro Observatory (EO) between 1952 and 1963. The meridional and zonal components of the wind profile from ground level up to 10 km altitude were computed by applying the model using EO digitized data for a sample of this period. The obtained results correlate very well with those obtained from the ERA5 reanalysis. A very small thermal jump with a weak effect on the computed ascent rate was found. This ascent rate is consistent with the values assigned in that period to the balloons filled with hydrogen used at the Ebro Observatory and to the 30 g balloons filled with helium used by the US National Weather Service.

Mineral compositions and thermobarometry of basalts and boninites recovered during IODP Expedition 352 to the Bonin forearc

Central aims of IODP Expedition 352 were to delineate and characterize the magmatic stratigraphy in the Bonin forearc to define key magmatic processes associated with subduction initiation and their potential links to ophiolites. Expedition 352 penetrated 1.2 km of magmatic basement at four sites and recovered three principal lithologies: tholeiitic forearc basalt (FAB), high-Mg andesite, and boninite, with subordinate andesite. Boninites are subdivided into basaltic, low-Si, and high-Si varieties. The purpose of this study is to determine conditions of crystal growth and differentiation for Expedition 352 lavas and compare and contrast these conditions with those recorded in lavas from mid-ocean ridges, forearcs, and ophiolites. Cr# (cationic Cr/Cr+Al) vs. TiO2 relations in spinel and clinopyroxene demonstrate a trend of source depletion with time for the Expedition 352 forearc basalt to boninite sequence that is similar to sequences in the Oman and other suprasubduction zone ophiolites. Clinopyroxene thermobarometry results indicate that FAB crystallized at temperatures (1142–1190 °C) within the range of MORB (1133–1240 °C). When taking into consideration liquid lines of descent of boninite, orthopyroxene barometry and olivine thermometry of Expedition 352 boninites demonstrate that they crystallized at temperatures marginally lower than those of FAB, between ~1119 and ~1202 °C and at relatively lower pressure (~0.2–0.4 vs. 0.5–4.6 kbar for FAB). Elevated temperatures of boninite orthopyroxene (~1214 °C for low-Si boninite and 1231–1264 °C for high-Si boninite) may suggest latent heat produced by the rapid crystallization of orthopyroxene. The lower pressure of crystallization of the boninite may be explained by their lower density and hence higher ascent rate, and shorter distance of travel from place of magma formation to site of crystallization, which allowed the more buoyant and faster ascending boninites to rise to shallower levels before crystallizing, thus preserving their high temperatures.

Sounding Characteristics that Yield Significant Convective Inhibition Errors due to Ascent Rate and Sensor Response of In Situ Profiling Systems

Accurate measurements of the convective inhibition (CIN) associated with capping inversions are critical to forecasts of deep convection initiation. The goal of this work is to determine the sounding characteristics most vulnerable to CIN errors arising from hysteresis associated with sensor response and ascent rate of profiling systems. This examination uses 5058 steady-state analytic soundings prescribed using three free parameters that control inversion depth, static stability, and moisture content. A theoretical well-aspirated first-order sensor mounted on a platform that does not disturb its environment is “flown” in these soundings. Sounding characteristics that result in the largest relative CIN errors are also the characteristics that result in the smallest CIN. Because they are more likely to support deep convection initiation, it is particularly critical that environments with small CIN are represented accurately. The relationship between relative CIN error and CIN exists because sounding characteristics that contribute to large CIN do not proportionally increase the CIN error. Analysis also considers CIN intervals with (operationally important) CIN on the threshold between environments that will and will not support deep convection initiation. For these soundings, CIN error is found to be largest for deep, dry inversions characterized by small static stability.

On the estimation of vertical air velocity and detection of atmospheric turbulence from the ascent rate of balloon soundings

Vertical ascent rate VB of meteorological balloons is sometimes used for retrieving vertical air velocity W, an important parameter for meteorological applications, but at the cost of crude hypotheses on atmospheric turbulence and without the possibility of formally validating the models from concurrent measurements. From simultaneous radar and unmanned aerial vehicle (UAV) measurements of turbulent kinetic energy dissipation rates ε, we show that VB can be strongly affected by turbulence, even above the convective boundary layer. For “weak” turbulence (here ε≲10−4 m2 s−3), the fluctuations of VB were found to be fully consistent with W fluctuations measured by middle and upper atmosphere (MU) radar, indicating that an estimate of W can indeed be retrieved from VB if the free balloon lift is determined. In contrast, stronger turbulence intensity systematically implies an increase in VB, not associated with an increase in W according to radar data, very likely due to the decrease in the turbulence drag coefficient of the balloon. From the statistical analysis of data gathered from 376 balloons launched every 3 h at Bengkulu (Indonesia), positive VB disturbances, mainly observed in the troposphere, were found to be clearly associated with Ri≲0.25, usually indicative of turbulence, confirming the case studies. The analysis also revealed the superimposition of additional positive and negative disturbances for Ri≲0.25 likely due to Kelvin–Helmholtz waves and large-scale billows. From this experimental evidence, we conclude that the ascent rate of meteorological balloons, with the current performance of radiosondes in terms of altitude accuracy, can potentially be used for the detection of turbulence. The presence of turbulence complicates the estimation of W, and misinterpretations of VB fluctuations can be made if localized turbulence effects are ignored.