the age of the solar system redefined by the oldest pb pb age of a meteoritic inclusion the age of the solar system is defined by the formation of the first solid grains in the solar nebula pb pb age dating of these solids which were later trapped in a meteorite indicates that the solar system is 0 34 1 91 million years older than previously thought the age of the solar system can be defined as the time of formation of the first solid grains in the nebular disc surrounding the proto sun this age is estimated by dating calcium aluminium rich inclusions in meteorites these inclusions are considered as the earliest formed solids in the solar nebula their formation marks the beginning for several long and short lived radiogenic clocks that are used to precisely define the timescales of solar system events such as the formation and evolution of planetary bodies1 2 3 here we present the 207pb 206pb isotope systematics in a calcium aluminium rich inclusion from the northwest africa 2364 cv3 group chondritic meteorite which indicate that the inclusion formed 4 568 2 million years ago this age is between 0 3 refs 4 5 and 1 9 refs 1 6 million years older than previous estimates and is the oldest age obtained for any solar system object so far we also determined the 26al 26mg model age of this inclusion and find that it is identical to its absolute pb pb age implying that the short lived radionuclide 26al was homogeneously distributed in the nebular disc surrounding the proto sun from the consistently old ages in the studied inclusion we conclude that the proto sun and the nebular disc formed earlier than previously thought

evaluation of recent tectonomagmatic discrimination diagrams and their application to the origin of basic magmas in southern mexico and central america discrimination diagrams to decipher tectonic settings have been in use for nearly 40 years although old diagrams have been extensively used the recent ones based on discriminant functions of ratio variables with or without log transformation proposed during 2004 2010 for the discrimination of four tectonic settings of island arc continental rift ocean island and mid ocean ridge were newly evaluated to show their high success rates of 57 3 100 and 58 5 100 for major element and immobile element based diagrams respectively for the continental arc of the andes evaluated for its similarity to island arc these four sets of diagrams showed success rates of 62 1 83 8 these four sets of five diagrams per set were therefore used to infer tectonic setting of the mexican volcanic belt mvb los tuxtlas volcanic field ltvf and central american volcanic arc cava using this approach the mvb especially its western central and eastern parts and the ltvf of southern mexico show a dominantly continental rift setting and the cava shows an arc setting the west central part of the mvb is consistent with dual tectonics of arc and rift these results confirm the application of an unusual mantle upwelling rift model for the mexican on land volcanism whereas the conventional plate tectonic subduction model seems to be applicable for the cava from guatemala to north western costa rica

delineation of biogeomorphic land units across a tropical natural and humanized terrain in los tuxtlas veracruz mexico abstract this paper analyzes landscape in a rainforest region integrating geomorphologic and ecosystem analysis methods major landscape elements geology geomorphology soils vegetation and land use were mapped as biogeomorphic land units using remote sensing and geographic information systems gis associations between these variables were analyzed by map fusion and the results were expressed on a land unit map scale ca 1 100 000 volcanic structures are dominant in the study area lava flows cover most of the region mainly as basalt several pyroclastic cones are widely distributed over the area but are dominant in the southeast region a hypsometric map derived from a digital elevation model dem and intersected with a land cover use map showed an altitudinal gradient of vegetation rainforest grows at lower altitudes 0 to 700 m and virola juglans and chionantus are distributed from 700 to 900 m these species are located on slopes of basalt and andesites intercalated with tephra recent isolated stratovolcano structures and an erosive flood plain at higher altitudes 900 to 1100 m the forest identified as chionantus ulmus randia is associated with cinder cones a quercus ulmus forest 900 to 1400 m covers the slopes of the highest volcano san martin and surrounding areas while evergreen elfin forest is at the top of this mountain 1660 m

oxygen isotope studies of carbonaceous chondrites abstract the carbonaceous chondrites display the widest range of oxygen isotopic composition of any meteorite group as a consequence of the interaction of primordial isotopic reservoirs in the solar nebula these isotopic variations can be used to identify the reservoirs and to determine conditions and loci of their interactions we present a comprehensive set of whole rock analyses of cv co ck cm cr ch and ci chondrites as well as selected components of some of these meteorites a simple model is developed which describes the isotopic behavior during parent body aqueous alteration processes the process of thermal dehydration also produces a recognizable effect in the oxygen isotopic composition

the relationship between quaternary volcanism in central mexico and the seismicity and structure of subducted ocean lithosphere late quaternary volcanism in central mexico is related to the subduction of young ocean lithosphere at the middle america trench along arc variations in seismicity volcano structure and composition of volcanic products bear a remarkable correlation with the age and structural framework of the downgoing slab morphological and petrographic characteristics of major volcanoes within the trans mexican volcanic belt tmvb serve to distinguish two calc alkaline subprovinces 1 a western arc averaging 60 km in width associated with aseismic subduction of the rivera plate the main cones of this region are dominated by two pyroxene andesites comprise volumes 70 km 3 and stand less than 3 000 m above sea level 2 a broad central and eastern arc related to subduction of a gently inclined segment of the cocos plate bounded by the rivera transform and the tehuantepec ridge major volcanic edifices possess summit elevations in the range 4 000 to 6 000 m have appropriately larger volumes typically 200 km 3 and are constructed with a high proportion of amphibole bearing lavas the boundary between these subprovinces is marked by a north south oriented structural depression the colima graben and it coincides with a 100 km offset in the volcanic front extensional tectonism in the colima graben accompanied by mixed calc alkaline and alkaline volcanism of potassic affinity is likely related to a hinge type transform fault which marks the cocos rivera plate juncture in the downgoing slab a third segment of ocean floor is presently interacting with continental lithosphere south of the gulf of tehuantepec where quaternary volcanism is weakly developed within a tectonically complex region that marks the diffuse cocos noam caribbean triple junction the northern limit of this triple junction is defined by the seismically active isthmus fault which may be related to alkaline volcanic activity at san andres tuxtla a tectonic reconstruction based on the evolution of oceanic crust reveals that the distribution of intermediate depth earthquakes along the arc is directly dependent upon the age of the subducted slab ocean lithosphere younger than approximately 20 m y is subducted aseismically at convergence rates approaching 9 cm yr the length of the inclined seismic zone indicates that the time constant for thermal relaxation in the slab is approximately 4 m y the tmvb overlies the aseismic extension of this young ocean lithosphere several aspects of this study have a bearing on the segmented nature of converging margins in general 1 the tectonic evolution of the ocean floor may determine the nature of segmentation at the site of subduction 2 the complete record of volcanism in the tmvb over the past million years can be related to the present plate configuration 3 alkaline and calc alkaline volcanism have developed contemporaneously at a converging plate margin 4 lineaments in volcanic arcs may reflect the structural complexity of the crust rather than segment boundaries in the subducted slab

