Volcanologists worldwide were shocked to learn of Jim Luhr's sudden death at age 53 on 1 January 2007, at the zenith of his career. Jim was director of the Global Volcanism Program at the Smithsonian Institution in Washington, D.C. He lived in University Park, Md., with his wife, Karen, and two daughters.

Within the Zitacuaro-Valle de Bravo (ZVB) region of tire central Mexican Volcanic Belt (MVB), three lava series have erupted during the Quaternary. (1) high-K2O basaltic andesites and andesites (2) medium-K2O basaltic audesites, andesites and dacites; (3) high-TiO2, basalts and basaltic andesites. The dominant feature of tire first two groups is the lack of plagioclase accompanying the various ferromagnesian phenocrysts (olivine, orthopyroxene, augite, and hornblende) in all but tire dacites. This absence of plagioclase in the phenocryst assemblages of the high-K2O and medium-K2O intermediate lavas is significant because it indicates high water contents during the stage of phenocryst equilibration. In contrast, the high-TiO2 group is characterized by phenocrysts of plagioclase and olivine. The spatial distribution of these three lava series is systematic. The southern section of the ZVB transect, 280-330 km from the Middle America Tench (MAT), is characterized by high-K2O melts that are relatively enriched in fluid-mobile elements and have the highest Sr-87/Sr-86 ratios. Medium-K2O basaltic andesite and andesite lavas are present throughout the transect, but those closest to the MAT are MgO-rich (3.5-9.4 wt%) and have phenocryst assemblages indicative of high magmatic water contents (3.5-6.5 wt % water) and relatively low temperatures (950-1000 degrees C). In marked contrast, the northern section of the ZVB transect (380-480 km from the MAT) has high-TiO2 high field strength element (HFSE) -enriched magmas that have comparatively dry (< 1.5 wt % magmatic water) and hot (1100-1200 degrees C) phenocryst equilibration conditions. The central section of the ZVB transect (330-380 km from the MAT) is a transition zone and produces moderately light rare earth element (LREE) and large ion lithophile element (LILE)-enriched, medium-K2O lavas with phenocryst assemblages indicative of iutermediate (1.5-3.5 wt %) water contents and temperatures. The high-K2O series compositions are the most enriched in LILE and LREE with a narrow range of radiogenic Sr-87/Sr-86 froth 0.704245 to 0.704507, Nd-143/Nd-144 values ranging from 0512857 to 0.512927 (epsilon(Nd)=4.27-5.63), and Pb-208/Pb-204 values from 38.248 to 38.442, Pb-207/Pb-204 values from 15.563 to 15.585 and Pb-206/Pb-204 values from 18.598 to 18.688. The medium-K2O series compositions are one, moderately enriched in the LILE and LREE, with a broader range of Sr-87/Sr-86; but similar Nd-143/Nd-144 and Pb-208/Pb-204 values to those of the high-K2O series. In contrast, the high-TiO2 series compositions have little enrichment in LILE or LREE and instead are enriched in the HFSE and heavy rare earth elements (HREE). The high-TiO2 lavas are isotopically distinct in their lower and narrower range of Nd-143/Nd-144. The isotopic variations are believed to reflect the upper mantle magma source regions as the low content of phenocrysts in most lavas precludes significant upper crustal assimilation or magma mixing; other than that represented by the presence of quartz xenocrysts (< 2 vol.%) with rlyolitic glass inclusions, which are found in many of these lavas.The systematic spatial variation in composition of the three lava series is a rflection of the underlying subduction-modified mantle and its evolution.

The Tepic-Zacoalco rift, a NW-trending corridor similar to 50 x similar to 250 km, is one arm of a triple-rift system in western Mexico. Together with the Colima rift and the Middle America Trench, it bounds the Jalisco block, a portion of western Mexico that may be moving independently of North America. The predominant basement rock types in the Tepic-Zacoalco rift are rhyolitic ash-flow tuffs and lavas, which were previously assumed to be Oligocene-Miocene in age, related to the Sierra Madre Occidental volcanic province, or older. New Ar-40/Ar-39 dates on 41 volcanic samples reveal a previously unrecognized, voluminous flare-up of rhyolitic ignimbrites between 5 and 3 Ma throughout the entire corridor of the Tepic-Zacoalco rift; they are often associated with Pliocene high-Ti basalts. The eruption rate during this Pliocene time period was an order of magnitude higher (hundreds of m/m.y.) than that documented in the Tepic-Zacoalco rift over the last 1 m.y. The Pliocene ash-flow tuffs have been faulted along NW-trending lineaments, producing vertical offsets up to at least 500 m. The voluminous ignimbrite flare-up in the Tepic-Zacoalco rift at 5-3 Ma may reflect the initial stages of rifting of the Jalisco block away from North America, analogous to what occurred in the proto-gulf region at 12-6 Ma,

