Sources of Sr and S In Aluminum-Phosphate-Sulfate Minerals In Early-Middle Triassic Sandstones (Iberian Ranges, Spain) and Paleoenvironmental Implications for the West Tethys

Abstract

Aluminum-phosphate-sulfate (APS) minerals, formed during early diagenesis in relation to acid meteoric waters, are the main host of Sr and S in the Early-Middle Triassic continental sandstones of the Iberian Ranges (east of the Iberian Peninsula). The sources of these elements and the effects of paleoenvironmetal changes on these sources and on the formation of APS minerals during Early-Middle Triassic times, were established on the basis of Sr and S isotopic analyses.The S and Sr data (d 34 S V-CDT = +11 to +14% and 87 Sr/ 86 Sr = 0.7099-0.7247, respectively) can be interpreted as resulting from mixing of different sources. Strontium was sourced from the dissolution of pre-existing minerals like K-feldspar and clay minerals inherited from the source areas, causing high radiogenic values. However, the isotopic signal must also be influenced by other sources, such as marine or volcanic aerosol that decreased the total 87 Sr/ 86 Sr ratios. Marine and volcanic aerosols were also sources of sulfur, but the d 34 S was lowered by dissolution of pre-existing sulfides, mainly pyrite. Pyrite dissolution and volcanic aerosols would also trigger the acid conditions required for the precipitation of APS minerals.APS minerals in the study area are found mainly in the Cañizar Formation (Olenekian?-Aegian), which has the lowest 87 Sr/ 86 Sr ratios. The lower abundance of APS minerals in the Eslida Formation (Aegian-Pelsonian) may indicate change in the acidity of pore water towards more alkaline conditions, while the increased 87 Sr/ 86 Sr ratios imply decreased Sr input from volcanic activity and/or marine aerosol inputs during Anisian times. Therefore, the decrease in abundance of APS minerals from the Early to Middle Triassic and the variations in the sources of Sr and S are indicative of changes in paleoenvironmental conditions during the beginning of the Triassic Period.These changes from acid to more alkaline conditions are also coincident with the first appearance of carbonate paleosols, trace fossils, and plant fossils in the upper part of the Cañizar Formation (and more in the overlying Eslida Formation) and mark the beginning of biotic recovery in this area. The presence of APS minerals in other European basins of the Western Tethys (such as the German Basin, the Paris Basin and the southeastern France and Sardinia basins) could thus also indicate that unfavorable environmental conditions caused delay in biotic recovery in those areas. In general, the presence of APS minerals may be used as an indicator of arid, acidic conditions unfavorable to biotic colonization.

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“…Nonetheless, it is important to notice that the strontium isotopes have a strong radiogenic influence from the source areas that also mark a change in provenance between Lower and Upper Units (Galán-Abellán, 2011; Galán-Abellán et al, 2013c), which is in concordance with other mineralogical and isotopic data such as Pb-Pb isotopes in detrital zircons, that point to a possible change in the source areas and provenance of the lower and upper parts of the Lower Unit, probably related to the tectonic pulses as consequence of Pangea Break up (Sánchez-Martínez et al, 2012). These changes could have triggered shifts in the water circulation and in its oxygenation and a decrease in the acidity conditions, in agreement with the APS minerals decrease and the reappearance of life sings (Galán-Abellán et al, 2013c;Borruel-Abadía et al, 2016).…”

“…Acid conditions also favoured phosphorous dissolution of preexisting phosphate minerals. The Lower Triassic Unit is richer in phosphorus than the Upper Unit, which has been attributed to the occurrence of secondary APS minerals formed by dissolution and reprecipitation of detrital phosphate (mainly apatite, monazite and lesser proportions of xenotime), preventing the P leaching during the acid rock alteration (Galán-Abellán et al, 2013c;Borruel-Abadía et al, 2016). In Meridiani Planum, the P source of secondary phosphates is likely weathering of the Ca-phosphates of igneous rocks Greenwood and Blake, 2006).…”

“…The disappearance of APS minerals during the Middle Triassic throughout the Upper Unit indicates higher pH conditions and therefore low phosphorous mobilization, coinciding with a reduction in arid conditions and reappearance of fossil plant remains and tetrapod trace marks, among other signs of life (Galán-Abellán et al, 2013c;Borruel-Abadía et al, 2015). These environmental changes are associated with a major tectonic pulse affecting the European plate and recorded by a regional unconformity in several basins of the western Tethys, likely reflecting the opening of new faunal corridors and better water oxygenation, marking the beginning of life recovery during the Middle Triassic in Iberia (Bourquin et al, 2009;.…”

“…Sedimentological, mineralogical and geochemical data all point to arid and acid environmental conditions during the Early Triassic progressing to more seasonal and humid conditions along with an increasing pH of meteoric waters during the Middle Triassic (López- Gómez et al, 2012;Galán-Abellán et al, 2013c;Borruel-Abadía et al, 2015). The appearance of the first fossil record in this area is consistent with this palaeoclimate change (Fig.…”

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“…Similar processes have been invoked to explain the genesis of APS concentrations in recent paleosurfaces in Sudan (Dill et al, 1991) or in the basement below the Permo-Triassic weathering profile in the Iberian Ranges (Borruel-Abadía et al, 2016). In addition, APS minerals were also formed by diagenetic processes in the Triassic cover (Galán-Abellán et al, 2013). Thus, when the P-rich acidic fluids reached the Silurian-Devonian carbonates they produced Ca-and Fe-rich phosphates, such as apatite, strengite, dufrenite, tintic-ite, and calcioferrite (Costa et al, 1993), and were accompanied by the formation of large amounts of "secondary" Fe oxides and hydroxides.…”