imaging the moho and subducted oceanic crust at the isthmus of tehuantepec mexico from receiver functions using teleseismic data recorded along a transect which we call veox for veracruz oaxaca seismic line of 46 broadband stations installed across the isthmus of tehuantepec in southern mexico we obtained receiver functions and stacked them to study the moho topography and back projected them to visualize the subducted slab geometry beneath the isthmus we observed a back azimuth dependent moho thickness across the transect particularly beneath the los tuxtlas volcanic field also we observed the cocos plate which subducts with an angle of 26 between 140 and 310 km from the trench comparison with regional seismicity indicates that it occurs below the oceanic crust

carbonaceous achondrites northwest africa 6704 6693 milestones for early solar system chronology and genealogy abstract northwest africa nwa 6704 6693 are medium to coarse grained achondrites with unique petrologic and geochemical traits that are distinct from the currently established meteorite groups here we report on the extinct 26al 26mg and 53mn 53cr systems to establish fine scale chronology of its formation and cr and ti isotopic anomalies to constrain the composition of the source reservoir of nwa 6704 6693 excesses in the neutron rich 54cr and 50ti isotopes due to nucleosynthetic anomalies separate nwa 6704 6693 from the vast majority of established achondrites and instead resemble the excesses seen among the carbonaceous chondrites specifically the cr type chondrites the excesses in these isotopes indicate a common feeding zone during accretion in the protoplanetary disk between the source of nwa 6704 6693 and that of the carbonaceous chondrites the 26al 26mg data for pyroxene and plagioclase from nwa 6704 yield a 26al 27al 0 3 15 0 38 10 7 mswd 0 49 and an initial 26mg 0 004 0 005 at the time of isotopic closure this initial 26al 27al 0 translates to an absolute age of 4563 14 0 38 ma relative to the d orbigny angrite however given the potential heterogeneity of 26al the d orbigny angrite might not be a good age anchor for the purpose of calculating 26al 26mg ages the 26al 26mg age relative to another carbonaceous achondrite nwa 2976 is 4562 66 0 60 ma the 53mn 53cr systematics of nwa 6704 6693 indicate a 53mn 55mn 0 of 2 59 0 34 10 6 mswd 1 2 with an evolved initial e53cr of 0 14 0 03 the 53mn 55mn 0 yields an 53mn 53cr age of 4562 17 0 76 ma relative to the d orbigny angrite concordant ages determined using the short lived 26al 26mg and 53mn 53cr systems and extant 207pb 206pb system 4562 60 0 30 ma for nwa 6704 6693 amelin et al 2019 indicate rapid cooling and nearly contemporaneous closing of multiple isotope systems the ancient crystallization ages and positive 54cr and 50ti anomalies of nwa 6704 6693 indicate widespread melting and differentiation processes occurring in both non carbonaceous nc and carbonaceous chondrite cc regions of the protoplanetary disk additionally we report the cr and ti isotopic composition for a petrologic range of cr type materials cr2 cr6 and achondrites the additional cr and ti isotopic data for these cr type materials indicates a range in isotopic composition not previously observed based on cr2 chondrites alone

cerro grande volcano the evolution of a miocene stratocone in the early trans mexican volcanic belt abstract cerro grande volcano is a well preserved low angle composite volcano that evolved between 11 and 9 ma it represents the beginning of volcanism in the eastern sector of the early trans mexican volcanic belt extensive field stratigraphic and petrologic work revealed a complex volcanic evolution that can be summarized in six principal stages i construction of a shield like lava cone ii generation of unusual lithic rich ignimbrites apparently related to powerful eruptions that caused rapid and significant vent erosion these eruptions could have resulted from shallow magma water interactions iii eruption of peripheral fissural lava flows following a nnw sse trending fissure system iv period of dome growth and explosive collapse v short period of repose followed by open vent explosive magmatic and hydromagmatic eruptions vi radial eruption of lava flows related to a ring fissural system geochemistry data of late miocene volcanics show a continental arc magmatic origin and show that crystal fractionation contributed strongly to magma differentiation several lava flows were erupted with a persistent nnw sse trending orientation and the entire late miocene geologic record is affected by highly dislocated normal faults with the same orientation this may indicate that late miocene volcanism was under tectonic control finally late miocene volcanics and the inception of the early trans mexican volcanic belt could be related to a shallow subduction angle influenced by a significant increase in the convergence rate along the middle american trench at about 20 ma

internal architecture of the tuxtla volcanic field veracruz mexico inferred from gravity and magnetic data abstract the tuxtla volcanic field tvf is a basaltic volcanic field emerging from the plains of the western margin of the gulf of mexico in the mexican state of veracruz separated by hundreds of kilometers from the trans mexican volcanic belt to the nw and the chiapanecan volcanic arc to the se it stands detached not only in location but also in the composition of its rocks which are predominantly alkaline these characteristics make its origin somewhat puzzling furthermore one of the large volcanoes of the field san martin tuxtla underwent an eruptive period in historical times ce 1793 such volcanic activity conveys particular importance to the study of the tvf from the perspective of volcanology and hazard assessment despite the above circumstances few investigations about its internal structure have been reported in this work we present analyses of gravity and aeromagnetic data obtained from different sources we present the complete bouguer anomaly of the area and its separation into regional and residual components the aeromagnetic data were processed to yield the reduction to the pole the analytic signal and the upward continuation to complete the interpretation of the gravity analyses three dimensional density models of the regional and residual anomalies were obtained by inversion of the gravity signal adding the response of rectangular prisms at the nodes of a regular grid we obtained a body with a somewhat flattened top at 16 km below sea level from the inversion of the regional three separate slender bodies with tops 6 km deep were obtained from the inversion of the residual the gravity and magnetic anomalies as well as the inferred source bodies that produce those geophysical anomalies lie between the sontecomapan and catemaco faults which are proposed as flower structures associated with an inferred deep seated fault termed the veracruz fault these fault systems along with magma intrusion at the lower crust are necessary features to understand the origin and structure of the tvf

