Thermal sensitivity analysis of emplacement of the magma chamber in Los Humeros caldera,Puebla, Mexico

We present results of our simulation study of the effects of the depth (top of the magma chamber at 5–10 km) and volume (1000–1400 km³) of the primary heat source beneath the Los Humeros caldera. The thermal gradient in the vicinity of the magma chamber calculated from the temperature excess (difference between the simulated and the initial temperatures prior to emplacement of the magma) is more sensitive to its depth of intrusion than to its volume. This relationship was quantified from multiple linear regression equations. The temperature excess at 2–3 km depth due to the emplacement of magma and its conductive cooling is also more dependent on the chamber depth than on its volume. Therefore, in the study of calderas, volcanoes, and geothermal fields, constraining the chamber depth is more important than its volume. Similarly, comparison of the thermal regime inferred along vertical and horizontal profiles shows the importance of solving the thermal transport equations in three dimensions instead of one or two dimensions.     

The volcanic and geothermally active Campi Flegrei caldera: an integrated multidisciplinary image of its buried structure

The Campi Flegrei caldera in southern Italy is one of the greatest geohazard areas on Earth. Evidence of an active magmatic and geothermal system is provided by ongoing ground uplift, with volcano-tectonic and long-period (LP) seismicity, the persistent degassing of ~1500 tonnes of CO₂ per day, the presence of hot fumaroles at temperatures of 90–150 °C, brine-rich aquifers (with total dissolved solids up to 33 g l^?1) and high thermal gradients in the crust (with temperatures reaching 420 °C at 3,050 m b.s.l.). Since the 1940s, more than 100 exploratory boreholes have been drilled in the area to depths of 80–3,100 m by the Azienda Geologica Italiana Petroli (AGIP) and the Società Anonima Forze Endogene Napoletane (SAFEN). To date, however, no systematic reanalysis of the drilling data has been carried out, and the buried volcanic structure has not been updated using the most recent scientific results and previous findings. By integrating unpublished data from the AGIP and SAFEN reports with published information from geological, volcanological, petrological, petrophysical and geophysical studies, this paper presents an improved picture of the Campi Flegrei caldera that will be useful for volcanic hazard assessment and mitigation in the Naples area and for future research planning. The results suggest that intra-caldera activity has been influenced by how the magmatic system at depths greater than about 4 km has determined the transfer of magma, volatiles, and heat to the overlying geothermal system and, ultimately, to the surface. In particular, intriguing is that the most volcanically active central-eastern sector of the caldera, which is subject to intense bradyseismic ground movement and gas emission, coincides with a structurally delimited subsurface rock volume characterized by an uprising of the 100 °C isotherm, a deep water supply to the shallower aquifer, the early disappearance of secondary calcite, LP seismicity and high seismic S-wave attenuation. In this area, we also document evidence of repeated injection at depths of c. 1.5–3.0 km of isolated and small-volume batches of magma, where occurred their crystallization and degassing. Shallow intrusions and degassing of magma are thus identified as two of the key processes that drive unrest in Campi Flegrei.     

Electric structure of the Copahue Volcano (Neuquén Province,Argentina), from magnetotelluric soundings: 1D and 2D modellings

Four magnetotelluric soundings were carried out in 1993 in the region of the Copahue active volcano located at the border between Chile and Argentina (37°45′S, 71°18′W). Three soundings were located inside the caldera of the ancient stratovolcano (east of Copahue) and the fourth outside it. The soundings inside the caldera were situated at about 6, 11, and 14 km from the volcano. Digital data were obtained covering the range of periods from 1 sec to 10,000 sec using induction coils and a flux-gate magnetometer to obtain the magnetic data and Cu-SO₄Cu electrodes for electric field measurements. The apparent resistivity curves corresponding to principal directions were analyzed in conjunction with the geological background in order to eliminate distortion — which is very important in this hot volcanic region. Then, 1D modellings were performed using the “normal” curves — i.e., curves without distortions. Using the apparent resistivity curves with distortions, 2D modelling was also performed along a profile perpendicular to the regional tectonic trend suggested by MT soundings into the caldera. Results show low resistivity values of about 3-15 Ωm between 9 km to 20 km depth in the crust, suggesting high temperatures, with minimum values of about 700°C with partially melted zones in the upper crust between 9 km to 20 km depth under the caldera. The presence of a possible sulphide-carbonaceous layer (SC layer) in the upper basement could play an important role in lowering the electrical resistivities because of its high electronic conductivity.     

