PT Unknown
AU Gilbert, DJ
TI Pre-eruptive conditions at Lonquimay and Puyehue-Cordón Caulle Volcanoes, Chile: Framework for tectonic influences
PY 2012
PU Christian-Albrechts-Universität zu Kiel
WP https://macau.uni-kiel.de/receive/diss_mods_00009432
LA en
DE Geologie; Vulkanologie; Magmatische Volatile; Geology; volcanology; magmatic volatiles
AB Vivid tectonic conditions - typical for subduction zones - affect the associated volcanism in many ways. Upper plate tectonic faults control not only the location of volcanoes but magnitude and orientation of tectonic stress along fault systems particularly control magma plumbing dynamics, i.e. the ascent, storage and eruption of magmas. For instance, bimodal volcanism in the central Andes is closely linked to the orientation and kinematics of regional faults and time-predictability may be a fundamental property of volcanism controlled by regional tectonic strain. While some eruption records show fairly uniform behavior through time, erratic evolution at other volcanoes may be controlled by either inherent change in the magmatic system or external forcing, or by complex coupling of both. Tectonic influence on volcanism on longer time scales is commonly implied from observed spatial correlations between tectonic and volcanic structures but may also be inferred from the pattern of temporal evolution of a volcano. On much shorter time scales, it is commonly believed that regional earthquakes are capable of triggering volcanic eruptions even though the particular processes involved are still under debate.
The aim of this PhD thesis is to unravel a possible linkage between magmatic processes, regional tectonic stress and volatile inventories of two exemplary magmatic systems in the Southern Volcanic Zone (SVZ) of the Andes: Lonquimay (LVC; 38°23’S, 71°36’W) and Puyehue-Cordón Caulle (PCCVC; 40°35’S, 72°7’W) volcanic complexes. While the pre-eruptive inherent conditions of magmatic systems are in principle "visible" in the eruption products, the possible influence of tectonic stresses is commonly inferred by indirect evidence. Therefore, my work has focused on the control of post-glacial volcanism by petrogenesis (e.g. fractional crystallization, magma mixing) and stress regime; and in particular how the resulting volatile budgets ruled the eruptive activity by controlling the criticality of the investigated volcanic systems, i.e. their susceptibility to external forcing.
The Lonquimay Volcanic Complex (LVC) in the high Southern Andes comprises a stratocone and two NE-trending flank-cone alignments that are associated with the arc-wide Liquiñe-Ofqui Fault Zone. Numerous effusive and explosive volcanic eruptions characterize its post-glacial magmatic activity. LVC’s tephrostratigraphy that comprises 22 dated pyroclastic deposits, was studied in order to investigate eruptive behavior through time. Statistical examination of the eruption time series yields two distinct repose time regimes. This change in recurrence times around 6000 b2k (years before 2000 AD) is also reflected in a change of magmatic products tapped during eruptions. In general, the LVC tephra deposits can be subdivided into three petrographic groups: a felsic group (Lonquimay Colored Pumice Tephra, LCPT), an intermediate population (Lonquimay Grey Pumice Tephra, LGPT) and a mafic member (Lonquimay Dark Scoria Tephra, LDST). LDST deposits as well as deposits compositionally zoned from LCPT to LGPT dominate the lower part of the stratigraphy for which recurrence times are short (RTmean=417±169 a). Deposits younger than 6000 b2k have dominantly LCPT and minor LDST compositions, no longer contain LGPT, and repose times are significantly longer (RTmean=1350±310 a). This change in eruption regime is interpreted to result from a tectonically driven rearrangement in the magma storage and plumbing system.