26 al 26 mg and 207 pb 206 pb systematics of allende cais canonical solar initial 26 al 27 al ratio reinstated abstract the precise knowledge of the initial 26 al 27 al ratio 26 al 27 al 0 is crucial if we are to use the very first solid objects formed in our solar system calcium aluminum rich inclusions cais as the time zero age anchor and guide future work with other short lived radio chronometers in the early solar system as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation new high precision multi collector inductively coupled plasma mass spectrometry mc icp ms measurements of 27 al 24 mg ratios and mg isotopic compositions of nine whole rock cais six mineralogically characterized fragments and three micro drilled inclusions from the cv carbonaceous chondrite allende yield a well defined 26 al 26 mg fossil isochron with an 26 al 27 al 0 of 5 23 0 13 10 5 internal mineral isochrons obtained for three of these cais a44a ajef and a43 are consistent with the whole rock cai isochron the mineral isochron of ajef with 26 al 27 al 0 4 96 0 25 10 5 anchored to our precisely determined absolute 207 pb 206 pb age of 4567 60 0 36 ma for the same mineral separates reinstate the canonical 26 al 27 al 0 of 5 10 5 for the early solar system the uncertainty in 26 al 27 al 0 corresponds to a maximum time span of 20 ka thousand years suggesting that the allende cai formation events were culminated within this time span although all allende cais studied experienced multistage formation history including melting and evaporation in the solar nebula and post crystallization alteration likely on the asteroidal parent body the 26 al 26 mg and u pb isotopic systematics of the mineral separates and bulk cais behaved largely as closed system since their formation our data do not support the supra canonical 26 al 27 al ratio of individual minerals or their mixtures in cv cais suggesting that the supra canonical 26 al 27 al ratio in the cv cais may have resulted from post crystallization inter mineral redistribution of mg isotopes within an individual inclusion this redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk cai and bulk mineral data obtained by mc icp ms the radiogenic 208 pb 206 pb ratio obtained as a by product from the pb pb age dating is used to estimate time integrated 232 th 238 u ratio value of cais limited variations among the minerals within a single cai contrasted by much larger variations among the bulk cais suggest th u fractionation occurred prior to crystallization of igneous cais if interpreted as primordial heterogeneity the value can be used to calculate the mean age of the interstellar dust from which the cais condensed

radar enabled recovery of the sutter s mill meteorite a carbonaceous chondrite regolith breccia doppler weather radar imaging enabled the rapid recovery of the sutter s mill meteorite after a rare 4 kiloton of tnt equivalent asteroid impact over the foothills of the sierra nevada in northern california the recovered meteorites survived a record high speed entry of 28 6 kilometers per second from an orbit close to that of jupiter family comets tisserand s parameter 2 8 0 3 sutter s mill is a regolith breccia composed of cm mighei type carbonaceous chondrite and highly reduced xenolithic materials it exhibits considerable diversity of mineralogy petrography and isotope and organic chemistry resulting from a complex formation history of the parent body surface that diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to c class asteroids are interpreted

heterogeneous distribution of 26al at the birth of the solar system evidence from refractory grains and inclusions we review recent results on o and mg isotope compositions of refractory grains corundum hibonite and calcium aluminum rich inclusions cais from unequilibrated ordinary and carbonaceous chondrites we show that these refractory objects originated in the presence of nebular gas enriched in 16o to varying degrees relative to the standard mean ocean water value the 17osmow value ranges from approximately 16 to 35 and recorded heterogeneous distribution of 26al in their formation region the inferred 26al 27al 0 ranges from approximately 6 5 10 5 to 2 10 6 there is no correlation between o and mg isotope compositions of the refractory objects 26al rich and 26al poor refractory objects have similar o isotope compositions we suggest that 26al was injected into the 26al poor collapsing protosolar molecular cloud core possibly by a wind from a neighboring massive star and was later homogenized in the protoplanetary disk by radial mixing possibly at the canonical value of 26al 27al ratio approximately 5 10 5 the 26al rich and 26al poor refractory grains and inclusions represent different generations of refractory objects which formed prior to and during the injection and homogenization of 26al thus the duration of formation of refractory grains and cais cannot be inferred from their 26al 26mg systematics and the canonical 26al 27al 0 does not represent the initial abundance of 26al in the solar system instead it may or may not represent the average abundance of 26al in the fully formed disk the latter depends on the formation time of cais with the canonical 26al 27al ratio relative to the timing of complete delivery of stellar 26al to the solar system and the degree of its subsequent homogenization in the disk the injection of material containing 26al resulted in no observable changes in o isotope composition of the solar system instead the variations in o isotope compositions between individual cais indicate that o isotope composition of the cai forming region varied because of coexisting of 16o rich and 16o poor nebular reservoirs gaseous and or solid at the birth of the solar system or because of rapid changes in the o isotope compositions of these reservoirs with time e g due to co self shielding in the disk

210pb chronology and trace metal geochemistry at los tuxtlas mexico as evidenced by a sedimentary record from the lago verde crater lake lago verde is a fresh water maar found on the lower slopes of san martin volcano at the sierra de los tuxtlas mexico currently the northernmost remnant of the tropical rain forest in america 210 pb and 137 cs analyzed in a sediment core were used to reconstruct the historical fluxes of ag cd cu pb hg and zn to the site during the last 150 yr the 210 pbxs derived sediment accumulation rates the magnetic susceptibility c n ratios and 13 c data evidenced background conditions at the lake until 1960s when enhanced erosion related to the clearing of large forested areas at los tuxtlas promoted higher accumulation rates of a heavier and more magnetic sedimentary material recent sediments from lago verde were found enriched by pb 26 fold natural concentration level ncls and moderately enriched by cd cu zn and hg 6 5 4 and 4 fold corresponding ncls respectively the fluxes of cu hg pb and zn have significantly increased since 1940s with peak ratios of total modern to pre industrial fluxes of 11 11 19 and 49 respectively the lake occupies a relatively pristine non industrialized basin and therefore the increased metal fluxes might be related to long distance aeolian transport of trace metals