Crystallisation of nepheline syenite in a subvolcanic magma system: Tenerife,canary islands

The late Quaternary phonolitic pumice deposits of Tenerife, Canary Islands, are the product of a periodically-tapped, periodically-replenished, zoned alkaline magma system. Nepheline syenite blocks occur as lithics in the deposits, and provide solidified representatives of the phonolite magmas. The blocks are considered to have crystallised in the roof zone of the active magma system at a depth of roughly 4 km beneath the Las Cañadas caldera. Conversion of highly-differentiated phonolitic magma to solid nepheline syenite was achieved between 770° and 680°C. Quenched glass of extreme composition in one erupted block provides a sample of the last interstitial liquid; comparison of this sample with phonolitic pumice, and with fully crystallised syenites, allows reconstruction of the magmatic, interstitial and subsolidus crystallisation histories of nepheline syenite. Phases represented by the phenocryst assemblage of the most differentiated phonolitic pumices (sanidine, sodalite, nepheline, Na-poor pyroxene, biotite, sphene and magnetite) formed an initial crystal “mush”, containing abundant trapped liquid, on the walls of the magma chamber. Interstitial crystallisation, involving the conversion of pyroxene to Na-rich compositions, continued growth of felsic phases, and partial to complete consumption of biotite, sphene, magnetite and apatite, was accompanied by extreme sodium enrichment in the residual liquid. Stellate aegirine, lavenite, loparite and Mn-rich ilmenite are among the final products of magmatic crystallisation. Modification of feldspar primocrysts persisted into subsolidus conditions.It is suggested that complete solidification of an open-system alkaline magma chamber results in the formation of an alkaline alkaline ring complex.     

A cascade of magmatic events during the assembly and eruption of a super-sized magma body

We use comprehensive geochemical and petrological records from whole-rock samples, crystals, matrix glasses and melt inclusions to derive an integrated picture of the generation, accumulation and evacuation of 530 km³ of crystal-poor rhyolite in the 25.4 ka Oruanui supereruption (New Zealand). New data from plagioclase, orthopyroxene, amphibole, quartz, Fe–Ti oxides, matrix glasses, and plagioclase- and quartz-hosted melt inclusions, in samples spanning different phases of the eruption, are integrated with existing data to build a history of the magma system prior to and during eruption. A thermally and compositionally zoned, parental crystal-rich (mush) body was developed during two periods of intensive crystallisation, 70 and 10–15 kyr before the eruption. The mush top was quartz-bearing and as shallow as ~3.5 km deep, and the roots quartz-free and extending to >10 km depth. Less than 600 year prior to the eruption, extraction of large volumes of ~840 °C low-silica rhyolite melt with some crystal cargo (between 1 and 10%), began from this mush to form a melt-dominant (eruptible) body that eventually extended from 3.5 to 6 km depth. Crystals from all levels of the mush were entrained into the eruptible magma, as seen in mineral zonation and amphibole model pressures. Rapid translation of crystals from the mush to the eruptible magma is reflected in textural and compositional diversity in crystal cores and melt inclusion compositions, versus uniformity in the outermost rims. Prior to eruption the assembled eruptible magma body was not thermally or compositionally zoned and at temperatures of ~790 °C, reflecting rapid cooling from the ~840 °C low-silica rhyolite feedstock magma. A subordinate but significant volume (3–5 km³) of contrasting tholeiitic and calc-alkaline mafic material was co-erupted with the dominant rhyolite. These mafic clasts host crystals with compositions which demonstrate that there was some limited pre-eruptive physical interaction of mafic magmas with the mush and melt-dominant body. However, the mafic magmas do not appear to have triggered the eruption or controlled magmatic temperatures in the erupted rhyolite. Integration of textural and compositional data from all available crystal types, across all dominant and subordinate magmatic components, allow the history of the Oruanui magma body to be reconstructed over a wide range of temporal scales using multiple techniques. This history spans the tens of millennia required to grow the parental magma system (U–Th disequilibrium dating in zircon), through the centuries and decades required to assemble the eruptible magma body (textural and diffusion modelling in orthopyroxene), to the months, days, hours and minutes over which individual phases of the eruption occurred, identified through field observations tied to diffusion modelling in magnetite, olivine, quartz and feldspar. Tectonic processes, rather than any inherent characteristics of the magmatic system, were a principal factor acting to drive the rapid accumulation of magma and control its release episodically during the eruption. This work highlights the richness of information that can be gained by integrating multiple lines of petrologic evidence into a holistic timeline of field-verifiable processes.     