Located at the volcanic front in the western Mexican arc, in the Colima Rift, is the active Volcán Colima, which lies on the southern end of the massive (∼450 km 3 ) Colima-Nevado volcanic complex. Along the margins of this andesitic volcanic complex, is a group of 11 scoria cones and associated lavas, which have been dated by the 40 Ar/ 39 Ar method. Nine scoria cones erupted ∼1.3 km 3 of alkaline magma (basanite, leucite-basanite, minette) between 450 and 60 ka, with >99% between 240 and 60 ka. Two additional cones (both the oldest and calc-alkaline) erupted <0.003 km 3 of basalt (0.5 Ma) and <0.003 km 3 of basaltic andesite (1.2 Ma), respectively. Cone and lava volumes were estimated with the aid of digital elevation models (DEMs). The eruption rate for these scoria cones and their associated lavas over the last 1.2 Myr is ∼1.2 km 3 /Myr, which is more than 400 times smaller than that from the andesitic Colima-Nevado edifice. In addition to these alkaline Colima cones, two other potassic basalts erupted at the volcanic front, but ∼200 km to the ESE (near the historically active Volcán Jorullo), and were dated at 1.06 and 0.10 Ma. These potassic suites reflect the tendency in the west-central Mexican arc for magmas close to the volcanic front to be enriched in K 2 O relative to those farther from the trench. Ferric-ferrous analyses on pristine samples from the alkaline cones adjacent to V. Colima and V. Jorullo indicate that their oxygen fugacities relative to the nickel-nickel oxide buffer are significantly higher (ΔNN0=2–4) than those for the calc-alkaline magma types (0–1.5). These ΔNNO values correlate positively with Ba concentrations and likely reflect the influence of a slab-derived fluid. As a result of the high oxidation states, the solubility of sulfur in these potassic magmas is enhanced. Indeed the sulfur content of both the whole rock and the apatite phenocrysts (and in olivine melt inclusions reported in the literature) suggest that part of their pre-eruptive sulfur gas (SO 2 ) concentrations could have been discharged to the atmosphere in amounts comparable to the 1982 eruption of El Chichón, although over a prolonged period spanning thousands of years (not per eruption).

Late Miocene-Pliocene (8-3Ma) olivine basalt lavas, dated in this study by the Ar-40/Ar-39 method, have been faulted and tilted on both the east and west sides of the Warner Range of NE California, which is itself a tilted block rising to 2960 m at its crest that is composed of Miocene-Oligocene lavas and volcaniclastic rocks. The late Miocene-Pliocene lavas, distinctively poor in K 2 0 and rich in MgO, are called low-K olivine tholeiites and have a different mantle source region than that of the older subduction-related lavas of the main Warner Range. Hays Canyon Range (max. elev. 2400 m) lies to the east of the Warner Range, and the broad Surprise Valley separates the two fault-bounded ranges. Middle Miocene (ca. 15 Ma) basic lavas, with a small easterly dip, cap the Hays Canyon Range and overlie Oligocene silicic ash-flow deposits and a basaltic andesite spatter volcano. Middle Miocene basic lavas also form the crest of the Warner Range and its westerly dip slope (similar to 15 degrees). Nearly horizontal basic lavas of the same age are also found on both sides of the Warner Range, and it is a plausible conclusion that these middle Miocene basalts were a contiguous group before faulting and uplift of the Warner Range.Derived estimates of uplift rates (similar to 1 mm/yr) of the Warner Range indicate that uplift could have been initiated at ca. 4 Ma, a period of the most voluminous eruption of low-K olivine tholeiite lavas. If the slower Cretaceous exhumation rate of the Sierra Nevada (0.5-1.0 mm/yr) is applied to the total offset of the Warner Range (4270 m), and it did not vary with time, then the uplift of the Warner Range was initiated at ca. 8 Ma, which coincides with the age of the oldest low-K olivine tholeiite lava (8 Ma). Low-K olivine tholeiites require a hot shallow asthenospheric source, and it is the rise of this hot mantle that is presumed to have caused the uplift of the Warner Range. Whether or not the widespread eruption of small volumes of Pliocene low-K olivine tholeiites in central and eastern Oregon is associated with crustal uplift is unknown.