geology and k ar dating of the tuxtla volcanic field veracruz mexico the tuxtla volcanic field tvf is located on the coast of the gulf of mexico in the southern part of the state of veracruz mexico volcanism began about 7 my ago in the late miocene and continued to recent times with historical eruptions in ad 1664 and 1793 the oldest rocks occur as highly eroded remnants of lava flows in the area surrounding the historically active cone of san martin tuxtla between about 3 and 1 my ago four large composite volcanoes were built in the eastern part of the area rocks from these structures are hydrothermally altered and covered with lateritic soils and their northern slopes show extensive erosional dissection that has widened preexisting craters to form erosional calderas the eastern volcanoes are composed of alkali basalts hawaiites mugearites and benmoreites with less common calc alkaline basaltic andesites and andesites in the western part of the area san martin tuxtla volcano and its over 250 satellite cinder cones and maars produced about 120 km3 of lava over the last 0 8 my a ridge of flank cinder cones blocked drainage to the north to form laguna catemaco lavas erupted from san martin and its flank vents are restricted to compositions between basanite and alkali basalt the alignment of major volcanoes and flank vents along a n55 w trend suggests an extensional stress field in the crust with a minimum compressional stress orientation of n35 e in total about 800 km3 of lava has been erupted in the tvf in the last 7 my this gives a magma output rate of about 0 1 km3 1000 year a value smaller than most composite cones but similar to cinder cone fields that occur in central mexico individual eruptions over the last 5000 years had volumes on the order of 0 1km3 with average recurrence intervals of 600 years the alkaline compositions of the tvf lavas contrast markedly with the calc alkaline compositions erupted in the subduction related mexican volcanic belt to the west leading previous workers to suggest that the tvf is not related to subduction trace element signatures of tvf lavas indicate however that they are probably related to subduction we suggest that the alkaline character of the tvf lavas is the result of low degrees of melting of a mantle source coupled with a stress regime that allows these small volume melts to reach the surface in the tvf

rare earth element distributions in uraninites implications for ore genesis abstract analysis of the rare earth element ree distributions in uraninites from a variety of hydrothermal mineral deposits indicates that wide variations are to be anticipated although crystal structure must exert a fundamental influence upon ree substitution in uraninite the present study suggests that other factors in particular hydrothermal fluid chemistry may be of equal importance total ree ree contents range from 15 000 ppm in uraninites from ultrametamorphic deposits to eu eu lree hree 0 08 1 10 they are interpreted to result from variations in the oxygen fugacity and the nature of the anionic complexes in the hydrothermal fluids responsible for ore transport additionally in uraniferous quartz pebble conglomerates the ree patterns of the detrital uraninites provide direct evidence regarding the provenance of these sedimentary u ores

late quaternary secular variation data from mexican volcanoes abstract we present 32 new paleomagnetic directions and 21 absolute paleomagnetic intensities pi from 33 volcanoes sampled at 66 sites and covering the last 46 ka of these 29 were radiocarbon dated 3 by thermo luminescence and one is of historical age ad 1793 rock magnetic experiments show that the dominant minerals are magnetite and titanomagnetite of low to intermediate titanium content and of pseudo single domain size paleodirections were determined using stepwise demagnetization protocols and pi by the izzi or thellier coe protocols and strict selection criteria were applied to ensure the reliability of the data previously published data were evaluated to fulfill similar quality criteria as our data resulting in 65 directional data point 18 paleointensity entries from lavas 59 archeointensities are found to be acceptable all accepted data come from a region within an 860 km radius around mexico city paleosecular variation psv curves of the past geomagnetic full vector were established by using the bootstrap resampling algorithm combined with cubic p splines for smoothing and interpolation because of the uneven time distribution of the data this was done for two successive periods between ad 2000 1600 bc and 1600 bc 44000 bc the full vector psv curves are unique for the american continents because of their temporal extension end because they include both directions and absolute pi the recent psv curves the last 3600 years are of high resolutions which certainly have implication for both regional and global geomagnetic field models fast psv change centered around ad 700 and ad 900 separated by a period of much slower psv were observed and rapid pi changes were found around 400 bc and ad 1000 in the period between 1600 bc and 44000 bc psv curves are well defined only within some periods and otherwise only define general psv trends several intensity peaks with values up to 65 t vadm 15 1022 am2 were observed around 250 bc 12000 bc 15000 bc and 27000 bc and intensity lows around ad 700 1200 bc 24000 17000 bc and 34000 bc abnormal directions were found around 26000 bc with negative inclinations down to about 30 which may be related to the mono lake or rockall event a comparison with other psv curves is complicated by the large distance to europe and the hawaiian islands with longitude differences of 99 and 57 respectively lake sediment data from southern texas show more differences than agreements suggesting that either of the data may be affected by systematic errors

isotopic and trace element constraints on the origin and evolution of alkaline and calc alkaline magmas in the northwestern mexican volcanic belt a wide variety of rock types has been produced by pliocene to recent volcanism in the northwestern portion of the mexican volcanic belt composite cones erupted rocks typical of calc alkaline volcanic belts associated with subduction monogenetic cinder cones surrounding the composite cones erupted mildly alkaline basalts and related rocks one larger center las navajas produced basalts trachytes and peralkaline rhyolites such alkaline rocks are typically associated with crustal extension related to rifting both subduction and rifting appear to be taking place in western mexico the most basic calc alkaline rocks although somewhat fractionated show depletion in high field strength elements hfse and enrichment in alkaline earth elements relative to other incompatible elements a characteristic typical of magmas in subduction related volcanic arcs the basic alkaline rocks show no depletion in hfse and show higher concentrations of incompatible elements than the basic calc alkaline rocks the calc alkaline rocks show a tight cluster of 87sr 86sr and 143nd 144nd ratios both of which correlate weakly with sio2 basic alkaline rocks have lower sr and higher nd isotopic ratios whereas the more differentiated alkaline rocks have isotopic ratios that overlap with and extend beyond the range of values found in calc alkaline rocks trace element data indicate that calc alkaline magmas could not have been derived by crystal fractionation from the alkaline magmas nor by assimilation of crustal materials by alkaline magmas the evolution of both calc alkaline and alkaline rocks requires a process of crystal fractionation accompanied by assimilation afc of crustal rocks to account for changes in isotopic ratios and trace element concentrations in the more differentiated members of each suite assuming a contaminant located in the lower or middle crust afc modeling shows that the amounts of assimilation required for alkaline magmas are somewhat higher than those for calc alkaline magmas calc alkaline and alkaline rocks appear to have resulted from melting of two distinct sources alkaline from an oib type source and calc alkaline from a mixed mantle and slab derived source