河间潜山地热资源开发方案数值模拟

古潜山地热资源具备岩溶孔隙发育程度高、热储面积厚度大、地热水储量大的优点。冀中坳陷内古潜山分布密集且地热资源丰富, 河间潜山位于冀中坳陷饶阳凹陷中东部, 具有良好的地热地质条件, 开发潜力巨大。本文基于河间潜山及其周缘地区测井资料、岩石热物性并进行了计算, 发现其地温梯度为29.8 ℃/km到44.5 ℃/km之间, 平均值为40.7 ℃/km。大地热流值介于64.8~80.6 mW/m2之间, 平均值为 73.4 mW/m2。通过水热耦合模拟方法模拟选定的地热资源有利区的温度变化, 结果发现河间潜山合理的开采井距为800 m, 合理开采量为60 L/s, 回灌温度为35 ℃, 总可开采量为6.32×1016 J, 单年可开采量为 6.32×1014 J, 可供暖面积为1.22×106 m2, 对于冀中坳陷潜山地热资源的开发利用具有一定的指导意义。     

基于钻井温的长白山天池火山北坡浅部岩浆房赋存深度研究

长白山火山区地壳热结构尚未建立,目前基于地球物理探测手段获得的天池火山浅部岩浆房赋存深度存在差异.通过对天池火山北坡CZK07钻孔测温情况的研究,在资料评价与地温梯度计算的基础上,结合全新世岩浆房温度资料,估算了北坡浅部岩浆房的赋存深度. CZK07钻孔位于地球物理探测所推测的浅部岩浆房正上方,靠近历史时期火山喷发火口,在孔深约610 m处地温较高且稳定（102.5～106.8℃）.连续测温资料显示,钻孔地温随深度呈一次正相关变化,地温梯度主要变化于134～178℃/km之间（平均为153℃/km）,可大致代表浅部岩浆房上覆地壳的地温梯度.基于前人浅部岩浆房的温度研究,本次定量估算的天池火山北坡浅部岩浆房的赋存深度,为天池水面下5.25～7.21 km,与地球物理探测的反演结果相近.     

The geothermal gradient map of Central Tunisia: Comparison with structural,gravimetric and petroleum data

Five hundred and fifty temperature values, initially measured as either bottom-hole temperatures (BHT) or drill-stem tests (DST), from 98 selected petroleum exploration wells form the basis of a geothermal gradient map of central Tunisia. A “global-statistical” method was employed to correct the BHT measurements, using the DST as references. The geothermal gradient ranges from 23° to 49°C/km. Comparison of the geothermal gradient with structural, gravimetric and petroleum data indicates that:

• 1.(1) the general trend of the geothermal gradient curves reflects the main structural directions of the region,
• 2.(2) zones of low and high geothermal gradient are correlated with zones of negative and positive Bouguer anomalies and
• 3.(3) the five most important oil fields of central Tunisia are located near the geothermal gradient curve of 40° C/km.

