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Deep CO2 in the end-Triassic Central Atlantic Magmatic Province - PubMed

Deep CO2 in the end-Triassic Central Atlantic Magmatic Province

Manfredo Capriolo et al. Nat Commun. .

Abstract

Large Igneous Province eruptions coincide with many major Phanerozoic mass extinctions, suggesting a cause-effect relationship where volcanic degassing triggers global climatic changes. In order to fully understand this relationship, it is necessary to constrain the quantity and type of degassed magmatic volatiles, and to determine the depth of their source and the timing of eruption. Here we present direct evidence of abundant CO2 in basaltic rocks from the end-Triassic Central Atlantic Magmatic Province (CAMP), through investigation of gas exsolution bubbles preserved by melt inclusions. Our results indicate abundance of CO2 and a mantle and/or lower-middle crustal origin for at least part of the degassed carbon. The presence of deep carbon is a key control on the emplacement mode of CAMP magmas, favouring rapid eruption pulses (a few centuries each). Our estimates suggest that the amount of CO2 that each CAMP magmatic pulse injected into the end-Triassic atmosphere is comparable to the amount of anthropogenic emissions projected for the 21st century. Such large volumes of volcanic CO2 likely contributed to end-Triassic global warming and ocean acidification.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Map of CAMP in central Pangea at about 200 Ma.

The black symbols indicate the provenance of the studied samples: triangle for Portugal, circle for Morocco, square for New Jersey, USA, and diamond for Nova Scotia, Canada. The figure is modified after ref. .

Fig. 2
Fig. 2. Representative bubble-bearing melt inclusions at transmitted light optical microscopy.

The black arrows indicate the bubble-bearing melt inclusions. a Single-bubble MI hosted in orthopyroxene (Opx; sample NS21, Nova Scotia, Canada). b Single- and multi-bubble MIs, with very irregular shapes, hosted in augitic clinopyroxene (Cpx; sample AN137A, Morocco). c Multi-bubble MI, partially crystallized (containing also opaque mineral phases), hosted in calcic plagioclase (Pl; sample NS9, Nova Scotia, Canada). d Multi-bubble MI hosted in augitic clinopyroxene (Cpx; sample NS9, Nova Scotia, Canada).

Fig. 3
Fig. 3. Chemical maps of glomerocrystic clinopyroxene aggregates.

Backscattered electrons (BSE) image a and corresponding scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDS) maps b–f of a thin section area including MIs and the hosting glomerocrystic clinopyroxene aggregates. In the BSE image the brighter portions of clinopyroxene have augitic (Aug) composition and the darker ones have pigeonitic (Pgt) composition. In the SEM–EDS maps the brighter regions correspond to higher concentrations of the analysed element. These maps were acquired on sample NEW31 (New Jersey, USA). The scale bar is shown in a. a BSE image, b Al map, c Ca map, d Fe map, e Mg map, and f Ti map.

Fig. 4
Fig. 4. Bubble-bearing melt inclusions at transmitted light optical microscopy and confocal Raman microspectroscopy.

Left column: transmitted light photomicrographs at optical microscope of the analysed areas, bordered by dotted lines. Right column: Raman hyperspectral maps of the corresponding areas. a, c, e Photomicrographs of elemental carbon-bearing single- and multi-bubble MIs (a and c: sample NEW31, New Jersey, USA; e: sample AN39, Morocco). b, d, f Raman hyperspectral maps of the same samples area. g Photomicrograph of an irregular-shaped CO2-bearing multi-bubble MI (sample NS12, Nova Scotia, Canada). h Raman hyperspectral map of the same sample area. The Raman signal of CO2 is weak due to its low density. However, spot analyses confirmed the presence of CO2 in all bubbles.

Fig. 5
Fig. 5. Raman spectra of CO2 and elemental carbon.

a Raman spectrum of CO2, acquired on sample NS9 (Nova Scotia, Canada). The Fermi diad is represented by sharp bands, at 1285 cm−1 and at 1388 cm−1, and the hot bands are represented by symmetrical weak bands, below 1285 cm−1 and above 1388 cm−1. b Raman spectra of elemental carbon: amorphous carbon acquired on sample NEW31 (New Jersey, USA), disordered graphite acquired on sample AN39 (Morocco), and ordered graphite (detail on the G band) acquired on a common pencil. Compared to the ordered graphite Raman spectrum, our Raman spectra of disordered graphite and amorphous carbon always have one or more D peaks between 1200 and 1400 cm−1. The G band lower than 1590 cm−1 indicates disordered graphite, while the G band higher than 1590 cm−1 indicates amorphous carbon (see “Methods” section). The D band is often composed by two peaks (D1 at ca. 1350 cm−1 and D5 at ca. 1270 cm−1) for both disordered graphite and amorphous carbon.

Fig. 6
Fig. 6. Crossplot of the Raman spectra of elemental carbon.

This crossplot displays the Raman spectra of elemental carbon with the peak position of the G band ranging from ca. 1575 cm−1 to ca. 1605 cm−1. The line in correspondence of 1590 cm−1 peak position value separates the disordered graphite data (below) from the amorphous carbon data (above) according to the present study (see “Methods” section). The areas bordered by dashed lines distinguish the graphite field from the kerogens and coals field according to ref. . The error on PP and therefore also on FWHM is considerably smaller than the spectral resolution for the Raman spectra displayed in the crossplot (0.8 cm−1), and thus is much smaller than the plotted symbols.

Fig. 7
Fig. 7. Sketch of the transcrustal plumbing system of CAMP basaltic magmas from the mantle to the surface.

The evolution of basaltic magmas occurs at variable depth by crystallization of minerals, which then form aggregates in crystalline mushes, and entrain bubble-bearing melt, forming MIs. Different volatile species exsolve at variable depth. In particular, CO2-rich fluids (white bubbles) start exsolving at great depth, whilst H2O-rich fluids (blue bubbles) and S-rich fluids (yellow bubbles) start exsolving at shallow depth. The black dashed arrows indicate the potential sources for the carbon in CAMP magma: the mantle, the deep crust and the Palaeozoic or Triassic sedimentary basins in which CAMP sills intruded. The carbon within the here studied MIs derives from the deep sources as demonstrated with clinopyroxene geobarometry data. Clinopyroxene crystallization pressures of this study have been calculated using ref. (Supplementary Note 2). Clinopyroxene crystallization pressures of bibliography are from ref. for Morocco, ref. for Portugal, and ref. for USA. The error (±0.2 GPa) takes into account the uncertainties from both the geobarometry model (±0.1 GPa) and the electron microprobe analyses (±0.1 GPa, deriving from the ±10% accuracy on measured Na concentration).

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