pb pb dating of individual chondrules from the cb a chondrite gujba assessment of the impact plume formation model the cb chondrites are metal rich meteorites with characteristics that sharply distinguish them from other chondrite groups their unusual chemical and petrologic features and a young formation age of bulk chondrules dated from the cba chondrite gujba are interpreted to reflect a single stage impact origin here we report high precision internal isochrons for four individual chondrules of the gujba chondrite to probe the formation history of cb chondrites and evaluate the concordancy of relevant short lived radionuclide chronometers all four chondrules define a brief formation interval with a weighted mean age of 4562 49 0 21 myr consistent with its origin from the vapor melt impact plume generated by colliding planetesimals formation in a debris disk mostly devoid of nebular gas and dust sets an upper limit for the solar protoplanetary disk lifetime at 4 8 0 3 myr finally given the well behaved pb pb systematics of all four chondrules a precise formation age and the concordancy of the mn cr hf w and i xe short lived radionuclide relative chronometers we propose that gujba may serve as a suitable time anchor for these systems

rock magnetic and geochemical proxies for iron mineral diagenesis in a tropical lake lago verde los tuxtlas east central mexico magnetic and non magnetic mineral analyses were conducted on a lacustrine sequence from lago verde in the tropical coast along the gulf of mexico that covers the last 2000 years the site witnessed the transformation of the environment since the early olmec societies until forest clearance in the last century through these analyses we investigated the processes that affected the magnetic mineralogy in order to construct a model of past environmental changes and compare this model with the archeological record and inferred climatic changes in the northern hemisphere of tropical america volcanic activity has played a major influence on sediment magnetic properties as a purveyor of ti magnetites ti maghemites and as a factor of instability in the environment anoxic reductive conditions are evident in most of lago verde s sedimentary record direct observations of magnetic minerals and ratios of geochemical fe ti and ferrimagnetic f and paramagnetic p susceptibility data are used as parameters for magnetite dissolution p fe f and precipitation f ti of magnetic minerals intense volcanic activity and anoxia are recorded before a d 20 leading to the formation of framboidal pyrite increased erosion higher evaporation rates lower lake levels anoxia and reductive diagenesis in non sulphidic conditions are inferred for laminated sediments between a d 20 850 this deposit matches the period of historical crisis and multiyear droughts that contributed to the collapse of the maya civilization dissolution of magnetite a high organic content and framboidal pyrite point to anoxic sulphidic conditions and higher lake levels after a d 850 higher lake levels in lago verde broadly coincide with the increased precipitation documented during the medieval warm period a d 950 1350 in the northern tropical and subtropical regions of the american continent for the little ice age a d 1400 1800 the relatively moist conditions inferred are in concordance with the glacial advances recorded in central mexico higher erosion rates reflect destruction of the rainforest over the last 40 years

seismicity and tectonics of the subducted cocos plate we have examined teleseismic earthquake locations reported by the international seismological centre isc for the middle america region and selected 220 as the most reliable these hypocenters and other data are used to delineate the deep structure of the subducted cocos plate the results indicate that the subducted plate consists of three major segments segment i extends from the panama fracture zone to the nicoya peninsula the structure of this segment is poorly defined segment ii is the largest and best defined segment this segment consists of two parts iia and iib part iia extends from the nicoya peninsula to western guatemala and is very well defined and continuous in structure its strike follows the curvature of the trench and dips at about 60 part iib extends from western guatemala to orizaba mexico the dip of this part of the segment decreases slightly toward the northwest and its strike is more northward than that of the trench segment iii extends from orizaba to the rivera fracture zone and is not well defined due to a lack of earthquake activity beneath about 100 km its orientation differs markedly from segment ii and strikes somewhat more westward than the trench between parts iia and iib of segment ii the subducted plate seems to be continuous bending smoothly to accommodate the change in geometry local network data from costa rica suggest there may be a tear between segments i and ii between segments ii and iii there is a gap in the hypocenters which makes it difficult to define the boundary the change in geometry between these two segments indicates that there may be a tear and two strike slip focal mechanisms in the region support this conclusion we find no convincing evidence supporting the existence of segments smaller than the three described above if there is smaller scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths there is no evidence for any major tear in the subducted plate associated directly with either the tehuantepec ridge or the orozco fracture zone the shallow subduction at the northwestern end of segment ii may be related to the bouyancy of the tehuantepec ridge the cocos ridge is probably directly responsible for the change in geometry between segments i and ii and may even be slowing or stopping subduction in segment i the structure of the subducted plate in segment ii and the changes in the character of volcanism along the arc can be related to the relative motion of the north american and caribbean plates the present geometry of part iib of segment ii is more consistent with the probable configuration of the trench about 7 ma ago than with the present configuration indicating that the north american plate is overriding the subduction zone appendices 2 3 and 4 are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 009 2 50

crustal thickness at the tuxtla volcanic field veracruz mexico from receiver functions abstract the tuxtla volcanic field tvf is a structure of basaltic rocks on the western margin of the gulf of mexico in the mexican state of veracruz located some 150 km from the easternmost tip of the mexican volcanic belt its tectonic relationship is still unclear the volcanism mostly alkaline is younger than 7 ma and has given origin to hundreds of cinder and scoria cones maars and four large composite volcanoes one of which san martin tuxtla erupted explosively in 1793 due to its volcanological importance it has been the subject of several geological studies none of which focused on its crustal structure moreover because the seismicity level in the area is relatively low no broadband seismometers of mexico s national seismological service are currently installed in the area in this paper we present the results of the analyses of 24 teleseismic events occurring between 2004 and 2008 recorded in two broadband stations deployed around san martin volcano the aim of this study was to determine the depth to the moho any major intracrustal interface in the area and a velocity model by means of receiver function analysis the results show that the crustal thickness in the area varies between roughly 28 and 34 km the receiver functions at one station suggest a second interface at a depth between 10 and 14 km this interface is probably the contact between an upper sedimentary layer and the transitional crust found elsewhere in the margins of the gulf of mexico the determination of the crustal thickness in the tvf is of importance to characterize the area and as a framework to pursue further studies of this volcanic field