Trachybasalt scoria from a cinder cone near the Mexican volcanic front contain phenocrysts of olivine with chromite inclusions, apatite, augite and hornblende, with microphenocrysts of plagioclase. The water-saturated phase relations reproduce the phenocryst assemblage between 1040degreesC and 970degreesC with water contents of between 2.5 and 4.5% (50-150 MPa). The absence of biotite phenocrysts in the scoria places a tight constraint on the pressure-temperature conditions of phenocryst equilibration, as there is only a small zone where biotite does not accompany hornblende in the experiments. Diluting the fluid phase with CO2 changes the composition of the olivine, indicating that CO2 was only a minor component of the fluid of the scoria. Hornblende is stable to 1040degreesC at oxygen fugacities of NNO + 2 (where NNO is the nickel-nickel oxide buffer), but at lower oxygen fugacities, the upper limit is 990degreesC. There is a progressive increase in crystallinity in experimental runs as both pressure and temperature decrease. Isobaric plots of crystallinity show that the onset of hornblende crystallization involves a reaction relation, and also results in a marked similar to15-40 vol. % increase in crystallinity. Ascending hydrous magmas intersecting the cooler crust could be trapped there by the large increase in crystallinity accompanying the isobaric crystallization of hornblende.

Our Dana Medal awardee was born and educated in San Francisco, and went to Berkeley as an undergrad in 1972, and graduated in 1976. He evinced an intense interest in hot springs, and his honors thesis was on the Hot Springs of the Devil’s Kitchen of Lassen Park. Thereafter his interest in geothermal waters prospered, particularly in the acid sulfate hot springs, and he became adept at all sorts of wet chemical techniques to measure the metals Fe, Ni, Zn, Cu as well as As, Ge, B, F, etc., and …

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Intermediate calc-alkaline magma (52–65% SiO2) in western-central Mexico is the focus of this paper, and the typically porphyritic andesites (57–65% SiO2) form large central volcanoes, whereas basaltic andesites (52–57% SiO2) are less porphyritic, and they are found as cones and flows but are absent from central volcanoes. Several studies of experimental phase equilibria on these lavas relate water concentration to the phenocryst assemblages and to the degree of crystallinity, so that the abundance, composition and variety of phenocrysts can be used to constrain the amount of water dissolved in the magmas. Thus, the plagioclase-rich andesites of Volcan Colima, Mexico, become so as a result of decompressional crystallisation at ~950 °C (the pyroxene phenocryst temperature), and lose their dissolved water (2.5 to 4.5 wt% H2O) which is inversely proportional to the modal abundance of plagioclase. The feeding magma to V. Colima, North America's most productive central volcano, is represented by hornblende lamprophyre, a lava type without plagioclase phenocrysts which requires at least 6 wt% water to reproduce the phenocryst assemblage. Thus, degassing of the V. Colima magmas, and of those of the other central volcanoes in the western-central Mexican volcanic belt, contributes essentially all their dissolved water to the conduit or to the atmosphere. The source of this magmatic water is related to the source of the intermediate magmas. For some this must lie in the mantle, as the incorporation of hornblende-lherzolite nodules in a hydrous andesite with hornblende phenocrysts could only have occurred while ascending through the mantle. Consistent with a mantle source is the composition of the olivine phenocrysts in Mexican lavas with 10 to 5% MgO, which is in the mantle range of Fo88–92. Accordingly, basaltic andesites and andesites with >5% MgO are candidates for a mantle source. The equilibration of intermediate magmas with the mantle, as illustrated by the experiments of various workers, requires that the magmas be hydrous at pressure. An additional constraint is that the activity of silica in the mantle must be equal to that in the hydrous magma at equilibrium. Using published and new experiments to define RTlnγSiO2 in hydrous liquids, this quantity is shown to vary as a function of liquid composition (H2O, MgO, Na2O+K2O), and it approaches zero for quartz-saturated hydrous liquids. Using appropriate values of RTlnγSiO2 for three intermediate lavas, the amount of water required to equilibrate with an olivine-orthopyroxene mantle source is calculated, and within error indicates that only the most silica-rich magma is at water saturation in the mantle, in agreement with published experimental work. Hydrous intermediate magmas, ascending from their hornblende-lherzolite source regions (~1 to 1.5 GPa) along the hydrous adiabat, may not encounter any phase boundaries until 0.2–0.4 GPa because of the increase in the thermal stability of hornblende in water-undersaturated magmas. Therefore, the phenocryst assemblages of hornblende-free andesites equilibrate at low pressures. The virtual absence of basalt in west-central Mexico (<4 Ma) is considered to be related to the large increase in crystallinity found in isobaric hydrous experiments crystallising hornblende at pressures close to those at the base of the crust. As a large proportion of the ferromagnesian components of basalt is acceptable to hornblende, it does not take a significant cooling interval (~40–50 °C) below the liquidus for hydrous basaltic magma to acquire >50% crystallinity, evidently also an eruptible limit for V. Colima andesitic lavas. If the lower limit of water dissolved in Mexican intermediate magmas is accepted as that required for phenocryst equilibration (~6 wt% water), and the upper limit as saturation in the mantle source at 1 GPa (~16 wt%) then, with an estimate of the volcanic and plutonic magma delivery rate (km3/106 year) per km of volcanic arc, the flux of water returned from the mantle along the 35,000-km, global subduction-related arc system can be estimated. Measurements of the volcanic flux are woefully few, and estimates from Mexico, the Lesser Antilles and central America show a range from 4 to 20 km3/106 year×km which, if subtracted from the isotopically constrained continental growth rate, gives the plutonic flux rate. This suggests that, of the magma flux ascending to the continental crust, only about a fifth reaches the surface. If the dissolved magmatic water limits are coupled with the volcanic and plutonic emplacement rates, then the amount of water returned by magmatism to the crust is crudely in balance with that subducted.