Such associations could have practical importance in petroleum exploration, but their significance must first be established through further investigation and additional data.     

Evolution of silicic magmas in the Kos-Nisyros volcanic center,Greece: a petrological cycle associated with caldera collapse

Multiple eruptions of silicic magma (dacite and rhyolites) occurred over the last ~3 My in the Kos-Nisyros volcanic center (eastern Aegean sea). During this period, magmas have changed from hornblende-biotite-rich units with low eruption temperatures (≤750–800°C; Kefalos and Kos dacites and rhyolites) to hotter, pyroxene-bearing units (>800–850°C; Nisyros rhyodacites) and are transitioning back to cooler magmas (Yali rhyolites). New whole-rock compositions, mineral chemistry, and zircon Hf isotopes show that these three types of silicic magmas followed the same differentiation trend: they all evolved by crystal fractionation and minor crustal assimilation (AFC) from parents with intermediate compositions characterized by high Sr/Y and low Nb content, following a wet, high oxygen fugacity liquid line of descent typical of subduction zones. As the transition between the Kos-Kefalos and Nisyros-type magmas occurred immediately and abruptly after the major caldera collapse in the area (the 161 ka Kos Plateau Tuff; KPT), we suggest that the efficient emptying of the magma chamber during the KPT drew out most of the eruptible, volatile-charged magma and partly solidified the unerupted mush zone in the upper crust due to rapid unloading, decompression, and coincident crystallization. Subsequently, the system reestablished a shallow silicic production zone from more mafic parents, recharged from the mid to lower crust. The first silicic eruptions evolving from these parents after the caldera collapse (Nisyros units) were hotter (up to >100°C) than the caldera-forming event and erupted from reservoirs characterized by different mineral proportions (more plagioclase and less amphibole). We interpret such a change as a reflection of slightly drier conditions in the magmatic column after the caldera collapse due to the decompression event. With time, the upper crustal intermediate mush progressively transitioned into the cold-wet state that prevailed during the Kefalos-Kos stage. The recent eruptions of the high-SiO₂ rhyolite on Yali Island, which are low temperature and hydrous phases (sanidine, quartz, biotite), suggest that another large, potentially explosive magma chamber is presently building under the Kos-Nisyros volcanic center.     

Numerical simulations to assess thermal potential at Tauranga low-temperature geothermal system,New Zealand

Tauranga low-temperature geothermal system (New Zealand) has been used for the last 40 years for direct uses including space heating, bathing and greenhouses. Warm-water springs in the area are between 22 and 39 °C, with well temperatures up to 67 °C at 750 m depth. A heat and fluid flow model of the system is used to determine reservoir properties and assess thermal potential. The model covers 130 km by 70 km to 2 km depth, and was calibrated against temperatures measured in 17 wells. Modelling shows that to maintain the observed primarily conductive heat flow regime, bulk permeability is ≤2.5?×?10^?14 m² in sedimentary cover and ≤1?×?10^?16 m² in the underlying volcanic rocks. The preferred model (R ²?=?0.9) corresponds to thermal conductivities of 1.25 and 1.8 W/m² for sedimentary and volcanic rocks, respectively, and maximum heat flux of 350 mW/m². The total surface heat flow is 258 MW over 2,200 km². Heat flux is highest under Tauranga City, which may be related to inferred geology. Model simulations give insights into rock properties and the dynamics of heat flow in this low-temperature geothermal system, and provide a basis to estimate the effects of extracting hot fluid.     

