# What, if any, paleoclimate data can be derived from igneous rocks?

Paleoclimate data often derives from sedimentary rocks. Metamorphic rocks can also contribute to paleoclimate information in a wide variety of ways.

What about igneous rocks? I guess that this can question be divided into two, about intrusive and extrusive rocks.

Extrusive rocks will have direct contact with the climate. They may exchange geochemical signals with the atmosphere or water they've erupted to. The cooling rate may be dependent on ambient temperature. Lava flows may fossilise the soil.

Intrusive rocks may be harder to interpret in that manner, if at all. The time resolution will obviously not be as good as dating individual beds from the lake floor, but I still think something can be gained from it. Maybe some igneous rocks can form only after a special sedimentary rock (derived from a specific climate) has been buried and remelted? What about isotopes? Sedimentay systems often fractionate various isotopic systems. Can this variability be later traced in igneous rocks?

• Possibly a dumb question - but would ash fall deposits (e.g. tuff etc) be alright to include? – user889 Nov 28 '14 at 11:57
• @SabreTooth not dumb. Although I've read somewhere that these rocks are "igneous going up, sedimentary doing down", they still represent some kind of interaction occurring between a magmatic system (especially when they're still hot in the "up" stage) and the atmospheric system. – Gimelist Nov 28 '14 at 13:07

Stable isotope signatures in igneous rock (primarily $\delta$18O) can be affected by climate, but the effect is complicated and unreliable enough that I doubt it can really be used to say much about palaeoclimate. You might be interested in my friend Lara Owens' dissertation in which she considers the influence of climate on $\delta$18O in lava from Mt Erebus, Antarctica. Oxygen isotopes in Erebus precipitation are some of the lightest in the world, and she suggests that this water, after incorporation into a hydrothermal system whose rocks were later assimilated into magma, caused the anomalous $\delta$18O in some of the samples.

It would be interesting to see if there is a global geographic trend in similar anomalies, but I haven't heard of anyone trying that.

A more direct way that igneous rocks can record paleoclimate is through the effects that the atmosphere can have on pyroclastic deposits. Isopleth maps often can reveal the direction the wind was blowing during the eruption. Clast morphologies might even tell you the weather -- I'm thinking of accretionary lapilli, which form when an ash-rich eruption plume encounters a rain storm.

In conclusion, you might be able to glean a few data points here and there, but nothing comparable to the amazing records available in sedimentary rocks.

Edit to add: I just went to a lecture by Jay Quade about figuring out the timing of uplift in the Andes, and whether it came before or after the Atacama became hyperarid. Among many other geochemical tools, he used $\delta$2H in hydration waters on tephra, primarily to figure out palaeoaltitudes. He said he thinks the relationship between $\delta$2H and altitude has been fairly constant over the period of investigation (back to 50 mya).

Edit to add: I had forgotten to point out that subglacial eruption deposits are often distinctive (tuyas, hyaloclastites, pillow basalts). Wherever these deposits are found, you know a glacier or ice sheet was present and if you can date the rock then you know when there was a glacier there. Some recent work has suggested that you can even figure out the thickness of the glacier using the concentration of dissolved gasses in melt inclusions in the subglacially erupted lava flow.

Potassium–argon (K-Ar) dating specifically and only works with igneous rocks. The idea being that when molten magma solidifies into, say, granite or basalt, any potassium ends up locked into crystaline forms. Some of that potassium will be the unstable K-40 isotope, decaying into A-40 with a half-life of 1.26 billion years. Since virtually no A-40 would have been present at time of crystalisation, the K-40:A-40 ratio gives a good indication of when solidification occurred.

There are of course other "radioactive clocks" based on other elements/isotopes with longer or short half-lives, which may be more suitable depending on how far back you're trying to look. And although we're not likely to find much in the way of organic material (fossils, etc.) in igneous rocks, information from different sources can usefully be collated.

I don't know of any specific case, but I'd have thought there would be many situations where sedimentary rock has been laid down over precisely-dated igneous rocks. And I don't know the exact limitations of dendrochronology, but it seems at least possible that you could detect identifiable "pattern sequences" in ancient annual tree-trunk ring growth patterns that suddenly become far more meaningful if you know when those patterns were formed.

Some seasonal growth variation will be down to local factors, and some will reflect "global" events. But given enough data, they could be distinguished. It should be possible to collate information from, say, K-Ar dating and dendrochronology to derive more information that is directly available from either source. And the whole is greater than the sum of the parts, as they say.

• While I see your point here that we can use dating of volcanic beds to constrain sedimentary rocks in time, this is not what I was asking. I was asking about directly deriving paleoclimate data from igneous rocks. – Gimelist Nov 28 '14 at 17:33
• @Michael: oic. Well, I only joined earthscience yesterday, and it just so happened when I saw your question on my second visit today, it was moments after reading a passage in Richard Dawkins The Greatest Show on Earth where he specifically makes the point that such crystalisation in igneous rocks is invaluable for cross-referencing across broader timeframes than can be accessed using direct C-14 dating of organic material. Your the time resolution [from igneous rocks] will obviously not be as good as dating individual [sedimentary] beds from the lake floor doesn't match what he says. – FumbleFingers Nov 28 '14 at 18:11
• Welcome to the site! Notice that I was referring to intrusive rocks, for example granites. While dating volcanic beds can be used to constrain the timing of sedimentation (given that said beds are interbedded), this is no possible in the case of plutonic rocks. – Gimelist Nov 28 '14 at 18:37
• @Michael: Thank you. The honest truth is I wouldn't have dreamt of posting an answer ordinarily (this stuff is way past my pay grade! :) But despite having no God to believe in or ascribe it to, I was lulled into supposing there must be some connection between the stuff I was reading by Dawkins and the question I saw immediately afterwards here. My god-that-I-don't-believe-in compelled me to assume it was my duty to share what little knowledge I had. – FumbleFingers Nov 28 '14 at 19:47
• ... and this is exactly what I am asking. However, your answer is not related to my question. – Gimelist Nov 28 '14 at 21:02

Atmospheric pressure affects the bubbles that form in lava, and this effect can be used to estimate paleopressure at the time of emplacement of a lava flow. You might need some special conditions, but it is possible to make such a study. The technique is described in Sahagian et al. (2002).

Sahagian, D. L., Proussevitch, A. A., & Carlson, W. D. (2002). Analysis of vesicular basalts and lava emplacement processes for application as a paleobarometer/paleoaltimeter. The Journal of Geology, 110(6), 671-685. [JSTOR link, free PDF link]