The Valley of Mexico and surrounding regions of Mexico and Morelos states in central Mexico contain more than 250 Quaternary eruptive vents in addition to the large, composite volcanoes of Popocatépetl, Iztaccíhuatl, and Nevado de Toluca. The eruptive vents include cinder and lava cones, shield volcanoes, and isolated andesitic and dacitic lava flows, and are most numerous in the Sierra Chichináutzin that forms the southern terminus of the Valley of Mexico. The Chichináutzin volcanic field (CVF) is part of the E-W-trending Mexican Volcanic Belt (MVB), a subduction-related volcanic arc that extends across Mexico. The crustal thickness beneath the CVF (∼50 km) is the greatest of any region in the MVB and one of the greatest found in any arc worldwide. Lavas and scoriae erupted from vents in the CVF include alkaline basalts and calc-alkaline basaltic andesites, andesites, and dacites. Both alkaline and calc-alkaline groups contain primitive varieties that have whole rock Mg#, MgO, and Ni contents, and liquidus olivine compositions (≤Fo90) that are close to those expected of partial melts from mantle peridotite. Primitive varieties also show a wide range of incompatible trace element abundances (e.g. Ba 210–1080 ppm; Ce 25–100 ppm; Zr 130–280 ppm). Data for primitive calc-alkaline rocks from both the CVF and other regions of the MVB to the west are consistent with magma generation in an underlying mantle wedge that is depleted in Ti, Zr, and Nb and enriched in large ion lithophile (K, Ba, Rb) and light rare earth (La, Ce) elements. Extents of partial melting estimated from Ti and Zr data are lower for primitive calc-alkaline magmas in the CVF than for those from the regions of the MVB to the west where the crust is thinner. The distinctive major element compositions (low CaO and Al2O3, high SiO2) of the primitive calc-alkaline magmas in the CVF indicate a more refractory mantle source beneath this region of thick crust. In contrast, primitive alkaline magmas from the CVF and other regions of the MVB show compositional similarities to intraplate-type alkali basalts erupted behind the arc in the Mexican Basin and Range province. These similarities are consistent with the hypothesis that slab-induced convection in the mantle wedge beneath the MVB causes advection of asthenospheric mantle from behind the arc to the region of magma generation. Trace element systematics of primitive magmas in the MVB reveal substantial variability in both the extent of mantle wedge enrichment by subduction processes and in the composition of mantle heterogeneities that are related to previous extraction of alkaline to sub-alkaline basaltic melts.