Magmatic gases in well fluids aid the mapping of the flow pattern in a geothermal system

Gas composition and silica concentrations of well fluids are used in conjunction with pressure, temperature and enthalpy data to obtain a model of the drilled part of the Krafla geothermal field (Northeast Iceland).A magma chamber is located at 3–8 km depth under the field. Magmatic gases emanate from the chamber and travel via a channel reaching the surface at the Hveragil eruptive fissure. The composition of the gases is apparently modified on the way, in that sulphur, which presumably is in the form of sulphur dioxide to begin with, is removed on the way, and what remains is in the form of hydrogen sulphide at the end. It is suggested that the major removal mechanism is the deposition of pyrite and pyrrhotite during the passage of the gases through the hydrothermal system.The hydrothermal system is divided into a lower part whose temperature exceeds 300°C and whose fluids are to a varying extent affected by the magmatic gases, and an upper part whose temperature is ca 200°C. This upper part is a run-off from the lower part but the magmatic effects have mostly disappeared there.     

Mineralogical study of spinel-lherzolite inclusions from basalts in southern and eastern China

Ten spinel-lherzolite inclusions and one olivine-websterite inclusion, which were collected from Fujian, Jiangsu, Hebei and Yunnan Provinces, consist of olivine (FΦ_87.7–91.2), enstatite (En_87.3–89.7), Cr-diopside and spinel. According to the Mg/(Mg+Fe²⁺) ratios in the rocks and their mineralogies, they are designated to the Cr-diopside type. The websterite is composed of bronzite (En_71.9) and augite, while the gabbro-norite consists of hypersthene (En_68.9) and augite, belonging to the Al-augite type. The geothermos of spinel-lherzolites were calculated with four geothermometric methods, giving a temperature range of 925°–1,072°C. However, according to P. R. A. Wells' method, temperatures range from 845δ to 1,014°C, and by D. H. Lindsly's approach, from 716°–974°C. Pressures range from 15.1 to 19.8 kb. Genetically, Spinel-lherzolites and olivine-websterite are thought to have been derived from residual mantle material by partial melting at approximately 1,000°C and at a depth of about 50–70 km. Websterite and gabbro-norite may be products of the crystallization-differentiation of alkali basaltic magma.     

The geochemical and petrological characteristics of prenatal caldera volcano: a case of the newly formed small dacitic caldera,Hijiori, Northeast Japan

Evaluating the magma depth and its physical properties is critical to conduct a better geophysical assessment of magma chambers of caldera volcanoes that may potentially cause future volcanic hazards. To understand pre-eruptive conditions of a magma chamber before its first appearance at the surface, this paper describes the case of Hijiori caldera volcano in northeastern Japan, which emerged approximately 12,000 years ago at a place where no volcano ever existed. We estimated the depth, density, bulk modulus, vesicularity, crystal content, and bulk H\(_2\)O content of the magma chamber using petrographic interpretations, bulk and microchemical compositions, and thermodynamic calculations. The chemical mass balance calculations and thermodynamic modeling of the erupted magmas indicate that the upper portion of the Hijiori magmatic plumbing system was located at depths between 2 and 4 km, and had the following characteristics: (1) pre-eruptive temperature: about 780 \(^{\circ }\)C; (2) bulk magma composition: 66 ± 1.5 wt% SiO\(_{2}\); (3) bulk magmatic H\(_2\)O: approximately 2.5 wt%, and variable characteristics that depend on depth; (4) crystal content: \(\le\)57 vol%; (5) bulk modulus of magma: 0.1–0.8 GPa; (6) magma density: 1.8–2.3 g/cm³; and (7) amount of excess magmatic H\(_2\)O: 11–32 vol% or 48–81 mol%. The range of melt water contents found in quartz-hosted melt inclusions (2–9 wt%) suggests the range of depth phenocrysts growth to be wide (2\(\sim\)13 km). Our data suggest the presence of a vertically elongated magma chamber whose top is nearly solidified but highly vesiculated; this chamber has probably grown and re-mobilized by repeated injections of a small amount of hot dacitic magma originated from the depth.     