To better constrain the plumbing system dynamics, thermobarometric calculations were performed using several complementary mineral liquid equilibria. The obtained P-T-X results reveal distinct magma storage levels for each petrographic group: the mafic LDST derive from mid crustal storage (Pmean≈440±100 MPa, Tmean≈1065±25°C), felsic LCPT mainly erupted from upper-crustal level (Pmean≈80±25 MPa, Tmean=935±20°C) whereas LGPT samples yield intermediate storage depths (Pmean=240±110 MPa, Tmean=1010±15°C). Magma contributions from this intermediate reservoir are restricted to >6000 b2k when the Lonquimay plumbing system was in a regime of short repose times; disappearance of the intermediate reservoir coincides with the change to longer repose times between eruptions. Despite the distinct storage levels, LVC’s magmatic products record a complex history of component magmas interacting within the plumbing system, thereby affecting the involved magmas and their volatile inventories and hence LVC’s eruptive behavior.
Two independent levels of volatile saturation are documented in LVC magmatic products. A first deep level (~14 km) of volatile-saturation was reached by LDST melts during ascent, leading to the formation of a S-rich fluid phase and therefore strong depletion of S in the melt. A second level of H2O-saturation was reached between 1.5 and 6 km in the shallow reservoir. The stagnation of LCPT melts at this level led to their H2O-oversaturation and hence exsolution of an aqueous fluid phase. The frequent injections of primitive melt batches into the LDST reservoir, as documented in the pheno- and xenocryst assemblages, may have triggered the mafic LDST eruptions. The occurrence of compositionally zoned, amphibole-bearing deposits that is limited to LCPT deposits older 6000 b2k indicates the occurrence of compositionally zoned magma bodies and hence stronger interaction between the three distinct reservoirs prior to this threshold. In contrast, the matrix glass and crystal records of most felsic eruptions younger than 6000 b2k do not show such a strong influence of mafic replenishment. I therefore propose that these eruptions may have been triggered by H2O-saturation, giving a possible explanation for the longer repose times between these eruptions.
While statistically significant correlations between major regional earthquakes and eruptions within days or even months have been reported by several studies, the 1960 eruption of the Puyehue-Cordón Caulle Volcanic Complex (PCCVC) is one of the most renowned examples thought to be susceptible to seismic triggering, because it occurred only 37h after the largest ever instrumentally recorded earthquake (MS=9.5). Its recent eruption that started on June 4th 2011 raised the question, if it was possibly related to the preceding MW 8.8 Maule earthquake of 2010. To determine the pre-eruptive criticality, thermobarometric and volatile studies were performed for the remarkably homogeneous rhyodacitic 2011, 1960 and four older post-glacial eruptions. The use of several independent methods allows us to give robust estimates of pre-eruptive P-T-X-conditions and how the volatile saturation state evolved over time. Several independent estimates for crystallization temperatures and pressures as well as pre-eruptive H2O-contents yield shallow magma storage (P=50-250 MPa, T=885-915°C) for all PCCVC eruptions studied. Pre-eruptive H2O contents of 4-5 wt% and the occurrence of water-dominated magmatic fluid inclusions indicate the exsolution of an aqueous fluid phase (salinitymean=3.2±0.1 wt% NaCl equivalent) at pressures of ~135 MPa for the 2011 eruption. Such pre-eruptive H2O-saturation in the upper part of the plumbing system appears to be typical for explosive eruptions at this volcanic system. I propose that due to volatile-supersaturation the system was due to erupt when it was eventually triggered by seismic activity. PCCVC’s position astride a NW fissure zone of the arc-wide Liquiñe Ofqui Fault Zone system (along which seismic energy/strain may be transmitted particularly effectively) as well as its ability to repeatedly reach critical magmatic conditions probably are the two factors that facilitate tectonic triggering of eruptions at this particular volcanic system.
Although presenting case studies based on conditions valid for LVC and PCCVC, the general implication from this thesis may be applicable to other volcanic systems with similar prerequisites. Particularly in evolved subduction-related systems shallow storage and hence volatile saturation are common features, as are arc-wide fault systems in the upper plate due to oblique convergence in other subduction zones. I therefore infer that tectonic forcing of volcanic systems may be more common than usually assumed and is probably not only confined to dictating the structural features but also affects eruptive behavior. But more studies of the petrogenetic processes determining the saturation state and finally the criticality of volcanic systems that are discussed to be tectonically influenced are needed to better constrain the actual effects of internal and external forcing.
PI Kiel
ER