manganese chromium isotope systematics of carbonaceous chondrites abstract in this article we present the results of cr isotope investigations of different types of carbonaceous chondrites and of the pallasite eagle station the 53 cr 52 cr ratios in the bulk samples of carbonaceous chondrites are correlated with 55 mn 52 cr ratios the slope of the correlation line yields a 53 mn 55 mn ratio of 8 5 1 5 10 6 at the time of mn cr fractionation mapping this ratio onto an absolute time scale yields a time for this event of 4568 1 0 8 1 1 ma ago this time is very similar to the formation age of efremovka cais of 4567 2 0 6 ma y amelin a n krot i d hutcheon a a ulyanov lead isotopic ages of chondrules and calcium aluminum rich inclusions science 297 2002 1678 1683 to a time of the chondrule formation of 4568 1 ma ago l e nyquist d lindstrom d mittlefehldt c y shih h wiesmann s wentworth r martinez manganese chromium formation intervals for chondrules from the bishunpur and chainpur meteorites meteorit planet sci 36 2001 911 938 which most likely constrains early global high temperature mn cr fractionation in a nebular setting the bulk samples of carbonaceous chondrites exhibit clear 54 cr excesses 54 cr that are correlated with the 53 cr excesses 53 cr and also with mn cr ratios one possible explanation of this correlation is that 54 cr is also radiogenic like 53 cr and was formed by the decay of the short lived parent radionuclide 54 mn the very short half life of 54 mn of 312 days would require that both short lived radionuclides 53 mn and 54 mn were generated locally in spallation reactions during the early period of an active sun the alternative and possibly more plausible explanation is the heterogeneous addition of presolar material the presolar component enriched in 54 cr is mostly contained in the matrix of carbonaceous chondrites the relative amount of matrix decreases in the sequence ci cm co cv a large proportion of mn is associated with the matrix while cr preferentially resides in the chondrules thus the mn cr ratio also follows the sequence ci cm co cv and is correlated with 54 cr the acid resistant residues of carbonaceous chondrites are characterized by relatively large excesses of 54 cr and moderate deficits of 53 cr the magnitude of these excesses and deficits decreases in the sequence ci cm cv and may imply that cr in the cm and cv residues is increasingly more equilibrated with cr from the rest of the meteorites the 53 cr 52 cr and 54 cr 52 cr ratios in the residues are anti correlated indicating that there are at least two cr components of possibly presolar origin all residues have large excesses of the most neutron rich ti isotope 50 ti the pattern of 50 ti does not exactly follow that observed for 54 cr but it is generally similar the cr isotope systematic of the pallasite eagle station indicates that the precursor of this meteorite was a cv type material the 53 mn 53 cr system indicates that the cr isotopes equilibrated in this meteorite 4557 5 0 6 ma ago

miocene to holocene marine tephrostratigraphy offshore northern central america and southern mexico pulsed activity of known volcanic complexes we studied the tephra inventory of fourteen deep sea drill sites of three dsdp and odp legs drilled offshore guatemala and el salvador legs 67 84 138 and one leg offshore mexico leg 66 marine tephra layers reach back from the miocene to the holocene we identified 223 primary ash beds and correlated these between the drill sites with regions along the volcanic arcs and to specific eruptions known from land in total 24 correlations were established between marine tephra layers and to well known quaternary eruptions from el salvador and guatemala additional 25 tephra layers were correlated between marine sites another 108 single ash layers have been assigned to source areas on land resulting in a total of 157 single eruptive events tephra layer correlations to independently dated terrestrial deposits provide new time markers and help to improve or confirm age models of the respective drill sites applying the respective sedimentation rates derived from the age models we calculated ages for all marine ash beds hence we also obtained new age estimates for eight known but so far undated large terrestrial eruptions furthermore this enables us to study the temporal evolution of explosive eruptions along the arc and we discovered five pulses of increased activity 1 a pulse during the quaternary 2 a pliocene pulse between 6 and 3 ma 3 a late miocene pulse between 10 and 7 ma 4 a middle miocene pulse between 17 11 ma and 5 an early miocene pulse 21 ma

the tafassasset primitive achondrite insights into initial stages of planetary differentiation abstract tafassasset is an exceptional meteorite that has been linked to both the cr chondrites and the primitive achondrites because previous evidence suggests it might be a primitive achondrite from a known chondrite type we have undertaken a study of the petrology geochemistry and formation history of the meteorite tafassasset is predominantly feo rich olivine 58 yet contains abundant fe ni metal 10 vol and sulfide 3 vol other phases include high and low ca pyroxene plagioclase chromite and phosphate it has a recrystallized texture containing equigranular grains that often meet at 120 triple junctions there are no relict chondrules in the thin sections examined although they have been reported previously electron microprobe analyses reveal homogeneous olivine fa 28 6 both low and high ca pyroxene fs 23 6 wo 3 7 and fs 12 2 wo 39 3 1 a range of plagioclase composition an 23 47 fe ni metal with 5 3 36 6 wt ni and 0 1 0 8 wt co troilite chromite and ca phosphate bulk composition analyses reveal two chips depleted in refractory lithophile and some siderophile elements compared to ci chondrites exceptions are enrichments in fe ni and co a third chip is essentially chondritic in bulk composition different stones of the meteorite have slightly different oxygen isotope composition yet all lie in the cr chondrite trend with one in the acapulcoite lodranite field thermodynamic calculations show that tafassasset equilibrated at a temperature above the fe ni fes eutectic and at an oxygen fugacity of iw 1 the texture heterogeneous distribution of mineral phases plagioclase composition two mineral closure temperatures and bulk composition all provide evidence that tafassasset partially melted on its parent body a comparison with the cr chondrites the brachinites and two anomalous achondrites indicates that tafassasset is most similar to ungrouped primitive achondrites lewis cliff lew 88763 and divnoe and to the brachinites in overall petrography modal mineralogy mineral compositions oxidation state and bulk composition the comparison also excludes the possibility that tafassasset formed by partial melting of a cr chondrite tafassasset is a primitive achondrite and likely evolved on a parent body that experienced incomplete melting never reached isotopic homogeneity and was from the same oxygen isotopic reservoir as the cr chondrite parent body