Petrology of Rhyolitic and Mixed Magma Ejecta from the 1875 Eruption of Askja, Iceland

Volcanic activity in Askja central volcano and its fissure swarmin 1875 occurred in response to a crustal rifting episode inIceland, resulting in up to 70 km lateral flow of magma withinthe crust, caldera collapse and a plinian eruption of acid magma(0·2 km³ dense-rock equivalent). Petrologic studies ofthe predominantly rhyolitic and crystal-poor ejecta reveal thata complex array of other liquid compositions was also present,including icelandite (0.75 per cent) and basalt (1.9 per cent),as well as leucocratic xenoliths of trondhjemite type. Mineralgeothermometers indicate that the rhyolite evolved at 990 to1010 °C and 0·5 Kb P_H2O, the icelandite at 1005 to1020 °C and at f_O2 10^–10 atm. and the basalt at 1140to 1170 °C. A petrologic model of Askja in 1875 consists of a density-stratifiedmagma chamber with a rhyolitic upper part and a lower part offerrobasalt, with an intervening layer of icelandite. The modelcalculations show that the icelandite can be derived from ferrobasaltby 50 per cent fractional crystallization, but one-stage fractionalcrystallization models cannot account for generation of theacid magma. Simple partial or complete fusion of the field-associatedtrondhjemite xenoliths cannot produce the acid magma. Instead,a more complex fusion, hybridization and fractional crystallizationmodel is presented, which is consistent with the available petrologicevidence. This model involves large-scale fusion of pre-existingtrondhjemite intrusions or reactivation of previously consolidatedroof-rock in the magma chamber followed by hybridization ofthe acid magma with 7 to 14 per cent basaltic magma. Finally,10 to 11 per cent fractional crystallization of the dacite hybridis required to produce the observed compositional range withinthe rhyolite ejecta. The 1875 explosive eruption was causedby the ascent of tholeiitic basalt magma from depth during crustalrifting. Influx of new basalt magma in 1874–75 triggeredconvective mixing and hybridization in the compositionally zonedmagma chamber.     

河北汤泉地热田地温场分布及其控制因素研究

汤泉位于河北省遵化市西北部,为山前丘陵地貌,地热资源丰富。本文对汤泉地热田内分布的诸多基岩热水井进行了钻孔测温,利用测温结果对该地热田地温场分布特征及控制因素进行了研究。研究发现:钻孔温度明显受地下水流的影响,绝大部分测温井表现为对流传热特征,个别表现为传导为主的传热特征;地温异常区域位于汤泉福泉宫和疗养院一带,异常中心呈两极分布,地热异常中心50 m埋深水温为51~54℃,100 m埋深水温可达60~67℃;该地热系统中地热水系地下水在深循环过程中,在正常的大地热流背景下被围岩逐渐加热所致;由花岗岩隔水底板构造形态所形成的隐伏盆地,构成了福泉宫至疗养院一带的蓄水构造;由于断裂构造切割花岗岩体,造成深部的热流沿断裂上升,混合并加热赋存于福泉宫至疗养院一带蓄水构造中的片麻岩裂隙水,形成该地段的地热异常现象;福泉宫地区和疗养院地区片麻岩含水层裂隙发育,使得深部热量能够快速到达浅部地层,并在浅部出现局部异常高温;汤泉地热田片麻岩热储层地热流体属于含岩盐地层溶滤的陆相沉积水,主要来源于大气降水。     

Improved Preproduction Conceptual-Numerical Model of the Roosevelt Geothermal Field using Geological,Geochemical and Structural Information

The Roosevelt Hot Springs Known Geothermal Resource Area (KGRA) is a Basin and Range-type geothermal resource, which is located in southwestern Utah. The integrated multicomponent geothermometry (IMG) approach is used to estimate the reservoir temperature at the Roosevelt Hot Springs KGRA. Geothermometric modeling results indicate the deep reservoir temperature is approximately 284.6°C. A conceptual model of the Roosevelt Hot Spring KGRA is provided through integrating the various pieces of exploration information, including the geological data, geothermometric results, temperature well log and field evidence. A two-dimensional cross-sectional model was thus built to quantitatively investigate the coupled thermal-hydraulic processes in the Roosevelt geothermal field. By matching the preproduction temperature log data of deep wells, parameters controlling flow and heat transport are identified. The method and model presented here may be useful for other geothermal fields with similar conditions.     

