A congressional subcommittee voted unanimously yesterday to subpoena Mobil Corp.'s records relating to its attempted acquisitions of Conoco and Marathon Oil as the first step in an inquiry into concentration in the energy industry.

In a related development, Senate and House Commerce committees also began making plans for joint hearings Thanksgiving week on Mobil's takeover attempts. Defeated in its attempt to acquire Conoco, Mobil announced Oct. 30 that it planned to spend $5.1 billion to take over Marathon, the nation's 16th-largest oil company. Mobil is the nation's second-largest oil company.

The announcement of the proposed takeover bid provoked an almost immediate fight from Marathon. The Ohio-based oil company sought and received a court order temporarily barring the takeover and began rallying support both in the Congress and in Findlay, Ohio, where it is the major employer.

Reacting to some of the criticism of Mobil's takeover attempt, the company noted yesterday that it has been mindful of concerns about the impact a Mobil acquisition might have on Findlay, its residents and Marathon employes and executives.

Because of those concerns, Rawleigh Warner Jr., chairman of Mobil, called Marathon Chairman Harold Hoopman on Oct. 30, simultaneously with the announcement of the takeover bid, and offered to meet with him to explore the potential benefits that might come from the merger, the company said in a prepared statement.

That offer has never been accepted, Mobil noted yesterday.

Yesterday, several members of Congress wrote the Federal Trade Commission, which is studying the merger, protesting a reported attempt by Mobil to win agreement to go forward with the acquisition under a "hold separate" proposal. Such agreements basically allow the takeover to proceed but require that the acquired firm be kept intact and separate in case divestiture is ordered later.

"It is bad enough that Mobil should even consider such an acquisition seriously, for the many competitive and other reasons set out in our earlier letters. It is even more distressing that the commission should consider permitting it to go through," four members of the Ohio delegation wrote.

Noting that the Congress had restricted some FTC activities last year, the letter warned that "if the commission allows this merger, Congress may well again be forced to act." The letter was signed by Sens. Howard Metzenbaum and John Glenn and Reps. Clarence Brown and Michael Oxley.