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New thermobarometric and U/Pb and 40Ar/ 39 Ar geochronologic data coupled with ages obtained from the Acatlán Complex, the basement of the Mixteco terrane of southern Mexico, reveal the existence of three distinctive high-pressure metamorphic events of early to middle Paleozoic age, each recorded in a separate lithological suite. Xayacatlán suite eclogites with oceanic affi nity underwent peak metamorphism at 609-491 °C and 13-12 kb during the Early Ordovician (ca. 490-477 Ma, U-Pb zircon), followed by a partial overprint at 600 °C and ~9.6 kb and then at 500 °C and ~6.7 kb. An overprinting event at 525-500 °C and ~9.5 kb is ascribed to the Devonian. The pressure-temperature (P-T) path of the Xayacatlán suite indicates a subduction-exhumation process followed by tectonically related reburial. Ixcamilpa suite blueschists with oceanic affi nity underwent epidote-blueschist metamorphism (T, 200-390 °C; P, 6-9 kb) and then epidoteamphibolite (T, 390-580 °C; P, 9-6 kb) events ascribed to the Late Ordovician-Early Silurian. Esperanza suite eclogites with continental affi nity underwent peak metamorphism at 830-730 °C and 17-15 kb. Amphibole from eclogite yields a 430 ± 5 Ma 40 Ar/ 39 Ar age, dating the high-pressure (HP) event. P-T paths of high-temperature (HT) eclogites like those of the Esperanza suite have been related to the collision of continental blocks. Partial overprinting occurred at 690-640 °C and 14-10 kb prior to 374 ± 2 Ma ( 40 Ar/ 39 Ar, phengite). The three HP suites were tectonically juxtaposed at different times before the Mississippian Period, resulting in the closure of the Iapetus Ocean. Phengite 40 Ar/ 39 Ar geochronology reveals the existence of a widespread tectonothermal event between 345 and 323 Ma, which may be related to the juxtaposition of the HP-composed block and the Gondwanan-affi nity Cosoltepec suite, causing the closure of the Rheic Ocean. The tectonothermal events in the Acatlán Complex coincide in time, physical conditions, and tectonic setting with events in the Appalachian-Caledonian orogen, suggesting their relation. On that basis the geology of the Acatlán Complex can lead to a more comprehensive understanding of the tectonic evolution of the Appalachian orogen and of the Gondwana-Laurentia interactions preceding the Pangean assembly.

Past mining activities in northern Mexico left a legacy of delerict landscapes devoid of vegetation and seasonal formation of salt efflorescence. Metal content was measured in mine tailings, efflorescent salts, soils, road dust and residential soils to investigate contamination. Climatic effects such as heavy wind and rainfall events can have great impact on the dispersion of metals in semi-arid areas, since soils are typically sparsely vegetated. Geochemical analysis of this site revealed that even though total metal content in mine tailings was relatively low (e.g. Cu = 1000 mg kg-1), metals including Mn, Ba, Zn, and Cu were all found at significantly higher levels in efflorescence salts formed by evaporation on the tailings impoundment surface following the rainy season (e.g. Cu=68000 mg kg-1). Such efflorescent fine-grained salts are susceptible to wind erosion resulting in increased metal spread to nearby residential soils. Our results highlight the importance of seasonally dependent salt-formation and wind erosion in determining risk levels associated with potential inhalation or ingestion of airborne particulates originating from contaminated sites such as tailings impoundments. In low metal-content mine tailings located in arid and semi-arid environments, efflorescence salts could represent a human health risk and a challenge for plant establishment in mine tailings.

People spend increasing amounts of time at home, yet the indoor home environment remains understudied in terms of potential exposure to toxic trace metals. We evaluated trace metal (and metalloid) concentrations (As, Cu, Cr, Mn, Ni, Pb, and Zn) and health risks in indoor dust from homes from 35 countries, along with a suite of potentially contributory residential characteristics. The objective was to determine trace metal source inputs and home environment conditions associated with increasing exposure risk across a range of international communities. For all countries, enrichments compared to global crustal values were Zn > Pb > Cu > As > Cr > Ni; with the greatest health risk from Cr, followed by As > Pb > Mn > Cu > Ni > Zn. Three main indoor dust sources were identified, with a Pb−Zn−As factor related to legacy Pb sources, a Zn−Cu factor reflecting building materials, and a Mn factor indicative of natural soil sources. Increasing home age was associated with greater Pb and As concentrations (5.0 and 0.48 mg/kg per year of home age, respectively), as were peeling paint and garden access. Therefore, these factors form important considerations for the development of evidence-based management strategies to reduce potential risks posed by indoor house dust. Recent findings indicate neurocognitive effects from low concentrations of metal exposures; hence, an understanding of the home exposome is vital.