Chemical evolution of a pleistocene rhyolitic center: Sierra La Primavera,Jalisco, México

The late Pleistocene caldera complex of the Sierra La Primavera, Jalisco, México, contains well-exposed lava flows and domes, ash-flow tuff, air-fall pumice, and caldera-lake sediments. All eruptive units are high-silica rhyolites, but systematic chemical differences correlate with age and eruptive mode. The caldera-producing unit, the 45-km³ Tala Tuff, is zoned from a mildly peralkaline first-erupted portion enriched in Na, Rb, Cs, Cl, F, Zn, Y, Zr, Hf, Ta, Nb, Sb, HREE, Pb, Th, and U to a metaluminous last-erupted part enriched in K, LREE, Sc, and Ti; Al, Ca, Mg, Mn, Fe, and Eu are constant within analytical errors. The earliest post-caldera lava, the south-central dome, is nearly identical to the last-erupted portion of the Tala Tuff, whereas the slightly younger north-central dome is chemically transitional from the south-central dome to later, moremafic, ring domes. This sequence of ash-flow tuff and domes represents the tapping of progressively deeper levels of a zoned magma chamber 95,000 ± 5,000 years ago. Since that time, the lavas that erupted 75,000, 60,000, and 30,000 years ago have become decreasingly peralkaline and progressively enriched only in Si, Rb, Cs, and possibly U. They represent successive eruption of the uppermost magma in the post-95,000-year magma chamber.Eruptive units of La Primavera are either aphyric or contain up to 15% phenocrysts of sodic sanidine quartz >ferrohedenbergite >fayalite>ilmenite±titanomagnetite. Whereas major-element compositions of sanidine, clinopyroxene, and fayalite phenocrysts changed only slightly between eruptive groups, concentrations of many trace elements changed by factors of 5 to 10, resulting in crystal/glass partition coefficients that differ by factors of up to 20 between successively erupted units. The extreme variations in partitioning behavior are attributed to small changes in bulk composition of the melt because major-element compositions of the phenocrysts and temperature, pressure, and oxygen fugacity of the magma all remained essentially constant.Crystal settling and incremental partial melting by themselves appear incapable of producing either the chemical gradients within the Tala Tuff magma chamber or the trends with time in the post-caldera lavas. Transport of trace metals as volatile complexes within the thermal and gravitational gradient in volatilerich but water-undersaturated magma is considered the dominant process responsible for compositional zonation in the Tala Tuff. The evolution of the post-caldera lavas with time is thought to involve the diffusive emigration of trace elements from a relatively dry magma as a decreasing proportion of network modifiers and/or a decreasing concentration of complexing ligands progressively reduced trace-metal-site availability in the silicate melt.     

An insight into the unrest phenomena at the Campi Flegrei caldera from Vp and Vp/Vs tomography

High resolution Vp and Vp/Vs tomography of the Campi Flegrei caldera is obtained using active and passive seismic data. We find a continuous ring of high Vp anomaly that defines the caldera rim associated to the last collapse. A sharp Vp/Vs decrease is observed between 2 and 4 km depth, suggesting the absence of magmatic fluids and the presence of rock volumes with over‐pressured gas within the source region of uplift. Atmospheric water penetrating within the caldera and deep CO₂ fluids are presumably heated by a magmatic body located at depth greater than 4 km nested within the limestone layer. Along the fractures bordering the shallow high Vp rim, deep gas and CO₂ fluids up‐raise and are released in the Pozzuoli solfatara. We hypothesize that the past unrest episode is more likely due to pressure changes within the shallow geothermal reservoir located at the top of the magma intrusion.     