Approximately 150 km west of Mexico City in the central part of the Mexican Volcanic Belt (MVB) near Zitácuaro, Mexico, young volcanism has produced shield volcanoes, large volume silicic deposits, and fault-related basalt and andesite lava flows and cinder cones. This paper concerns a small cluster of Pleistocene andesite cones and flows which can be separated into two distinct groups: high-magnesium andesites (>6% MgO, 57–59% SiO 2 ), conveniently called basaltic andesites, with phenocrysts of orthopyroxene and augite, or augite and olivine; and andesites (60–62% SiO 2 , <4.6% MgO), which have phenocrysts of orthopyroxene and augite, and ghosts of relict hornblende. Remarkably, plagioclase phenocrysts are absent, and evenly distributed but sparse (0.5–3.5%) quartz xenocrysts are present in all the lavas. In order to establish the conditions under which early crystallizing plagioclase is suppressed in these lavas, water saturated experiments up to 3 kbars were performed on one of the basaltic andesites. The conditions required to reproduce the phenocryst assemblages (either olivine + augite or opx + augite) are temperatures in excess of 1000 °C, with water saturated liquids (>3 wt%) at pressures of about 1 kbar. Compared to basaltic andesites of western Mexico, the Zitácuaro basaltic andesites have ∼2 wt% lower Al 2 O 3 concentrations, which causes plagioclase to precipitate at significantly lower temperatures, and it therefore follows the crystallization sequence: olivine, augite, and orthopyroxene. Based on ubiquitous quartz xenocrysts, with glassy rhyolitic inclusions, a reasonable conclusion is that substantial mixing of a quartz-bearing rhyolitic magma with a parental basaltic andesite has occurred at low pressure (shallow depth), and this would account for the low Al 2 O 3 concentrations in the Zitácuaro basaltic andesites. Whatever the mechanism of incorporation, the quartz xenocrysts are evidence of contamination of basaltic magma with more siliceous material, thus making it difficult to use these magmas as indicators of mantle melting processes.

We have conducted high pressure (to 3 kbar), water saturated melting experiments on an andesite (62 wt% SiO2) and a basaltic andesite (55 wt% SiO2) from western Mexico. A close comparison between the experimental phase assemblages and their compositions, and the phenocryst assemblages of the lavas, is found in water saturated liquids, suggesting that the CO2 content was minimal in the fluid phase. Thus the historic lavas from Volcan Colima (with phenocrysts of orthopyroxene, augite, plagioclase, and hornblende) were stored at a temperature between 950–975 °C, at a pressure between 700–1500 bars, and with a water content of 3.0–5.0 wt%. A hornblende andesite (spessartite) from Mascota, of nearly identical composition but with only amphibole phenocrysts, had a similar temperature but equilibrated at a minimum of 2000 bars pressure with a dissolved water content of at least 5.5 wt% in the liquid. Experiments on the basaltic andesite show that the most common natural phenocryst assemblages (olivine, ±augite, ±plagioclase) could have precipitated at temperatures from 1000–1150 °C, in liquids with a wide range of dissolved water content (∼2.0–6.0 wt%) and a corresponding pressure range. A lava of the same bulk composition with phenocrysts of hornblende, olivine, plagioclase, and augite is restricted to temperatures below 1000 °C and pressures below 2500 bars, corresponding to <5.5 wt% water in the residual liquid. Although there is some evidence for mixing in the andesites (sporadic olivine phenocrysts), the broad theme of the history of both lava types is that the phenocryst assemblages for both the andesitic magmas and basaltic andesitic magmas are generated from degassing and reequilibration on ascent of initially hydrous parents containing greater than 6 wt% water. Indeed andesitic magmas could be related to a basaltic andesite parent by hornblende-plagioclase fractionation under the same hydrous conditions.

We have designed and built a new apparatus for measuring the thermal conductivity of molten silicates, an adaptation of the transient hot‐wire technique to high temperature. The method is the most suitable for high temperatures, as it suppresses convection and minimizes radiative heat transfer. Because of the simple geometry, the radiative transport that does occur can be estimated and subtracted from the total response, yielding true phonic conductivities. Measurements of molten CaMgSi 2 O 6 at approximately 1673, 1773, and 1873 K yield thermal conductivities of 0.31, 0.17 and 0.04 W m −1 K −1 respectively. These values are considerably lower than previous estimates for molten silicates and indicate that the current estimates for the cooling times of magma bodies are probably too short.