a quaternary monogenetic volcanic field in the xalapa region eastern trans mexican volcanic belt geology distribution and morphology of the volcanic vents abstract the most abundant volcanic manifestations along the east west trending trans mexican volcanic belt tmvb are the scoria cones these have been grouped by other authors in extended monogenetic volcanic fields such as michoacan guanajuato chichinautzin apan and los tuxtlas here we present geological and morphological data of a relatively unknown group of monogenetic volcanoes located on the east flank of the cofre de perote volcano cp around the city of xalapa in the state of veracruz mexico within an area of about 2400 km2 the xalapa monogenetic volcanic field xmvf contains over 50 late quaternary volcanoes most of them are scoria cones but small shield volcanoes and tuff rings also occur the lava flows produced by these volcanoes are constrained by an abrupt topography and cover a great percentage of the surface on the eastern and northeastern flanks of cp between 3000 and 500 m a s l the representative rocks of the different volcanic centers include olivine basalt basaltic andesite with phenocrysts of plagioclase pyroxene and minor olivine and andesite with phenocrysts of plagioclase and pyroxene sio2 and al2o3 contents of the rocks vary between 45 and 62 wt and 15 to 18 wt respectively most of the basaltic rocks have mgo contents between 4 2 and 9 wt ni and cr concentrations between 23 and 180 and 10 to 380 ppm respectively with a typical calc alkaline behavior trace elements suggest two types of magmas the most abundant are characterized by an enrichment of lile and lree with negative anomalies of nb and ti which denote a calc alkaline affinity others are lile depleted and show high concentrations of mgo cr and ni which is typical of primary calc alkaline magmas the mean scoria cone morphological values are cone height hco 90 8 m cone diameter wco 686 38 m crater diameter wcr 208 49 m and 0 12 km3 for the cone volume we dated twelve different scoria cones using the 40ar 39ar method for the other cones we used morphologic indicators such as the ratio of the scoria cone height to basal diameter hco wco and maximum slope angle additionally vegetation type and cone and lava flow morphologies were also considered in order to determine rough relative ages in general the ages fall into three groups those older than 2 0 ma those between 0 25 0 40 ma and those less than approximately 0 1 ma

sediment chemistry and detrital zircon record in the bosque and paseo del mar coastal areas from the southwestern gulf of mexico abstract in this study we analyzed the mineralogy and bulk chemical composition of 20 sediment samples as well as trace rare earth element ree concentrations and u pb ages of 400 detrital zircons retrieved from the bosque pb and paseo del mar pm coastal sediments southwestern gulf of mexico to investigate the provenance the xrd study revealed that the pb sediments are associated with quartz muscovite cordierite albite and anorthite the pm sediments are associated with calcite aragonite and biogenic materials which derived probably within the coastal environment the weathering indices like chemical index of alteration cia chemical index of weathering ciw and plagioclase index of alteration pia revealed a moderate intensity of weathering for the pb sediments the chondrite normalized ree patterns of the pb and pm sediments are with negative and positive europium anomalies eu eu suggesting that the sediments were derived by the combination of felsic and mafic source rocks the th u ratios 0 3 of the pb and pm zircons indicated an igneous origin the chondrite normalized ree patterns of the pb and pm zircons are depleted in low ree lree and enriched in heavy ree hree with positive cerium and negative europium anomalies a wide variation in chondrite normalized ree patterns of zircons with respect to ages was observed the pb is dominated by proterozoic 551 2500 ma and paleozoic zircons 445 480 ma ordovician whereas pm is dominated by cenozoic zircons 5 21 ma miocene indicating that the zircons were derived from different source terranes the chiapas massif cmc and chuacus complexes are identified as the potential source areas which contributed proterozoic zircons to pb on the other hand cuicateco terrane cui and tuxtla volcanic field tvf are considered as the potential source areas which supplied significant number of cenozoic zircons to the pm coastal area the zircon u pb ages and the bulk sediment geochemistry together indicate that the source terranes which supplied sediments to the pb and pm coastal areas consist of mafic and felsic rocks the results of this study further illustrated the importance of littoral currents in distributing sediments although rivers are considered as a carrier and agent of distributing sediments along the coastal areas

the 1793 eruption of san martin tuxtla volcano veracruz mexico abstract san martin tuxtla n18 562 w95 199 1659 masl is a basaltic volcano located in southern veracruz a mexican state bordering the gulf of mexico it rises in a volcanic field strewn with monogenetic volcanic cones maars and three other large volcanoes mostly dormant since the late pliocene santa marta san martin pajapan and cerro el vigia the latest eruptive event of san martin occurred in 1793 and was described by don jose mariano mozino a naturalist under the commission of the viceroy of the then new spain in this work we present results of the study of this eruption based on historical accounts and field observations we identified an ash deposit around the volcano related to the 1793 eruption mapped its distribution and determined its granulometric petrographic and geochemical characteristics these studies suggest that the volcano began its activity with explosive phreatomagmatic explosions which were followed by strombolian activity this period lasting from march to october 1793 the activity continued with an effusive phase that lasted probably 2 years the eruption covered an area of about 480 km2 with at least 1 cm of ash the fines reaching distances greater than 300 km from the crater a total mass of about 2 5 1014 g was ejected and the volcanic columns probably reached altitudes of the order of 10 km during the most explosive phases the lava emitted formed a coulee that descended the northern flank of the volcano and has an approximate volume of 2 0 107 m3

constraints on the origin of alkaline and calc alkaline magmas from the tuxtla volcanic field veracruz mexico lavas erupted in the tuxtla volcanic field tvf over the last 7 ma include primitive basanites and alkali basalts mildly alkaline hy normative mugearites and benmoreites and calc alkaline basalts and basaltic andesites the primitive lavas are silica undersaturated with high concentrations of both incompatible and compatible trace elements variable la yb with constant yb at 6 to 8 times chondritic and low sr and o and variable pb and nd isotopic ratios the primitive magmas originated by increasing degrees of melting with pressure decreasing from greater than 30 kbar to 20 kbar in the garnet stability field another group of alkali basalts and hawaiites has lower ni and cr concentrations and higher fe mg ratios and was derived from the primitive group by crystal fractionation at pressures of several kbar incompatible trace elements in these silica undersaturated lavas show depletion in high field strength elements hfse relative to large ion lithophile elements similar to subduction related basalts ba nb ratios are nearly constant and thus the hfse depletion cannot be the result of a residual hfse bearing phase in the source but could be the result of generation from a source contaminated by fluids or melts from the subducted lithosphere the silica saturated mugearites and benmoreites and the calc alkaline basalts and basaltic andesites were erupted only between 3 3 and 1 0 ma these have incompatible element concentrations generally lower than in the silica undersaturated lavas and thus could not have been derived by crystal fractionation from the silica undersaturated alkaline magmas magmas parental to the silica saturated magmas originated by higher degrees of melting at lower pressures than the primitive magmas melting may have been promoted by an influx of fluid from the subducted lithosphere trace element and sr nd pb and o isotopic data suggest that three components are involved in the generation of tvf magmas the mantle a fluid from the subducted lithosphere and continental crust tvf alkaline lavas are similar to those erupted in the back arc region of the mvb and japan and show characteristics similar to alkaline magmas erupted in the southern andean volcanic arc these low degree melts reach the surface along with calc alkaline lavas in the tvf due to an extensional stress field that allows their passage to the surface