Petrology of a Subduction-related Caldera and Post-Collisional,Extension-related Volcanic Cones from the Early Cambrian and Middle Ordovician (?) of the Camaquã Basin,Southern Brazil

The region studied, located in southern Rio Grande do Sul State, Brazil, 25 km to the northeast of Lavras do Sul, records two volcanic events. The first is the eruption of pyroclastic material and localized flows of alkali-basalt, trachyandesite and andesite composition, corresponding to the early Cambrian Hilário Formation. This event in the region, through tectonic reactivation and explosive expulsion of a large amount of material from the magmatic reservoir chamber, which later faulted and collapsed, has generated an elliptical caldera of 7.2 km × 3.0 km. The rocks in the caldera have derived from partial melting of a spinel lherzolitic or garnet lherzolitic mantle, in a typically orogenic, calc-alkaline environment. They were generated during the final phase of subduction of the Adamastor plate beneath the Rio de la Plata plate in the early Cambrian. The second event occurred in a post-collisional tectonic setting in the Middle Ordovician?, when alkaline magma was emplaced through deep fractures, generating four cones within the collapsed caldera. The rocks in the cones bear the geochemical signature of a more evolved magma when compared to the rocks in the caldera, having been formed through low fusion rates of a garnet-depleted source in the upper mantle. It represents the last phase of the Pan-African-Brasiliano Orogeny, which occurred right after the collision of the Rio de La Plata and Kalahari continental plates, in a retroarc setting.     

Evaluation of the solute geothermometry of thermal springs and drilled wells of La Primavera (Cerritos Colorados) geothermal field,Mexico: A geochemometrics approach

A detailed study on the solute geothermometry of thermal water (18 springs and 8 drilled wells) of La Primavera geothermal field (LPGF) in Mexico has been carried out by employing a geochemical database compiled from the literature and by applying all the available solute geothermometers. The performance of these geothermometers in predicting the reservoir temperatures has been evaluated by applying a geochemometrics (geochemical and statistical) method. The springs of the LPGF are of bicarbonate type and the majority have attained partial-equilibrium chemical conditions and the remaining have shown non-equilibrium conditions. In the case of geothermal wells, water is dominantly of chloride-type and, among the studied eight geothermal wells, four have shown full-equilibrium chemical conditions and another four have indicated partial-equilibrium conditions. All springs of HCO₃^−​ type water have provided unreliable reservoir temperatures, whereas the only one available spring of SO₄²⁻ type water has provided the reservoir temperature nearer to the average BHT of the wells. Contrary to the general expected behavior, spring water of non-equilibrium and geothermal well water of partial-equilibrium chemical conditions have indicated more reliable reservoir temperatures than those of partially-equilibrated and fully-equilibrated water, respectively. Among the chemical concentration data, Li and SiO₂ of two springs, SO₄²⁻ and Mg of four springs, and HCO₃ and Na concentrations of two geothermal wells were identified as outliers and this has been reflected in very low reservoir temperatures predicted by the geothermometers associated with them (Li–Mg, Na–Li, Na–K–Mg, SiO₂ etc.). Identification of the outlier data points may be useful in differentiating the chemical characteristics, lithology and the physico-chemical and geological processes at the sample locations of the study area.In general, the solute geothermometry of the spring waters of LPGF indicated a dominantly (94%) of underestimated deep reservoir temperatures, whereas in the case of the geothermal wells, many temperatures (54%) are underestimated, some are (43%) overestimated and a very small number (3%) are similar to an average bottom-hole temperatures (BHT) of the wells. 28 out of the total applied 29 geothermometers for spring waters have predicted the deep reservoir temperatures that are characterized by statistically significant large differences compared to the average BHTs of the geothermal wells. In the case of waters of the geothermal wells, 23 out of the total applied 28 geothermometers have predicted the reservoir temperatures similar (statistically no significant differences) to the BHTs of the corresponding geothermal wells.