primitive calc alkaline and alkaline rock types from the western mexican volcanic belt since the early pliocene small volumes of alkaline magmas have erupted in the western mexican volcanic belt mvb in close association with the volumetrically dominant calc alkaline magmas both suites include relatively rare primitive types characterized by high mg cr and ni contents primitive hypersthene normative basalts parental to the calc alkaline suite are found along the volcanic front but are absent at greater distances from the trench the volcanic front calc alkaline suites are typically hornblende bearing indicating relatively high water contents associated primitive alkaline magmas at volcanic front locations also contain hydrous minerals these nepheline normative suites include basanites phlogopite bearing minettes and other hornblende bearing lamprophyres these are probably the youngest and freshest lamprophyres yet discovered on earth and ideal samples for addressing the origin of this exotic class of rocks on extended morb normalized elemental plots all lamprophyres show patterns similar to the calc alkaline basalts but have overall enrichments of 2 to 25x in ti k p ba sr light rare earth elements and other incompatible elements the lamprophyres show the same strong relative enrichments of ba k and sr and depletions in ti and nb that characterize the calc alkaline rocks of western mexico and all subduction zones these similarities in relative element abundances support a common source region for all primitive magmas of the western mvb the primitive calc alkaline and alkaline rocks of western mexico show narrow overlapping ranges in sr nd and pb isotopic ratios also consistent with derivation of all magmas from a common source the presence of the relatively cold subducted cocos and rivera plates below the volcanic front demands that all primitive magmas originated in the depth range 30 to 75 km these magmas are enriched in ba k sr and other elements probably transported by hydrous fluids rising from the subducted slab several lines of evidence support the presence of phlogopite in the source region of western mvb magmas probably in the form of phlogopite rich veins cutting asthenospheric peridotite the first liquids formed upon partial melting of this veined source region will concentrate the vein component and generate the lamprophyres these lamprophyric melts rarely erupt in arc settings more commonly forming dikes in the arc crust eruption of lamprophyric magmas in the western mvb is favored by through crustal extensional fracture systems related to active rifting of the jalisco block from the n american plate larger volumes of calc alkaline basalt are generated by melting of the same source which dilutes the vein component with larger proportions of peridotitic wall rock

geology geochronology and tectonic setting of late cenozoic volcanism along the southwestern gulf of mexico the eastern alkaline province revisited a nnw trending belt of alkaline mafic volcanic fields parallels the gulf of mexico from the u s border southward to veracruz state in eastern mexico previous studies grouped this volcanism into the so called beastern alkaline provinceq eap and suggested that it resulted from gulf parallel extensional faulting migrating from north to south from oligocene to present on the basis of new geologic studies forty nine unspiked k ar and two 40 ar 39 ar ages we propose a new geodynamic model for the volcanism along the southwestern gulf of mexico we studied in detail four of the six recognized fields of mafic alkaline volcanism in veracruz state 1 the lavas flows of tlanchinol area 7 3 5 7 ma 2 the alamo monogenetic field and sierra de tantima 7 6 6 6 ma 3 the poza rica and metlatoyuca lava flows 1 6 1 3 ma and 4 the chiconquiaco palma sola area 6 9 3 2 ma other two mafic volcanic fields may represent the continuation of alkaline volcanism to the southeast the middle miocene lavas at anegada high offshore port of veracruz and the middle to late miocene volcanism at the los tuxtlas the existence of major neogene extensional faults parallel to the gulf of mexico i e n s to nnw sse proposed in previous works was not confirmed by our geological studies elongation of volcanic necks vent alignment and faults mapped by subsurface data trend dominantly ne to ene and nw to nnw these directions are parallel to transform and normal faults that formed during the late jurassic opening of the gulf of mexico ascent of mafic magmas was likely facilitated and controlled by the existence of these pre existing basement structures

slab detachment control on mafic volcanic pulse and mantle heterogeneity in central mexico seismic tomography studies and plate reconstructions suggest that the farallon slab broke off shortly before subduction ended off southern baja california however the progress of detachment in time and space and its consequences on the volcanism of central mexico have not so far been considered here i use the neogene geologic record of central mexico to propose a lateral propagation of slab detachment beneath the trans mexican volcanic belt during the late miocene i suggest that the trace of the detachment is expressed by a short 2 3 m y eastward migrating pulse of mafic volcanism that took place from ca 11 5 to ca 6 ma to the north of the pliocene quaternary volcanic arc as hot subslab material flowing into the slab gap produced a transitory thermal anomaly in the mantle wedge slab detachment of the deeper and denser part of the plate was initiated in the southern gulf of california area by the incoming of progressively younger oceanic lithosphere at the paleotrench that produced an increasing coupling between the magdalena microplate and the overriding north american plate the tear in the slab propagated eastward from the gulf of california to the gulf of mexico paralleling the southern mexico trench system the decrease in the rivera north america convergence rate between ca 9 and 7 ma appears to be related to the loss of slab pull after the detachment sparse oceanic island type basalts emplaced since the end of the miocene in the trans mexican volcanic belt are located above a trench parallel slab window between the inferred detachment trace and the leading edge of the present slab which has been detected seismically in this context the occurrence of these unusual intraplate magmas is easily explained by the infiltration of enriched asthenosphere into the subarc mantle

water chemistry of lakes related to active and inactive mexican volcanoes abstract water chemistry of crater lakes maars and water reservoirs linked to some mexican volcanoes within and outside the mexican volcanic belt has been determined for several years and examined regarding environmental and volcanic factors all the analyzed lakes are relatively small with a maximum depth of 65 m and are located in regions with different climates from semi arid to very humid with altitudes ranging from 100 to more than 4000 m a s l crater lakes in active volcanoes el chichon popocatepetl have very low ph moderate to high temperatures and major ion concentrations varying with the level of volcanic unrest lakes in sub arid and temperate arid regions like maars in puebla and guanajuato states show high alkalinity and ph with bicarbonate carbonate chloride sodium and magnesium as predominant ions lakes located in humid climates central michoacan and veracruz state have low mineralization and near neutral ph values in general conservative dissolved ions and conductivity appear to be mostly controlled by precipitation evaporation and by the ionic concentration of groundwater inputs calcium magnesium sulfate concentrations and ph are strongly influenced by volcanic rock or volcanic gas interactions with water the influence of low level volcanic activity on crater lakes may be obscured by water rock interactions and climatic factors one of the aims of this paper is to define the relative influence of these factors searching for a reference frame to recognize the early volcanic precursors in volcano related lakes